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Carbon availability and microbial biomass in soil under an irrigated wheat-maize cropping system receiving different fertilizer treatments

Mahmood, T., Azam, F., Hussain, F., Malik, K.A.
Biology and fertility of soils 1997 v.25 no.1 pp. 63-68
soil microorganisms, biomass, carbon, nutrient availability, wheat soils, nitrogen fertilizers, urea, mineralization, biological activity in soil, biogeochemical cycles, seasonal variation, animal manures, application rate, semiarid zones, soil respiration, corn soils
Seasonal changes in carbon availability and microbial biomass were studied in soil under an irrigated wheat-maize cropping system receiving different fertilizer treatments over the past 10 years. Treatments included N-100 and N-200 (urea at 100 and 200 kg N ha-1 year-1, respectively), FYM-16 and FYM-32 (farmyard manure at 16 and 32 t ha-1 year-1, respectively) and a control (unfertilized). Aerobically mineralizable carbon (AMC; C mineralized after 10 days aerobic incubation at 30 degrees C) increased (13-16%) under wheat at both rates of urea whereas under maize it increased (22%) only with the lower rate of urea. Farmyard manure also increased the content of soil AMC under both crops, the effect being two- to threefold higher under wheat than under maize. Urea application caused an 32-78% increase in the specific respiratory 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, microbial biomass C (MBC) decreased in urea-treated plots but showed a slight increase at the higher rate of FYM. During the maize season, MBC was higher under both urea (42-46%) and FYM (36-47%) treatments as compared to the control. Microbial biomass turnover rate was highest for FYM-32 (2.08), followed by FYM-16 and urea treatments (1.35-1.49); control plots showed a turnover rate of 0.82. The higher AMC and SRA during the active growth period of wheat than that of maize indicated that root-derived C from wheat was higher in amount and more easily degradable.