Soil nitrogen transformations in response to farming practices and the presence of roots.

摘要

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.