Modelling Long‐Term Carbon and Nitrogen Dynamics in an Arable Soil Receiving Organic Matter

Abstract

Predictions of a model based on a general theory of decomposition of organic 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 with biannual additions of straw, peat, sawdust, farmyard manure, green manure, and sewage sludge in order to study which litter characteristics determine decomposition and accumulation rate of soil organic matter. The central concepts of the theory are a continuously changing substrate quality, a constant decomposer efficiency, and a climatically controlled decomposer growth rate. The model requires seven parameters to describe C dynamics and two more to describe N dynamics. Substrate qualities were estimated from measured decomposition rates, but no single characteristic of the examined substrates could be considered as a general key indicator on decomposition rate. Predicted values were within a few percent of the measured values for C and within 13% of the measured 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 the plots receiving peat and lowest in the plots receiving straw and green manure. Intermediate accumulations were predicted in plots receiving sewage sludge, sawdust, and farmyard manure. The model also predicted that the net contribution of N to the crops from straw, peat, and sawdust would be small or negligible. By contrast, contributions of N from sewage sludge and from green and farmyard manure were predicted to be considerable.