Temperature and stubble management influence microbial CO2-C evolution and gross N transformation rates

Few studies have examined the kinetics of gross nitrogen (N) mineralization, immobilization, and nitrification rates in soil at temperatures above 15 degrees C. In this study, N-15 isotopic pool dilution was used to evaluate the influence of retaining standing crop residues after harvest versus burning crop residues on short-term gross N transformation rates at constant temperatures of 5, 10, 15, 20, 30, and 40 degrees C. Gross N mineralization rates calculated per unit soil organic carbon were between I and 7 times lower in stubble burnt treatments than in stubble retained treatments. In addition, significant declines in soil microbial biomass (P = 0.05) and CO2-C evolution (P < 0.001) were associated with stubble burning. Immobilization rates were of similar magnitude to gross N mineralization rates in stubble retained and burnt treatments incubated between 5 and 20 degrees C, but demonstrated significant divergence from gross N mineralization rates at temperatures between 20 and 40 degrees C. Separation in the mineralization immobilization turnover (MIT) in soil at high temperatures was not due to a lack of available C substrate, as glucose-C was added to one treatment to test this assumption. Nitrification increased linearly with temperature (P < 0.001) and dominated over immobilization for available ammonium in soil incubated at 5 degrees C, and above 20 degrees C indicating that nitrification is often the principal process controlling NH4+ consumption in a semi-arid soil. These findings illustrate that the MIT at soil temperatures above 20 degrees C is not tightly coupled, and consequently that the potential for loss of N (as nitrate) is considerably greater due to increased nitrification. (c) 2005 Elsevier Ltd. All rights reserved.