MODELING SOIL ORGANIC-MATTER IN ORGANIC-AMENDED AND NITROGEN-FERTILIZED LONG-TERM PLOTS

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 bannual 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.