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DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION

Soil Carbon and Nitrogen Organic Fractions in Degraded vs. Non-Degraded Mollisols in Argentina

^a Fellow Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Facultad de Ciencias Agrarias, Universidad Nacional de Mar del Plata, Instituto Nacional de Tecnología Agropecuaria (INTA), Unidad Integrada Balcarce, C.C. 276 (7620) Balcarce, Argentina
^b Current address: Department of Agronomy, Kansas State University, Manhattan, KS 66506-5501
^c Estación Experimental La Estanzuela, Instituto Nacional de Investigaciones Agropecuarias (INIA), Colonia, Uruguay
^d INPOFOS Cono Sur, PPI-PPIC, Av. Santa Fe 910, (1641) Acassuso, Buenos Aires, Argentina

^* Corresponding author (fgarcia{at}ppi-ppic.org).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The use of no-tillage has notably increased in the Pampas region of Argentina during the last 10 yr. Two tillage experiments with contrasting previous agricultural use, degraded and non-degraded soils, were evaluated in the southeast of Buenos Aires province, Argentina. The objectives were to: (i) quantify the effects of tillage and N fertilization on quantity and vertical distribution of C and N in the soil organic matter (SOM) and particulate organic matter (POM) fractions as well as potentially mineralizable N (PMN), and (ii) evaluate these fractions as indicators of soil quality. Tillage systems were conventional tillage (CT), minimum tillage (MT), and no-tillage (NT) (main plots), and N fertilization rates were 0, 120, and 150 kg ha^-1 (subplots). Total organic C (TOC), total N (TN), POM-C, POM-N, and PMN were measured at 0- to 7.5- and 7.5- to 15-cm soil depth. In Exp. I (degraded soil) TOC was greater under NT (27 g kg^-1) than under CT (24 g kg^-1) in the 0-N treatments. No differences in TOC and TN were found in Exp. II at 0 to 7.5 cm (non-degraded soil). Carbon in POM and POM-N were greater under NT in the fractions of 212 to 2000 and 53 to 212 µm at 0 to 7.5 cm, but they were similar or greater under CT at 7.5- to 15-cm depth in Exp. I. Stratification of TOC, TN, and POM were observed under NT in Exp. I. Potentially mineralizable N was greater under NT (62 mg kg^-1) in Exp. I, however, no differences in PMN were observed in Exp. II. Carbon in POM 212 to 2000 µm and PMN were the more sensitive indicators of tillage effects, mainly in Exp. I.

Abbreviations: CT, conventional tillage • MAOM, mineral-associated organic matter • MT, minimum tillage • NT, no-tillage • PMN, potentially mineralizable N • POM, particulate organic matter • SOM, soil organic matter • TN, total N • TOC, total organic C

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
NO-TILLAGE HAS DEVELOPED into a successful management system in the Pampas region of Argentina. Introduction of the herbicide glyphosate [N-(phosphonomethyl) glycine], the development of planters and drills, and the technologies developed and adapted through research and experimentation have allowed the sustained increase of the area under NT since the early 1990s (García et al., 2000). Presently, the total area under NT is estimated at 12 million ha (Peiretti, 2002), which represents >30% of the total annually cropped area. Soils in the southern Pampas region (13 million ha) developed from eolic sediments of the quaternary period and are classified as Typic Argiudoll, Litic Argiudoll, or Petrocalcic Argiudolls (USDA classification). These soils are moderately acid, low in available P, and have high OC content (29–46 g kg^-1). Water erosion is a major problem on soils between "sierras" (low mountains, 300–600 m over sea level). Also, the increase in row crop production observed in the last 20 yr has resulted in decreasing of SOM and available P (Echeverría and Ferrari, 1993). The adoption of NT by farmers under these conditions will improve soil quality by increasing SOM and reducing soil water erosion.

No-tillage management improves soil aggregation (Havlin et al., 1990; Carter, 1992; Beare et al., 1994) and reduces loss of SOM because of less soil disturbance and reduced litter decomposition rates (Paustian et al., 1997). Most simulations of tillage effects on SOM sequestration predict that NT will have greater C sequestration than CT (Parton et al., 1987; Lee et al., 1993; Paustian et al., 1997), which has a positive influence on the quality of agricultural soil (Doran and Linn, 1994; McCarty and Meisinger, 1997).

Even though SOM is a key desirable component, it is not an adequate indicator of soil quality by itself (Bezdicek et al., 1996). Several studies found that the different SOM pools could better reflect changes in soil quality than SOM alone (Cambardella and Elliott, 1992; Wander et al., 1994). One of these pools that could be considered is POM that is composed of sand-sized particles, primarily of partially decomposed root fragments (Cambardella and Elliott, 1992), and it accounts for the majority of the SOM initially lost as a result of conversion of grassland soils to cropping (Parton et al., 1987; Cambardella and Elliott, 1992; Chan, 1997). Also, PMN and soil microbial biomass have been proposed as soil quality indicators (Turco et al., 1994).

The objectives of this study were to: (i) quantify the effects of tillage and N fertilization practices on the quantity and vertical distribution of C and N in the SOM and POM fractions as well as PMN, and (ii) evaluate the use of these fractions as indicators of soil quality, to determine the most sensitive for detecting changes due to management practices. This study was conducted with soils under contrasting quality conditions: degraded and non-degraded.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Site Description and Soils
Soils were sampled from two experimental sites in the southeastern area of Buenos Aires province, Argentina. The climate is humid-subhumid mesothermal, and the 30-yr average annual precipitation is 928 mm, 80% of which occurs in the spring-summer period.

The experimental design for both experiments was a split-plot arrangement set as a randomized complete block with four replications. Tillage systems were main plots, and N fertilization subplots. Individual plots were 50-m long and 15-m wide (750 m²). Experiment I was located at the Balcarce Experimental Station of the National Institute of Agricultural Technology (INTA) (37° 45' S lat., 58° 18' W long.). The soil is a fine, mixed, thermic Petrocalcic Paleoudoll, and the slope was 0.5%. The trial was established in 1992, on soil considered degraded because the site had been under cultivation for 25 yr. This soil had only 40% of the original structural stability and a high bulk density of 1.45 Mg m^-3 at both soil depths (3–8 and 15–20 cm) (Ferreras et al., 2000). Crop rotation was: spring wheat (Triticum aestivum L.) (1992), soybean [Glycine max (L) Merr] (1993), spring wheat (1994), corn (Zea mays L) (1995), corn (1996), sunflower (Heliantus annus sp) (1997), and corn (1998).

Tillage systems were CT and NT. The CT treatment consisted of disking to mix residues with soil, one moldboard plow tillage operation at 0- to 20-cm depth, and one to three diskings to a depth of 8 to 10 cm each year for seedbed preparation, depending on weather conditions and on the previous crop species (Rizzalli, 1998). The NT treatment consisted of chemical weed control during the fallow period, and seeding directly into the standing residue of the previous crop with the NT drill (Semeato, Agrometal or Tanzi drills depending on the year and crops). Nitrogen fertilizer urea [CO (NH₂)₂] was broadcasted at 120 kg N ha^-1.

Experiment II was located on a farm at Tandil, Buenos Aires, Argentina (37° 34' S lat., 59° 04' W long.). Soil was a complex of fine, mixed, thermic Typic Argiudolls and a fine, illitic, thermic Petrocalcic Paleudolls (USDA soil classification), and the slope was 2.5%. The trial was established in 1995 on soil considered non-degraded, because it had been under pasture for 6 yr and NT management. This soil had a low bulk density of 1.19 and 1.26 Mg m^-3 under MT and NT at the 3- to 8- and the 13- to 18-cm depth, respectively (Fabrizzi, 2000), and high structural stability (66–71%) (Elissondo et al., 2001). Crop rotation was corn-wheat. Tillage systems were MT and NT. Minimum tillage consisted of two tillage operations with a chisel plow at the 5- to 10-cm depth, and two diskings to a depth of 8 to 10 cm each year for seedbed preparation. No-tillage consisted of chemical weed control during the fallow period, and seeding directly into the standing residue of the previous crop. Nitrogen rates were 0 and 150 kg ha^-1. Nitrogen as urea was incorporated into the soil under the residue layer with the NT drill (John Deere 750 drill, John Deere Argentina S.A., G. Baigorria, Sta. Fe, Argentina).

An undisturbed soil located at the Balcarce Experimental Station was sampled as a reference condition for both Exp. I and II (grassland reference). The soil is a fine, mixed, thermic Petrocalcic Paleoudoll, which had never been cultivated, and it was under grassland for more than 30 yr.

Soil Sampling
Thirty soil samples were taken from each plot at 0- to 7.5- and 7.5- to 15-cm depth increments. Samples were collected on 15 and 22 Oct. 1999 for Exp. I and II, respectively, after tillage operations but before planting. Soil samples from the grassland reference were taken at the same date of Exp. I, also at 0- to 7.5- and 7.5- to 15-cm depth. Soil samples were sieved through 2-mm grid screens, and stored at 4°C until use.

Laboratory Methods
Potentially mineralizable N was determined by anaerobic incubation (Bundy and Meisinger, 1994). Equivalent of 5 g dry soil was placed in a test tube, and saturated with 12.5 mL of deionized water. The tubes were stopped and placed in an incubator at 40°C for 7 d. After 7d, 12.5 mL of 4 M KCl were added, and the tubes were mechanically shaken for 1 h. The supernatant was filtered through Whatman no. 42 filter paper (Whatman International Ltd., Maidstone, England). The same procedure was used for non-incubated samples. Ammonium was determined colorimetrically (Mulvaney, 1996). Potentially mineralizable N was determined as the NH₄–N recovered from the incubated soil minus the NH₄–N recovered from the non-incubated soil.

Carbon and N in POM were determined using methods adapted from Cambardella and Elliott (1992). Soil wet samples sieved through 2-mm grid screens were separated by dispersing the equivalent of 3.33 and 6.66 g dry soil with 10 and 20 mL sodium hexametaphosphate (5%) for POM-C and POM-N, respectively. The suspensions were shaken on a reciprocal shaker for 16 h, and then passed through 212- and 53-µm sieves to remove the POM. The material collected on each sieve, sand and POM, was retained and dried at 80°C. Then, the fractions recovered were POM-C and POM-N in 212- to 2000- and 53- to 212-µm size fractions.

The soil slurry that passed through 53-µm sieve contained the mineral-associated organic matter (MAOM). Carbon and N in MAOM were determined by difference between TOC and TN contents, and POM-C and POM-N in the measured fractions.

Total organic C and POM-C contents were determined according to the method described by Tinsley (1967). Total N and POM-N contents were determined by Kjeldahl digestions (Bremner, 1965).

The proportion of mineralizable N (PMN/TN ratio) was determined as the ratio between PMN and TN, where:

Statistical Analysis
Analysis of variance was performed using SAS PROC MIXED (SAS Institute, 1996) to assess treatments differences on TOC, TN, POM, MAOM, and C/N ratio.

Tillage, N fertilization, and soil depth were the class variables. Results were considered statistically significant at P < 0.05, except where noted. Two and three-way interactions were Tillage x Depth (T x D), Nitrogen x Depth (N x D), Tillage x Nitrogen x Depth (T x N x D). Correlation analyses were performed using PROC CORR (SAS Institute, 1996).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Grain Yields
Grain yields from 1992 through 1998 in Exp. I and II are presented in Table 1. Variations in grain yield could be attributed to differences in weather conditions, crop sequence, and soil type between both experiments. Grain yield data for Exp. I has been reported by Rizzalli and Berardo (R. Rizzalli and A. Berardo, personal communication, 1993); Rizzalli and García (R. Rizzalli and F. O. Garcia, personal communication. 1994); Bergh (1997); Rizzalli (1998); and Larrosa (2000). Grain yields were lower under NT for the 0-N treatments, but N fertilization reduced the differences between both tillage systems indicating a greater N deficiency under NT (Table 1). In Exp. II, no differences were observed during 1995 and 1996. In 1997, Tillage x N interaction was observed with significant differences among tillage treatments in the 0-N treatments (CT: 7870 kg ha^-1; NT: 5859 kg ha^-1) (Table 1). Nitrogen effects were observed in 1998, being grain yield greater under 150-N treatments (3786 kg ha^-1) than 0-N treatments (2965 kg ha^-1).

View larger version (43K): [in this window] [in a new window]   Fig. 1. Effect of conventional tillage (CT), minimum tillage (MT), and no-tillage (NT) systems on (a) total organic C (TOC) and (b) total N (TN) contents and in (c) particulate organic matter (POM)-C and (d) POM-N contents in the 212 to 2000 and 53 to 212 µm at 0- to 15-cm soil depth. The same letters above bars from each location are not significantly different at P < 0.05.

Carbon and Nitrogen in Particulate Organic Matter
In Exp. I, POM-C and POM-N concentrations in both size fractions were significantly greater under NT (POM-C: 1.58 and 2.05 g kg^-1; POM-N: 0.101 and 0.173 g kg^-1, for 212 to 2000 and 53 to 212 µm, respectively) than under CT (POM-C: 0.69 and 1.11 g kg^-1; POM-N: 0.045 and 0.107 g kg^-1, for 212 –2000 and 53–212 µm, respectively) at 0- to 7.5-cm soil depth (Table 2). Conversely, at 7.5 to 15 cm, POM-C and POM-N were greater under CT (POM-C: 0.94 and 1.11 g kg^-1 for 212–2000 and 53–212 µm, respectively; POM-N 212-2000: 0.05 g kg^-1) than under NT (POM-C: 0.54 and 0.97 g kg^-1 for 212 to 2000 and 53 to 212 µm respectively; POM-N 212–2000: 0.034 g kg^-1), except for POM-N in 53- to 212-µm size fraction. Also, POM-N concentrations in 212- to 2000-µm size fraction were greater under 120-N treatments (0.045 g kg^-1) than 0-N treatments (0.036 g kg^-1) at this depth (Table 2).

Carbon in MAOM was higher under NT than CT, but this increment was greater for 120-N (NT: 25.1 g kg^-1; CT: 21.6 g kg^-1) than for 0-N treatment (NT: 23.3 g kg^-1; CT: 22.1 g kg^-1) at 0- to 7.5-cm depth (T x N interaction) (Table 2). Also, MAOM-N was greater under NT (2.14 g kg^-1) than under CT (1.96 g kg^-1). At 7.5- to 15-cm depth, there were no differences between tillage and N treatments in MAOM-C and MAOM-N contents (Table 2).

In Exp. II, data were not as consistent as in Exp. I. Carbon in POM 212 to 2000 µm was significantly affected by N fertilization under NT (0-N: 1.47 g kg^-1; 150-N: 2.02 g kg^-1), but there was no N effect under MT (1.8 g kg^-1) at 0- to 7.5-cm depth (T x N interaction) (Table 3). Carbon in POM 53-212-µm was greater under MT (2.36 g kg^-1) than under NT (2.13 g kg^-1) at this depth. Nitrogen in POM only had differences in the fraction 212 to 2000 µm, being greater for the 150-N treatments (0.113 g kg^-1) than for the 0-N treatments (0.094 g kg^-1) at 0 to 7.5 cm. At 7.5- to 15-cm soil depth, POM-C and POM–N were greater under MT (POM-C: 0.62 and 1.35 g kg^-1; POM-N: 0.04 and 0.14 g kg^-1, for 212 to 2000 and 53 to 212 µm respectively) than under NT (POM-C: 0.48 and 1.02 g kg^-1; POM-N: 0.03 and 0.11 g kg^-1, for 212 to 2000 and 53 to 212 µm, respectively). Also, POM-N 53 to 212 µm was greater in 150-N treatment (0.130 g kg^-1) than in 0-N treatments (0.117 g kg^-1) at this depth.

Carbon in MAOM was similar between treatments at 0 to 7.5 cm, but it was significantly greater under MT (28.5 g kg^-1) than under NT (26.3 g kg^-1) at 7.5- to 15-cm depth (Table 3). Nitrogen in MAOM was similar between treatments at both depths.

Carbon in POM and POM-N concentrations were highly stratified under NT (Table 4). In Exp. I, POM-C under NT were 1.04 to 1.08 g kg^-1, and POM-N were 0.06 to 0.08 g kg^-1 greater at 0 to 7.5 cm than at 7.5 to 15 cm in 212- to 2000- and 53- to 212-µm size fractions, respectively (Table 2). Carbon in MAOM and MAOM-N concentrations were slightly higher under NT at 0 to 7.5 cm than 7.5 to 15 cm (1.43 and 0.13 g kg^-1 for MAOM-C and MAOM-N, respectively). In Exp. II, POM-C and POM-N concentrations were stratified in both tillage systems (Table 3), but this effect was stronger under NT than MT. Also, a slight stratification of MAOM-C and MAOM-N was observed (Table 3).

In Exp. I, POM-C and POM-N contents were greater under NT (POM-C: 230 and 328 g C m^-2; POM-N: 14.6 and 28.9 g N m^-2, for 212 to 2000 and 53 to 212 µm, respectively) than CT (POM-C: 178 and 241 g C m^-2; POM-N: 9.9 and 22.2 g N m^-2, for 212 to 2000 and 53 to 212 µm, respectively) at the 0- to 15-cm soil depth at both size classes (Fig. 1c and 1d).

The POM-C and POM-N concentrations under CT and NT were 18 to 48% and 26 to 48% of those observed in the grassland reference in the fractions 212 to 2000 and 53 to 212 µm, respectively, at 0 to 7.5 cm. At 7.5 to 15 cm, these percentages were 55 to 95% and 72 to 84% in 212- to 2000- and 53- to 212-µm size fractions, respectively. Carbon in MAOM and MAOM-N under CT and NT represented 62 to 72% and 75 to 81% of the concentrations in the grassland reference at 0- to 7.5- and 7.5- to 15-cm depth, respectively.

In Exp. II, POM-C and POM-N in 212- to 2000-µm size fraction were similar between tillage systems, but they were greater under MT (POM-C: 339 g C m^-2; POM-N: 31.2 g N m^-2) than NT (POM-C: 302 g C m^-2; POM-N: 28.3 g N m^-2) in the 53- to 212-µm fraction at 0- to 15-cm soil depth (Fig. 1c and 1d). Carbon in POM and POM-N under MT and NT represented 45 to 51% and 50 to 55% of those in the grassland reference in the 212- to 2000- and 53- to 212-µm size fractions, respectively at 0 to 7.5 cm. At 7.5- to 15-cm depth, these values were 49 to 78% and 63 to 108% for 212- to 2000- and 53- to 212-µm size fractions, respectively. Carbon in MAOM and MAOM-N represented 84 to 88% and 93 to 102% under MT and NT of the concentrations of the grassland reference at 0 to 7.5 and 7.5 to 15 cm, respectively.

Potentially Mineralizable Nitrogen
Potentially mineralizable N was significantly higher under NT (61.5 mg kg^-1) than under CT (24.4 mg kg^-1) at 0 to 7.5 cm in the Exp. I (Table 5). Strong stratification of PMN was observed under NT systems (D x T interaction) (Table 5). Potentially mineralizable N under NT was 42.5 mg kg^-1 greater at 0- to 7.5-cm than 7.5- to 15-cm soil depth. Data show no differences in PMN because of N fertilization. A strong stratification of PMN was observed in Exp. II, but there were no significant effects of tillage or N treatments (Table 5).

Relationships between Pools of Carbon and Nitrogen
In Exp. I, the TOC/TN ratio for whole soil content was significantly different between tillage systems (P = 0.04), being greater under NT (11.5) than under CT (11.2) (Fig. 2a) at 0- to 7.5-cm depth. There were no differences in the TOC/TN ratio at 7.5- to 15-cm soil depth (Fig. 2b). Particulate organic matter C/N ratio in the fraction 53 to 212 µm appeared to be enriched in C relative to N when comparing NT and CT treatments at 0 to 7.5 cm, while POM C/N in the 212- to 2000-µm size fraction was similar between tillage treatments (Fig. 2a). Mineral-associated organic matter C/N ratio was similar between treatments at both depths. No differences between tillage systems were found in TOC/TN ratio for whole soil content and POM in Exp. II (Fig. 2c and 2d). In POM 212- to 2000-µm size fraction, the grassland reference had a higher C/N ratio than MT and NT systems.

View larger version (39K): [in this window] [in a new window]   Fig. 2. Carbon/N relation for total organic C (TOC) and total N (TN) concentrations, particulate organic matter (POM) concentrations in the 212- to 2000-µm and 53- to 212-µm size fractions, and mineral associated organic matter (MAOM) concentrations for conventional tillage (CT) and no-tillage (NT) in Exp. I and at (a) 0- to 7.5- and (b) 7.5- to 15-cm depth, and minimum tillage (MT) and no-tillage (NT) in Exp. II at (c) 0- to 7.5- and (d) 7.5- to 15-cm depth.

View larger version (20K): [in this window] [in a new window]   Fig. 3. Relationships between potentially mineralizable N (PMN) and (a) C and (b) N in particulate organic matter in the fraction 212 to 2000 µm (POM-C and POM-N) in both experiments.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

The grassland reference had greater TOC and TN contents than tillage systems in both experiments, but the introduction of NT management reduced these differences. The 25 yr under CT management in Exp. I could explain the decline of soil C and N compared with the grassland reference. Non-degraded soil conditions in Exp. II may explain the lower differences in TOC and TN contents with the grassland reference, and between NT and MT treatments. These results are in accord with those reported by Dalal and Mayer (1986), Chan (1997), and Chan et al. (1998).

The increase in TOC and TN contents under NT in Exp. I at 0 to 15 cm indicates that C and N sequestration were observed under this tillage system, which agrees with results found by Cambardella and Elliott (1994), and Six et al. (1999). However, there were no differences in Exp. II at 0- to 15-cm depth. As in this experiment, several studies found that the use of NT practices did not increase TOC contents compared with CT soils (Angers et al., 1997; Franzluebbers et al., 1999; Needelman et al., 1999). The comparison between our two experiments indicates that the C and N sequestration capability of NT systems depends on initial SOM status. Similar to the TOC and TN results, tillage affected POM and MAOM-C and-N contents in Exp. I. In Exp. II, tillage mainly affected the POM-C and–N in the 53- to 212-µm size fraction. Since both size classes were affected by tillage practice in Exp. I, they could be the source of SOM lost under CT. Beare et al. (1994) found that macroorganic matter (>50 µm) was not the only source of SOM lost under CT compared with NT, since microorganic matter decreased in the same proportion.

Crop yield could be another factor that influences tillage impacts in both experiments. In Exp. I, grain yields were lower under NT than under CT in the 0-N treatments, but N fertilization reduced the differences between both tillage systems (Table 1), however, TOC, TN, POM-C, and POM-N were greater under NT than under CT. Thus, the main effect of NT on the C and N organic fractions could be attributed to surface residue accumulation and lower rates of residue decomposition (Sanchez et al., 1998; Wander et al., 1998; Sá et al., 2001). In Exp. II, grain yields were similar or slightly greater under MT and plant productivity cannot explain the differences observed in POM-C and POM-N.

Carbon in POM in the 212- to 2000-µm plus 53- to 212-µm size fractions accounted for 19.8% of TOC in the grassland reference, but only 8 and 13% of TOC under CT and NT in Exp. I, respectively, and 13 and 12% of TOC under MT and NT in Exp. II, respectively, at 0 to 7.5 cm (Tables 2 and 3). Carbon in MAOM accounted for 81% of TOC in the grassland, 92 and 87% of TOC under CT and NT in respectively, Exp. I, and for 88% of TOC under MT and NT in Exp. II. These results could indicate that POM-C was the C form that has preferentially been lost as a result of cropping. These results agree with those reported by Cambardella and Elliott (1992), Chan (1997) and Chan et al.(2002).

In degraded soils (Exp. I), the decrease in TOC respect to the grassland reference (17.5 and 13.7 g kg^-1 under CT and NT, respectively) is greater than in non-degraded soils (Exp. II, 9 and 8.1 g kg^-1 under MT and NT, respectively) at 0- to 7.5-cm soil depth. For instance, in Exp. I, 36 and 34% of the differences in TOC between the grassland reference and CT and NT, respectively, could be explained by the decline of POM at this depth (Table 2). The corresponding values for Exp. II were 45 and 54% under MT and NT, respectively (Table 3). The loss of soil C not only includes the most active fractions (35 and 49% of POM in the Exp. I and II, respectively), but also the most stable ones (68 and 56% of MAOM in Exp. I and II, respectively).

Experiment II had a greater loss percentage of TOC in the form of POM, as POM-C was lost more rapidly than other fractions (MAOM-C). Chan (1997) and Dalal and Mayer (1986) also reported rapid loss of sand size organic matter (POM) during the initial period of cultivation as occurred in Exp. II. Sá et al. (2001) reported that conversion of native grassland to cropland caused a significant decrease in TOC in the 200- to 2000-µm size fraction after just 1 yr by plow tillage at all depths.

The greatest PMN contents under NT than CT in Exp. I could be related with greater immobilization and reduced rate of mineralization, or both, and also lower losses by erosion under NT than under CT (Franzluebbers et al., 1995; Doran et al., 1998; Needelman et al., 1999). Lack of differences in PMN between tillage systems in Exp. II reflects shorter previous agricultural use of soil and high soil fertility conditions. When comparing the PMN content under Exp. II with the grassland soil, the differences were smaller than those for Exp. I.

The greater PMN/TN ratio under NT systems in Exp. I reflects the influence of tillage on SOM quality, with a highest proportion of total N present in the active fractions. Franzluebbers et al. (1994) also found that mineralized N was greater under NT than CT, and they suggest that the greater amount of N in active fractions could be related to the conservation of active and passive SOM pools under NT. Similar to our results, Chan (1997) observed that permanent pastures had the highest mineralizable N.

Vertical Distribution of Soil Organic Matter Fractions Under Different Soil Management
Redistribution of TOC and TN in the 0- to 15-cm layer could be attributed to changes in residue management that occurred with introduction of NT. Our results indicate that POM stratification was more extreme than stratification of TOC and TN, and they were consistent with those of Wander et al. (1998) and Needelman et al. (1999).

When comparing tillage treatments between experiments, stratification of POM-C and POM-N showed a similar behavior under NT in both experiments. These results agree with general findings of Beare et al. (1994), Wander et al. (1998), Needelman et al. (1999), and Bayer et al. (2001). However in Exp. II, MT presented stratification of POM, and this was not observed under CT in Exp. I. The difference could be explained by the intensity of tillage; whereby most of the residue incorporated into the surface 20 cm under CT, while MT left more residue on the surface (average of 27% residue cover at planting time).

Evaluation of Soil Quality Indicators
Our results showed a high correlation between PMN and POM-C and POM-N fractions, indicating that most of the mineralizable N comes mainly from the POM fraction. Others authors have reported that POM is associated with mineralizable N (Franzluebbers and Arshad, 1997; Chan, 1997). Considering that methods to determine N mineralization through incubations are tedious and time-consuming, and the great easiness of POM determination, POM could be used as a reliable indicator to predict N mineralization.

In this study, C and N in POM and PMN constituted a more sensitive SOM fraction than MAOM to reflect the effects of soil management changes, mainly in Exp. I (Fig. 4) . Several authors found that POM was more sensitive than soil organic C for detecting changes due to tillage management (Elliott et al., 1994, Franzluebbers et al., 1999; Needelman et al., 1999; Wander and Bollero, 1999).

View larger version (23K): [in this window] [in a new window]   Fig. 4. Relative values of total organic C (TOC), total N (TN), potentially mineralizable N (PMN), C and N in particulate organic C (POM-C 212–2000 µm, POM-N 212–2000 µm, POM-C 53–212 µm, POM-N 53–212 µm), and mineral associated organic matter (MAOM-C, MAOM-N) at 0- to 7.5-cm soil depth in Exp. I. The reference for Exp. I was CT 0-N = 100.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Redistribution and stratification of TOC, TN, POM, and PMN were observed under NT, with greater contents at the soil surface in the degraded soil. Residue placement and the slower decomposition may explain why the use of NT increased accumulation of SOM in the soil surface at the expense of SOM retained in depth. Potentially mineralizable N was significantly correlated with POM. This suggests that POM could be a good predictor of PMN. Particulate organic matter and PMN were the most sensitive indicators of soil quality and soil management. These fractions could be used under situations where TOC and TN contents cannot detect differences among tillage systems or other soil management practices.

The introduction of NT management reduced the differences in SOM fractions, but it did not allow soil to reach the same C and N values presented in the grassland reference. In this study, POM did not appear to be the only source of SOM lost under CT compared with NT practices, and stable fractions like MAOM could be lost. We should consider that these experiments have been evaluated in the short-term (8 yr after the introduction of NT). Future evaluations will be needed to determine the effects on SOM in the long-term.

	ACKNOWLEDGMENTS

 
This study was made possible with financial support of EEA INTA Balcarce, Facultad de Ciencias Agrarias (UNMdP), INIA La Estanzuela, and the International Foundation for Science (Research Grant C/2239-1). The authors are grateful to technical and field staff of EEA INTA-FCA Balcarce, Argentina, and to personnel of the Soils Laboratory of INIA La Estanzuela, Uruguay for their help and valuable technical assistance. We also thank Roberto Rizzalli for allowing us to sample the Exp. I.

Received for publication December 5, 2001.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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