Our national nutrient reduction needs: Applying a conservation prioritization framework to US agricultural lands
Targeted conservation approaches seek to focus resources on areas where they can deliver the greatest benefits and are recognized as key to reducing nonpoint source nutrients from agricultural landscapes into sensitive receiving waters. Moreover, there is growing recognition of the importance and complementarity of in-field and edge-of-field conservation for reaching nutrient reduction goals. Here we provide a prioritization framework that can help with spatial targeting: It begins with identifying areas with high agricultural nutrient surplus, i.e. where the most nitrogen (N) and/or phosphorus (P) inputs are left on the landscape after crop harvest. Subwatersheds (eight-digit hydrologic unit code or HUC8) with high surplus included almost half of the conterminous US subwatersheds, and were located predominantly in the Midwest for N, in the South for P, and in California for both N and P. Then we identified most suitable conservation strategies using a hierarchy of measures including nutrient use efficiency (proportion of nutrient inputs removed in crop harvest), tile drainage, existing buffers for agricultural run-off, and wetland restoration potential. In-field nutrient input reduction emerged as a priority because nutrient use efficiency fell below a high but achievable goal of 0.7 (30% of nutrients applied are not utilized) in 86% and 88% of high surplus subwatersheds for N and P, respectively. In many parts of the southern and western US, in-field conservation (i.e. reducing inputs + preventing nutrients from leaving fields) alone was likely the optimal strategy as agriculture was already well-buffered. However, additional edge-of-field buffering would be important to conservation strategies in 67% of high N and 58% of high P surplus subwatersheds nationwide. Nutrient efficiencies were often high enough in the Midwest that proposed strategies focused more on preventing nutrients from leaving fields, managing tile effluent, and buffering agricultural fields. Almost all HUC2 river basins would benefit from a variety of nutrient reduction conservation strategies, underscoring the potential of targeted approaches to help limit excess nutrients in surface and ground waters.