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Long-term acidification of a Brazilian Acrisol as affected by no till cropping systems and nitrogen fertiliser.

Abstract. Cropping systems and N fertilisation affect soil acidification mainly due to the removal of alkaline plant material from the field and nitrate leaching. The study evaluated the acidification of a subtropical soil under no till cropping systems with different C and N addition rates for 19 years. The contributions of leguminous and non-leguminous crops (fallow/maize, black oat/maize, black oat + vetch/maize, black oat + vetch/maize + cowpea, lablab + maize, pigeon pea + maize, and digitaria) and mineral N fertiliser (0 and 180 kg N/ha.year as urea) to total acidification were estimated. Cropping systems and N fertilisation significantly affected soil pH, which ranged from 4.3 to 5.1. The presence of leguminous species and mineral N promoted greater decreases in soil pH and net soil acidification, which resulted in increases in exchangeable A1 content and A1 saturation. Black oat + vetch/maize with N fertilisation promoted the highest soil net acidification rate (2.65 kmol [H.sup.+]/ha.year), while digitaria had the lowest (1.07 kmol [H.sup.+]/ha.year). Leguminous species and N fertilisation increased soil acidification through changes in the C cycle associated with the removal of alkaline plant material by grains. Leguminous-based cropping systems promoted higher maize yields than those comprising essentially gramineous species, indicating an opportunity for a reduction in N fertiliser rates. With N application, however, maize yield did not differ among cropping systems, despite differences in soil pH and exchangeable Al.

Additional keywords: soil acidification, cropping systems, nitrogen, no till, chemical characteristics, buffering capacity.

Introduction

Soil acidification occurs under natural and agricultural ecosystems and is directly related to the carbon (C) and nitrogen (N) cycles in the soil. Agriculture usually increases soil acidification rates, which are affected by soil management practices due to their effects on C and N cycles in the soil. The C and N cycles are the main agents responsible for soil acidification in most cases, with acidification depending predominantly on the forms and quantities in which each element comes or leaves the system. Under agriculture, soil acidification results mainly from the removal of plant materials through grain yields, hay, and silage, leaching of nutrients (specially nitrate), accumulation of organic matter, erosion of fertile soil, and fertiliser reactions (van Breemen et al. 1983; Helyar and Porter 1989; Fenton and Helyar 2002; Verburg et al. 2003).

Cropping rotation systems have different effects on C and N inputs to soil, which depend on the biomass production and biological fixation of nitrogen by leguminous species (Burle et al. 1997; Bayer et al. 2000) and, therefore, can affect soil acidification rates. In addition, intrinsic characteristics of crop species with respect to organic anion content and their ability to intercept nitrate influence soil acidification under different cropping systems (Helyar 1976; Slattery et al. 1998; Franchini et al. 2001).

In addition to the biological fixation of N, this element can be added to the system through application of N fertilisers, which can have a significant effect on soil acidification. In this case, 2 aspects are fundamental in relation to soil acidification. One aspect is related to the source of mineral N applied to the soil, as each form has an impact on potential acid generation (Bouman et al. 1995; Barak et al. 1997; Fenton and Helyar 2002). The other aspect is linked to the N losses by nitrate leaching. Nitrogen fertilisation should be done according to the plant requirement in order to decrease nitrate leaching and, consequently, minimise soil acidification (Helyar 1976; Tang et al. 2000). In this way, nutrient efficiency use is improved, farmer profit is increased, and environmental risks of nitrate contamination are reduced.

Understanding soil acidification processes and determining the most relevant cause factors in each agricultural system is fundamental to the development of strategies to decrease soil acidification rates. Several studies have focused on the C cycle and the contribution to soil acidification, mainly using pastures and forages (Ridley et al. 1990a; Coventry 1992; Loss et al. 1993). However, the long-term effect of no till cropping systems on soil acidification is still not well understood. Similarly, previous studies have compared different sources of mineral N and their effect on soil acidification (Bouman et al. 1995; Barak et al. 1997), but few have attempted to compare mineral N fertilisation to biologically fixed N (Burle et al. 1997). This study evaluates the rates of soil acidification under no till cropping systems with different addition rates of C and N over 19 years, and estimates the contribution of leguminous and non-leguminous crops and mineral N fertiliser to soil acidification.

Material and methods

Site characteristics

The study was based on a long-term experiment (19 years) established at the Agronomic Experimental Station of the Federal University of Rio Grande do Sul, in Eldorado do Sul (RS), Southern Brazil (30[degrees]51'S, 51[degrees]38'W). The soil is classified as sandy clay loam Acrisol by FAO classification and as Typic Paleudult by the American classification. When the experiment was initiated in 1983, the soil showed visible signs of physical degradation caused by conventional tillage that had been adopted over the previous 13 years. The soil particle size distribution is 540 g/kg of sand, 240 g/kg of silt, and 220 g/kg of clay, with the clay fraction composed mainly by kaolinite (720 g/kg) and iron oxides (109 g/kg of [Fe.sub.2][O.sub.3]). The local climate is subtropical humid, Cfa type, according to Koppen classification, with annual mean temperature of 19.4[degrees]C and rainfall of 1440mm evenly distributed throughout the year.

The experiment comprises 10 no till cropping systems in the main plots (8 by 5 m) and 2 N fertilisation levels, 0 kg N/ha.year (ON), and mean application of 145 kg N/ha.year (145N) applied to the maize crop as urea, in the subplots (4 by 5 m), according to a split-plot randomised block design with 3 field replications. Between 1983 and 1994, the N fertilisation rate was 120 kg N/ha.year and thereafter increased to 180 kg N/ha. year, resulting in a mean of 145 kg N/ha.year. In order to obtain a wide range of C and N additions by plant residues and maize yield, 7 cropping systems were selected (winter/summer cropping system species): fallow/maize (Zea mays L.) (F/M), black oat (Avena strigosa Schreb)/maize (O/M), black oat + vetch (Vigna sativa L.)/maize (OV/M), lablab (Dolichos lablab) + maize (L + M), black oat + vetch/maize + cowpea (Vigna unguiculata [L.] Walp.) (OV/MC), pigeon pea (Cajanus cajan L. Millsp.) + maize (P + M), and digitaria (Digitaria decumbens) (Di) (Table 1). The first 4 cropping systems were evaluated under the 2N rates, while the last 3 systems were evaluated only without N application. The OV/M system consisted of oat + subterranean clover (Trifolium subterraneum L.)/maize from 1983 to 1992, when the clover was replaced by vetch.

Maize is usually sown in October, and black oat and vetch in April, both mechanically. Cowpea and lablab are sown about 30-40 days after maize is sown, using a manual drill. Pigeon pea was re-established every 4 years, but the plants are trimmed each year at about 1 m height, before sowing the maize. In the F/M system, weeds were allowed to establish during the winter. Since the establishment, the soil has not been limed. Annually, 50 kg/ha of [P.sub.2][O.sub.5] (as single or triple superphosphate) and 50kg/ha of [K.sub.2]O (as KC1) are applied to the maize crop a depth of 0.10 m below the plating line. Supplementary irrigation was applied to the maize crop. Plant residues were managed with a cutter-roller before maize was sown and, if necessary, weeds were eliminated with glyphosate application. In the Di system, maize was sown only in 4 years (1989, 1993, 1995, and 2000) during the experimental period, while in the other cropping systems maize was cropped each year. Digitaria was never grazed and its biomass was retained on the plot.

Soil sampling and analysis

Soil samples from the layers 0-0.025, 0.025-0.05, 0.05-0.075, 0.075-0.125, 0.125-0.175, and 0.175-0.30m depth were manually collected in October 2002, air-dried, ground, and sieved (<2 mm). They were analysed for soil pH in water (1 : 1); exchangeable Ca, Mg, K, and Al; H + A1 titratable to pH 7.0, according to Sparks (1996); and the A1 saturation and base saturation were then calculated. The soil organic C concentration was determined by dry combustion, using a TOC analyser (Shimadzu VCSH). In order to correct soil compaction and differences in soil bulk densities among treatments, the organic C content was calculated using the equivalent mass approach (Ellert and Bettany 1995; Sisti et al. 2004) by taking the highest mass of soil (soil under O + V/M) as reference. Soil bulk densities were determined by Pillon (2000) using the core method (Table 1).

The pH buffer capacity of the soil from each cropping system was determined by curves of pH after addition of alkaline or acid solutions at known concentrations. This analysis was performed only in soils with no mineral N fertilisation. Aliquots of 20 g of soil (7 replicates) from the 0-0.025 m layer (having a wide range of organic C content in soil samples) were placed in snap caps containing 40 mL of HCl (3.9, 7.8, and 17.2 mmol/L), NaOH (4.1, 8.7, and 17.4 mmol/L), or only distilled water. Samples were stirred with a glass rod and hermetically closed, and pH values were determined after 24 h. The soil buffering capacity (pHBC) was assumed as the amount of alkali or acid (converted to cmol [H.sup.+])required to change the pH value of 1 kg of soil by 1 unit. The estimated buffering capacity refers to the pH 4.0-6.5 range, where it fits a linear equation, and pHBC was taken as the slope of the line, as in this pH range the pHBC is supposed to be approximately linear (Helyar and Porter 1989). The results of pHBC were correlated with carbon content and the equation was used to estimate the pHBC for each soil layer, considering that texture and mineralogy are very similar in the 0-0.30 m soil depth. The pHBC was then used to estimate the total acidification in each soil layer by using the equation adapted from Helyar and Porter (1989):

Acidification (kmol [H.sup.+]/ha) = [DELTA]pH x pHBC x MR (1)

where: pHBC is soil buffering capacity, in kmol [H.sup.+]/kg.(pH unit); MR is soil mass of reference, in kg/ha, which corresponds to the soil mass in the treatment O + V/M in each of the 6 layers at 0-0.30m depth. The soil pH in 1983 was 5.8 in the 0-0.175m layer and 5.2 at 0.175-0.30 m layer. The total acidification in the 0-0.30 m layer was obtained from the sum of acidifications in each of the 6 layers.

Once the buffer capacity for each soil treatment was obtained and the total acidification in the 0-0.30m layer during the period of 19 years was estimated, the framework of Helyar and Porter (1989) for modelling soil acidification was used to estimate the absolute contribution of the N cycle in total soil acidification, as well as to estimate the relative contribution of each component to soil acidification:

Acid addition (mol/ha.period)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where OA represents organic anions in organic matter, residues, and removed plant products; HC[O.sub.3] represents the bicarbonate products in solution; and L represents lime or other alkali additions. The subscripts ac, ex, and ad represent, respectively, material accumulated in, exported from, or added to the system. The units of all terms are mol(+) or mol(-)/ha.period. Further details are provided in Helyar and Porter (1989) and Verburg et al. (2003).

The acidification related to the C cycle in the present study was estimated taking into account the acidification due to the removal of corn grain from the field and the shifts in organic matter contents in soil during the 19 years. The net balance of bicarbonate fluxes was not taken into account in the calculation as it was assumed to be negligible under the prevailing conditions in the acidification of this subtropical acid soil.

In order to estimate the alkalinity of corn grain, samples (2 g dry mass grains) were ashed at 550[degrees]C for 4 h in a muffle furnace. Upon cooling, 20 mL of 1 mol/L HCI was added to the ashed sample and stirred. A subsample (5 mL) was then titrated to end point using 0.25 mol/L NaOH and phenolphthalein as indicator. The results indicated no significant difference in ash alkalinity among cropping systems and N fertiliser rate. It was found that the acidity generated by the harvest of each Mg of corn grain from field would require ~11.9 kg CaC[O.sub.3] for its neutralisation. The results are slightly higher than the value of 9 kg CaC[O.sub.3]/Mg of corn grain obtained by Slattery et al. (1991). The acidification associated with grain removal was converted to CaC[O.sub.3] equivalent by assuming 0.5 Mg CaC[O.sub.3] is required to neutralise 10 kmol [H.sup.+] (Fenton and Helyar 2002).

The acidification due to organic matter accumulation (O[A.sub.ac]) was estimated on the soil layer 0-0.175 m depth interval because there are no available C stock data for deeper layers at the beginning of the experiment and because in no till systems the accumulation of organic matter occurs predominantly in surface layers. According to Fenton and Helyar (2002), a buildup of soil organic matter partially decomposed or of live and dead biomass above the soil has the same effect on soil acidity as removing products from field. However, the organic anion content of soil organic matter is not easily measured because of problems of extracting all the soil organic matter and changes during extraction. Thus, Eqn 3, suggested by Helyar and Porter (1989), was used in the present study for estimating the acidification due to organic matter accumulation, which assumes a cation exchange capacity value (mean of 60 soils) of 32 cmol/kg of organic matter and a pH for zero net charge of 1.5:

[OA.sub.ac](kmol/ha)-{[[OC.sub.t2]([pH.sub.t1] - 1.5)]-[[OC.sub.t1]([pH.sub.t1]-l.5)]} x 0.32 (3)

where [OC.sub.t2] is total organic carbon stock in 2002 (Mg/ha) calculated by equivalent soil mass; and [OC.sub.t1] is initial stock of total organic carbon in the soil (33.4 Mg/ha).

The loss of ammonium and organic anions in runoff and leaching was neglected, and the effect of inorganic C pool was not accounted for (input of HC[O.sub.3.sup.-] by rainfall and its loss by leaching or run-off) on soil acidification. Lime was not applied to the soil since the establishment of the experiment, and acid rainfall was not detected in the locale. It was considered that acidification due to biogeochemical cycles of elements other than C and N was negligible. The acidification due to application of single and triple superphosphate, however, was considered in the estimates using a value of 8 and 15 kg CaC[O.sub.3], respectively, to neutralise the acidification generated from application of 100 kg fertiliser (Bolan and Hedley 2003). The cropping systems had the same annual rate of fertilisation (with exception of digitaria, which was not fertilised with NPK), and therefore the annual acidification due to the phosphate fertiliser was equivalent to a mean value of 10 kg CaC[O.sub.3]/year.

By difference between the total net acidification and the acidification attributed to the aforementioned components, the effect of nitrate leaching on soil acidification was estimated. Within the N cycle, the single major cause of acidification is usually the leaching of nitrate following nitrification of ammonium, which was either applied as fertiliser (urea) or mineralised from soil organic N in the experiment. In most aerated subtropical soils at pH 5-8, accumulation of ammonium and nitrate ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) is limited by nitrification and plant uptake (Helyar and Porter 1989; Verburg et al. 2003), and therefore, the acidification due to their accumulation was disregarded in the acidification estimation.

Statistical analysis

Results of soil analysis were statistically analysed by 1- and 2-way analysis of variance (ANOVA) with 3 replicates, using SAS version 8.02 (Statistical Analysis System Institute, Cary, NC). Treatment means were separated by Tukey Test (P < 0.05 and P < 0.10). Relations among variables were evaluated by means of determination coefficient ([r.sup.2]).

Results and discussion

Cropping systems and acidification

The values of pH-[H.sub.2]O, exchangeable Al, base saturation, and A1 saturation over 4 periods (0, 5, 10, and 19 years) during the study are presented in Figs 1 and 2. Significant declines in soil pH were observed over the 19 years among cropping systems (Fig. 1a) in all layers in the 0-0.30 m depth (Fig. 2a). Leguminous-based cropping systems (P + M, L + M, O + V/ + C, and O + V/M) had a greater impact on decrease in pH (about 1 unit of pH decline in 19 years) than cropping systems composed essentially of gramineous species (O/M, F/ M, and Di), resulting in the lowest pH values in the soil profile. These systems promote higher N inputs to the soil, thus increasing the potential for nitrate loss by leaching and the removal of alkaline vegetative material, as the higher N availability increases maize grain yield. However, differences in pH decrease between cropping systems containing subtropical legumes (vetch) and tropical legumes (cowpea, lablab, and pigeon pea) were not evidenced.

[FIGURE 1 OMITTED]

As a consequence of the low pH values, soils under leguminous-based cropping systems had the highest values of exchangeable A1 (Figs 1b and 2b), as this attribute increases significantly as pH decreases below 5.5 (Marion et al. 1976), promoting a concomitant decrease in base saturation (Figs 1c and 2c) and increase in A1 saturation (Figs 1d and 2d). In general, the higher rates of pH decrease occurred in the first years of evaluation, mainly until the fifth year, with about a 50% drop in pH in some cases and a tendency of decreasing rates in the subsequent period (Table 1). On average, the rate of soil pH decrease was 0.076, 0.053, and 0.041 pH unit/year in the 1st-5th, 6th-10th, and 11th-19th years, respectively. As mentioned above, the O + V/M cropping system was composed of oat+ clover/maize from 1983 to 1992, when the clover was substituted by vetch. Coincidently, this cropping system resulted in the greatest drop in soil pH values during the first years of the experiment (0.117 pH unit/year) compared with the other cropping systems, and after the switch to vetch, the rate of pH decrease declined in such a way that, in 2002, P + M and O + V/M + C had pH values similar to that from O + V/M. The effect of clover on soil acidification is highlighted in the literature (Ridley et al. 1990b; Coventry 1992). However, in case of the present study, the decline in the rate of pH decrease after the introduction of vetch may be due to the pH in this treatment reaching a critical value, below which pH is strongly buffered by mineral dissolution and leaching of the ionic products from this dissolution (Helyar 2003; Lesturgez et al. 2006). In this way, the pH tends to reach a stable value (steady-state), as the acidification caused by the C and N cycles tends to equilibrate with the alkalinisation due to mineral dissolution (Helyar 2003).

The smallest pH drop over the 19 years in the soil was under digitaria (Fig. la and Table 1) and its highest pH values down the soil profile after the period (Fig. 2a) suggested that this is a more closed system regarding C and N losses. This is consistent with the fact that, in this system, there was little removal of alkaline vegetative material, as there was exportation of grains only in 4 of the 19 years. In addition, minimal nitrate leaching is expected due to the dense and abundant root system of this grass species, promoting an efficient nitrogen uptake.

The L + M system promoted a distinct pattern of pH in the soil profile, with the highest pH value in soil surface and the lowest in deeper layers compared with the other cropping systems (Fig. 2a). According to Tang et al. (2000), it is due to the spatial separation of alkalis and acids generation in the C and N cycles. In the C cycle, while acids exudated by roots due to an excess of cation uptake over anions during plant growth are distributed in all soil layers explored by roots, plant residues are mainly oxidised at the soil surface, thus promoting acidification in subsoil and alkalinisation in topsoil. In the N cycle, mineralisation of organic N occurs mainly at the soil surface, where ammonification+nitrification generates net acidification. This is neutralised if the nitrate is absorbed by the root system due to the exudation of [OH.sup.-]UHC[O.sub.3.sup.-] that occurs in order to maintain the neutrality of the root cells. Therefore, if acidification due to nitrate leaching is predominant, lower pH values would be expected in surface layers than in subsoil. As this was not verified, the results suggest that acidification due to the C cycle was more effective than that due to the N cycle, especially in the L+M system. Further discussion about the relative contribution of C and N cycles to soil acidification will be presented later.

[FIGURE 2 OMITTED]

Base saturation values (Fig. 2c) ranged mostly between 30 and 50% and A1 saturation values (Fig. 2d) were elevated (>10%). These indicators, in addition to the pH and exchangeable Al values, indicate that liming is advisable on this soil according to the current criteria for liming in the south of Brazil (CQFS RS/SC 2004).

Nitrogen mineral fertilisation and soil acidification

Application of mineral N promoted soil pH values significantly lower than non fertilised soils (Fig. 3a), with a concomitant increase in exchangeable A1 (Fig. 3b). The effect of N fertilisation on pH decline can be attributed to both N[O.sub.3]-N leaching and higher export of organic anions in the corn grain. Annual application of 145 kg urea-N/ha resulted in a mean (0-0.30 m depth) decrease of 0.25 pH unit over 19 years, when compared to the treatments receiving no mineral N fertilisation. Conyers et al. (1996) observed that, after 12 years, annual applications of 100 kg urea-N/ha (split in 3 applications) to wheat led to a mean decrease of 0.4 pH unit in the surface soil (0-0.10 m) compared with treatments receiving no mineral N.

In this experiment, mineral N was always applied as urea. According to Fenton and Helyar (2002), although urea hydrolysis generates [H.sup.+] and, therefore, is acid in the short-term (Eqn 4), N added to the soil as urea or through biological fixation causes net acidification in medium- and long-terms only if there is N loss as nitrate leaching. As previously mentioned, if all nitrate is absorbed by plants, the exudation of 1 mol of [OH.sup.-] (or HC[O.sub.3.sup.-]) per tool of N[O.sub.3.sup.-] uptake by the root cells neutralises the reaction. The authors reported that when N is applied as urea, it is necessary to add 3.6 kg of lime to neutralise the acidity generated for each kg of N lost as nitrate through leaching. This behaviour is different when other forms of N fertiliser are used, as in case of ammonium sulfate or mono-ammonium phosphate, in which even without loss by leaching, there is net soil acidification. In contrast, the application of N as calcium nitrate or sodium nitrate alkalinises the soil if there is no loss by nitrate leaching and is neutral if 100% of applied N is lost by nitrate leaching (Fenton and Helyar 2002):

CO[(N[H.sub.2]).sub.2] + 4[O.sub.2] = [2H.sup.+] + 2N[O.sub.3.sup.-] + [H.sub.2]O + C[O.sub.2] (4)

[FIGURE 3 OMITTED]

Mineral N application, in affecting soil acidification, also affected exchangeable A1 (Fig. 3b), with significant differences in all soil layers analysed in the study. In some soil layers, exchangeable A1 levels were approximately doubled in N fertilised treatments compared with non N fertilised soils. In addition, in soils that received no N fertiliser additions, exchangeable A1 was >1 cmolc/kg, considered high for plant roots (CQFS RS/SC 2004), only in the deepest layer of 0.175 0.30m, while in N-fertilised soil this value was surpassed in almost all soil layers evaluated, with exception of the layer 0-0.025 m.

pH buffer capacity and net soil acidification

The net soil acidification is a function of the change in pH and the pH buffer capacity of the soil. Figure 4 presents the values of buffer capacity obtained from pH titration curves in surface soil (0-0.025 m depth), which ranged from 1.01 to 1.75 cmol/kg. (pH unit) for F/M and L + M, respectively. These values are included in the range 0.2 3.0 cmol/kg.(pH unit) usually found in literature for Australian agricultural soils (Coventry and Slattery 1991; Loss et al. 1993; Conyers et al. 1996). The pHBC showed a significant relationship to the content of soil organic carbon ([r.sup.2] = 0.98, P < 0.001), which is due to the high density of functional groups in organic matter that act as buffer by association/dissociation of these groups. The equation in the Fig. 4 suggests that the buffer capacity of this soil is mainly dependent on organic matter, as the pHBC is low when organic carbon is absent. According to the equation, the pHBC by mineral constituents in this soil was approximately 0.3 cmol [H.sup.+]/kg soil.(unit pH).

[FIGURE 4 OMITTED]

The equation obtained in Fig. 4 (pHBC = 0.305+0.046 TOC) was used to estimate the pHBC of the other soil layers based on their organic carbon contents (Table 2) calculated in equivalent soil mass. Afterwards, the net soil acidification (kmol [H.sup.+]/ha) in the layer 0-0.30 m was assessed for the period of 19 years using Eqn 1, in addition to the data for pH-[H.sub.2]O and bulk density from Table 2, and the results were converted to equivalent CaC[O.sub.3] (Table 3). The contribution of different sources of acids and alkalis to the soil acidification was also estimated, according to the methods described above.

Net acidification occurred in all treatments, ranging from 20.4 to 50.4 kmol [H.sup.+]/ha (equivalent to 1.02 Mg CaC[O.sub.3]/ha and 2.52 Mg CaC[O.sub.3]/ha, respectively) for the period of 19 years, which corresponded to rates from 1.07 to 2.65 kmol [H.sup.+]/ha.year. The leguminous-based cropping systems had higher total acidification than those based only on gramineous species. Besides, the application of N fertiliser promoted soil acidification, with exception of lablab+maize system, in which differences due to N fertiliser on soil acidification were not observed. The possible reason for this behaviour is that the lablab likely had great ability to minimise the leaching of nutrients in the soil profile. The application of 180 kg urea-N/ ha.year in F/M, O/M, and O + V/M increased the total acidification 45, 58, and 35%, respectively. The highest acidification occurred in soil under O + V/M 180 N and O/M 180 N. However, the lime requirement to raise the soil pH to the same value as 1983 in the 0-0.30 m layer is relatively low, as the results indicated a maximum requirement of 2.52 Mg CaC[O.sub.3]/ha.

The acidification rates estimated in the present study are consistent with those usually observed in agricultural ecosystems, from 2 to 5 kmol [H.sup.+]/ha.year, according to Helyar (2003). However, the range can be wider according to the cropping system or agricultural activity. Coventry and Slattery (1991) reported a net acid input of 3.22, 4.11, and 5.26 kmol [H.sup.+]/ha.year for continuous wheat, wheat-lupin sequence, and continuous lupin, respectively, in the 0-0.20 m soil layer. Lesturgez et al. (2006) found 7.6 kmol [H.sup.+]/ha.year in limed soil (0-0.50 m depth) under cowpea--maize cropping system. In pasture paddocks, low rates of 0.16-0.21 kmol [H.sup.+]/ha.year are cited by Loss et al. (1993) in the 0-0.60 m soil depth, while Ridley et al. (1990b) reported rates of 0.16 to 2.37 kmol [H.sup.+]/ha. year in fertilised and non-fertilised pastures.

The C cycle contributed to the soil acidification mainly via the removal of alkaline products from the field, through the harvest of maize grain, which was higher in leguminous-based cropping systems and in N-fertilised soils. Accordingly, the harvest of corn grain in N-fertilised soils during 19 years promoted an extra acidification that would require about 0.8 Mg CaC[O.sub.3]/ha more than in non fertilised soils, in order to neutralise this source of acidification (Table 3).

The changes in soil organic matter stock promoted distinct effects on soil acidification. According to the model, alkalinisation occurred in soils where organic carbon stocks decreased (negative values in Table 3), mainly in the F/M system, while acidification occurred in soils where the stocks increased. Therefore, changes in organic carbon stocks promoted soil acidification ranging from -0.39 to 1.44 equivalent Mg CaC[O.sub.3]/ha for the systems F/M 0 N and L + M 180N, respectively. Apart from the L + M system, however, the [OA.sub.ac] made a relatively low contribution to the soil acidification, representing, on average, only 12% of the total. It is of note that the effect of organic matter stocks on soil acidification was estimated by extrapolation using the equation suggested by Helyar and Porter (1989), which is not calibrated to the characteristics of organic matter of the present study, and therefore, these results should be treated with caution. However, it might provide an estimate of the magnitude of the soil acidification due to C cycle.

The effect of C cycle (removal of alkaline products from field plus the effect of changes in soil carbon stocks considered in this study) on soil acidification resulted in contrasting contributions to the total acidification. In soil under digitaria, the C cycle had no effect on soil acidification, which is due to the low exportation of organic anions from field by maize grains, as maize was cultivated in this system only for 4 years during the 19 years of the experiment, while in the other cropping systems maize was cultivated each year. The highest acidification values due to C cycle were found in soil under lablab+maize with 0 and 180 kg N/ha.year, which had the highest acidification due to accumulation of organic matter in addition to the acidification promoted by the exportation of maize grains. The fact that the lablab + maize system had the highest relative contribution of C cycle to net soil acidification and the lowest relative contribution of N cycle among the cropping systems is in agreement with the greatest spatial separation of alkalis and acids in soil under this cropping system, which led to the highest pH value at topsoil and the lowest in subsoil layers, as discussed previously. The lablab crop might have relocated the alkalis from deep layers to surface layers, decreasing the total acidification in the 0-0.30 m layer.

The acidification due to the N cycle (nitrate leaching), which was obtained by difference between the total acidification and the sum of the other factors of soil acidification, ranged from null in soil under L + M 0 and 180 N to about 1.07 Mg CaC[O.sub.3]/ha in soil under F/M 0 N. The acidification due to the C cycle is related to that due to the N cycle. The hypothesis that the presence of leguminous species in the cropping systems and fertilisation with mineral N would increase soil acidification due to higher N availability and consequent higher N loss by leaching was contested by the results of the present study. The presence of leguminous species and the mineral N fertilisation increased the soil acidification, but it happened mainly due to the higher relative participation of the C cycle in the acidification, as consequence of the increase in maize grain yield promoted by those treatments.

Observing the effect of leguminous crops in absence of N fertilisation (0 N), the inclusion of vetch to the O/M system increased the relative acidification due to C cycle from 25.8% to 46.9%, reaching 72.1% under vetch and cowpea (O + V/M + C). Similar analysis can be done with the mineral N fertilisation, in which, within each cropping system, the application of 180 kg N/ha.year decreased the relative contribution of the N cycle on soil acidification in comparison to the corresponding non fertilised soil. Evidently, it does not mean that the quantity of N lost by nitrate leaching was smaller in soils under leguminous-based cropping systems and mineral N fertilisation than in those soils under gramineous-based cropping systems and without N fertilisation. It does mean that the proportion of acidification due to C cycle was increased by the higher contents of N in the soil-plant system.

The reliability of the results and discussion is dependent on the level of veracity of the assumptions adopted and the estimation of the pHBC utilised. As the acidification due to nitrate leaching in N cycle is obtained by difference, this approach assumes that all acidification that is not accounted for by other processes was due to nitrate leaching. Therefore, the over- or under-estimation of these processes, as well as in pHBC, implies a concomitant error in the estimation of soil acidification due to N leaching. As pointed out by Porter et al. (1995) and Verburg et al. (2003), uncertainty in pH buffer capacity may be a source of error in the framework of Helyar and Porter (1989) for estimating soil acidification. In case of a higher total acidification than that found, it would imply a greater relative contribution of N cycle to the soil acidification concomitantly to a smaller relative participation of C cycle.

Because some soil layers achieved relatively low pH values, it is probable that the weathering of alumino-silicate minerals prevented a further decrease in soil pH, and it should be considered in the assessment of the net acidification found. According to Lesturgez et al. (2006), the net acid addition rate estimated from changes in soil pH and pHBC is not an appropriate method for assessing acidification in soils under pH([CaCl.sub.2]) < 4.0 (approximately pH([H.sub.2]0) < 4.5). By evaluating the disorder and crystallinity of kaolinite, the main clay mineral in the Thai soil studied, the authors suggested that the dissolution of this secondary mineral was responsible for buffering soil pH at 4.0, and that the pH([CaCl.sub.2]) 4.0 is a threshold value in that soil. In the present study, similarly, kaolinite is the main clay mineral (720 g/kg clay). In Fig. 1a, the drop in pH values in most of cropping systems showed a tendency of stabilisation in soil pH (steady-state) with time, which can be due to a buffering mechanism from the dissolution of minerals. If the dissolution was really effective, the net acidification was underestimated, as well as the acidification caused by nitrate leaching.

Two other possible mechanisms that could buffer soil pH in acid soils are the restrictions on plant yield and on nitrification rates. However, apparently nitrification has not been restricted in the soil of the experiment, as suggested by the absence of ammonium accumulation (data not shown). Similarly, the maize yield seems to be not affected by the differences in soil pH and exchangeable A1 (Fig. 5), as the cropping systems did not differ in maize grain yield when mineral N was applied. It is important to note that, although in most of the cropping systems the soils had pH values <5.0 and elevated exchangeable Al, in the last years, the grain yield reached more than 8000 kg/ha when mineral N was applied. The long-term impact of high annual C input by leguminous-based cropping systems associated with no tillage might have promoted the alleviation in A1 toxicity due to its complexation with organic matter and the adequate level of nutrients in the surface soil (Burle et al. 1997). In addition, maize irrigation probably contributed to minimise the injurious effects of acidity on plants, by improving water and nutrient supply. Future studies are required to confirm these observations.

[FIGURE 5 OMITTED]

In the absence of mineral N fertilisation, cropping systems involving leguminous species had higher grain yield, highlighting the importance of the inclusion of these species in a crop rotation preceding maize, in order to diminish the requirements of mineral N and production costs (Fig. 5). The presence of leguminous species promoted maize grain yield up to 3 times higher than that in the F/M system. In spite of that, however, results presented in Fig. 5 suggest that the N fertilisation must be done in all cropping systems, in such way that the mineral N complements the quantity of N from soil and biologic fixation. Improvements in physical, biological, and chemical conditions of the no-till soil, the use of maize varieties of higher yield potential, and the increase in mineral N rate help to explain the higher maize grain yield in the latter years compared to the former years of experiment.

Conclusions

Soil acidification was intensified by leguminous species in notill cropping systems and by N fertilisation, associated to a decrease in pH values and base saturation and an increase in exchangeable A1 and A1 saturation in soil. The presence of leguminous species and mineral N fertiliser increased soil acidification attributed to the C cycle due to the increment in removal of alkaline plant material by grain yield. The pH buffer capacity was significantly and directly related to the total organic carbon content in the soil. Leguminous-based cropping systems promoted higher maize yields than the other systems, in spite of more acidic soil conditions than in soils under cropping systems constituted essentially by gramineous species, indicating a potential to decrease the use of mineral N fertiliser. When the mineral N was applied, however, the cropping systems did not differ in maize yield, even with differences in soil pH and exchangeable A1.

Manuscript received 28 June 2007, accepted 7 December 2007

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F. C. B. Vieira (A), C. Bayer (A,B), J. Mielniczuk (A), J. Zanatta (A), and C. A. Bissani (A)

(A) Department of Soil Science, Federal University of Rio Grande do Sul, PO Box 15100, 91.501-970 Porto Alegre, RS, Brazil.

(B) Corresponding author. Email: cimelio.bayer@ufrgs.br
Table 1. Rates of soil pH decrease (pH unit/year) under no till
cropping systems for 19 years

Means followed by the same upper case letter in rows and lower case
letter in columns do not differ (Tukey's multiple range test P > 0.10)

 Period
 1-5 6-10 11-19

F/M 0.083Aab 0.03713ab 0.03513a
O/M 0.093Aab 0.05013ab 0.03413a
O + V/M 0.117Aa 0.06513ab 0.03413a
O + V/M + C 0.075Aab 0.078Aa 0.044Aa
L + M 0.044Ab 0.068Aab 0.042Aa
P + M 0.089Aab 0.060Aab 0.05113a
Digitaria 0.035Ab 0.012Ab 0.047Aa

Table 2. Values of pH-[H.sub.2]O, total organic carbon, and bulk density
employed for estimating net acidification of an Acrisol under no
till cropping systems and nitrogen fertilisation for 19 years

F, Fallow; M, maize; O, black oat; L, lablab; V, vetch; C, cowpea;
P, pigeon pea; Di, digitaria. Original pH in 1983: 5.8 in the soil
layer 0-0.175 m and 5.2 in the layer 0.175-0.30 m. Total organic
carbon in 1983: 13.3 g/kg soil in the layer 0-0.175 m, and unknown
in the 0.175-0.30 m soil layer. In each soil layer, means followed
by the same lower case letter do not differ by Tukey's multiple
range test (P > 0.05) (effect of crop systems without mineral N).
In each soil layer and each crop system, means followed by the lower
case letter do not differ by Tukey's multiple range test (P > 0.05)
(effect of mineral N)

 F/M O/M
Layer
(m) 0 N 180 N 0 N 180 N

 pH-[H.sub.2]O (2002)

0-0.025 5.16ab (A) 4.84 (B) 5.l0abc (A) 4.58 (B)
0.025-0.05 4.89ab (A) 4.58 (B) 4.86abc (A) 4.36 (B)
0.05-0.075 4.88ab (A) 4.54 (B) 4.85ab (A) 4.30 (B)
0.075-0.125 4.86ab (A) 4.39 (B) 4.70ab (A) 4.32 (B)
0.125-0.175 4.79b (A) 4.56 (B) 4.63b (A) 4.41 (B)
0.175-0.30 4.76ab (A) 4.59 (A) 4.84a (A) 4.55 (B)

 Total organic carbon, g/kg (2002)

0-0.025 15.64b (A) 17.10 (A) 16.52b (A) 17.31 (A)
0.025-0.05 11.70b (A) 12.63 (A) 13.O1b (A) 14.39 (A)
0.05-0.075 9.93b (A) 10.51 (A) 10.83b (B) 13.54 (A)
0.075-0.125 8.78b (A) 10.16 (A) 9.83ab (A) 11.19 (A)
0.125-0.175 9.32a (A) 10.21 (A) 9.02a (B) 10.13 (A)
0.175-0.30 10.31ab (A) 10.67 (A) 9.66b (A) 9.91 (A)

 Bulk density (A) (g/[cm.sup.3])

0-0.025 1.54 1.54 1.54 1.54
0.025-0.05 1.56 1.56 1.56 1.56
0.05-0.075 1.56 1.56 1.56 1.56
0.075-0.125 1.62 1.62 1.62 1.62
0.125-0.175 1.62 1.62 1.62 1.62
0.175-0.30 1.62 1.62 1.59 1.59

 O+V/M O+V/M+C

Layer
(m) 0 N 180 N 0 N 180 N (B)

 pH-[H.sub.2]O (2002)

0-0.025 4.70c (A) 4.51 (B) 4.97bc --
0.025-0.05 4.56bc (A) 4.30 (B) 4.576c --
0.05-0.075 4.47c (A) 4.22 (B) 4.57bc --
0.075-0.125 4.49b (A) 4.28 (B) 4.57b --
0.125-0.175 4.68b (A) 4.37 (B) 4.63b --
0.175-0.30 4.69ab (A) 4.34 (B) 4.76ab --

 Total organic carbon, g/kg (2002)

0-0.025 18.03b (A) 20.36 (A) 21.02b --
0.025-0.05 14.17ab (A) 14.45 (A) 15.58ab --
0.05-0.075 11.78b (A) 12.73 (A) 13.40ab --
0.075-0.125 10.27ab (A) 10.72 (A) 10.78ab --
0.125-0.175 9.21a (A) 9.41 (A) 9.88a --
0.175-0.30 9.53b (B) 10.28 (A) 10.32ab --

 Bulk density (A) (g/[cm.sup.3])

0-0.025 1.54 1.54 1.54 --
0.025-0.05 1.58 1.58 1.58 --
0.05-0.075 1.58 1.58 1.58 --
0.075-0.125 1.66 1.66 1.66 --
0.125-0.175 1.66 1.66 1.66 --
0.175-0.30 1.66 1.66 1.66 --

 L+M P+M

Layer
(m) 0 N 180 N 0 N 180 N (B)

 pH-[H.sub.2]O (2002)

0-0.025 5.50a (A) 5.53 (A) 4.72bc --
0.025-0.05 5.15a (A) 5.OO (A) 4.54c --
0.05-0.075 4.88ab (A) 4.73 (A) 4.55bc --
0.075-0.125 4.71ab (A) 4.62 (A) 4.58b --
0.125-0.175 4.53b (A) 4.51 (A) 4.60b --
0.175-0.30 4.45b (A) 4.62 (A) 4.67ab --

 Total organic carbon, g/kg (2002)

0-0.025 30.77a (A) 31.05 (A) 24.41b --
0.025-0.05 18.51a (A) 20.55 (A) 16.78ab --
0.05-0.075 15.41a (B) 16.50 (A) 14.15ab --
0.075-0.125 12.54a (A) 13.71 (A) 11.84ab --
0.125-0.175 11.06a (A) 11.03 (A) 10.04a --
0.175-0.30 11.08a (A) 10.55 (A) 10.13ab --

 Bulk density (A) (g/[cm.sup.3])

0-0.025 1.55 1.55 1.44 --
0.025-0.05 1.61 1.61 1.65 --
0.05-0.075 1.67 1.67 1.65 --
0.075-0.125 1.66 1.66 1.65 --
0.125-0.175 1.63 1.63 1.65 --
0.175-0.30 1.63 1.63 1.65 --

 Di
Layer
(m) O N 180 N (B)

 pH-[H.sub.2]O (2002)

0-0.025 5.12abc --
0.025-0.05 5.12a --
0.05-0.075 5.14a --
0.075-0.125 5.10a --
0.125-0.175 5.21a --
0.175-0.30 4.92a --

 Total organic carbon, g/kg (2002)

0-0.025 19.12b --
0.025-0.05 13.89b --
0.05-0.075 12.38ab --
0.075-0.125 10.25ab --
0.125-0.175 8.94a --
0.175-0.30 10.22ab --

 Bulk density (A) (g/[cm.sup.3])

0-0.025 1.14 --
0.025-0.05 1.36 --
0.05-0.075 1.57 --
0.075-0.125 1.55 --
0.125-0.175 1.61 --
0.175-0.30 1.64 --

(A) Pillon (2000); replicate values were not available.

(B) Values not determined for this treatment.

Table 3. Total soil acidification and sources of acid or alkali
(equivalent Mg CaC[O.sub.3]/ha) for the layer 0-0.30 m of an Acrisol
under cropping systems in no tillage and N fertilisation for 19
years

Note: 1 Mg CaC[O.sub.3] is required to neutralise 20 kmol [H.sup.+].
Negative values indicate alkalinisation, and positive, acidification.
F, Fallow; M, maize; O, black oat; L, lablab; V, vetch; C, cowpea;
P, pigeon pea; Di, digitaria; 0 N and 180 N = 0 and 180 kg
N-urea/ha.year

Acidification F/M O/M

 0 N 180 N 0 N 180 N

C cycle
 [OA.sub.ac] (A) -0.39 -0.07 -0.20 0.17
 [OA.sub.exp] (B) 0.49 1.51 0.57 1.62
Subtotal 0.10 1.44 0.37 1.79
Superphosphate 0.19 0.19 0.19 0.19
N cycle (C) 1.07 0.34 0.88 0.30
Total (19 years) 1.36 1.97 1.44 2.28

% due to C cycle (7.7) (73.0) (25.8) (78.6)
% due to N cycle (78.4) (17.4) (61.0) (13.0)

 O+V/M O+V/M+C

 0 N 180 N 0 N 180 N (D)

C cycle
 [OA.sub.ac] (A) 0.00 0.16 0.31 --
 [OA.sub.exp] (B) 0.87 1.58 1.00 --
Subtotal 0.87 1.74 1.31 --
Superphosphate 0.19 0.19 0.19 --
N cycle (C) 0.80 0.59 0.32 --
Total (19 years) 1.86 2.52 1.82 --

% due to C cycle (46.9) (69.0) (72.1)
% due to N cycle (42.9) (23.5) (17.5)

 L+M P+M

 0 N 180 N 0 N 180 N (D)

C cycle
 [OA.sub.ac] (A) 1.09 1.44 0.57 --
 [OA.sub.exp] (B) 0.94 1.42 0.84 --
Subtotal 2.03 2.26 1.41 --
Superphosphate 0.19 0.19 0.19 --
N cycle (C) 0.00 0.00 0.44 --
Total (19 years) 1.90 1.89 2.04 --

% due to C cycle (100.0) (100.0) (69.1)
% due to N cycle (0.0) (0.0) (21.6)

 Di

 0 N 180 N (D)

C cycle
 [OA.sub.ac] (A) 0.03 --
 [OA.sub.exp] (B) 0.07 --
Subtotal 0.10 --
Superphosphate 0.04 --
N cycle (C) 0.88 --
Total (19 years) 1.02 --

% due to C cycle (10.4)
% due to N cycle (85.7)

(A) [OA.sub.ac] = change in total organic carbon stocks in soil;
[OA.sub.ac] (kmol/ha) = {O[C.sub.t2] ([pH.sub.t1] - 1.5)] -
[[OC.sub.t1] ([pH.sub.t1] - 1.5)]} x 0.32.

(B) [OA.sub.exp] = remove of alkaline vegetal material by maize
harvest (11.9 kg CaC[O.sub.3]/Mg of maize grain).

(C) Estimated by difference between total acidification and the
other factors that not the N cycle.

(D) Values not determined for this treatment.
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Author:Vieira, F.C.B.; Bayer, C.; Mielniczuk, J.; Zanatta, J.; Bissani, C.A.
Publication:Australian Journal of Soil Research
Geographic Code:3BRAZ
Date:Feb 1, 2008
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