Influence of Nitrogen Input on Active Soil Carbon in a Long-Term Crop Rotation Experiment.

Increased nitrogen input generally increases crop yields, which could lead to an increase in Active Soil Carbon, also known as Permanganate Oxidizable Carbon (POXC).  This study attempted to determine if a positive correlation exists between increased nitrogen application and active soil carbon in a long-term cotton-corn-soybean rotation.  Soil samples were taken from various treatments from Cullars Rotation, which is one of the oldest soil fertility experiments in the nation. Our results showed that there was significantly less POXC in the subsurface soil than in the surface soil.  Also, crop rotations where no fertilizer was used had drastically low POXC.  However, our results also indicated POXC patterns changed depending on what residues or crop was planted at the time of sampling.  The plots with Crimson clover tended to have more POXC when N fertilizer was not applied.  Those plots with soybean residues had relatively steady POXC values for the surface but for the subsurface POXC increased with nitrogen application.  Finally those plots with wheat in corn residues had generally the same POXC levels in the surface and sub-surface samples for the treatment with NPK fertilizer and lime.  Thus POXC could be affected by nitrogen application in some crops and not in others.