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 o ver 37 days in an incubation experiment, at constant temperatures of 3, 9 a nd 15 degreesC. With the use of a N-15 label in the form of ((NH4)-N-15)(2) SO4 and employing the pool dilution principle, the short-term dynamics of t he gross N transformation rates were estimated using both the classical ana lytical equations formulated by Kirkham and Bartholomew (Soil Sci. Sec. 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 con cerning homogeneity of labelling, disturbance of processes upon labelling a nd reliability of measurements were evaluated. Gross transformation rates c alculated with the numerical model FLUAZ were considered superior to those calculated analytically, because with the FLUAZ model data variability coul d be taken into account, statistical measures corresponding to calculated r ates 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 temper ature. Initially, there was high microbial activity in the Italian ryegrass treatment, followed by a decline in the second half of the incubation, ref lecting changes in the quality of substrate being decomposed. The Q(10) rel ationship was used to shed light on this effect and a comparison of Q(10) v alues indicated that the breakdown of recalcitrant substances was more limi ted at low temperature than that of the more easily degradable substances. Decreases in the gross N mineralisation-to-immobilisation ratio with increa sing temperature suggested that gross N immobilisation may be more sensitiv e to low temperatures than gross N mineralisation. That this may be the cas e was indicated by a positive net mineralisation rate for Italian ryegrass at 3 degreesC, versus a net immobilisation in the short term at 9 and 15 de greesC, 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. ( C) 2001 Elsevier Science Ltd. All rights reserved.