In Situ Measurement of Nitrate Flux and Attenuation Using a Soil Passive Flux Meter

Abstract

This work enhances our understanding of catchment‐scale N budgets by demonstrating the modification and application of a simple method for direct in situ measurements of vadose zone nitrate leaching and attenuation. We developed a soil passive flux meter (SPFM) to measure solute leaching based on a modified design of ion‐exchange resin columns, and we tested the design in numerical simulations, laboratory experiments, plot‐scale field experiments, and a catchment‐scale field deployment. Our design minimized flow divergence around the resin column to attain nearly 100% capture of surface applied tracers in plot‐ and catchment‐scale deployments. We found that mixing resin with native soil and extending the column height 10 cm above the resin layer minimized divergence of soil water around the column, resulting in a field‐measured convergence factor (χ) of 1.3 that was consistent with numerical simulations. For catchment‐scale testing, SPFMs were used at nine sites in three dominant land uses (crop, pasture, and turf) with known N inputs in two deployments, one during the 4‐mo wet season and an additional set during the 8‐mo dry season, to obtain integral annual measures of soil nitrate fluxes. In situ measured nitrate leaching determined from the SPFMs was positively correlated with known N inputs (R2 = 0.55, p < 0.05) and attenuation averaged 67% (± 24% SD) of inputs across all sites. Although N inputs explain a large portion of the variability, our results emphasize the importance of both inter‐ and intra‐land use variability in landscape‐scale N budgets. Core Ideas We developed a soil passive flux meter (SPFM) to measure solute leaching in situ. Our design minimized flow divergence around the resin column (nearly 100% capture). The SPFMs provided robust and accurate measurements of soil nitrate leaching. The SPFMs, together with N input data, enabled assessment of nitrate attenuation.

Disaggregating Landscape‐Scale Nitrogen Attenuation Along Hydrological Flow Paths

“…The SPFM results from Desormeaux et al (2019) suggest that turf systems are highly efficient at attenuating N inputs from urban fertilizer, which is in line with previous research reporting N attenuation of 80-100% across both fertilized (Trenholm & Sartain, 2010) and unfertilized (Herrmann & Cadenasso, 2017) turf systems. Our estimates of N attenuation in crop and pasture systems are close to the global average N use efficiency of 47% (Lassaletta et al, 2014) and agree with estimates for Florida production systems (Prasad & Hochmuth, 2016).…”

“…Natural attenuation in the soil represents the total attenuation from crop uptake and gaseous losses, and we used the same values for soil attenuation of manure N in livestock production systems and fertilizer N in crop production. Previous work has suggested that manure‐based systems result in greater N attenuation (Xia et al, ), and the absolute value of the average N recovery at the pasture sites was greater than crop sites (Desormeaux et al, ). Land management practices varied substantially within land use categories and exerted a strong control on net N inputs below the root zone; however, evaluating the influence of site‐specific land management practices at the catchment scale remains for future work.…”

“…Our estimates of N attenuation in crop and pasture systems are close to the global average N use efficiency of 47% (Lassaletta et al, ) and agree with estimates for Florida production systems (Prasad & Hochmuth, ). While legumes are often identified as a more sustainable N source (Schultze‐Kraft et al, ), the results from Desormeaux et al () suggest that conventional cultivation of legumes can potentially contribute to net N inputs leached below the root zone. While legumes account for only 2% of N inputs in the springshed and therefore did not contribute significantly to spring N export, this has implications for catchments with higher contributions from agricultural BNF (e.g., Sobota et al, ).…”

“…The uncertainty of N S for manure, agricultural fertilizer, and urban fertilizer was estimated from SPFM‐measured soil attenuation reported in Desormeaux et al (). In addition to the variance in measured data, an additional standard deviation of 10% was added to reflect the unaccounted‐for uncertainty of representing the entire springshed with a finite number of sample sites, suggesting that N S is one of the least certain model parameters in the analysis.…”

“…Measured soil attenuation in fertilized urban land uses across the Silver Springs springshed was 90% ± 4%, which was significantly greater than attenuation measured in crop (60% ± 36) and pasture (53% ± 25) sites (Desormeaux et al, 2019). As crop and pasture attenuation was not significantly different, we used the average attenuation across pasture and crop sites to estimate attenuation for manure, agricultural fertilizer, and biological fixation from cultivated legumes.…”