Evaluation of carbon and nitrogen mineralization rates in meadow soils from dairy farms under transit to biological cropping systems

Mineralized soil N from meadow soils will become an important source of N to following crops in low-input biological cropping systems. The C and N mineralization rates of soils from 34 sites situated on dairy farms recently converted to biological cropping systems were evaluated in a 56-wk incubation at 25degreesC. Data on C and N mineralization were fitted to first-order kinetic models. Carbon and N generally presented similar patterns of mineralization. Total mineralized N (N.) fanged between 88 and 235 mg N kg(-1) soil, which represented 6.6 to 22% of total N. Carbon mineralization (C-m) rate was between 11 and 17 times greater than N mineralization (1523-2638 mg C kg(-1) soil) and C mineralized represented 9 to 19% of soil organic C. The rate constant was between 0.05 and 0.123 wk(-1) for C (k(C)) and ranged from 0.032 to 0.088 wk(-1) for N (k(N)); The half-life for C (T(C)0) and for N (T(N)0) varied, respectively, between 5.6 and 13.3 wk and between 15 and 28 wk. Results show that about 80% of total mineralized C and N were mineralized during the first 25 wk of incubation, corresponding to the mineralizable fraction of soil organic matter (OM). Data on C and N mineralization have been adjusted using a bicompartmental model (active and recalcitrant pools), which corresponded, respectively, to first-order and exponential equations. Total mineralizable C and N (C-m and N-m), and the C and N rate constants (k(C) and k(N)) were strongly related, whereas the rate constants of the recalcitrant pools (h(C) and h(N)) were negatively related to these parameters. This suggests that C and N mineralizable pools were independent of the humified stable OM (recalcitrant pool). Carbon and N mineralization parameters were positively related to the soil clay and silt contents, but inversely to the sand levels. This study indicates that when ploughed, meadow soils contain large mineralizable N pools, which could sustain following crops with N nutrition in low-input biological cropping systems.