Low soil temperature effects on short-term gross N mineralisation–immobilisation turnover after incorporation of a green manure

The decomposition of Italian ryegrass (Lolium multiflorum L.) was studied over 37 days in an incubation experiment, at constant temperatures of 3, 9 and 15°C. With the use of a 15N label in the form of (15NH4)2SO4 and employing the pool dilution principle, the short-term dynamics of the gross N transformation rates were estimated using both the classical analytical equations formulated by Kirkham and Bartholomew (Soil Sci. Soc. Am. Proc., 18 (1954) 33) and the numerical model FLUAZ (Mary et al., Soil Biol. Biochem., 30 (1998) 1963). The assumptions of the pool-dilution method concerning homogeneity of labelling, disturbance of processes upon labelling and reliability of measurements were evaluated. Gross transformation rates calculated with the numerical model FLUAZ were considered superior to those calculated analytically, because with the FLUAZ model data variability could be taken into account, statistical measures corresponding to calculated rates were given, and nitrate immobilisation and nitrification kinetics were considered. The effect of temperature on the C mineralisation and gross N transformation rates was clear, all rates increasing with increasing temperature. Initially, there was high microbial activity in the Italian ryegrass treatment, followed by a decline in the second half of the incubation, reflecting changes in the quality of substrate being decomposed. The Q10 relationship was used to shed light on this effect and a comparison of Q10 values indicated that the breakdown of recalcitrant substances was more limited at low temperature than that of the more easily degradable substances. Decreases in the gross N mineralisation-to-immobilisation ratio with increasing temperature suggested that gross N immobilisation may be more sensitive to low temperatures than gross N mineralisation. That this may be the case was indicated by a positive net mineralisation rate for Italian ryegrass at 3°C, versus a net immobilisation in the short term at 9 and 15°C, as would also normally be expected for a green manure material with a C-to-N ratio above 20 such as the Italian ryegrass used in this study.