Phosphorus use efficiency and crop production: Patterns of regional variation in the United States, 1987-2012.

Crop N use efficiency (NUE) and P use efficiency (PUE) might be expected to exhibit different patterns across agricultural regions due to their very different environmental dynamics and management strategies. Here, following our previous work on regional patterns of NUE, we review patterns of PUE and related variables, including major inputs of P to US crops over 1987-2012, based on the Farm Resource Regions developed by the Economic Research Service (USDA-ERS). Unlike N, P inputs to cropland only occur in the forms of P fertilizer, which has generally changed little over time relative to N fertilizer, and manure P, which has increased. Expressed as percentages of total P inputs, they necessarily have opposite impacts on PUE because of the stronger relationship of crop production to fertilizer compared to manure produced in a region. Across the US, PUE trends have varied significantly, increasing in some regions, in contrast to NUE which has generally remained constant or declined on decadal time scales. As with N, the Heartland region dominates national patterns due to the magnitude of crop production, showing a significant relationship with fertilizer P but none with manure P on a cropland area basis. Most other regions show similar responses, but the Northern Crescent, Eastern Uplands and Southern Seaboard regions shows a negative response to fertilizer on the same basis. The regional response of production to P inputs on a cropland area basis differs from that on a total area basis, suggesting that the type of scaling used is critical under changing cropland area. In the US, manure is still treated largely as a waste to be managed rather than a nutrient resource. Differences between P and N need to be considered in the context of management of environmental quality and food security.