Nutrient cycling through microbial biomass under rice-pasture rotations replacing native savanna

An area of native savanna on an acid, strongly P-sorbing Oxisol in the Eastern Plains of Colombia was opened and sown to various rotations of grass pasture with rice, grass-legume pasture with rice or rice monocrop. After 4.5 y, the soil under each cropping system was sampled and analysed for total organic matter, microbial C, N and P content, and mineralization rate of C and N. Microbial biomass C did not vary much among treatments, whereas the N and P contents of the biomass were considerably lower in rice monocrop than in any crop-pasture treatment. Biomass P was also low under native savanna. The contribution of microbial N or P to soil-organic-matter N or P was lowest under the rice monocrop and highest under the grass-legume pasture. Microbial C-to-N ratios fell in ranges commonly reported, but C-to-P ratios were rather wide (34-50), indicating that the microbes may have adapted themselves to the low-P conditions of these soils. The contribution of microbial P to soil-organic-matter P, however, was about the same as usually found in soils of much higher P fertility. As microbial biomass nutrients cycle relatively rapidly and P availability in these strongly P-sorbing soils is low, the microbial biomass may play an important role in supplying P to plants growing in these soils. A similar conclusion was reached for N. The microbial respiration rate per unit of microbial biomass C (qCO2), as determined during an 11-day incubation, was higher under rice-pasture rotations--particularly in the presence of a legume--than under rice monocropping. This suggests that a greater fraction of microbes was active under rice-pasture rotations, probably because of a more continuous and higher input of fresh organic matter. The fraction of organic matter mineralized during the incubation was also highest for the rice-pasture treatments.