Low soil temperature effects on short-term gross N mineralisation± immobilisation turnover after incorporation of a green manure M.K. Andersen, L.S. Jensen * Plant Nutrition and Soil Fertility Laboratory, Department of Agricultural Sciences, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark Received 20 January 2000; received in revised form 11 July 2000; accepted 27 July 2000 Abstract The decomposition of Italian ryegrass (Lolium multi¯orum L.) was studied over 37 days in an incubation experiment, at constant temperatures of 3, 9 and 158C. With the use of a 15 N label in the form of ( 15 NH 4 ) 2 SO 4 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 nitri®cation 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, re¯ecting changes in the quality of substrate being decomposed. The Q 10 relationship was used to shed light on this effect and a comparison of Q 10 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 38C, versus a net immobilisation in the short term at 9 and 158C, 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. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: 15 N Pool-dilution; Decomposition; Gross N turnover; MIT; Plant residue; Temperature 1. Introduction In stockless organic farming systems, the soil receives most of its nutrient input from plant residues, in the form of catch crops or green manure. Knowledge of the nutri- ent mineralisation dynamics immediately following the incorporation of plant residues into the soil in spring, is a prerequisite for ensuring an optimal timing of nutri- ent release with the demands of a subsequent crop. The availability of crop residue N to succeeding crops is determined by the outcome of the mineralisation± immobilisation turnover (MIT) and the stabilisation of residue N in soil organic matter pools after its incorpora- tion into the soil (Jansson and Persson, 1982). Much research in the area of plant residue decomposition in soil has had its focus on N dynamics (Thorup-Kristensen, 1993; Jensen, 1994; Mary et al., 1996) and the somewhat static net-mineralisation parameter has often been used as a measure of plant-available N in the system. A more detailed picture of the dynamics of plant available N in soil would be gained from an understanding of factors controlling the balance between the gross-mineralisation and -immobilisation processes over time. Little research has been done to quantify the effect of temperature on the MIT in the low temperature range prevailing during winter and spring, relevant for decomposition of incorpo- rated catch crops and green manures. Net N mineralisa- tion has commonly been assumed to decline strongly at low temperatures to become nil at 08C (Van Veen and Frissel, 1981; Hansen et al., 1991). However, Van Scholl et al. (1997) found that considerable mineralisation of added plant residues can take place at temperatures close to 08C and in a ®eld study. Breland (1994) showed Soil Biology & Biochemistry 33 (2001) 511±521 0038-0717/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0038-0717(00)00192-9 www.elsevier.com/locate/soilbio * Corresponding author. Tel.: 145-3528-3460; fax: 145-3528-3470. E-mail address: lsj@kvl.dk (L.S. Jensen).