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Decomposition of Nitrogen-15-Labeled Wheat and Cellulose in Soil: Modeling Tracer Dynamics

Inst. of Soils and Water, ARO, The Volcani Center, Bet Dagan 50250, Israel

J. A. E. Molina

Dep. of Soil Science, Univ. of Minnesota, St. Paul, MN 55108

C. E. Clapp

USDA-ARS, St. Paul, MN 55108

*Corresponding author.

ABSTRACT

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 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 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.

Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 3509-E, 1992 series. This research was supported by a grant from the US-Israel Binational Agricultural Research and Development Fund (BARD).

Received for publication April 13, 1992.