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

The decomposition of heterogeneous plant material could be described m ore generally if it were based on decomposition rates of defined mater ials. In this study, mineralization of N-15-labeled wheat (Triticum ae stivum L.) and N-15 turnover linked with the decomposition of cellulos e 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 w ere measured and compared with simulated data. Cellulose induced net i mmobilization of 70 mg N kg-1 within 2 wk; thereafter, net N mineraliz ation was greater than for untreated soils. The decomposition rate con stant 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 biom ass than simulated. Wheat decomposition was divided into three stages, corresponding to soluble, cellulose-like, and resistant fractions tha t 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 ad ded organic N, whereas N-15 recovery as inorganic N was 40 to 50%, dep ending on the soil. The difference was attributed to concurrent assimi lation of labeled N by soil microbial biomass that depended partly on native soil N concentrations and should be considered in interpreting tracer experiments.