Nitrate losses across 29 Iowa watersheds: Measuring long‐term trends in the context of interannual variability

Abstract

In the U.S. Corn Belt, annual croplands are the primary source of nitrate loading to waterways. Long periods of fallow cause most nitrate loss, but there is extreme interannual variability in the magnitude of nitrate loss due to weather. Using mean annual (2001–2018) flow‐weighted nitrate‐N concentration (FWNC; mg NO3––N L–1), load (kg NO3––N), and yield (kg NO3––N ha–1 cropland) for 29 watersheds, our objectives were (a) to quantify the magnitude and interannual variability of 5‐yr moving average FWNC, load, and yield; (2) to estimate the probability of measuring 41% reductions in nitrate loss after isolating the effect of weather on nitrate loss by quantifying the interannual variability of nitrate loss in watersheds where there was no trend in 5‐yr moving average nitrate loss (Iowa targets a 41% nitrate loss reduction from croplands); and (c) to identify factors that, in the absence of long‐term trends in nitrate loss, best explain the interannual variability in nitrate loss. Averaged across all watersheds, the mean probability of measuring a statistically significant 41% reduction in FWNC across 15 yr, should it occur, was 96%. However, the probabilities of measuring 41% reductions in nitrate load and yield were only 44 and 32%. Across watersheds, soil organic matter, tile drainage, interannual variability of precipitation, and watershed area accounted for interannual variability in these nitrate loss indices. Our results have important implications for setting realistic timelines to measure nitrate loss reductions against the background of interannual weather variation and can help to target monitoring intensity across diverse watersheds.

Benefitting productivity and the environment: Current and future maize cropping systems improve yield while reducing nitrate load

“…There was a significant year effect on nitrate load ( p < 0.001). Year affected N loads more than treatment which is consistent with the large interannual variability in N loads (Danalatos et al., 2022). Given that the specific timing of spring nitrate loading has the highest correlation with the size of the Gulf of Mexico hypoxic zone (Scavia et al., 2017), we also explored the Jan–May treatment effect (Figure S6) and found similar patterns to the overall nitrate load.…”

Linking Water Age, Nitrate Export Regime, and Nitrate Isotope Biogeochemistry in a Tile‐Drained Agricultural Field

“…This study indicates that surface‐applied fertilizer N (including those incorporated or knifed into surface soil) is particularly prone to loss via preferential flows initiated in near‐surface soil layers. As understanding the effects of agricultural management practices and changing climate on nutrient loadings in tile‐drained landscapes is intensifying as a priority across the Midwestern U.S. (Danalatos et al., 2022), more research is needed to better understand the effects of tillage practices specifically and soil physical conditions in general on $${{\text{NO}}_{3}}^{-}$$ export from tile‐drained fields.…”

Linking water age, nitrate export regime, and nitrate isotope biogeochemistry in a tile-drained agricultural field

“…This study indicates that surface-applied fertilizer N (including those incorporated or knifed into surface soil) is particularly prone to loss via preferential flows initiated in near-surface soil layers. As understanding the effects of agricultural management practices and changing climate on nutrient loadings in tiledrained landscapes is intensifying as a priority across the Midwestern U.S. [Danalatos et al, 2022], more research is needed to better understand the effects of tillage practices specifically and soil physical conditions in general on NO 3 export from tile-drained fields.…”