Assessment of Soil Organic Carbon, Nitrogen and C/N Ratios As Key Components for Evaluating Nitrate Reduction Strategies at a Subbasin Scale.

In the U.S. Midwest, many states, including Iowa, Ohio, Illinois and Minnesota are developing strategies to reduce export of nitrate-N (NO₃-N) from agricultural nonpoint sources to the Mississippi River. Watershed projects implemented through U.S. Department of Agriculture (USDA) assistance programs typically include a variety of in-field and edge-of-field best management practices (BMPs) to reduce NO₃-N loss. Many of these conservation practices are intended, in part, to improve soil quality because of its critically important role in influencing nutrient cycling, water holding capacity and overall soil sustainability. Management of soil organic carbon (SOC) and nitrogen (N) is considered a key component to prevent N loss to groundwater and artificial drainage. Our study intensively sampled the top soil (top 20 cm) in a 780 ha subbasin of Rapid Creek watershed, Iowa to evaluate and quantify landscape-scale storage of SOC and N and to identify potential groundwater vulnerability for these soils via NO₃-N leaching. Results indicated variability in N and SOC concentrations in the subbasin soils within a normal distribution and spatial analysis did not show any systematic field by field variations that would suggest potential hot spots for NO₃-N loss. Significantly lower SOC concentrations were measured in shoulder landscape positions, but no other variability was detected for N or C/N ratios. We estimated the total mass of SOC and N present in the subbasin using both slope class and soil series classifications and found that both quantification methods provided similar estimates (6.22 x 10³ to 5.88 x 10³ Mg SOC and 5.57 x 10² to 3.71 x 10² Mg N). C/N ratios ranged from 1-37 with a mean of 18 +/-6, but consistent with SOC and N, no field-level variation in the ratios was readily apparent. Most of the soils in the subbasin had C/N ratios less than 20 which suggests potential for soil N mineralization. Without a hot-spot for soil N loss identified, we propose a basin-wide approach to reduce NO₃-N export through incorporation of cover crops to reduce nitrification and increase SOC and C/N ratios.