A global insight on sensitivity of nitrate leaching to drainage in arable cropping systems

Nitrate (NO3-) leaching leads to widespread deterioration of water quality. NO3- leaching can be regulated by both biochemical and hydrological processes. However, the extent to which drainage accounts for the variation in NO3- leaching and the sensitivity of NO3- leaching to drainage remain unclear, particularly at the global scale. This raises uncertainties in prioritizing the regulation of hydrological (e.g., drainage) processes for reduced NO3- leaching. A global meta-analysis was conducted for arable cropping systems to test two hypotheses: (1) Global variations in NO3- leaching are explained more by drainage and associated influencing factors (e.g., water input); (2) NO3- leaching is more sensitive to the changes in drainage under conditions with lower risks of preferential flow. In this meta-analysis, the average NO3- leaching loss from arable cropping systems was 22.2 kg N ha(-1) year(-1). Water input, followed by N input, were the most influential in explaining global variations in NO3- leaching. Compared with NO3- concentration, drainage and associated influencing factors explained more of the global variations in NO3- leaching losses. NO3- leaching was more sensitive to drainage in soils with finer texture and higher soil organic carbon content, in regions with low to medium mean annual temperature (<15 degree celsius) and medium mean annual precipitation (600-1000 mm), where water input during the measurement period was low (<2 mm day(-1)), nitrogen fertilizer rates were high (>200 kg N ha(-1)), and where conservation tillage systems (no-tillage or non-inversion tillage) were adopted. Therefore, more attention should be given to avoiding excessive water application in soils characterized by low risk of preferential flow. Preferential flow reduces the chance for drainage water to interact with NO3- in the soil matrix, particularly in areas with high N inputs. While this study has successfully tested our hypotheses, it further highlights the importance of water input management for mitigating global NO3- leaching risks, particularly under environments and changing climate conditions more conducive to drainage events.