Nitrogen availability effects on carbon mineralization, fungal and bacterial growth, and enzyme activities during decomposition of wheat straw in soil

Our objectives were (1) to investigate the response of wheat (Triticum aestivum L.) straw decomposition to increasing nitrogen availability (0.5, 0.8, 1.2 and 1.9% N of straw dry matter) in a microcosm experiment at controlled temperature (15 degrees C) and moisture (-10 kPa), (2) to elucidate underlying mechanisms at the microbial-community level, and (3) to develop a model according to the measurements. Concentrations of available N (straw N and soil inorganic N) below 1.2% of straw dry matter significantly reduced the rate of carbon mineralization from straw residues and the growth of total soil microbial biomass (chloroform fumigation-extraction). The negative effects appeared shortly after the pool of soil mineral N had been depleted. The N effect on the microbial community was mainly caused by reduced fungal growth (ergosterol), while total bacterial biomass (epifluorescence microscopy) was not significantly affected. However, number of cellulase-producing, colony-forming bacterial units decreased with decreasing N availability. In straw-amended soil, decreasing N concentrations reduced activities of exocellulase, endocellulase and hemicellulase, while in unamended control soil the effects were opposite. We developed a model in which three fractions of straw residues, determined by proximate analysis (Van Soest), and a humus pool were assumed to decay according to first-order rate kinetics. In accordance with the microbial measurements, the microbial community was subdivided into three groups consuming readily decomposable, structural and humus materials respectively. When holocellulose decay rate was modified as a function of N availability, the model simulated N effects on C mineralization and microbial biomass growth very well. Our experiment showed that wheat straw mineralization may be retarded at N concentrations well above those frequently found after grain harvest in the field and that effects of N availability should be taken into account when modelling C and N turnover in agricultural soils. Moreover, the trial exemplified a situation where studies of functional groups of decomposer organisms were required to explain and model satisfactorily differences observed at the process level.