Saturday, 15 July 2006

Influence of No Tillage and Conventional Tillage and Crop Rotation on Soil Carbon and Nitrogen Accumulation and Soil Aggregation in a Rhodic Ferralsol.

Lincoln Zotarelli1, Eleno Torres2, Robert Boddey3, Segundo Urquiaga3, and Bruno J. R. Alves3. (1) Univ of Florida, Agronomy Dept, 304 Newell Hall, Gainesville, FL FL32611, (2) Embrapa Soja, Caixa Postal 231 - CEP 86001-970, Londrina, Brazil, (3) EMBRAPA-Agrobiologia, Caixa Postal 74.505, Caixa Postal 74.505, Seropedica, RJ 23890-000, BRAZIL

The use of conservation tillage with crop rotation can contribute to increase soil organic matter (SOM) and improve aggregate stability. The aim of this study was to quantify the soil C and N accumulation and the stability of soil aggregates influenced by different crop rotations with and without leguminous green-manures under No Tillage (NT) and Conventional Tillage (CT). The field experiment was carried out between 1997 and 2003 at Londrina, PR, with three crop rotations: R1 lupin(L)/maize(M) oat(O)/soybean(S) wheat(W)/soybean(S); R2 W/S L/M L/M W/S (high frequency of lupin as green manure) and R3 O/M W/M O/M W/M (dominance of gramineae). All cereals were fertilized with N except in the case of maize after lupin. The biomass and total N accumulation of the crops along with N2 fixation and N fertiliser inputs to the system were monitored. Soil samples from different depths up to 80 cm deep were taken to measure soil C and N accumulation. Soil samples taken in 2001 and 2003 were separated into aggregate size classes (53-250; 250-2000 and >2000 m) by wet sieving. The Light Fraction (LF) and intra-aggregate particulate organic matter were isolated. Crop yields and amounts of residues were not influenced by soil tillage. However, maize planted under NT in R3 had a lower yield, probably due to soil N deficiency provoked by high C/N ratio residues from the prior graminaceous crops. When lupin was included in the rotation before maize, maize yield was still high without N-fertilizer. For wheat, the rotation with soybean also promoted an improvement in crop yield. The amount of C in residues deposited on the soil was 36; 45 and 41 Mg ha-1 for R1, R2, R3, respectively. The system N balance was directly related to the presence of the winter legume. In R3 where there was no N2 fixation contribution, the N balance was -118 kg N ha-1 but for R2 and R1 the N balance was positive at approximately 40 kg N ha-1 which illustrates the importance of the legume for soil N accumulation. In general, NT improved C and N concentration at 0-5 and 5-20 cm soil depth. Soil C stock was significantly higher under NT at 0-5 cm depth for all crop rotations in 2003, six years after NT adoption. When C stocks were calculated for the 0-80 cm soil layer using the correction for equal soil mass, there were no significant differences between NT and CT, except when the comparison was within R1. In R1, NT accumulated 6.8 Mg ha-1 more C than the soil under CT. Under NT the accumulation of C in the 0-80 cm depth interval was greater under R1 than in other rotations by approximately 10.5 Mg ha-1. The higher accumulation of C under R1 associated with NT was due to the greater equilibrium between inputs and outputs of N when the gramineae and legume crops were alternated in the same area. However, the high occurrence of leguminous green manure in R2 seems to hinder C accumulation in soil probably due to a high occurrence of more labile residues. In the case of R3, the lack of a positive N balance would explain the lack of C accumulation in soil. The study of soil aggregates revealed NT had a greater mean weight diameter than the soil under CT due to a larger proportion of macroaggregates (>2000 m). The results of the soil organic matter fractionation showed that there were enhanced C concentrations in larger aggregate size classes. There were no differences in mineral-associated C under the different soil preparation systems or crop rotations, which was also observed in relation to Fe and Al concentrations of the soil. There was enhanced C concentration in LF in larger aggregate size classes, being greatest at 5-20 cm soil depth under CT. Within macroaggregates (250-2000 m), around 75% of their mass was constituted by microaggregates under CT and NT, and the C concentration of these microaggregates was 3 times greater than C concentration found in free microaggregates under NT, but no differences on C concentration were found under CT. Thus, the results confirm the theory that fresh residues act as a nuclei for macroaggregate formation and subsequently microaggregates. From the predominance of macroaggregates over microaggregates we can conclude that under NT the aggregation process was more active than under CT, which would ensure soil C stability. Also, variations in soil aggregation between 2001 and 2003 showed that crop rotation plays an important role on soil aggregation, and the data suggested that legumes would reduce macroaggregate formation.

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