Soil Microbial Biomass and Mineralizable Carbon As a Function of Crop Rotation and Soil Acidity Amendment in a No-Tillage System in Brazil.

Tropical climate and weathered soil conditions create significant challenges for increasing soil organic matter content. However, crop management strategies could affect short-term dynamics of active fractions of soil organic matter. Thus, our aim was to evaluate the microbial biomass and mineralizable carbon in the profile of a Rhodic Hapludox as a function of crop rotation system and surface application of soil amendments to alleviate acidity constraints.This experiment was set up in October 2006 in Botucatu, State of Sao Paulo, Brazil. A randomized block design consisted of four production systems, varying by the type of crop during the dry season, i.e. forage crop, grain-producing crop, cover crop, or bare fallow.  The main crop was either soybean, maize, rice, or common bean, with the same crop within a year during the rotation. Split-plots were two sources of soil amendment and a control (i.e. limestone, silicate, and no amendment) for a total of 12 treatments with four replications. Soil samples were collected in 2013 at depths of 0-5, 5-10, and 10-20 cm for evaluation of microbial biomass (chloroform fumigation-incubation) and mineralizable carbon (aerobic incubation at 50% water-filled pore space and 25 °C for 24 days).In general, soil microbial biomass to a depth of 10 cm was greater when forage was in the crop rotation, while mineralizable carbon was similar among crop rotations at all depths. Particularly at a depth of 0-5 cm, soil microbial biomass and mineralizable carbon were greater with application of limestone than with no amendment. Further analyses will help us evaluate the short-term effectiveness of crop rotation and soil acidity correction to alter the active fractions of soil organic matter.

Keywords: organic matter, crop rotations, soil acidity.