Soil Microbial Dynamics: Short‐ and Long‐Term Effects of Inorganic and Organic Nitrogen

Abstract

Abstract 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 NH 3 ‐lyase, and β‐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 NH 4 NO 3 ‐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.