Unexpected release of phosphate and organic carbon to streams linked to declining nitrogen depositions

Abstract

Reductions in emissions have successfully led to a regional decline in atmospheric nitrogen depositions over the past 20 years. By analyzing long-term data from 110 mountainous streams draining into German drinking water reservoirs, nitrate concentrations indeed declined in the majority of catchments. Furthermore, our meta-analysis indicates that the declining nitrate levels are linked to the release of dissolved iron to streams likely due to a reductive dissolution of iron(III) minerals in riparian wetland soils. This dissolution process mobilized adsorbed compounds, such as phosphate, dissolved organic carbon and arsenic, resulting in concentration increases in the streams and higher inputs to receiving drinking water reservoirs. Reductive mobilization was most significant in catchments with stream nitrate concentrations <6 mg L . Here, nitrate, as a competing electron acceptor, was too low in concentration to inhibit microbial iron(III) reduction. Consequently, observed trends were strongest in forested catchments, where nitrate concentrations were unaffected by agricultural and urban sources and which were therefore sensitive to reductions of atmospheric nitrogen depositions. We conclude that there is strong evidence that the decline in nitrogen deposition toward pre-industrial conditions lowers the redox buffer in riparian soils, destabilizing formerly fixed problematic compounds, and results in serious implications for water quality.

Carbon and nutrient export regimes from headwater catchments to downstream reaches

“…However in the present study, high SRP during low flow was also observed in headwater catchments free of point-source contributions, and this phenomenon was more prominent in the headwater catchments with the presence of riparian hydromorphic soils (US-Agr and US-For). In independent (mostly forested) catchments located in low mountain areas in Germany, Musolff et al (2016b) have observed that summer SRP release was associated with iron release, suggesting that iron colloids could be a vector of SRP. Musolff et al (2016b) also showed that DOC was closely associated with SRP transfer, similar to observations in US-Agr and US-For.…”

“…In independent (mostly forested) catchments located in low mountain areas in Germany, Musolff et al (2016b) have observed that summer SRP release was associated with iron release, suggesting that iron colloids could be a vector of SRP. Musolff et al (2016b) also showed that DOC was closely associated with SRP transfer, similar to observations in US-Agr and US-For. Hence, a biogeochemical process taking place in the riparian zone could release SRP during the summer low flow in headwater catchments, mimicking a point-source signal in downstream reaches.…”

Nitrate decline unlikely to have triggered release of dissolved organic carbon and phosphate to streams

“…Musolff et al. () recently offered another, mono‐causal explanation for the increase in DOC in reservoirs of Germany. They suggest that decreasing atmospheric N deposition has triggered increasing dissimilatory reduction and dissolution of Fe oxides in riparian zone soil, which has caused the release of sorbed organic matter (as well as release of sorbed phosphate).…”

“…The study by Musolff et al. () is based on the statistical evaluation of data compiled from 110 streams draining into 36 German reservoirs, showing parallel increases in Fe and DOC. The authors proposed that the mobilization of 1 mg C results from the reductive dissolution of 0.12 mg Fe (molar OC:Fe ratio: 39).…”

“…Iron rich, low‐density structures resulting from co‐precipitation and chelation of organic compounds and Fe might result in higher molar OC:Fe ratios of the solid phase but mostly do not exceed 6–10 (Lalonde et al., ; Wagai & Mayer, ), which is still far smaller than the OC:Fe ratio of 39 in surface waters observed by Musolff et al. (). There is no experimental evidence that co‐precipitated and chelated organic matter can be mobilized by reduction of Fe.…”