UC Davis

In temperate regions, the adoption of no-tillage (NT) often stimulates the sequestration of soil C and N and improves soil structural stability. The aim of this study was to investigate if NT has similar impacts on the stability of water-stable aggregates and C and N stabilization in two Oxisols (Typic Haplorthox) under different crop rotations. Slaking-resistant aggregates were isolated by wet sieving and analyzed for C and N concentrations at two agricultural experiment sites (Passo Fundo and Londrina) in southern Brazil. At both sites, the total organic C and N in the 0- to 5-cm depth interval, decreased in order native vegetation (NV) > NT > conventional tillage (CT). The mean weight diameter (MWD) of the aggregates was on average 0.5 mm greater under NT compared with CT, and was also greater (approximately 0.2 mm) under more diverse rotations, which included a leguminous green-manure crop, in comparison with continuous wheat (Triticam aestivum)-soybean (Glycine Max L.). The aggregate-size distribution was dominated (60-90%) by macroaggregates (> 250 mu m). At both sites, CT decreased the proportion of the largest macroaggregate class (> 2000 mu m) by approximately 10% in comparison with NT management, and there was a corresponding increase in the proportion of the 53- to 250-mu m aggregate class. In the 0- to 5-cm soil layer of both sites, the C and N concentrations were significantly higher in the macroaggregates of the NT than of the CT systems. The lack of differences in C, N content, and ON ratio across aggregate-size classes indicated that these soils dominated by 1:1 clays and Fe/Al oxides do not express an aggregate hierarchy and that an increase in aggregation does not explain the increase in C and N under NT. In conclusion, CT decreased aggregation similarly in Oxisols as in temperate soils, but this decrease does not explain the loss of C and N because the tight feedback between soil organic matter (SOM) and aggregate stability (i.e., SOM being a major binding agent) observed for temperate soils was not found for Oxisols.