Hadas, A., ARO, Bet Dagan, Israel
Feigenbaum, S., ARO, Bet Dagan, Israel
Sofer, M., ARO, Bet Dagan, Israel
Molina, J.A.E., ARO, Bet Dagan, Israel
Clapp, C.E., ARO, Bet Dagan, Israel

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 15N-labeled wheat (Triticum aestivum L.) and 15N 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 (15NH4)2SO4 at the same C rate and C/N ratio, were added to two soils and incubated for 32 wk at 30°C and 60% water-holding capacity. Inorganic and Kjeldahl N and 15N 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 × 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 15N 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.