Biochemical composition and mineralization kinetics of organic inputs in a sandy soil

The carbon mineralization of added organic materials (AOM) in soil was assessed by combining laboratory and modeling approaches. The AOM used in the organic fertilizer industry included: plant residues from agri-food origin, animal wastes, manures, composts, and organic fertilizers. They were fractionated by sequential analyses of fibers and analyzed for C, N and ash contents. A previous kinetic study permitted to select two predictive models for AOM C mineralization in a sandy soil. These models, m4 and m6, were respectively defined by (i) two compartments (labile L and very resistant R) with three parameters: PL (proportion of L), and kmL, kmR (kinetic constants of L and R); (ii) three compartments (very labile L', resistant R' and stable S), with two parameters: P'L and PS (proportions of L' and S) with fixed kinetic constants at 28 degrees C, 75% WHC. We tested for the best prediction of the above parameters with the analytical data. These predictions were significant for the whole AOM set, but to a lesser degree for the C mineralization of AOM with contrasted characteristics. A Principal Component Analysis (PCA) was used to classify the AOM according to their biochemical contents into two groups: (+) ligneous ones with relatively high C and low N contents (mostly plant-originated AOM), and (-) more nitrogenous ones, poorer in organic C and ligno-cellulosic fibers (mostly animal-originated or partially composted AOM). The classification improved the predictive equations, which use one to three biochemical variables in agreement with the conceptual definition of the parameters. P'L, PL and PS were more accurately estimated than kmL, and kmR. For most of the AOM, m6 gave better simulations than m4. From m6 equations, the conceptual compartments L', R' (with P'R = 1 - P'L - PS) and S appeared to correspond to (i) parts of soluble, nitrogenous and hemicellulosic compounds, (ii) cellulose and the remaining fraction of hemicelluloses, (iii) the ligneous fraction, respectively.