Seasonal changes in soil microbial biomass carbon under a wheat-maize cropping system receiving urea and farmyard manure in different combinations

Seasonal changes in carbon (C) availability and soil microbial biomass carbon (C-mic) were studied under an irrigated wheat-maize system receiving urea at 50 or 100 kg N ha(-1) year(-1) in combination with 8 or 16 t ha(-1) year(-1) of farmyard manure (FYM). Treatment effects on C-mic were not visible during the wheat growing season, whereas fertilizer application significantly increased C-mic during the maize growing season. In unfertilized soil, C-mic was similar under two crops, whereas fertilized soils showed 11-56% higher C-mic under maize than under wheat. Under both crops, C availability, as assessed by aerobically mineralizable C (AMC) and total organic C, was generally higher in fertilized soils than in the unfertilized. Fertilized soils showed much higher AMC under wheat than under maize; the stimulatory effect being much more pronounced due to increasing application rate of FYM than that of urea. In fertilized soils, but not in the unfertilized, specific respiratory activity (SRA) of soil microbial biomass was twice higher under wheat as compared to that under maize. Results suggested that the soil microflora under wheat was probably dominated by 'r-stiategists', which respired more C as CO2 than that incorporated into microbial biomass. In contrast, 'k-strategists' dominated under maize, incorporating relatively more C into their biomass than that respired as CO2. Fertilizer application significantly increased C-mic turnover rate; while increasing the FYM application rate further increased the turnover rate, the increasing urea level had no effect. The overall high C-mic turnover rate, particularly in fertilized soils (1.69-2.29 year(-1)) indicated that nutrient cycling through soil microbial biomass may be substantial under agro-climatic conditions prevailing in this region.