CARBON AVAILABILITY AND MICROBIAL BIOMASS IN SOIL UNDER AN IRRIGATED WHEAT-MAIZE CROPPING SYSTEM RECEIVING DIFFERENT FERTILIZER TREATMENTS

Seasonal changes in carbon availability and microbial biomass were stu died in soil under an irrigated wheat-maize cropping system receiving different fertilizter treatments over the past 10 years. Treatments in cluded N-100 and N-200 (urea at 100 and 200 kg N ha(-1) year(-1), resp ectively), FYM-16 and FYM-32 (farmyard manure at 16 and 32 t ha(-1) ye ar(-1), respectively) and a control (unfertilized). Aerobically minera lizable carbon (AMC; C mineralized after 10 days aerobic incubation at 30 degrees C) increased (13-16%) under wheat at both rates of urea wh ereas under maize it increased (22%) only with the lower rate of urea. Farmyard manure also increased the content of soil AMC under both cro ps, the effect being two- to threefold higher under wheat than under m aize. Urea application caused an 32-78% increase in the specific respi ratory activity (SRA) under wheat but caused an 11-50% decrease during the maize season. Farmyard manure also resulted in a higher SRA under both crops but only at the higher application rate. Under wheat, micr obial biomass C (MBC) decreased in urea-treated plots but showed a sli ght increase at the higher rate of FYM. During the maize season, MBC w as higher under both urea (42-46%) and FYM (36-47%) treatments as comp ared to the control. Microbial biomass turnover rate was highest for F YM-32 (2.08), followed by FYM-16 and urea treatments (1.35-1.49); cont rol plots showed a turnover rate of 0.82. The higher AMC and SRA durin g the active growth period of wheat than that of maize indicated that root-derived C from wheat was higher in amount and more easily degrada ble.