SOIL MICROBIAL DYNAMICS - SHORT-TERM AND LONG-TERM EFFECTS OF INORGANIC AND ORGANIC NITROGEN

Soils with minimal long-term organic inputs typically have reduced biological activity, which has implications for current interests in shifting from inorganic to organic inputs and promoting efficient nutrient cycling in agroecosystems. A greenhouse experiment was conducted to investigate the long- and short-term effects of organic vs. inorganic N on microbial biomass, metabolic quotient, and key soil enzymes (protease, L-histidine NH3-lyase, and beta-glucosidase) involved in N and C cycles. Treatments applied factorially to four corn (Zea mays L.) crops grown for 306 d were: four soils from long-term field plots (beef manure, pea vine [Pisum sativum L.], 0, or 90 kg N ha-1, each applied biennially for 59 yr); four greenhouse organic residues (pea vine, beef manure, poultry manure, or control); and four rates of inorganic N fertilizer (0- 1600 mg NH4NO3-N pot-1 with 2 kg soil pot-1). In the long-term, soil microbial biomass and enzyme activity correlated with total C inputs. Recent organic inputs, regardless of long-term management, had a large effect on soil biological response, which was controlled by residue composition (lignin content) and supported 80 to 400% greater microbial biomass C than the control. Long-term inorganic N applications decreased organic matter and biological activity, whereas short-term inorganic N applications had limited effects on soil enzyme activities and microbial biomass C, suggesting that inorganic N can maintain plant productivity during a transition to organic N sources without inhibiting the buildup of microbial biomass. The metabolic quotient gave mixed results as a soil biological indicator, being high with long-term N or recent beef manure applications and low with recent N applications.