Multilevel Assessments of Contribution of Livestock Manure to Nitrogen Budget in Arid-land Ecosystems: The Case of Dairies in New Mexico
The livestock industry is one of the major contributors to nitrogen pollution. Nearly one-third of the global human-induced nitrogen emissions are contributed by this sector. The intensification of dairy farms has resulted in an oversupply of nitrogen in smaller land pockets causing degradation of environmental and human health. New Mexico leads the country in terms of the average number of cows per large dairy farms and has held this position since 2002. A study by Johnson et al. (2003) indicates that nearly two-thirds of the New Mexican dairies have improper management of manure leading to groundwater pollution. This study aims to quantify the contribution of large dairy farms towards the nitrogen budget of the arid land ecosystem and recommend optimal abatement strategies. The nitrogen budget is calculated as the difference between livestock manure nitrogen loadings and crop nitrogen requirements. First, we modeled the spatial distribution of large dairy farms in New Mexico based on herd and land sizes and performed a farm-level assessment. For this, circles corresponding to the application acreage of farms were drawn and land cover data were used to calculate total nitrogen utilization by crops. For the county-level assessment, county totals were obtained by aggregating over livestock farms and croplands/grasslands in a county. Based on the geocoded locations of dairy and the number of cows in each farm, nitrogen excretion values were assigned to each county. Then, we performed a water-shed level assessment by aggregating over livestock farms and croplands/grasslands in the vulnerable watersheds. Results showed that eight counties, six watersheds, and all the farms at the individual level had excess nitrogen beyond the assimilative capacity of croplands. Then we proposed to include rangelands as possible sinks and performed a sensitivity analysis to understand the variation in nitrogen budget at 20, 40, 60, and 80% willingness to accept levels. Findings indicated that at 40% and above acceptance rate, all the excreted nitrogen can be assimilated within the system. Therefore, the integration of the crop-livestock system with the rangelands can provide a viable solution to reduce the nutrient loadings in air, soil, and water and provide affordable commercial fertilizer substitutes for the farmers and ranchers.