Predicting N transformations from organic inputs in soil in relation to incubation time and biochemical composition

Seventeen different added organic materials (AOM) in a sandy soil were incubated under controlled laboratory conditions (28 °C, 75% WHC), and examined for C and N mineralisation. The transformation of added organics (TAO) model has been presented in previous work for predicting C mineralisation. The two variables (very labile and stable fractions of AOM) used in TAO have been related to the biochemical characteristics of the AOM. The transformed added organic N fraction (TAONF) was estimated from the remaining CAOM and NAOM linked by the C-to-N ratios. TAONF was split (Pim parameter) into immobilised N (imN) and inorganic N (inorgN). When necessary, an additional N mineralisation of imN was predicted by first order kinetics (constant kremin). The TAO version with the two parameters Pim and kremin allowed us to predict very different dynamics of N mineralisation and N immobilisation from the AOM. In a few cases, another first order kinetic law (constant kv) was used to predict N volatilisation from inorgN. mical characteristics of AOM were used for predicting N transformations. First, at each incubation date, inorgN was approximated to inorgNa=α(N-to-CAOM)+β by linear regression. The α, β and −β/α (C-to-NAOM threshold for mineralisation/immobilisation) were related to time. The TAO expression (1−Pim)TAONF was then replaced by the proposed approximation inorgNa as a function of incubation time and C-to-NAOM. Secondly, significant relationships were computed between kremin and organic fibre content of AOM. Finally, a TAO approximation was proposed for predicting the simultaneous transformations of C and N, only using biochemical data (plus the kv parameter in a few cases of N volatilisation). For all AOMs, the validity of the approximation and its borderline cases were examined by comparing the two TAO versions.