Soil organic matter and water-stable aggregates under different tillage and residue conditions in a tropical dryland agroecosystem

Changes in the proportions of water-stable soil aggregates, organic C, total N and soil microbial biomass C and N, due to tillage reduction (conventional, minimum and zero tillage) and crop residue manipulation (retained or removed) conditions were studied in a tropical rice-barley dryland agroecosystem. The values of soil organic C and total N were the highest (11.1 and 1.33 g kg(-1) soil, respectively) in the minimum tillage and residue retained (MT + R) treatment and the lowest (7.8 and 0.87 g kg(-1), respectively) in conventional tillage and residue removed (CT - R) treatment. Tillage reduction from conventional to minimum and zero conditions along with residue retention (MT + R, ZT + R) increased the proportion of macroaggregates in soil (21-42% over control). The greatest increase was recorded in MT + R treatment and the smallest increase in conventional tillage and residue retained (CT + R) treatment. The lowest values of organic C and total N (7.0-8.9 and 0.82-0.88 g kg(-1) soil, respectively) in macro- and microaggregates were recorded in CT - R treatment. However, the highest values of organic C and total N (8.6-12.6 and 1.22-1.36 g kg(-1), respectively) were recorded in MT + R treatment. The per cent increase in the amount of organic C in macroaggregates was greater than in microaggregates. In all treatments, macroaggregates showed wider C/N ratio than in microaggregates. Soil microbial biomass C and N ranged from 235 to 427 and 23.9 to 49.7 mg kg(-1) in CT - R and MT + R treatments, respectively. Soil organic C, total N, and microbial biomass C and N were strongly correlated with soil macroaggregates. Residue retention in combination with tillage reduction (MT + R) resulted in the greatest increase in microbial biomass C and N (82-104% over control). These variables showed better correlations with macroaggregates than other soil parameters. Thus, it is suggested that the organic matter addition due to residue retention along with tillage reduction accelerates the formation of macroaggregates through an increase in the microbial biomass content in soil. (C) 2001 Elsevier Science B.V. All rights reserved.