MODELING LONG-TERM - CARBON AND NITROGEN DYNAMICS IN AN ARABLE SOIL RECEIVING ORGANIC-MATTER

Predictions of a model based on a general theory of decomposition of o rganic matter were compared with measured changes in total soil C and N pools in a 35-yr field experiment on clay loam in central Sweden wit h biannual additions of straw, peat, sawdust, farmyard manure, green m anure, and sewage sludge in order to study which litter characteristic s determine decomposition and accumulation rate of soil organic matter . The central concepts of the theory are a continuously changing subst rate quality, a constant decomposer efficiency, and a climatically con trolled decomposer growth rate. The model requires seven parameters to describe C dynamics and two more to describe N dynamics. Substrate qu alities were estimated from measured decomposition rates, but no singl e characteristic of the examined substrates could be considered as a g eneral key indicator on decomposition rate. Predicted values were with in a few percent of the measured Values for C and within 13% of the me asured values for N, although the residual fractions of added C ranged from 14 to 69%, and the residual fractions of N varied from 24 to 400 % (net immobilization). In agreement with the field observations, the model predicted that the accumulation of soil C would be highest in th e plots receiving peat and lowest in the plots receiving straw and gre en manure. Intermediate accumulations were predicted in plots receivin g sewage sludge, sawdust, and farmyard manure. The model also predicte d that the net contribution of N to the crops from straw, peat, and sa wdust would be small or negligible. By contrast, contributions of N fr om sewage sludge and from green and farmyard manure were predicted to be considerable.