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

Soils with minimal long-term organic inputs typically have reduced bio logical activity, which has implications for current interests in shif ting from inorganic to organic inputs and promoting efficient nutrient cycling in agroecosystems. A greenhouse experiment was conducted to i nvestigate the long- and short-term effects of organic vs. inorganic N on microbial biomass, metabolic quotient, and key soil enzymes (prote ase, L-histidine NH3-lyase, and beta-glucosidase) involved in N and C cycles. Treatments applied factorially to four corn (Zea mays L.) crop s grown for 306 d were: four soils from long-term field plots (beef ma nure, pea vine [Pisum sativum L.], 0, or 90 kg N ha-1, each applied bi ennially for 59 yr); four greenhouse organic residues (pea vine, beef manure, poultry manure, or control); and four rates of inorganic N fer tilizer (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 b y residue composition (lignin content) and supported 80 to 400% greate r microbial biomass C than the control. Long-term inorganic N applicat ions decreased organic matter and biological activity, whereas short-t erm inorganic N applications had limited effects on soil enzyme activi ties and microbial biomass C, suggesting that inorganic N can maintain plant productivity during a transition to organic N sources without i nhibiting the buildup of microbial biomass. The metabolic quotient gav e mixed results as a soil biological indicator, being high with long-t erm N or recent beef manure applications and low with recent N applica tions.