Carbon Stabilization in Oxisol and Mollisol under Long-Term No-Till.

Soil C stabilization can be affected by soil management and soil type.  In agriculture soils, no-tillage is often considered a strategy to build up and preservation of soil organic matter. The objective of our study was to determine the influence of different long-term tillage and C inputs on SOC and total N, soil aggregation and aggregate associated C and N fractions in an Oxisol (Brazil), and Mollisol (Kansas, USA). Tillage systems were conventional tillage (CT) and no-tillage (NT) in both soils. In the Mollisol, different sources of N to the corn were the subplots: mineral fertilizer and cattle manure. In the Oxisol, two crop rotations were the subplots: R1 – soybean/wheat, and R2 – corn/radish/wheat/soybean/oat/soybean/oat+vetch. Soil samples were taken at 0-5 cm depth. The water-stable aggregates (WSA) and the aggregate associated C and N fractions were separated using a wet sieving method. Total C and N contents were determined by dry combustion. The NT showed larger stocks of SOC and total nitrogen in both soils. The CT neglects the effect of the higher input of C promoted by the cattle manure in the Mollisol and the crop rotation R2 in the Oxisol. The NT and the increase of C inputs leads to an increase of the amount of macroaggregates (> 2000 µm). Carbon and N concentrations in the aggregates differed according to the soil type in response to management. The data suggest the relative importance of the physical, chemical, and biological mechanisms for stabilization vary according to the soil type and that soil management can greatly impact those mechanisms.