DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY Biochemical Quality of Crop Residues and Carbon and Nitrogen Mineralization Kinetics under Nonlimiting Nitrogen Conditions I. Trinsoutrot, S. Recous, B. Bentz, M. Line `res, D. Che ` neby, and B. Nicolardot* ABSTRACT and recalcitrant fractions (e.g., Verberne et al., 1990). Models that simulate agronomic scenarios (e.g., Probert Statistical relationships were established between the fate of C and et al., 1998; Brisson et al., 1998) often describe the bio- N from 47 types of crop residues and their biochemical characteristics chemical quality of crop residues only by their relative during a soil incubation at 15C. The incubations were carried out under nonlimiting N in order to differentiate the effects of biochemical C to N contents (C/N ratio). Indeed the criterion of characteristics of residues from those of soil N availability. Depending quality used most often to predict mass loss or N miner- on the residue, the apparent mineralization of residue C after 168 d alization during crop residue decomposition is the C/N varied from 330 to 670 g kg -1 of added C. Mineralization kinetics ratio of the plant material (Taylor et al., 1989; Vanlauwe were described using a two-compartment decomposition model that et al., 1996). However, this ratio does not account for decomposes according to first-order kinetics. Amounts of C mineral- the availability of C and N, which is often essential to ized after 7 d and the decomposition rate coefficient of the labile describe the decomposition kinetics (Camire ´ et al., 1991; fraction were related mainly to the soluble C forms of the residue. No Recous et al., 1995). Many studies have been aimed at statistical relationship was established between the N concentration of finding other biochemical characteristics to predict the residues and their decomposition in the soil. The incorporation of decomposition of crop residues. Others have shown that crop residues into soil led to various soil mineral N dynamics. Two residues caused net N mineralization from the time of their incorpora- the initial residue N content (Frankenberger and Ab- tion, whereas all the others induced net N immobilization (1–33 g N delmagid, 1985), lignin (Mu ¨ ller et al., 1988), polyphenols kg -1 of added C). After 168 d, only residues with a C/N ratio 24 (Constantinides and Fownes, 1994), and soluble C con- induced a surplus of mineral N compared with the control soil. The centrations (Reinertsen et al., 1984; Oglesby and mineral N dynamics were related mainly to the organic N concentra- Fownes, 1992; Kachaka et al., 1993) are useful indicators tion of the residues and to their C/N ratio. At the start of incubation, of residue quality. Most of these studies have concerned these dynamics were also influenced by the presence of polyphenols residues of tropical legumes, generally rich in N and in the plant tissues. Finally, this study showed the need to include polyphenols (Oglesby and Fownes, 1992), which differ the biochemical quality of crop residues in any C and N transformation qualitatively from crop residues found in temperate cli- models that describe decomposition. In contrast, the N concentration matic conditions. Nitrogen availability may control the or C/N ratio of the residues are sufficient to predict the net effects of crop residues on soil mineral N dynamics. kinetics of decomposition of crop residues, particularly those with high C/N ratio such as cereals, when the N requirements of the soil decomposers are not fulfilled by the residue or soil N contents (Recous et al., 1995). T he decomposition of crop residues is the result of In this case the biochemical quality of the residue no complex microbial processes controlled by numer- longer controls the dynamics of C and the associated N ous factors (Swift et al., 1979). Among these, the bio- dynamics, and residues containing various amounts of chemical composition of the residues exerts an impor- N are no longer comparable, whatever the nature of tant influence (Heal et al., 1997). Most models that aim their constituents. It is therefore not surprising that in at simulating the decomposition of residues take into many studies the residue N content has been shown to be account the initial biochemical quality of residues, but the main factor predicting the kinetics of decomposition the description is fairly variable. Models that adopt a (Tian et al., 1992, 1995). Consequently, when N avail- mechanistic approach to describe C and N biotransfor- ability is a limiting factor of decomposition, which is mations have included biochemical, such as soluble, the case for most of the data published, the kinetics hemicellulose, cellulose, and lignin fractions (e.g., Selig- of decomposition or C mineralization observed do not man and Van Keulen, 1981), or easily decomposable allow the effect of biochemical quality to be assessed or distinguished from the effects of N availability on I. Trinsoutrot, B. Bentz, and B. Nicolardot, INRA - Unite ´ d’agronomie C decomposition. de Cha ˆ lons-Reims, Centre de Recherche Agronomique, 2 esplanade The objective of our work was to establish, under Roland Garros, BP 224, 51686 Reims cedex 2, France; S. Recous, conditions of nonlimiting N, the relationships between INRA - Unite ´ d’agronomie de Laon-Pe ´ ronne, rue Fernand Christ, 02007 Laon cedex, France; M. Line ` res, INRA - Station d’agronomie, the kinetics of C and N decomposition and the biochemi- 71 avenue Edouard Bourlaux, BP 81, 33883 Villenave d’Ornon cedex, cal characteristics of the residues produced by the main France; D. Che ` neby, INRA - CMSE Laboratoire de Microbiologie arable crops of temperate regions: oilseed rape (Brassica des Sols, 17 rue Sully, BP 1540, 21034 Dijon cedex, France. Received 23 Mar. 1999. *Corresponding author (Bernard.Nicolardot@reims. Abbreviations: CEL, cellulose fraction; d.m., dry matter; HEM, hemi- inra.fr) cellulose fraction; LIG, lignin fraction; POL, soluble polyphenols; S20, soluble C fraction; SOL, soluble fraction. Published in Soil Sci. Soc. Am. J. 64:918–926 (2000). 918