Do Nutrient Management Practices Reduce Nitrogen Use? Evidence from California’s Irrigated Lands Regulatory Program

This paper examines how nitrogen management practices (NMPs) affect nitrogen (N) use efficiency, which is measured as nitrogen applied (A) minus nitrogen removed (R) through crop uptake (A−R). The analysis uses unique field-level panel survey data from 11 water quality coalitions across California’s Central Valley, spanning the period 2019–2023. Because fertilizer application decisions are jointly determined with cropping management practices and field conditions, we employ an Instrumental Variables (IV) framework that uses soil, tissue, and irrigation-water testing practices as instruments for nitrogen application intensity. The results indicate that several management practices are associated with significant improvements in nitrogen efficiency outcomes. The largest gains are observed for split nitrogen applications, followed by fertigation and foliar nitrogen application. To further examine heterogeneity across growers, we estimate Quantile IV models across the distribution of A−R outcomes. The estimates reveal substantial variation in the effectiveness of management practices across such distribution. In particular, the practice of split nitrogen applications generates the largest efficiency gains among relatively less efficient fields, while fertigation and foliar applications exhibit heterogeneous effects across quantiles. The findings suggest that improvements in nitrogen efficiency are driven by a relatively narrow set of efficiency-oriented practices, highlighting the importance of distinguishing among management practices in both regulatory design and extension outreach efforts.