Modeling Soil Organic Matter in Organic‐Amended and Nitrogen‐Fertilized Long‐Term Plots

Abstract

Abstract To study the effects of organic matter and fertilizer additions on soil organic‐matter (SOM) dynamics, we analyzed results from a 30‐yr‐old Swedish field experiment, using the CENTURY model. Field treatments on a sandy clay loam included biannual addition (up to 4000 kg C ha −1 ) of straw, sawdust, and no organic additions, with and without N fertilizer (80 kg N ha −1 ), and green manure, farmyard manure, and bare fallow. Soil C and N, crop production (small‐grain and root crops), N uptake, and mineralization and immobilization of N were modeled and compared with field measurements. Changes in soil C and N, as much as 30% after 30 yr, were generally well represented by the model. Most of the treatment differences in SOM could be explained by the rate of organic‐matter input, its lignin content, and C/N ratio, plus the effect of N fertilizer on belowground C inputs. However, there appeared to be additional positive effects of N supply on SOM accumulation that were not fully explained by the model. The quality of organic amendments strongly influenced N uptake and crop productivity, through controls on N mineralization and immobilization. Mean annual N uptake ranged from a low of 3.1 g m −2 in sawdust‐amended soil to a high of 9.2 g m −2 in fertilized, straw‐amended soil. Simulated N losses accounted for 7 to 20% of total N inputs. Nitrogen‐balance estimates by the model were consistent with the observed data for four of the treatments, but treatments with low N availability appeared to have additional, unexplained N inputs.