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Burning crop residues under no-till in semi-arid land, Northern Spain--effects on soil organic matter, aggregation, and earthworm populations.

Introduction

Stubble burning has traditionally been used in semi-arid land for pest and weed control and to facilitate soil management. In the study area of Central Navarre, north-east Spain (as in other Mediterranean regions), it is, or was until recently, common practice, although strictly controlled regarding dates and fire control. However, this practice is currently under discussion in Europe, and in some countries stubble burning is banned or strongly restricted. In addition, conservation and sustainable agriculture regulations do not usually allow for stubble burning. Advantages of not burning, especially the long-term gain in soil organic carbon (SOC), its impact on aggregation, and the reduction of erosion, have been reported (e.g. Six et al. 2004; Zhang et al. 2007).

Nevertheless, the increase in the density and virulence of pests (Conway 1996), including diseases that carry over through residues, and initial weed infestations (Rasmussen 1995) are a matter of concern when implementing no-till (NT) techniques, especially when suitable crop rotations are lacking. In fact, these problems often are the major reason for farmers to reject the use of such systems.

An additional management problem related to an excessive stubble accumulation on the soil surface is the reduction of the period during which the soil condition is appropriate for sowing due to the greater amount of water retained in the soil under NT. This occurs in some cereal-producing areas in Spain, especially in years with above-average rainfall. An extra chisel ploughing operation before seeding is needed to allow soil water to drain and/or to evaporate. This implies a disruption of the continuity of the NT system, breaking up the typical structure of the soil which is prerequisite for proper functioning.

Stubble burning combined with NT is thus seen by some as an alternative management technique that allows for a reduction in the use of agrochemicals and for longer periods available for seeding. From a soil conservation perspective, however, the potential effect of this practice would be the loss of a major part of crop residues in the form of C[O.sub.2]. This would imply (i) a decrease of crop-derived C inputs to the soil and (ii) a higher risk of erosion and drought stress due to decreased soil cover. Erosion risks can be minimised when burning is done shortly before seeding in autumn, since the soil would remain covered between crops, except for a short period between seeding and plant emergence. Late burning would also allow for some residue to be incorporated by biological activity into the soil during summer (Heenan et al. 1995), especially when large earthworm populations are present. Scarce information, however, is available on the impact of stubble burning on the soil properties under NT.

Studies on the management of barley stubble in combination with different tillage practices in Australia showed different results in relation to SOC and its components. Carter and Mele (1992) found that stubble burning over direct seeding on a sandy Calcic Luvisol (pH 6.5, 10 years under NT) in a humid-temperate area reduced aggregate stability in the upper 25 mm of the topsoil (short wet sieving after pre-wetting), most likely as a consequence of a reduction in the microbial biomass, but did not reduce the total SOC stock. Studies of Chan et al. (2002) on an acid soil (pH 4.5) showed that stubble burning had much less effect in reducing SOC than tillage, after 19 years of NT. This was reflected in the C present in the particulate organic matter (POC). Within NT treatments, they observed a significant decrease in the amount of incorporated organic C (C in the <53 [micro]m fraction) when the stubble was burnt. This decrease also had an impact on aggregation, as evidenced by a lower stability of micro-aggregates (<53 [micro]m). Recently, Chan and Heenan (2005) reported on 2 studies on acid soils in north-east Australia. The loss of SOC due to stubble burning in a long-term field experiment amounted to less than one-third of that caused by tillage. A decrease in macro-aggregate stability was also observed. Long-term changes in soil strength under NT are of concern because of their implications on root growth and the crop's development potential (Chan et al. 2003).

Earthworm activity has been proposed as one of the most sensitive indicators of changes in soil quality as a consequence of tillage and residue management (Baker 1998). NT has a clear impact on earthworm populations mainly because of the reduction of mechanical damage and the increase in soil water content (Curry 1998). Another positive factor is the lower exposure of earthworms to predatory birds (Rovira et al. 1987). However, little is known about the effect of stubble burning on earthworms in NT systems.

The objective of this work was to study the consequences of stubble burning on SOC and soil quality indicators in a nontilled semi-arid Mediterranean agricultural soil. Conventional mouldboard tillage without burning of the residues was used as reference treatment.

Materials and methods

Soils and experimental site An experimental site was established in 1994 at Olite (Navarre, NE Spain) (42[degrees] 27' 19" N; 1[degrees] 41' 10" W; altitude 402 m a.s.l.). The soil is a fine-clayey Calcic Haploxerept (Soil Survey Staff 2003). Its characteristics are summarised in Table 1. The texture of the carbonate-rich surface horizon (0-0.30 m) is clay loam and contains 5.8 g/kg of organic C on average. Site and experimental characteristics have been reported by Bescansa et al. (2006). Only the most relevant characteristics are outlined here.

Climate in the area is semi-arid Mediterranean (dry subhumid (C1B' 2db'), in the Thornthwaite classification), with an average annual precipitation of 525 mm, only 18% of which falls in the summertime (July-September). Mean annual evapotranspiration is 740 mm, and mean monthly temperature is 13.5[degrees] C.

Experimental design and sampling

The experiment was designed as a randomised complete block with 4 replications. Plots were 9 by 24 m in size. Treatments were: no tillage without stubble burning (NT), no-tillage with stubble burning (NTSB), and conventional tillage with mouldboard plough and no stubble burning (MT).

Barley (Hordeum vulgate L. vat. Tipper) was planted each year in October, at a sowing rate of 158 kg/ha. For NT and NTSB, a direct seeder was used, which opened the seedrow 30-50 mm deep. MT consisted of 0.25-m-deep primary tillage with a 3-furrow mouldboard plough, followed by a smoothing pass with a float before seeding. Crop residues were incorporated into the arable layer during tillage. Seeding was accomplished using a coulter-seeder. For NTSB, stubble was burnt with a low-intensity fire in October, just before seeding.

For this study, soil was sampled in April 2004 (10 years after the start of the experiment) at 0-0.05, 0.05-0.15, and 0.15-0.30 m depth intervals, 6 months after sowing, when barley was at physiological maturity.

Field and laboratory methods

Disturbed soil samples were collected using an Edelman type auger. Five subsamples were collected per plot and depth, and combined to obtain a composite sample for chemical and physical analyses.

Part of the samples was air-dried and sieved to pass a 2-mm sieve. Total SOC was determined by wet oxidation (Walkley and Black) after grinding the sample to a powder and homogenisation. C in the form of POC was isolated by dispersion and sieving of 10 g of air-dried soil, using a method described by Marriott and Wander (2006). Briefly, samples were dispersed with 150 mL of 5% [(NaP[O.sub.3]).sub.6] and the fraction >53 [micro]m (which includes the POM and the sand-size mineral components of the soil) was collected on a 53-[micro]m-opening polycarbonate mesh (Gilson Co. Inc., Columbus, OH) after 3 rinses with distilled water, and then oven-dried at 50[degrees] C. Samples were then ground to a powdery consistency and POC was determined by wet oxidation. CaC[O.sub.3] content was measured in a Bernard's calcimeter by quantifying the C[O.sub.2] produced when attacking a soil sample (<2 mm) with HCl (Bonneau and Souchier 1987).

Undisturbed core samples were collected using beveledged steel rings, to determine soil bulk density ([[rho].sub.b]). All determinations of the organic components and carbonate analyses were carried out in triplicate and converted from g/kg to Mg/ha using bulk density.

Samples from the 0 to 0.05 m depth were incubated to determine their OC mineralisation rate. Ten g of air-dried and sieved (<2 mm) soil was weighed into vials and tapped to settle the material. Deionised water was added to bring samples to 55% of their field capacity, based on the mass of water determined in a previous study (Bescansa et al. 2006). Samples were incubated for 28 days at 25[degrees]C in sealed 1-L jars containing NaOH 0.2 M traps for respired C[O.sub.2]. Traps were changed on days 1, 3, 7, 14, 21, and 28 and the solutions titrated with standardised HCl to determine the C evolved as C[O.sub.2] (C[O.sub.2]-C). BaCl 0.5 N was added to the traps immediately after removal from the jars to avoid further enrichment with atmospheric C[O.sub.2]. Aeration was assured by opening the jars at every sampling date. This was sufficient in view of the small size of the samples in relation to the jars.

Immediately after sampling, a part of the composite soil sample was gently forced through an 8-mm-opening sieve. These aggregates were allowed to dry in the air and used to determine dry and wet aggregate stability of the soil; 100 g of these aggregates was placed in the top of a column of sieves of 6.3, 4, 2, 1, 0.5, 0.25 mm openings and shaken in a rotary movement at 60 strokes/min for 60 s in a Retsch VS 100 device (Retsch GmbH & Co. Haan, Germany) to determine the dry aggregate size distribution. For wet sieving, a constant shower-like flux (6 L/min) of distilled water was applied from the top of the same set of sieves while sieving (60 strokes/min, 60 s). In both cases, aggregates were homogenised before sieving using a Retsch DR 100 sample conditioner coupled to a Retsch PT 100 divisor (Retsch GmbH & Co. Haan, Germany) to ensure equal initial aggregate distributions among samples. Aggregate size distribution was expressed as the mean weight diameter (MWD) after dry and wet sieving, calculated by summing the products of aggregate fractions and mean diameter of aggregate classes.

Penetration resistance (PR) was measured in the field to a depth of 0.60 m using a field penetrometer (Rimik CP20, Agridy Rimik Pty Ltd, Toowoomba, Qld, Australia). Nine measurements per plot were done recording resistance at 15-mm depth intervals in April after an important rainy period to avoid differences in moisture content between treatments.

Two soil blocks (0.20 by 0.20 by 0.20 m) were taken in each plot in May (spring, physiological maturity of barley) and in November (autumn, soon after seeding) 2004. Earthworms were sampled by hand-sorting and counted in the field. Individuals were weighed (fresh weight basis) in the laboratory, fixed with ethanol-formalin, and preserved in 10% formalin (Baker and Lee 1993).

Statistical analysis

Data were analysed using ANOVA (univariate linear model). Treatment means were compared using significant differences (P < 0.05), and post-hoc analysis was performed by Duncan test (P < 0.05). Unless otherwise stated, significant results are based on a probability level of P = 0.05. All statistical analyses were performed using SPSS 12.1 software (SPSS Inc. 2003).

Results and discussion

Soil organic matter

Results of the determination of SOC and POC stock in all treatments are shown in Table 2. There was no effect of stubble burning under NT on total SOC, but effects of tillage were observed in the 0-0.05 m depth. SOC values were significantly higher under both NT and NTSB than under MT. No effects of tillage or stubble burning were detected below 0.05 m. Burning of crop residues did not thus affect the soil C stock; soil disruption and exposure of subsurface crop residues did. These results coincide with those reported by Carter and Mele (1992) in a Calcic Luvisol after 10 years of NT. Chan and Heenan (2005) also reported a similar behaviour in a short-term (5 years) experimental field on an Alfisol, and Rasmussen et al. (1980) reported no differences in soil C stock due to stubble burning after 50 years of cultivation in a wheat-fallow cropping system on semi-arid soils of north-west USA, compared with the effects of crop residue incorporation by tillage. The absence of differences in SOC between NT and NTSB can be explained by a persistence of carbonised C from straw under NTSB (Carter and Mele 1992), and by the low incorporation rates of stubble into the soil in some climates (Haines and Uren 1990). Lopez et al. (2005) found a major decomposition of stubble mulch before it entered the soil in our study region. The amount of root-derived SOC can be ~1.3 times greater than that of above-ground crop biomass-derived SOC in rainfed cereal crops such as barley and ryegrass (Rasse et al. 2005).

In contrast to SOC, differences were observed in the amount of POC between treatments in the 0-0.05 and 0.05-0.10 m layers. At 0-0.05 m depth, a greater amount of POC was found in the soil under NT than under MT, and soil under NTSB had an intermediate amount of POC (Table 2). In the 0.05-0.15 soil layer, soil under NT had the greatest POC content, but differences between NTSB and MT disappeared. This indicates that the accumulated SOC in the topsoil under NTSB was of a different nature than that under NT. POC is generally associated with plant residues in an early state of decomposition in the surface horizons (Cambardella and Elliot 1992). It constitutes the most labile pool of SOC, being a sensitive indicator for SOC losses and gains as a result of management (Skjemstad et al. 2006), and is generally more abundant under NT than when the soil is tilled (Six et al. 1999). The different depth distribution of POC in NT and NTSB suggests that even if more POC was accumulated in the topsoil when tillage was eliminated, the input of fresh aboveground crop residues was higher in NT than in NTSB, although this difference may be negligible compared with the differences in total SOC.

The mineralisation curves after 28 days of incubation (Fig. 1) corroborate the POC results, showing higher inputs of fresh crop residues in the upper 0.05 m of the soil under NT. Greater and earlier mineralisation was observed in NT, corresponding to a higher POC ('the epicenter for microbial activity', according to Six et al. 1999) content. Under NTSB, mineralisation started later than in other treatments, and was similar to MT from day 3 onwards. More C was respired by the NT samples after 1, 3, 7, and 14 (P < 0.05) and 21 and 28 (P < 0.10) days of incubation, due to the combination of higher inputs of fresh crop residues in the 0.05 m topsoil layer, and sudden soil disturbance by sampling, which does not occur under field conditions. This explains why respiration rates were higher in NT than MT, but C stocks were lower in MT than in NT.

Accumulated C[O.sub.2]-C was similar for NTSB and MT samples, indicating that the availability of SOC for microorganisms in the upper layer of the soil was similar. This does not necessarily mean that OC in both treatments was of similar nature, and might be related to the inherent recalcitrance of partially burnt and charred plant residues, illustrated by the slow start of the mineralisation for NTSB samples. Smaller C[O.sub.2]-C evolution under incubation conditions has also been reported for soils under long-term stubble burning and MT by Powlson et al. (1987). This suggests that it is more the nature of the burnt straw residues rather than the location in the soil that limits their mineralisation. Black carbon (BC) has been defined as a continuum of combustion products, ranging from slightly charred, degradable biomass to highly condensed, refractory soot (Masiello 2004). It comprises a wide range of vegetal materials generated after a fire, from partially burnt plant tissues to completely altered compounds, and receives increasing attention from soil scientists because it is an important component of SOC in burnt agro-ecosystems (Rumpel et al. 2006). The relative high recalcitrance of charcoal in relation to other soil organic components results in longer mean residence time in soil (Skjemstad et al. 2002), and thus it is considered a potential C sink. Skjemstad et al. (2002) also found that traditional methods for estimating SOC may overestimate the fraction available to microbial decomposition, because they detect and report the burnt material as 'soil organic carbon', even though remains of the burnt soil are biologically inactive.

[FIGURE 1 OMITTED]

We hypothesise that 2 processes occurred simultaneously. On one hand, stubble burning resulted in smaller POC stock in the topsoil due to less above-ground crop residue available at the soil surface after burning. On the other hand, recalcitrant forms of organic C accumulated after several years of stubble burning so that the total amount of SOC was similar in the 2 NT systems and higher than under MT. Thus, stubble burning (i) had a smaller impact on SOC (and POC) than tillage in the surface layer, and (ii) affected SOC quality but not its quantity (stock), as already suggested by Rasmussen and Collins (1991).

Soil physical properties and aggregation

Table 3 shows that [[rho].sub.b] under NT and NTSB was higher than under MT, which is commonly observed (Tebrugge and During 1999). This higher density was found only in the upper 50 mm of the soil, and not in the entire arable layer, in agreement with other studies on the spatial and temporary nature of compaction (Ismail et al. 1994) under NT. The absence of differences in [[rho].sub.b] between NT and NTSB is in line with Valzano et al. (1997), who reported differences as a consequence of stubble burning under NT in the soil hydraulic behaviour (sorptivity, infiltration and permeability), but no effect on soil bulk density ([[rho].sub.b]) and SOC.

As for the PR values, however, significant differences were observed between treatments at all depths (Table 3), related to tillage as well as to stubble burning. As expected, PR was lowest in the tilled layer under MT. PR was also significantly higher in NTSB than NT without stubble burning at all depths.

We associate the different PR values between NT and NTSB to the observed differences in pore-size distribution between NT and NTSB (Bescansa et al. 2006). The volume of water retained in the soil between -33 and -50 kPa, and the number of pores with an equivalent diameter of 6-9 [micro]m under NTSB, were smaller than under NT.

Similar to the results of total SOC, aggregate stability data indicated significant differences due to tillage in both the 0-0.05 and 0.05-0.15 m layers (Table 3), where NT treatments had a higher aggregate stability than MT, irrespective of residue management. No differences were found in the MWD after dry sieving (data not shown), which can be explained by the elevated content in CaC[O.sub.3] and clay (359 and 347 g/kg soil, respectively, in the 0-0.30 m upper layer; Bescansa et al. 2006), both components increasing the mechanical resistance of dry aggregates. The aggregate size distribution after wet sieving (Fig. 2), consistently showed larger amounts of water-stable macro-aggregates (0.250-2.0mm and >2.0mm) and fewer micro-aggregates (<0.250 mm) in the soil under NT and NTSB than under MT.

These results are similar to those found by Chan and Heenan (2005) in their short-term trial in Australia, which they explained as being a reflection of the differences in SOC. However, other long-term studies have reported lower aggregate stability under NTSB than NT (Carter and Mele 1992; Chan et al. 2002; Chan and Heenan 2005).

Aggregate stability is the result of the equilibrium reached within soil aggregates between the binding agents and the applied disruption forces. The role of SOC in water-stable aggregation was likely to be of greater importance in the soil of Chan et al. (2002) and Chan and Heenan (2005) (an acid Luvisol) and in the sandy Calcic Luvisol (66% sand) described by Carter and Mele (1992), than in our Ca-saturated, clay-rich soil (Muneer and Oades 1989; Chan et al. 2003; Six et al. 2004). In our soil, where no differences in CaC[O.sub.3] content or stock were found between treatments (Table 3), differences in aggregate stability can be explained by those found in SOC. This illustrates (i) the essential role of SOC in aggregation, even in Ca-rich soils, and (ii) the site-specific nature of the aggregation processes and its variability due to management. Physical protection of the organic matter within micro-aggregates may play a bigger role in NT and chemical protection a higher role in NTSB, but the effects of BC on macro and micro-aggregate formation remain unclear.

Earthworm activity

Burning of crop residues did not affect the number and total biomass of earthworms in spring and autumn (Fig. 3). In contrast, tillage caused a significant reduction in the population density and mass of earthworms in the upper 0.20 m of soil. This suggests that earthworms are likely to benefit more from the absence of mechanical disturbance, better temperature, and milder moisture regimes under NT than from food supply (Chan et al. 2003). We attribute the lack of differences between NT and NTSB in part to the low intensity and late occurrence of the burning, which left enough dead plant material for earthworms to live on, and also to the short duration of the fire, which did not imply an increase of soil temperature big enough to disturb their activity. These results are in agreement with those of Chart and Heenan (1993) on a Chromic Luvisol under a wheat/lupin rotation after 10 years of tillage and stubble burning. They contrast, however, with those reported by Mele and Carter (1999), who found lower earthworm densities under NT when wheat (Triticum aestivum L.) stubble was burnt in acidic Mediterranean soils, probably due to differences in soil and soil conditions, as illustrated by the effects of liming on earthworm populations in their studied soils.

[FIGURE 2 OMITTED]

Differences in the abundance of earthworms between spring and autumn were not observed under NT, regardless of the residue management, but under MT, earthworm abundance was significantly lower in autumn than in spring (Fig. 3). This may be attributed to direct effects of tillage with a mouldboard before seeding in autumn. Beyond the direct mechanical elimination of earthworms and their exposure to predators such as birds after tillage (Rovira et al. 1987), it has been suggested that the energy diverted into burrow reestablishment means less energy available for reproductive activity (Mele and Carter 1999). In Mediterranean semi-arid land, the lower soil water content under MT than under NT in spring and autumn (Bescansa et al. 2006) is also likely to affect earthworm activity, as suggested by Haines and Uren (1990). In a 7-year field trial on a sandy clay loam under semi-arid conditions in eastern Victoria, Australia, they found a stronger effect of burning than of tillage on the earthworm population, but the opposite was true for biomass per surface unit, which means bigger individuals under NTSB than under NT. Mele and Carter (1999) also reported a greater proportion of adult individuals in their NT burnt plots compared with no burning. Our data did also show a trend towards bigger earthworms under NTSB than NT in autumn. We explain this in relation to the differences observed in PR between NT and NTSB; the harder soil under NTSB in some periods would 'select' for thicker and bigger individuals.

In a detailed study on the role of earthworms in aggregation, Pulleman et al. (2005) have postulated the existence of an 'alternative pathway' to the microbial-mediated one (Six et al. 1999) for the formation of micro-aggregates in undisturbed soils. Organic residues would be directly incorporated into newly formed micro-aggregates as a result of earthworm transit in the soil. This might be the case in our soil as well, as suggested by the high amount of stable aggregates >2 mm (Fig. 2), especially under those treatments where earthworm numbers are high. The ability of earthworms to integrate BC and semi-burnt plant material into the soil matrix remains unclear and is a promising field of research in the light of these results.

[FIGURE 3 OMITTED]

Conclusions

The effect of burning barley residues on NT on a Calcic Haploxerept (Soil Survey Staff 2003) in semi-arid NE Spain affected the measured soil properties much less than the use of MT. The organic carbon stock, stable aggregation status, and earthworm numbers and biomass were similar under NT and NTSB after 10 years of yearly stubble burning in a continuous NT barley cropping system.

The most significant observed effect of stubble burning under NT was the alteration of penetration resistance and of SOC quality in the upper soil layers. Higher penetration resistance under NTSB was likely to be the explanation for thicker earthworms found under this treatment. SOC was found to be more easily mineralisable under NT than under NTSB, where less POC was also found. We attribute these differences to the accumulation of charred plant material and smaller aboveground biomass inputs under NTSB.

Our results on aggregate stability and earthworm activity were different from those reported from similar trials in other areas, where researchers have found a negative impact of stubble burning under NT. This may be due to differences in soil type and management, and in the timing of burns, which in our study took place after the summer period and shortly before seeding in fall. This also illustrates the site-specific nature of soil quality changes under controlled burning.

Our results indicate that the role of burnt plant residues and earthworms in SOC accumulation and soil aggregation in Mediterranean carbonated soils under NT, and the formation and accumulation of BC in these systems, merit further attention and require more research in the future.

Acknowledgments

The support of the Gobierno de Navarra and of the Instituto Nacional de Investigacirn y Tecnologia Agraria y Alimentaria INIA, Spanish Agency (Project no. SC98-020-C4-3) is acknowledged as well as the technical assistance of J. J. Perez de Ciriza and the staff of the Instituto Tecnico y de Gestirn Agricola-ITGA. We are grateful to Prof. M. J. Briones (Universidade de Vigo, Spain) for her assistance and suggestions on earthworm sampling and handling, and O. Fernandez and I. del Rio for carbonate analyses.

Manuscript received 6 February 2007, accepted 26 July 2007

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Valzano FP, Greene RSB, Murphy BW (1997) Direct effects of stubble burning on soil hydraulic and physical properties in a direct drill tillage system. Soil & Tillage Research 42, 209-219. doi: 10.1016/S0167-1987(96)01101-4

Zhang GS, Chan KY, Oates A, Heenan DP, Huang GB (2007) Relationship between soil structure and runoff/soil loss after 24 years of conservation tillage. Soil & Tillage Research 92, 122-128. doi: 10.1016/j.still.2006.01.006

Inigo Virto (A,C), Maria Jose Imaz (A), Alberto Enrique (A), Willem Hoogmoed (B), and Paloma Bescansa (A)

(A) Departamento Ciencias del Medio Natural, E.T.S.I. Agronomos, Universidad Publica de Navarra, Campus Arrosadia, 31006 Pamplona, Spain.

(B) Farm Technology Group, Wageningen University, The Netherlands.

(C) Corresponding author. Present address: Equipe M.O.S. Unite Mixte de Recherche 'Biogeochimie des Milieux Continentaux', Bat. EGER. Institut National Agronomique Paris-Grignon, 78850 Thiverval-Grignon, France. Email: inigo.virto@unavarra.es or ivirto@grignon.inra.fr
Table 1. General soil characteristics and particle size distribution
in the experimental plots

Data correspond to the soil characterisation pits dug before the
implementation of the experimental field (rainfed spring barley under
conventional tillage for decades)

Soil depth (m) 0-0.30 0.30-0.75 0.75-1.05

Particle size distribution (g/kg):
 Sand (50-2000 [micro]m) 291 315 277
 Silt (2-50 [micro]m) 362 322 328
 Clay (<2 [micro]m) 347 363 395
Bulk density (Mg/[m.sup.3]) 1.48 1.76 1.79
Available water capacity (A)
 ([m.sup.3]/[m.sup.3]) 0.169 0.191 0.193
Organic carbon (g/kg) 5.85 3.48 2.90
CaC[0.sub.3] (g/kg) 359 360 335
pH (water) 8.25 8.5 8.2
Electrical conductivity (dS/m) 0.58 1.52 4.54
Cation exchange capacity (cmol/kg) 19.7 20.7 21.6

(A) Soil moisture content between -33 and -1500 kPa.

Table 2. Stocks of total C (SOC) and C in the particulate organic
matter (POC), and average barley grain yield under different stubble
and tillage treatments (mean [+ or -] standard error)

NT, No tillage; NTSB, no tillage with stubble burning; MT, mouldboard
tillage. Within columns, values followed by different letters belong
to different Duncan's homogeneous groups (P < 0.05), for each depth

Depth Treatment SOC POC
(m) (Mg/ha) (Mg/ha)

0-0.05 NT 11.3 [+ or -] 1.19a 3.01 [+ or -] 0.13a
 NTSB 11.2 [+ or -] 0.55a 2.56 [+ or -] 0.13b
 MT 8.42 [+ or -] 0.17b 1.83 [+ or -] 0.07c

0.05-0.15 NT 16.6 [+ or -] 0.44 1.29 [+ or -] 0.06a
 NTSB 16.9 [+ or -] 0.37 1.06 [+ or -] 0.05b
 MT 16.0 [+ or -] 0.48 1.07 [+ or -] 0.06b

0.15-0.30 NT 23.6 [+ or -] 0.49 0.85 [+ or -] 0.12
 NTSB 23.0 [+ or -] 0.58 0.55 [+ or -] 0.03
 MT 22.5 [+ or -] 0.41 0.83 [+ or -] 0.07

Depth Treatment Yield (A)
(m) (Mg/ha)

0-0.05 NT 3.70 [+ or -] 0.27
 NTSB 3.64 [+ or -] 0.50
 MT 3.70 [+ or -] 0.44

0.05-0.15 NT
 NTSB
 MT

0.15-0.30 NT
 NTSB
 MT

(A) Average barley grain yield from 1995 to 2006.

Table 3. Bulk density ([[rho].sub.b]), penetration resistance (PR),
aggregate mean diameter after wet sieving ([MWD.sub.W]), and
CaC[O.sub.3] under different stubble and tillage treatments (mean
[+ or -] standard error)

NT, No tillage; NTSB, no tillage with stubble burning; MT, mouldboard
tillage. Within columns, values in the same column marked with
different letters belong to different Duncan's homogeneous groups
(P < 0.05), for each depth

Depth Treatment [[rho].sub.b] PR
(m) (Mg/[m.sup.3]) (MPa)

0-0.05 NT 1.67 [+ or -] 0.04a 0.98 [+ or -] 0.09b
 NTSB 1.73 [+ or -] 0.03a 1.35 [+ or -] 0.16c
 MT 1.55 [+ or -] 0.01b 0.38 [+ or -] 0.04a

0.05-0.15 NT 1.69 [+ or -] 0.02 1.49 [+ or -] 0.09b
 NTSB 1.71 [+ or -] 0.03 1.85 [+ or -] 0.12c
 MT 1.63 [+ or -] 0.03 0.82 [+ or -] 0.11a

0.15-0.30 NT 1.67 [+ or -] 0.02 1.49 [+ or -] 0.1Ob
 NTSB 1.62 [+ or -] 0.04 1.84 [+ or -] 0.13c
 MT 1.62 [+ or -] 0.03 0.85 [+ or -] 0.08a

Depth Treatment [MWD.sub.W] CaC[0.sub.3]
(m) (mm) (Mg/ha)

0-0.05 NT 1.33 [+ or -] 0.15b 267 [+ or -] 13
 NTSB 1.48 [+ or -] 0.16b 276 [+ or -] 14
 MT 0.54 [+ or -] 0.08a 252 [+ or -] 3

0.05-0.15 NT 1.58 [+ or -] 0.10b 542 [+ or -] 21
 NTSB 1.65 [+ or -] 0.09b 569 [+ or -] 23
 MT 0.87 [+ or -] 0.10a 538 [+ or -] 8

0.15-0.30 NT n.a. 798 [+ or -] 35
 NTSB n.a. 798 [+ or -] 22
 MT n.a. 822 [+ or -] 17

n.a., Not analysed.
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Author:Virto, Inigo; Imaz, Maria Jose; Enrique, Alberto; Hoogmoed, Willem; Bescansa, Paloma
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:4EUSP
Date:Sep 1, 2007
Words:6361
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