Estimating Net Nitrogen Mineralization from Carbon Dioxide Evolution

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Abstract
The ability to estimate net N mineralization from C decomposition data has the potential to improve our understanding of N dynamics in soil systems. It was the objective of this study to study this relationship using substrates with varying decomposition rates and C/N ratios. Five substrates including sewage sludge, alfalfa (Medicago sativa L.), clover (Trifolium sp.), bermudagrass [Cynodon dactylon (L.) Pers.] and ryegrass (Lolium multiflorum Lam.) were incubated in Crowley silt loam (Typic Albaqualfs) or Captina silt loam (Typic Fragiudults) soil at known soil temperatures and moistures. Concurrent CO2 evolution and soil inorganic N concentrations were measured periodically. A significant linear relationship between N mineralization and CO2 evolution was found experimentally for each substrate. A computer simulation model was developed which used first order kinetics for conversion of substrate C to CO2. Substrate C mineralization rate constants, substrate C/N ratios and microbial efficiency were primary inputs, while substrate, biomass, and soil organic matter were the major compartments of the model. Microbial efficiency was defined for any C pool undergoing decomposition as the ratio of assimilated C to assimilated C plus dissimilated C. An important feature of the approach was the introduction of a fraction with a C/N ratio of protein that decomposed very rapidly for those substrates where initial N mineralization was large while CO2 evolution was small. Model predictions of both CO2 evolution and net N mineralization were in good agreement with experimental results.