| The types and amounts of carbon (C) and nitrogen (N) inputs, and the presence of live roots, affect C and N turnover and retention in agricultural soils. Microbial responses to rewetting and subsequent drying in soil of irrigated Californian cropping systems receiving either high organic matter (OM) inputs (organic) or inorganic N fertilizer and low OM inputs (conventional) were studied. In the field and in microcosms, microbial biomass and activity, phospholipid fatty acids (PLFA), rates of inorganic N production and consumption, nitrous oxide (N2O) efflux and total denitrification were measured. In microcosms with tomato (Lycopersicon esculentum) plants, short-term N flow between inorganic and organic pools was estimated in soil with roots and in root exclosures. Microbial biomass C and C mineralization rates were greater in organic than conventional soil. At about 60% WFPS, the greatest CO2 efflux occurred, and some PLFA indicators suggested optimal conditions for microbial activity at this moisture level. Gross ammonification rates and potentially mineralizable N were about twice as high in organic than conventional soil. Rates of microbial nitrate (NO3 -) immobilization were equal to at least 37% and 32% of gross nitrification rates in organic and conventional soil, respectively. In both soils, high N2O emissions coincided with high ammonium (NH 4+) concentrations at WFPS of >60% and lasted <2 d following irrigation. Inorganic N rather than C availability appeared to limit N2O efflux and total denitrification. In soil microcosms with tomato plants, 15NO3- was converted to 15NH4+ within 24 hours. The amount of 15NO4+ in the soil with roots was >30 times greater than in root exclosures within the same microcosms. Either remineralization of microbial biomass that had taken up 15NO 3- or leakage of reduced 15N by tomato roots after assimilation of 15NO3 - may have occurred. Simulation modeling indicated that NH 4+ uptake by tomato plants was modest compared to NO 3- uptake. Total NH4+ immobilization rates by plants and microbes were equal to at least 35% of gross nitrification rates. Microbial NO3- immobilization and the rapid cycling between organic and inorganic pools in the presence of roots contribute to ecosystem N retention potential. |
