DECOMPOSITION OF NITROGEN-15-LABELED WHEAT AND CELLULOSE IN SOIL - MODELING TRACER DYNAMICS

The decomposition of heterogeneous plant material could be described more generally if it were based on decomposition rates of defined materials. In this study, mineralization of N-15-labeled wheat (Triticum aestivum L.) and N-15 turnover linked with the decomposition of cellulose in soil were measured and compared with simulated kinetics computed by the model NCSOIL. Dried wheat shoots (2 g C kg-1) with a C/N ratio of 14.4, or cellulose with ((NH4)2SO4)-N-15) at the same C rate and C/N ratio, were added to two soils and incubated for 32 wk at 30-degrees-C and 60% water-holding capacity. Inorganic and Kjeldahl N and N-15 were measured and compared with simulated data. Cellulose induced net immobilization of 70 mg N kg-1 within 2 wk; thereafter, net N mineralization was greater than for untreated soils. The decomposition rate constant of cellulose, computed by optimization of the model, was 0.024 d-1. The model underestimated N immobilization, the subsequent rate of net N mineralization, and the isotopic dilution of inorganic N. These discrepancies probably resulted from slower turnover of microbial biomass than simulated. Wheat decomposition was divided into three stages, corresponding to soluble, cellulose-like, and resistant fractions that decomposed with rate constants of 3.0, 0.024, and 4 x 10(-8) d-1 and accounted for 19, 45, and 36%, respectively, of organic wheat N. The computed gross mineralization of wheat N after 32 wk totaled 64% of added organic N, whereas N-15 recovery as inorganic N was 40 to 50%, depending on the soil. The difference was attributed to concurrent assimilation of labeled N by soil microbial biomass that depended partly on native soil N concentrations and should be considered in interpreting tracer experiments.