Soil carbon sequestration is a viable short-term option to mitigate increased atmospheric CO2. In agriculture, strategies to increase the soil carbon (C) sink include no-tillage, cover crops, and improved crop rotation. The objective of this study was to determine the influence of tillage systems on SOC and total N, soil aggregation and aggregate associated C and N in three soil types: Oxisol (Brazil), Vertisol (Argentina), and Mollisol (USA). Long-term tillage experiments included tilled (T) and no-till (NT) systems. A native grassland was included for comparison in each site. Soil samples were taken at 0-5, 0-15, and 15-30 cm depths. Water-stable aggregates (WSA) were separated using a wet-sieving method. Total C and total N were determined by dry combustion. A shift from native grassland to an agroecosystem decreased microbial biomass, but this decrease was less pronounced under NT. Cultivation reduced the mass of macroaggregates and the concentration associated C and N; however among agroecosystems, NT, regardless soil type, tended to be more similar to the native grassland sites. Agroecosystems reduced TOC and total N stocks, regardless of soil type, compared to the native grassland. This effect followed: Mollisol > Oxisol > Vertisol, and was more pronounced at the 0-5 cm soil depth than at deeper depths. This loss of C and N was associated with the decrease in the mass of macroaggregates and lower C and N concentrations of the aggregates. Macroaggregation was related to TOC and microbial biomass in the Mollisol, suggesting that the biological process of aggregate formation is the principal mechanism of C protection in these soils. The relationship between TOC and large macroaggregates showed lower values for the Oxisol and Vertisol, indicating that in these soils TOC has a complementary role in macroaggregation.
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Acknowledgments This material is based upon work supported by the Cooperative State Research, Education, and Extension Service, US Department of Agriculture, Under Agreement No. 2001-38700-11092. The authors want to thanks FUNDACEP, Federal University of Santa Maria, Brazil, and EEA INTA Parana, Argentina for soil sampling permission, assistance on the field work, and for the information provided. This is contribution no. 08-219-J from the Kansas Agricultural Experiment Station, Manhattan, KS, USA.
- Native grassland