Hydro-climatic controls explain variations in catchment-scale nitrogen use efficiency

Abstract

The efficiency of fertilizer conversion to harvestable products is often low in annual crops such that large amounts of nutrients are lost from fields with negative consequences for the environment. Focusing on nitrogen (N) use efficiency (NUE: the ratio of N in harvested products over the sum of all N inputs), we propose that hydrological controls can explain variations in NUE, because water mediates both the uptake of N by plants and N leaching. We assess these controls at the catchment scale, at which the water balance can be constrained by precipitation and runoff data and NUE can be quantified with census data. With this approach we test the hypotheses that a higher evaporative ratio (ET/P: the ratio of evapotranspiration over precipitation) increases N retention, thereby increasing NUE both across catchments at a given time and through time. With data from 73 catchments in the United States, encompassing a wide range of pedoclimatic conditions for the period 1988–2007, we apply a linear mixed effect model to test the effect of ET/P on NUE. Supporting our hypotheses, ET/P was positively related to NUE, and NUE increased through time. Moreover, we found an interaction between ET/P and time, such that the ET/P effect on NUE decreased in the period 1998–2007. We conclude that climatic changes that increase ET/P without negatively affecting yields, will increase N retention in the examined catchments.