Temperature and stubble management influence microbial CO2-C evolution and gross N transformation rates
2006
Hoyle, F.C. | Murphy, D.V. | Fillery, I.R.P.
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, 15N 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 1 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.
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