Microbial immobilization of ammonium and nitrate in relation to ammonification and nitrification rates in organic and conventional cropping systems

Abstract

Agricultural systems that receive high or low organic matter (OM) inputs would be expected to differ in soil nitrogen (N) transformation rates and fates of ammonium (NH 4 +) and nitrate (NO 3 −). To compare NH 4 + availability, competition between nitrifiers and heterotrophic microorganisms for NH 4 +, and microbial NO 3 − assimilation in an organic vs. a conventional irrigated cropping system in the California Central Valley, chemical and biological soil assays, 15N isotope pool dilution and 15N tracer techniques were used. Potentially mineralizable N (PMN) and hot minus cold KCl-extracted NH 4 + as indicators of soil N supplying capacity were measured five times during the tomato growing season. At mid-season, rates of gross ammonification and gross nitrification after rewetting dry soil were measured in microcosms. Microbial immobilization of NO 3 − and NH 4 + was estimated based on the uptake of 15N and gross consumption rates. Gross ammonification, PMN, and hot minus cold KCl-extracted NH 4 + were approximately twice as high in the organically than the conventionally managed soil. Net estimated microbial NO 3 − assimilation rates were between 32 and 35% of gross nitrification rates in the conventional and between 37 and 46% in the organic system. In both soils, microbes assimilated more NO 3 − than NH 4 +. Heterotrophic microbes assimilated less NH 4 + than NO 3 − probably because NH 4 + concentrations were low and competition by nitrifiers was apparently strong. The high OM input organic system released NH 4 + in a gradual manner and, compared to the low OM input conventional system, supported a more active microbial biomass with greater N demand that was met mainly by NO 3 − immobilization.