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The ameliorating effects of biochar and compost on soil quality and plant growth on a Ferralsol.

Introduction

Soil degradation processes caused by erosion, organic matter and plant nutrient depletion, and nutrient imbalances are among the major challenges affecting agricultural productivity and food security (Sanchez 2002; Foley et al. 2005; Lai 2009). The productivity of some lands has declined by 50% because of soil erosion and desertification (Eswaran et al. 2001). Annual global soil loss is estimated at 75 Gt, costing the world about US$400 billion per year (Eswaran et al. 2001). The deterioration of soil fertility is exacerbated by nutrient mining, unsuitable land use and management, competing uses of resources, and application of insufficient external inputs. For example, in Australia, crop production has led to a substantial loss of soil organic matter (SOM) from the cereal belt, where the long-term SOM loss often exceeds 60% from the top 0-10 cm of soil after 50 years of cereal cropping (Dalai and Chan 2001). Losses of labile components of SOM, microbial biomass and mineralisable nitrogen (N) have been higher, resulting in greater decline in soil productivity (Dalai and Chan 2001).

In the future, the long-term benefit of allocating more land to agriculture will not offset the negative environmental impacts of land degradation (Tilman et al. 2002). Instead, a more promising approach to ensuring food security is to increase yield from currently cultivated land where productivity is low (Foley et al. 2011). Sustainable agricultural intensification--increasing productivity per unit land area--is thus necessary to secure the food supply for the increasing world population (Tilman et al. 2011). In most tropical environments, sustainable agriculture faces significant constraints due to low nutrient status and rapid mineralisation of SOM (Zech et al. 1997). Decline in SOM content results in decreased cation exchange capacity (CEC). Under such circumstances, the efficiency of applied mineral fertilisers is low (Glaser et al. 2002; Troeh and Thompson 2005). In addition, most small-scale farmers cannot afford to apply mineral fertilisers regularly because of high costs. Therefore, nutrient deficiencies are prevalent in many crop production systems of the tropics and this constrains productivity.

Soils fertilised with compost or manure have higher contents of SOM and soil microorganisms than mineral-fertilised soils, and are more enriched in phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) in the topsoil and N[O.sub.3]-N, Ca and Mg in subsoils (Edmcades 2003; Quilty and Cattle 2011). Well-made composts are known to improve soil structure, resulting in improved air exchange, and water infiltration and retention (Fischer and Glaser 2012). Soils amended with organic fertilisers also have lower bulk density and higher porosity, hydraulic conductivity and aggregate stability than mineral-fertilised soils (Edmeades 2003; Lai 2009). Labile forms of SOM are of prime importance as a reserve and source of plant nutrients in tropical soils poor in minerals that can be weathered (Zech et al. 1997). However, the amount of the labile SOM is smaller and the turnover rate and release of nutrients in humid tropical soils is faster than in temperate soils. Accelerated mineralisation of SOM limits the practical application of organic fertilisers in the tropics (Zech et al. 1997; Kaur et al. 2008).

Biochar is charcoal produced by controlled pyrolysis for use as a soil amendment or in carbon (C) sequestration (Lehmann and Rondon 2006). Various studies have shown that application of biochar to soil can improve soil biophysical and chemical properties and nutrient supply to plants (Glaser et al. 2002; Sohi et al. 2010), enhance plant growth and yield (Lehmann and Rondon 2006; Chan et al. 2007; Major et al. 2010), and reduce greenhouse gas emissions through C sequestration (Van Zwieten et al. 2010; Ippolito et al. 2012; Zhang et al. 2012). Biochar helps to improve agricultural productivity by reducing soil acidity and by enhancing CEC and fertiliser-use efficiency (Lehmann et al. 2003; Steiner et al. 2008; Chan and Xu 2009), water retention capacity (Downie 2011) and plant-available water content (Tammeorg et al. 2014), and by creating a habitat for beneficial soil microorganisms (Thies and Rillig 2009). Biochar can be used to rejuvenate depleted soils, making more agricultural land available and increasing crop yields so that the need for expansion of agricultural land area is decreased (Blackwell et al. 2009; Barrow 2012). Biochar has significantly improved the efficiency of N fertilisers and increased plant growth and yield (Lehmann et al. 2003; Steiner et al. 2008). The long-term benefits of biochar for nutrient availability include greater stabilisation of SOM, slower nutrient release from added organic matter, and better retention of cations due to higher CEC (Lehmann et al. 2003; Steiner et al. 2008). The resultant change in soil nutrient status may affect both plant growth and productivity. Responses to biochar application will depend on the type and rate of biochar applied, as well as soil physico-chemical characteristics.

Some recent studies have indicated that the simultaneous application of biochar and compost could lead to enhanced soil fertility, improved plant growth and C sequestration potential (Fischer and Glaser 2012; Schulz and Glaser 2012). Liu et al. (2012) showed that the combined application of compost and biochar had a synergistic, positive effect on SOM content, nutrient contents and water-holding capacity of soil under field conditions. Information on the combined effects of biochar and compost on soil fertility and crop performance in tropical soils is generally not adequate. Different methods of producing and applying compost and biochar are hypothesised to differ in their effects on soil biophysical and chemical properties, and plant growth and yield. Therefore, the objectives of this study were to determine the effect of compost and biochar applied to an infertile tropical soil on: (z) growth and nutrient uptake of maize, (ii) soil water content and chemical characteristics, and (iii) nutrient retention and leaching.

Materials and methods

Experimental setup

The study was a pot trial designed to determine whether soil fertility and plant productivity could be enhanced by biochar and compost applied singly or together. Willow biochar (WB; Earth Systems Pty Ltd, Melbourne, Vic.) and acacia biochar (AB; Renewable Carbon Resources Australia Pty Ltd, Charleville, Qld) produced at 500[degrees]C were selected for the soil amendments. Both biochar types were characterised by JSM-6300 scanning microscope (JEOL Ltd, Tokyo) before application; WB had more pore spaces than AB. Compost was produced from a mix of bagasse, poultry litter and municipal waste, following the standard windrow procedures for compost preparation, by King Brown Technologies (Mareeba, Qld). Chemical characteristics of the compost, WB and AB used as soil amendments in this study are given in Table 1.

The products were screened through a 4-mm sieve before being mixed with the soil. Soil used in the trial was taken from a sugarcane field (17[degrees]1.23'S, 145[degrees]24.21'E; 0-10cm depth) in North Queensland, Australia. The soil was a Ferralsol (FAO classification, IUSS Working Group WRB 2007) developed on Quaternary basalt. Chemical properties of the experimental soil were determined for samples taken before planting (Table 1). The pot trial was conducted in a greenhouse from 7 June to 9 August 2013 at James Cook University, Cairns Campus, Queensland. During the growing period, the temperature ranged between 17.6[degrees]C and 27.1[degrees]C and the mean relative humidity was 62.5%. The plastic pots used had 16-cm upper and 14-cm lower diameter, height of 16 cm, and total volume of ~2750 [cm.sup.3]. A 2-kg subsample of air-dried soil, screened through a 4-mm sieve, was placed into each pot after mixing thoroughly with the amendments. All pots were then watered to approximately field capacity.

The experiment comprised the following seven combinations of compost, biochar and mineral fertiliser treatments: (1) untreated control; (2) mineral fertiliser only (F) at a rate of 280 mg N as urea, 70 mg P as triple superphosphate and 180 mg K as KC1 [pot.sup.-1], which is equivalent to 140 kg N, 35kg P and 90kg K [ha.sup.-1]; (3) 75% F + 40g compost [pot.sup.-1] (F + Com); (4) 100% F + 20g WB [pot.sup.-1] (F + WB); (5) 75% F+10g WB + 20g compost poC1 (F + WB + Com); (6) 100% F + 20g AB [pot.sup.-1] (F +AB); (7) 75% F+ 10 g AB + 20 g compost poC1 (F + AB + Com). The experiment was arranged in a randomised complete block design with four replications. Compost was applied at rates of 20 and 10t [ha.sup.-1], and biochar at rates of 10 and 5t [ha.sup.-1]. The N content of the compost was considered and fertiliser N rate was decreased by 25% when applied together with compost, assuming that only 10-20% of the total N content of the compost is mineralised in the first year (Fischer and Glaser 2012). Nitrogen was applied in two applications: 156 mg N [pot.sup.-1] at planting and 124 mg N [pot.sup.-1] as topdressing at 4 weeks after planting. Eight maize (Zea mays L.) seeds were planted at a depth of 3 cm in each pot 1 week after the amendments had been mixed with the soil. After emergence, four plants were maintained in each pot until harvesting. Each pot was given 150mL of water daily for 14 days after planting and 200 mL thereafter until the end of the experiment. The bottom of each pot had four holes and was lined with gauze to minimise loss of particulate matter but allow leaching of soil solution. Each pot was equipped with a sealable plastic bag at the bottom for leachate collection.

Measurements

Plant height was recorded just before harvesting. Chlorophyll content (greenness) was measured on days 34, 48 and 52 after planting, with four replicate measurements on three leaves (youngest fully expanded leaves) per plant, using a chlorophyll meter (SPAD-502; Konica Minolta, Tokyo). Leachate was collected every 2 weeks, its volume was measured, and subsamples were kept frozen for analysis. Soil water content (SWC) was measured every week after planting, using a HydroSense II probe (Campbell Scientific Inc., Logan, UT, USA). Specific leaf weight (SLW, expressed as dry weight cnT2) of plants was measured. One leaf from each plant of the same age and position was sampled 1 week before harvesting. Leaves were transferred immediately to individual, sealed plastic bags that were kept in an insulated box above ice packs until all leaves were harvested. In the laboratory, a hole-punch with a diameter of 8 mm was used to take a leaf disc from the middle of the leaf lamella. Fresh leaf discs were weighed and dried at 70[degrees]C for 48 h before reweighing them. SLW was calculated as dry weight of leaf disc per area of hole-punch.

The aboveground parts of four plants were harvested at soil level from each pot 62 days after planting. The root part of each plant from each pot was also separated judiciously from the soil, cleaned and weighed. Fresh shoots and roots were dried separately at 70[degrees]C for 72 h, and the shoot: root ratio (SRR) was determined. The shoot part of plants was used to determine content and uptake of nutrients by plants. Total plant N and C concentrations were determined using an elemental analyser (ECS 4010 CHNSO Analyzer) fitted with a zero blank auto-sampler (Costech Analytical Technologies Inc., Valencia, CA, USA). Total P, K and N[O.sub.3]-N concentrations in plants were quantified at the Analytical Research Laboratories (ARL), Awatoto, New Zealand. Nitrate-N in plant tissue was determined using 2% acetic acid (Miller 1998). Plant K content was determined by atomic absorption spectroscopy after wet digestion with sulfuric acid (Watson et al. 1990). Plant P content was determined photometrically with the molybdenum blue method (Mills and Jones 1996).

Total C and N contents in soil were determined by the method used for plants. Exchangeable cations and P contents, electrical conductivity (EC), N[O.sub.3]-N and N[H.sub.4]-N were determined by ARL. The EC was determined by a conductivity meter on a 1:2.5 soil:water suspension (Rayment and Lyons 2010). Available soil P was determined according to Colwell (1963). Exchangeable K, sodium (Na), Ca and Mg contents were determined by 1 m ammonium acetate extraction buffered at pH 7 for 30 min (Rayment and Lyons 2010). Soil N[H.sub.4]-N and N[O.sub.3]-N were determined colourimetrically after extraction with 1 m KC1 (Rayment and Lyons 2010). The chemical properties of compost, WB and AB were determined in the laboratory following methods similar to those used for soil analysis (Table 1). Pot leachate samples were analysed for N[O.sub.3]-N, P, K, Mg, Ca and Na concentration. Nitrate concentration of the leachate was measured using a nitrate meter (B-743, HORIBA Ltd, Kyoto, Japan). Phosphorus concentration was determined photometrically using the molybdate blue method, and K, Ca, Na and Mg concentrations were measured by inductively coupled plasma-atomic emission spectroscopy at the Advanced Analytical Center, James Cook University, Townsville, Queensland.

Data analyses

The data were subjected to analysis of variance using the general linear model procedure (PROC GLM) of SAS statistical package version 9.1 (SAS Institute, Cary, NC, USA). The total variability for each trait was quantified by using the following model:

[Y.sub.ij] = [mu] + [R.sub.i] + [T.sub.j] + [e.sub.ijk]

where [Y.sub.ij] is total observations, [mu] is the grand mean, [R.sub.i] is effect of the ith replication, [T.sub.j] is effect of the jth treatment, and [e.sub.ijk] is the variation due to random error. Means for the treatments were compared by using the MEANS statement with the least significant difference (l.s.d.) test at P=0.05. To perform the multivariate approach of correlation and principal component analysis (PCA), the data were standardised by removing treatment mean character values, followed by dividing by the corresponding character standard deviations. Correlation coefficients (r) were then calculated among plant parameters, nutrient uptake and soil nutrient contents by the SAS CORR procedure, and the PCA was performed by the SAS PRINCOMP procedure to distinguish the treatments as a function of soil management and to determine the most important parameters to characterise them.

Results

Plant growth and nutrient uptake

The different soil fertility treatments significantly increased above- and belowground biomass, SRR, SLW, chlorophyll content and plant height of maize (P<0.05 and P< 0.001; Fig. 1, Table 2). Shoot and root biomass were greater with F + Com than with other treatments. The shoot biomass recorded from F + Com and F + WB + Com treatments, respectively, was 4 and 3.6 times that of the control and 1.4 and 1.2 times that of the F treatment (Fig. 1). Root biomass was highest in the F + Com treatment, but the differences among F+ Com, F+ WB and F + WB + Com treatments were not statistically significant. Applications of F + Com and F + WB, respectively, produced root biomass 3.2 and 3.0 times that of the control and 1.3 and 1.3 times that of the F treatment. Comparing the two biochar types, WB addition increased root biomass over AB by 20%. However, applications of F + Com, F + WB + Com or F + AB + Com resulted in similar SRR. The shoot and root parts of plants constituted 73-79% and 21-27% of plant biomass, respectively. Plants grown with F + Com or F + WB + Com had greater chlorophyll content and plant height than the other treatments. Chlorophyll contents in the F, F + Com and F + WB + Com treatments were 1.3, 1.5 and 1.4 times that of the control (Table 2). The treatment F + WB + Com resulted in the highest SLW, with the lowest SLW in the control treatment.

Treatments significantly (P< 0.01) increased plant uptake of total N, C, P and N[O.sub.3]-N relative to the control (Table 2); F + Com, F + WB or AB, and F + WB or AB + Com significantly increased plant N uptake compared with the F treatment. Plant C, N, P and K uptakes were in the ranges 4.4-19.7g, 150-1117mg, 15-87mg and 354-1746mg pot respectively, with the highest values in F + Com and the lowest in the control treatment (Table 2). However, there was no statistically significant difference between F + Com and F + WB + Com for N, N[O.sub.3]-N and P uptake. Nitrogen and P contents of plants in the F + Com treatment were, respectively, 7.4 and 5.9 times those in the control and 1.7 and 1.5 times those in the F treatment. Applications of biochar and compost resulted in optimal N and P uptake by plants (within the sufficiency range). Fertiliser+Com resulted in the highest plant N[O.sub.3]-N uptake (41 mg pot '), whereas the lowest plant N[O.sub.3]-N content was in the control (Table 2). Significant differences were not observed for N[O.sub.3]-N uptake between organic amendments and mineral fertiliser except for the F + Com treatment. Mineral fertiliser alone, F + Com, F + WB + Com and F + AB + Com had plant N[O.sub.3]-N contents that were, respectively, 9.6, 14, 11.8 and 10 times that of the control treatment. Overall, application of compost and biochar increased plant growth, soil nutrient status and plant N content, with shoot biomass (as a ratio of control value) decreasing in the order F + Com (4.0) > F + WB + Com (3.6)>F + WB (3.3) > F + AB + Com (3.1) >F +AB (3.1)>F (2.9)>control (1.0).

Soil characteristics

The treatments with organic components significantly (P<0.001) improved SWC and decreased leachate volume (Table 3). Differences in leachate volume among treatments increased during the growing period, because the demand for water by plants depended on treatments. Therefore, the cumulative leachate volume was inversely related with the above- and belowground biomass. Soil water content was highest in soil treated with compost and biochar. Following harvest, soil organic C (SOC), total N, C : N ratio, N[O.sub.3]-N and NF14-N, available P, exchangeable cations, effective CEC (ECEC) and EC significantly responded to the treatments (Tables 3 and 4). Soil nutrient contents were higher when compost and biochar were added to soil than in the control and F treatments. The highest SOC content (33 g pot ') was obtained from F + WB, which is in agreement with the initial C content of WB. In F + WB-amended soil, SOC content increased by a factor of 1.4 and 1.5 compared with the control and F treatments, respectively (Table 3). There was a linear relationship between the amount of C added in the amendments and the SOC content at the end of the experiment. The maximum Colwell P value (108 mg [pot.sup.-1]) was obtained from F + Com soil. Flighest N[O.sub.3]-N and N[H.sub.4]-N contents were in F + Com and F + WB + Com treated soils (Table 3). Fertiliser + Com and F + WB + Com treated soils had higher contents of available nutrients such as K, Ca and Mg than the other treated soils (Table 4). Fertiliser+ WB + Com addition especially increased soil available K, Ca and ECEC after harvesting, whereas F + Com addition resulted in the highest exchangeable Mg and Na. The Ca: Mg ratios for the different treatments ranged from 4 to 5.4 (Table 4), within the recommended range (4-6).

Significant correlations were observed between plant growth parameters, plant nutrient concentrations, SWC and soil chemical properties (P< 0.05, P<0.01 and P<0.001). Maize shoot biomass was positively significantly correlated with leaf chlorophyll content, root biomass, plant height, SLW and SWC (r=0.99, 0.98, 0.96, 0.92 and 0.74, respectively) under different soil fertility treatments (Table 5). However, the amount of leachate collected as percolated water was negatively correlated with shoot and root biomass, leaf chlorophyll content, SLW and SWC (data not shown). Shoot and root biomass, chlorophyll content and plant height were significantly correlated with plant and soil contents of N, N[O.sub.3]-N and P (Table 5). Plant N content was positively correlated with SWC, soil N, P and K contents, and plant N[O.sub.3]-N, P and K concentrations, but soil K content was not significantly correlated with shoot biomass, plant height, SLW, plant N[O.sub.3]N concentration or SWC. The PCA revealed that the first two principal components (PC 1 and PC2) accounted for ~91 % of the total variation of the treatments, of which 84% was contributed by PCI (Table 6). The first eigenvector has similar weights on all of the characters. Thus, most characters in PCI individually contributed comparable effects (0.186-0.255) to the total variation of the treatments (Table 6). The second eigenvector has positive loadings on the variables soil N and K. contents, SOC, SWC and ECEC. Each vector corresponds to one of the analysis variables and is proportional to its component loading. For instance, the biplot of PCI and PC2 showed that the variables F + Com, F + WB + Com, F + WB, F+ AB + Com and control load heavily on the first component, whereas the variables F and F + AB load heavily on the second component (Fig- 2).

Nutrient leaching

Leaching of N[O.sub.3]-N, P, K, Ca, Mg and Na significantly (P<0.01) differed among the treatments (Table 7). Applications of F + WB + Com and F + WB significantly reduced the cumulative leaching of N[O.sub.3]-N, P, exchangeable K, Ca and Mg (Table 7). The greatest leaching of N[O.sub.3]-N and P was recorded from the F treatment. On the other hand, leaching of K, Ca, Mg and Na was greatest for the control, followed by the F treatment (Table 7). The treatments F + WB + Com and F + WB had lower cumulative leaching of nutrients than other treatments. Most of the native available N[O.sub.3]-N was leached from the control during the first crop-establishment period. Significantly higher ratios of N[O.sub.3]-N, P and K uptake to leaching were obtained from soil treated with F + WB + Com (Fig. 3). By contrast, F + Com resulted in the highest ratios of N[O.sub.3]-N and K uptake to initial soil content, and F + Com and F + WB + Com had similar ratios of P uptake to initial soil content. Ratios of nutrient uptake to leaching and to initial soil content were lowest for the control, followed by the F treatment (Fig. 3). There was a significant linear correlation between the amount of leaching of nutrients and the volume of leachate. Marked differences were observed among treatments in the magnitude of leaching of nutrients. Overall, as the growth of plants progressed, the leaching of nutrients was markedly reduced because of higher nutrient uptake by the plants, and hence smaller amounts left to leach.

Discussion

Plant growth and nutrient uptake

Our results showed significant increases in plant growth and biomass production with F + Com and F + WB + Com additions compared with the control and fertiliser only; the effects were due to improved availability of water and nutrients. By contrast, the F + AB treatment was no better than fertiliser alone with regard to biomass production or N, P and K uptake. Shoot and root biomass increments were higher as a result of compost addition than biomass addition with either biochar type, indicating that the amount applied and the nutrients supplied by compost were adequate. Other studies have shown that application of compost increased biomass of oats (Schulz and Glaser 2012), shoot and root biomass of ryegrass (Khan and Joergensen 2012), and biomass of rice and cowpea (Lehmann et al. 2003). There were also significant effects resulting from biochar type; higher total biomass was obtained from WB than from AB. This plant growth differential may be due to WB having a higher nutrient retention capacity because of greater pore spaces and ability to supply plants with nutrients. The effect of biochar on soil physical and chemical properties depends on feedstock type and pyrolysis conditions (Novak et al. 2009b). Singh et al. (2010) indicated that wood biochars had higher total C but lower contents of ash, total N, P, K, sulfur (S), Ca, Mg, aluminium (Al), Na, copper and CEC than manure-based biochars.

Both positive and negative yield responses have been reported for a wide variety of crops as a result of biochar application to soils (Chan and Xu 2009; Tammeorg et al. 2014). For instance, maize yield increased by 98--150% and water use efficiency by 91-139% as a result of manure biochar addition (Uzoma et al. 2011), wheat plant biomass increased by 250% following charred paper mill waste addition (Van Zwieten et al. 2010), and wheat grain yield increased by 18% from the use of oil mallee biochar (Solaiman et al. 2010). Plant growth and yield increases with biochar additions have, in most cases, been attributed to optimisation of the availability of plant nutrients (Gaskin et al. 2010; Glaser et al. 2002; Lehmann et al. 2003), increase in soil microbial biomass and activity (Biederman and Harpole 2013; Thies and Rillig 2009), and reduction of exchangeable [Al.sup.3+] (Glaser et al. 2002; Steiner et al. 2007). Likewise, wood biochar addition increased wheat yield by up to 30%, with no differences in grain N content, and sustained yield for two consecutive seasons without biochar addition in the second year (Vaccari et al. 2011). Major et al. (2010) reported that maize grain yield did not increase significantly in the first year following addition of 201 [ha.sup.-1] of biochar (biomass-derived black C), but increased by 28%, 30% and 140% over the control for the 3 years following. Gathome-Hardy et al. (2009) reported that biochar addition alone did not show a significant effect on barley yield, but applications of 501 biochar and 80 kg N fertiliser [ha.sup.-1] increased barley grain yield by 30%, which could be attributed to increased N-use efficiency.

Application of compost and biochar singly or together when combined with fertiliser enhanced chlorophyll content and SLW over mineral fertiliser alone, which could also indicate increased nutrient availability, vigorous plant growth and healthier plants, resulting in higher plant biomass. Chlorophyll content, an indicator of photosynthetic activity, is related to the N concentration in green plants and serves as a measure of the response of crops to N fertiliser application and soil nutrient status (Minotta and Pinzauti 1996). Hua et al. (2012) reported that application of bamboo biochar increased chlorophyll content of ryegrass by 20-32% compared with the control. Application of F + WB + Com resulted in the production of leaves with higher SLW than those in the control and fertiliser-only treatments (+0.68 and 0.32 mg [cm.sup.-2], respectively). SLW is related to leaf resistance or susceptibility to insect attack, with higher SLW conferring resistance (Steinbauer 2001).

Without amendment, the nutrient content of the soil used in this study was extremely low. Phosphorus content of the plants in the control and fertiliser-only treatments was lower than the established sufficiency ranges. Phosphorus content of maize plants in the F + WB + Com treatment was not higher than in the F + Com treatment, suggesting that WB had no effect on plant P concentration; F + Com and F + WB + Com both considerably improved nutrient content of maize plants per unit of root biomass. Higher P uptake by the crop implies that a higher concentration of P was maintained in the soil solution and available to the plants. The organic amendments in this study may have made the soil more porous and friable, which potentially enhanced root growth and development. This may have improved the root-nutrient contact in the soil and optimised nutrient availability and uptake by plants, because the effect of the amendments was particularly noticeable on root growth. Higher plant nutrient uptake was accompanied by increased shoot and root biomass.

Soil characteristics

Applications of biochar and compost had a significant influence on SWC and chemical characteristics. Post-harvest SOC content of the mineral-fertilised soil was lower than in the control, suggesting that application of mineral fertiliser alone may exacerbate the depletion of SOM through accelerated decomposition and mineralisation relative to organic inputs. Soils treated with biochar had higher SOC, and SOC remained more stable than in soils treated with manure (Sukartono et al. 2011). Similarly, charcoal amended soil lost 8% and 4% SOC for mineral-fertilised and unfertilised plots compared with losses of SOC from compost-amended (27%) and control plots (25%), as well as reduced exchangeable Al (Steiner et al. 2007). Although F + Com, F + WB + Com and F + AB + Com had a slight effect on soil C:N ratio, F + WB increased the C: N ratio the most compared with the initial value (14.2: 1), which was reflected in the high SOC in soil treated with F + WB. This might be due to insignificant mineralisation during the growing period, so the increase in SOC could be proportional to the amount added. By contrast, mineral fertiliser had negative effect on the C: N ratio compared with the initial value. Steiner et al. (2008) reported that biochar and compost amendment increased soil C: N ratio after two consecutive harvests. In this study, compost and biochar additions significantly improved soil quality, including increases in SOC, exchangeable cations and water retention.

Biomass yield was significantly lower in the F + WB + Com treatment than in F + Com. However, the SOC increase in soil treated with F + Com and F + WB + Com improved nutrient content and availability compared with other treatments, which was reflected in plant growth and biomass yield. A possible explanation for this could be that increasing total SOC by compost or compost and biochar addition increased reactive surfaces and stimulated microbial growth, which may lead to a short-term immobilisation of plant-available nutrients. In the course of plant growth, these nutrients might be released through mineralisation of compost and dead microorganisms, thus leading to improved plant growth over the course of the experiment. Schulz and Glaser (2012) reported that application of biochar with compost resulted in better plant growth and C sequestration than biochar with mineral fertiliser. With biochar and biochar + compost, a significant part of the initial total C content remained after the second harvest, whereas only 58% remained in the biochar + fertiliser treatment. Nevertheless, in contrast to total C, black C contents remained almost constant during two crop growth periods without further biochar additions, but the mineral fertiliser only reduced the black C content to 75% of the original amount (Schulz and Glaser 2012).

Compost and biochar additions directly influenced the availability of native or applied nutrients, which significantly contributed to the increase in available soil K and Mg, ECEC, N[O.sub.3]-N and N[H.sub.4]-N after harvest compared with the control and fertiliser only. The increase in total N would be simply due to the N in the compost. Enhanced crop growth in the compost-treated soil in this study was largely due to improved nutrient availability and uptake. Higher soil macro- and micronutrient contents from compost and biochar additions may have been beneficial to plant performance, because compost + WB contains significant amounts of essential elements. By contrast, despite low N content of biochar, effects on nitrification rates that are positive (Berglund et al. 2004; DeLuca et al. 2009) or negligible or negative (Dempster et al. 2012) have been reported depending on soil pH (Yao et al. 2011). Liu et al. (2012) showed that compost and biochar addition increased total SOC, and plant-available Ca, K, P and Na contents by 2.5, 2.2, 2.5, 1.2 and 2.8 times, respectively, and increased the soil pH by up to 0.6 and doubled plant-available SWC compared with the control.

Soils fertilised with compost or manure have higher SOM content, porosity, hydraulic conductivity and aggregate stability and are more enriched in P, K, Ca and Mg (Edmeades 2003), as well as having lower bulk density (Tammeorg et al. 2014), than fertilised soils. CEC is a measure of the soils ability to hold cations, which is associated with clay and SOM content (Troeh and Thompson 2005). The ECEC of the experimental soil in this study was low, which may have caused higher leaching in mineral-fertilised than organic-amended soil during the plant growth period because of lower retention capacity of applied nutrients. In this study, the Ca: Mg ratio showed variations for different treatments. The Ca: Mg ratio has a significant influence on soil chemical properties and nutrient availability (Hazelton and Murphy 2007).

Shoot biomass showed positive significant correlations with plant height, chlorophyll content, root biomass, SLW, SWC, plant-available nutrients and nutrient uptake. The direct effect of available soil nutrients and plant nutrient uptake from soil treated with F + Com, F + WB or F + WB + Com exceeded the direct effect of available nutrients and nutrient uptake from F + AB and fertiliser-only treatments on maize growth and biomass production. The highest correlation was of plant biomass with plant nutrient uptake, and the compost and biochar amendment facilitated availability of these nutrients in this study. Positive associations between soil available nutrients and nutrient uptake might enable the choice and preparation of appropriate soil amendments. Available soil P and N[O.sub.3]-N had a more significant influence on shoot and root biomass than other nutrients, meaning that these nutrients were most limiting and corresponded with the soil P and N[O.sub.3]-N content before planting and uptake by plants. Solaiman et al. (2012) also showed positive linear correlations between soil chemical characteristics (K, P, EC, CEC, N[O.sub.3], N[H.sub.4] and S), wheat seed germination, and root and shoot growth as a result of addition of different biochar types, but a negative correlation between seed germination and Al content. From this result, it can be inferred that high shoot and root biomass, high chlorophyll content and taller plants are the traits associated with the performance of maize.

The PCA indicated that the first component (PC 1) provided a reasonable summary of the data, accounting for ~84% of the total variance. That is, PCI explained most of the variation in the entire dataset, and the other three, most of the remaining variation. It is usually believed that characters with larger absolute values closer to unity within the first principal component influence the clustering more than those with lower absolute values closer to zero (Jolliffe 2002). In this study, however, almost all characters in the first eigenvector individually contributed similar effects to the total variation of the treatments, suggesting that the first component is primarily a measure of the whole characters. Thus, the differentiation of the treatments into different clusters was rather dictated by the cumulative effects of several characters. Similarly, Sena et al. (2002) compared conventionally managed plots that intensively utilised pesticides and chemical fertilisers with non-disturbed forest areas and alternatively managed plots using PCA to visualise the effects of alternative soil amendment.

Nutrient leaching

Differences in cumulative water percolation and extent of nutrient leaching among treatments were caused by variations in plant water uptake and efficiency of nutrient retention. Leaching of nutrients was significantly reduced from compost and biochar amendment compared with the control and mineral-fertilised soil, supporting our third objective. The treatment F + WB + Com had more impact in lowering the cumulative leaching of nutrients than F + AB or F + AB + Com, possibly because of more pore spaces and increased sorption capacity of biochars through oxidative reactions on the biochar surfaces over time. Singh et al. (2010) showed the reduction of leaching of N[H.sub.4]-N by 55-93% from the applications of manure and wood biochars on different soil types. Leaching loses of N[O.sub.3]-N range from 0% to 60% of the applied N fertiliser (Meisinger and Delgado 2002).

Marked differences were observed among treatments in the magnitude of leaching of nutrients at the end of the experiment. A similar study showed that charcoal application decreased the proportion of leached N and Ca on Ferralsols (Lehmann et al. 2003). Although N and K proved very mobile in soil, application of F + WB and F +WB + Com reduced leaching of N[O.sub.3]-N by 66% and 73%, and K by 68% and 69%, respectively, compared with soil treated with fertiliser only. Thus, the retention of N and K should be specifically targeted with additions of slow-releasing nutrients and/or soil amendments. Leaching occurs at a much slower rate when most of the ions are present in exchangeable form (Troeh and Thompson 2005), or when uptake by plants increases (Lehmann et al. 2003). Low leaching at high nutrient availability, as found in this study, ensures sustainable soil fertility, which coincides with the findings of Lehmann et al. (2003). Soils that have been strongly weathered and leached often have low levels of exchangeable Ca and Mg, and plant growth may be nutrient-limited as a result (Glaser et al. 2002). A study by Sika (2012) indicated that biochar significantly reduced the leaching of N[H.sub.4]-N (by 12-86%), N[O.sub.3]-N (by 26-95%), basic cations, P and certain micronutrients. By contrast, Novak et al. (2009a) reported higher EC and K and Na concentrations of leachates, but lower concentrations of Ca, P, Mn and zinc (Zn). The degree of leaching of cations such as Ca, Mg and Na was directly related to the differential nutrient retention capacity of treatments, because the availability of these nutrients for plants was directly dependent on the soil reserve. Although the initial soil K status was sufficient for plant growth, the ratio of K uptake to initial soil K was the lowest for the control, because K uptake might be limited by the low availability of other nutrients, such as N and P.

Conclusion

This study shows that the experimental soil was deficient in plant-available nutrients, consistent with the general observation that Ferralsols of the humid tropics are nutrient-depleted and suboptimal for plant growth without additions of organic and inorganic amendments. Applications of F + Com, F + WB or F + WB + Com were more efficient in improving SOC and soil water storage capacity, and nutrient-retention capacity and nutrient-use efficiency of maize than mineral fertiliser alone. The use of compost and biochar as soil amendments reduced the loss of some nutrients through leaching, with nutrient leaching of N[O.sub.3]-N, P and exchangeable bases significantly decreased as the growth of plants progressed. Although F + Com + WB or AB did not outperform F + Com in terms of biomass yield and nutrient uptake, the combined application of compost and biochar may enhance and sustain soil biophysical and chemical characteristics, because most of the compost will disappear over time through decomposition, whereas the biochar will stay in the soil for decades. Root biomass was significantly increased by F + Com, F + WB and F + WB + Com compared with other treatments. Further long-term research is required to evaluate and quantify the benefits and effects of these amendments in terms of improving and sustaining soil fertility, crop productivity and economic returns to users. Moreover, despite several positive, short-term research results, the amount of recalcitrant C supplied by biochar and compost and sequestered in the soil needs to be determined through long-term field experiments.

http://dx.doi.org/10.1071/SR14118

Acknowledgements

The authors express their sincere acknowledgement to the School of Earth and Environmental Sciences of James Cook University, and Department of Agriculture, Fisheries and Forestry for their financial support through the Carbon Farming Initiative Project. Editing of the manuscript by Dr John Armour is highly valued and appreciated. We are also very grateful to Dr Jen Whan for her assistance in the analyses of soil and plant samples at the Advanced Analytical Laboratory of James Cook University.

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Cetachew Agegnehu (A,B), Michael I. Bird (A), Paul N. Nelson (A), and Adrian M. Bass (A)

(A) School of Earth and Environmental Sciences, and Centre for Tropical Environmental and Sustainability Sciences, James Cook University, Cairns Campus, McGregor Road, Smithfield, Qld 4878, Australia.

(B) Corresponding author. Email: getachew.jenberu@my.jcu.edu.au

Table 1. Element contents and pH of compost, willow biochar, acacia
biochar and experimental soil before planting ECEC, Effective cation
exchange capacity; n.d., not determined

Element                          Unit        Compost     Willow
                                                         biochar

pH([H.sub.2]O)                                   8.1         9.5
pH(Ca[Cl.sub.2])                                --           8.3
Carbon                      g [kg.sup.-1]      233          92.4
Nitrogen (N)                g [kg.sup.-1]       11           0.36
Phosphorus                  g [kg.sup.-1]        3.3         1.96
Potassium                   g [kg.sup.-1]        8.9        15.4
Calcium                     g [kg.sup.-1]       14          15.2
Magnesium                   g [kg.sup.-1]        7.0         4.4
Sodium                      g [kg.sup.-1]        2.1         1.04
ECEC                        g [kg.sup.-1]        n.d.        n.d.
Sulfur (S)                  g [kg.sup.-1]        1.5         0.07
Ammonium-N (N[H.sub.4]-N)   mg [kg.sup.-1]       n.d.        n.d.
Nitrate-N (N[O.sub.3]-N)    mg [kg.sup.-1]       n.d.        n.d.
Copper                      mg [kg.sup.-1]      72.0       239.7
Zinc                        mg [kg.sup.-1]     181.0      2571
Manganese                   mg [kg.sup.-1]     421.5     14599
Iron                        mg [kg.sup.-1]   17877      125898
Boron                       mg [kg.sup.-1]      17.0         n.d.
Molybdenum                  mg [kg.sup.-1]       2.2         n.d.
Cobalt                      mg [kg.sup.-1]       9.8         n.d.
Aluminium                   mg [kg.sup.-1]    7596        3175

Element                     Acacia    Experimental
                            biochar       soil

pH([H.sub.2]O)                8.0          6.5
pH(Ca[Cl.sub.2])              7.5          5.6
Carbon                       59.0         11.4
Nitrogen (N)                  0.43         0.80
Phosphorus                    0.02         0.031
Potassium                     0.09         0.30
Calcium                       3.9          0.86
Magnesium                     0.06         0.11
Sodium                        0.09         0.01
ECEC                        n.d.           1.28
Sulfur (S)                    0.05         n.d.
Ammonium-N (N[H.sub.4]-N)   n.d.          11.3
Nitrate-N (N[O.sub.3]-N)    n.d.          14.0
Copper                        9.4          n.d.
Zinc                         38.0          n.d.
Manganese                    22.0          n.d.
Iron                         47.0          n.d.
Boron                         9.3          n.d.
Molybdenum                   <0.3          n.d.
Cobalt                       <0.4          n.d.
Aluminium                   400            n.d.

Table 2. Effects of treatments on plant parameters and nutrient
uptake into shoot parts of plants

F, Mineral fertiliser; Com, compost; WB, willow biochar; AB, acacia
biochar; SLW, specific leaf weight; l.s.d., least significant
difference; CV, coefficient of variation. Within columns, means
followed by the same letter are not significantly different at
P=0.05. ** P<0.01; *** P<0.001

Treatment             SLW (mg     Chlorophyll  Plant    C uptake (g
                    [cm.sup.-2])    content    height  [pot.sup.-1])
                                  (SPAD unit)   (cm)

Control                1.98c        26.9d       78.2b      4.41e
F                      2.34b        36.3c      111.0a     14.19d
F + Com                2.58ab       40.6a      115.0a     19.66a
F + WB                 2.58ab       39.0ab     112.0a     16.00c
F + WB + Com           2.66a        40.5a      114.3a     17.35b
F + AB                 2.56ab       37.6bc     110.5a     14.68d
F + AB + Com           2.36b        38.1bc     112.5a     14.89d
Significance level       **           ***       ***         **
l.s.d. (P=0.05)        0.28          2.1         5.8       0.98
CV (%)                 7.7           4.0         3.6       4.59

Treatment           Nutrient uptake (mg [pot.sup.-1])

                      N    N[O.sub.3]-N    P      K

Control             150d       2.9c      14.8c   354d
F                   648c      27.9b      60.1b  1148c
F + Com             1117a     40.8a      86.9a  1746a
F + WB              943b      28.6b      69.1b  1330bc
F + WB + Com        990ab     34.3ab     86.1a  1470b
F + AB              771c      29.1b      60.6b  1219c
F + AB + Com        904b      29.4b      69.6b  1316bc
Significance level   ***       ***        ***     **
l.s.d. (P=0.05)     129.4      7.9       10.5    226.0
CV (%)               11.2     10.4       11.1     12.4

Table 3. Effects of treatments on cumulative leachate, soil water
content and soil chemical properties after harvesting

F, Mineral fertiliser; Com, compost; WB, willow biochar; AB, acacia
biochar; SWC, soil water content; SOC, soil organic carbon; l.s.d.,
least significant difference; CV, coefficient of variation. Within
columns, means followed by the same letter are not significantly
different at P=0.05. ** P<0.01, *** P< 0.001

Treatment             Cumulative      SWC (%) (g     Nutrient
                     leachate (mL    [pot.sup.-1])   content       N
                     [pot.sup.-1])                     SOC

Control                  715a            17.6c        23.2c      1.40c
F                         18%            20.0c        22.4c      1.42c
F + Com                   58e            23.7b        30.0b      2.20a
F + WB                    68e            26.3ab       33.4a      2.0ab
F + WB + Com              54e            26.9a        28.7b      2.02ab
F + AB                   140c            26.5ab       27.4b      1.80b
F + AB + Com              81d            28.1a        27.8b      2.01ab
Significance level        ***             ***          ***        **
l.s.d. (P = 0.05)         18.1            3.1          3.4       0.22
CV (%)                     6.5            8.8          8.1       8.4

Treatment             C: N     Nutrient content (mg [pot.sup-1])
                     ratio
                                P      N[O.sub.3]-N   N[H.sub.4]-N

Control              16.6a     49.0d       1.01e         35.6c
F                    15.Sab    68.4c       4.02d         45.0c
F + Com              13.6c    107.6a      17.6a          85.2a
F + WB               16.7a     95.0ab     10.8c          61.4b
F + WB + Com         14.2b     97.6ab     15.8ab         86.0a
F + AB               15.2ab    88.6b      14.8b          77.8a
F + AB + Com         13.8c     94.2ab      8.6c          61.8b
Significance level     **      ***         ***            ***
l.s.d. (P = 0.05)     1.8      15.5        2.2           13.6
CV (%)                8.0      11.3       14.3           14.2

Table 4. Effects of treatments on soil chemical properties after
harvesting

F, Mineral fertiliser; Com, compost; WB, willow biochar; AB, acacia
biochar; ECEC, effective cation exchange capacity; EC, electrical
conductivity; l.s.d., least significant difference; CV, coefficient
of variation. Within columns, means followed by the same letter are
not significantly different at P=0.05. ** P<0.01; *** P<0.001

Treatment             Exchangeable cations (mg [pot.sup.-1])

                       K       Ca      Mg       Na       ECEC

Control              202c    1590d    190c    40.2d     2020d
F                    210c    1660cd   192c    43.6cd    2104cd
F + Com              306ab   1885ab   284a    72.4a     2516a
F + WB               304ab   1800b    216bc   56.4bc    2378b
F + WB + Com         342a    1950a    242b    64.4ab    2600a
F + AB               232c    1680cd   202c    43.8cd    2158c
F + AB + Com         288b    1760bc   236b    51.8bcd   2334b
Significance level   ***     ***      ***     **        **
l.s.d. (P=0.05)       41.6    119.0    31.0   13.0       127
CV (%)                10.4      4.6     9.3   16.5         3.5

Treatment            Ca:Mg         EC
                     ratio   (dS [m.sup.-1])

Control              8.4a          0.05c
F                    8.7a          0.07b
F + Com              6.5c          1.0ab
F + WB               8.4a          1.11a
F + WB + Com         8.1ab         0.08b
F + AB               8.3ab         0.08b
F + AB + Com         7.5b          1.0ab
Significance level   **            **
l.s.d. (P=0.05)      0.85          0.02
CV (%)               7.2          17.4

Table 5. Correlation coefficients among plant parameters, soil water
content, soil and plant nutrient contents

SWC, Soil water content; SLW, specific leaf weight; PHT, plant
height; CHLC, chlorophyll content; RB, root biomass; SB, shoot
biomass. * P< 0.05, ** P<0.01, *** P< 0.001; ns, not significant

Parameters            Soil K     Soil P     Soil N       Plant
                                                      N[O.sub.3]-N

SB                   0.73n.s.   0.93 **    0.80 *       0.99 ***
RB                   0.79 *     0.94 **    0.83 *       0.96 ***
CHLC                 0.77 *     0.94 **    0.81 *       0.97 ***
PHT                  0.63n.s.   0.85 *     0.68n.s.     0.96 ***
SLW                  0.75n.s.   0.89 **    0.77 *       0.89 **
SWC                  0.72n.s.   0.83 *     0.79 *       0.66n.s.
Plant N              0.83 *     0.98 ***   0.90 **      0.96 ***
Plant P              0.81 *     0.93 **    0.82 *       0.98 ***
Plant K              0.75 *     0.95 **    0.84 *       0.99 ***
Plant N[O.sub.3]-N   0.69n.s.   0.91 **    0.77 *
Soil N               0.89 **    0.96 ***
Soil P               0.85 *

Parameters           Plant K    Plant P    Plant N     SWC       SLW

SB                   0.99 ***   0.98 ***   0.98 ***   0.74 *   0.92 **
RB                   0.97 ***   0.97 ***   0.98 ***   0.75 *   0.92 **
CHLC                 0.98 ***   0.98 ***   0.98 ***   0.78 *   0.94 **
PHT                  0.94 **    0.94 **    0.93 **    0.74 *   0.88 **
SLW                  0.89 **    0.91 **    0.91 **    0.76 *
SWC                  0.69n.s.   0.72n.s.   0.78 *
Plant N              0.98 ***   0.98 ***
Plant P              0.98 ***
Plant K
Plant N[O.sub.3]-N
Soil N
Soil P

Parameters             PHT        CHLC        RB

SB                   0.96 ***   0.99 ***   0.98 ***
RB                   0.95 **    0.98 ***
CHLC                 0.97 ***
PHT
SLW
SWC
Plant N
Plant P
Plant K
Plant N[O.sub.3]-N
Soil N
Soil P

Table 6. Percentage, cumulative variances and eigenvectors on the
first four principal components (PC1-4) for 18 characters in seven
treatments

SOC, soil organic carbon; ECEC, effective cation exchange capacity

Parameter                           PC1      PC2      PC3      PC4

Eigenvalue                         15.1      1.17     0.65     0.51
%Variance                          84.00     6.49     3.62     2.84
Cumulative                         84.00    90.49    94.11    96.95

Character                                    Eigenvectors

Plant N concentration               0.255   -0.037    0.015    0.130
Soil P content                      0.254    0.119    0.015    0.011
Chlorophyll content                 0.252   -0.169   -0.040    0.087
Plant P concentration               0.251   -0.145    0.101    0.060
Plant K concentration               0.251   -0.160    0.103    0.026
Shoot biomass                       0.250   -0.214    0.040    0.019
Root biomass                        0.249   -0.142   -0.116    0.200
Plant C content                     0.249   -0.217    0.020    0.021
Plant N[O.sub.3]-N concentration    0.245   -0.256    0.125   -0.032
Specific leaf weight                0.240   -0.111   -0.328   -0.231
Plant height                        0.234   -0.324   -0.002    0.206
Soil N content                      0.234    0.358    0.059    0.048
Soil N[O.sub.3]-N content           0.230    0.115   -0.056   -0.589
Soil N[H.sub.4]-N content           0.228    0.125    0.064   -0.603
Soil K. content                     0.217    0.345   -0.022    0.176
Soil water content                  0.208    0.256   -0.111    0.192
SOC content                         0.194    0.394   -0.549    0.200
ECEC                                0.186    0.341    0.717    0.113

Table 7. Effect of treatments on cumulative loss of nutrients by
leaching at the end of the experiment

F, Mineral fertiliser; Com, compost; WB, willow biochar; AB, acacia
biochar; l.s.d., least significant difference; CV, coefficient of
variation. Within columns, means followed by the same letter are not
significantly different at P=0.05. ** P<0.0l; *** P<0.001

Treatment                  Nutrient leached (mg [pot.sup.-1])

                    N[O.sub.3]-N    P      K     Ca     Mg      Na

Control               102.1b      0.37d  22.8a  32.7a   7.2a   4.96a
F                     133.6a      1.23a  20.7b  24.1b   5.6b   2.34b
F + Com                85.7c      0.18f  10.8c  23.5b   4.4c   2.57b
F + WB                 44.7ef     0.26e   6.5e  10.3d   2.3d   0.75d
F + WB + Com           35.6f      0.14g   6.4e   9.4d   2.2d   1.04cd
F + AB                 69.4d      0.46c  11.0c  16.5c   3.7c   1.37c
F + AB + Com           47.9e      0.63b   8.4d  10.9d   2.6d   1.22cd
Significance level     ***         ***    **     ***    **      **
l.s.d. (p = 0.05)      10.1       0.04    1.7    3.0    0.86   0.41
CV (%)                  9.2       5.8     8.9   11.1   14.5   13.7
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Article Details
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Author:Agegnehu, Getachew; Bird, Michael I.; Nelson, Paul N.; Bass, Adrian M.
Publication:Soil Research
Article Type:Report
Geographic Code:8AUST
Date:Feb 1, 2015
Words:10589
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