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.

Water Availability and Land Management Control Catchment‐Scale Agricultural Nitrogen and Phosphorous Use Efficiencies

“…The slopes of the relations between nutrient use efficiencies and evaporative ratio decreased through time because the efficiencies were lower in the last 5 years in catchments with a high evaporative ratio. Therefore, NUE and PUE were less sensitive to the evaporative ratio in the more recent years, as already noted for NUE across two decades by Scaini et al (2020). Moreover, the increase in NUE and PUE through time is faster in catchments with a higher fraction of area under maize or soybean.…”

“…An effective indicator of nutrient use efficiency is the ratio of nutrient (e.g., nitrogen, N, or phosphorous, P) removed in harvested products to the total inputs of that nutrient (Lassaletta et al., 2014a; Scaini et al., 2020), hereafter NUE for nitrogen and PUE for phosphorous use efficiencies. These efficiencies can be defined at the field (Weih et al., 2018) or larger scales, such as counties (Swaney & Howarth, 2019b; Swaney et al., 2018b) or—our focus here—catchments.…”

“…However, reducing nutrient loads is costly and requires changes in land use practices and agronomic management (Christianson et al, 2018). To identify management approaches to reduce nutrient load to the environment, we need to quantify how environmental conditions and agricultural management jointly affect the efficiency of nutrient retention in harvested products (i.e., the crop nutrient use efficiency).An effective indicator of nutrient use efficiency is the ratio of nutrient (e.g., nitrogen, N, or phosphorous, P) removed in harvested products to the total inputs of that nutrient (Lassaletta et al, 2014a;Scaini et al, 2020), hereafter NUE for nitrogen and PUE for phosphorous use efficiencies. These efficiencies can be defined at the field (Weih et al, 2018) or larger scales, such as counties (Swaney & Howarth, 2019b;Swaney et al, 2018b) or-our focus here-catchments.…”

“…To reduce nutrient loads to the environment, we need to explore whether NUE and PUE vary in coordinated or contrasting ways at the catchment scale, in response to combined hydroclimatic, agricultural, and edaphic controls. Scaini et al (2020) discussed how ET/P and time affect NUE across catchments in the United States. Here, we explore how agricultural practices, climate, and edaphic conditions interact to determine catchment-scale agricultural NUE and PUE, as well as their ratio-a measure of contrasting N and P loss pathways.…”

Primary productivity in subsidized green-brown food webs

“…We thus use a measure of normalized productivity, given by u P P/I. We note that this normalized productivity term is dimensionless, representing an ecosystem-level nitrogen use efficiency if we assume that mortality is due to harvesting in analogy to agricultural ecosystems (Scaini et al, 2020). As we show in the results, it is also useful examine how P changes when the subsidies are more organic, i.e., d dψ P (u P and I are constant along the ψ axis, and we can therefore ignore them here).…”