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, tota l N and soil microbial biomass C and N, due to tillage reduction (conventio nal, minimum and zero tillage) and crop residue manipulation (retained or r emoved) conditions were studied in a tropical rice-barley dryland agroecosy stem. 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 retaine d (MT + R) treatment and the lowest (7.8 and 0.87 g kg(-1), respectively) i n conventional tillage and residue removed (CT - R) treatment. Tillage redu ction from conventional to minimum and zero conditions along with residue r etention (MT + R, ZT + R) increased the proportion of macroaggregates in so il (21-42% over control). The greatest increase was recorded in MT + R trea tment and the smallest increase in conventional tillage and residue retaine d (CT + R) treatment. The lowest values of organic C and total N (7.0-8.9 a nd 0.82-0.88 g kg(-1) soil, respectively) in macro- and microaggregates wer e recorded in CT - R treatment. However, the highest values of organic C an d 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 macro aggregates was greater than in microaggregates. In all treatments, macroagg regates showed wider C/N ratio than in microaggregates. Soil microbial biom ass 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 bi omass C and N were strongly correlated with soil macroaggregates. Residue r etention in combination with tillage reduction (MT + R) resulted in the gre atest increase in microbial biomass C and N (82-104% over control). These v ariables showed better correlations with macroaggregates than other soil pa rameters. Thus, it is suggested that the organic matter addition due to res idue retention along with tillage reduction accelerates the formation of ma croaggregates through an increase in the microbial biomass content in soil. (C) 2001 Elsevier Science B.V. All rights reserved.