Soil Organic C and Fertilizer N: What Relationship When Not Confounded?.

A recent report on the relationship of soil organic C to fertilizer N addition was confounded by the combined impacts of land use change, tillage, N fertilization and time. A long-term (37 yr) monoculture corn (Zea mays L.) tillage (moldboard plow, no-tillage) by N rate (0, 84, 168, 334 kg N per hectare) trial was sampled to a depth of 90 cm, as was the surrounding native grass sod (age greater than 80 yr), to determine the impact of N fertilization, without confounding, on soil organic C. Corn yield in both tillage systems rose with increasing fertilizer N rate, as did organic C, especially in the upper solum. Organic C depletion was most associated with tillage, though land use change (from sod to continuous corn) was also important. As expected, unfertilized sod exhibited greater organic C than unfertilized no-tillage or moldboard plowed soils. There was little tillage by N interaction, indicating that fertilizer N is positively related to soil organic matter levels and associated improvements in cropland soil productivity (grain yield), when considered without confounding.