Soil Microbial Biomass and Mineralizable Carbon Affected By Crop Rotations Under No-till System.

Crop rotations are a key feature of conservation agriculture under no-tillage to achieve adequate soil cover and diversity in a no-till system, all of which should enhance surface soil organic carbon (C). We tested the long-term effects of crop rotation on soil microbial biomass C (chloroform fumigation-incubation) and mineralizable C (aerobic incubation at 50% water-filled pore space and 25 °C for 24 days) in a Typic Rhodudalf under long-term no-till. The experiment was initiated in 2003 in Botucatu, Sao Paulo, Brazil, with variations in crops appearing in the dry season as main plots (triticale or sunflower) and in the spring cover crop season (pearl millet, sunn hemp, forage sorghum, or fallow with chisel) as sub-plots. Soybean was grown every year in the summer. Soil was collected in 2015 at depths of 0-10 and 10-20 cm. Mineralizable C was lower with fallow-chisel than with other spring cover crops at 0-10 cm, but not different among cover crops at 10-20 cm. The flush of CO2 following rewetting of dried soil during 3 days was greater with triticale than sunflower as winter crop at 0-10 cm, but not different among winter crops at 10-20 cm. Soil microbial biomass C was unaffected by winter crop and spring cover crop treatment at both depths. Carbon input and lack of soil disturbance were reasons for enrichment of soil C fractions at the soil surface.  These results emphasize the importance of crop rotations in affecting soil biological properties.