The Upper Mississippi River Basin is the largest source of reactive nitrogen (N) to the Gulf of Mexico. Concentration‐discharge (C‐Q) relationships offer a means to understand both the terrestrial sources that generate this reactive N and the in‐stream processes that transform it. Progress has been made on identifying land use controls on C‐Q dynamics. However, the impact of basin size and river network structure on C‐Q relationships is not well characterized. Here, we show, using high‐resolution nitrate concentration data, that tile drainage is a dominant control on C‐Q dynamics, with increasing drainage density contributing to more chemostatic C‐Q behavior. We further find that concentration variability increases, relative to discharge variability, with increasing basin size across six orders of magnitude, and this pattern is attributed to different spatial correlation structures for C and Q. Our results show how land use and river network structure jointly control riverine N export.

In many temperate forested watersheds, hydrologic nitrogen export has declined substantially in recent decades, and many of these watersheds show enduring effects from historic acid deposition. A watershed acid remediation experiment in New Hampshire reversed many of these legacy effects of acid deposition and also increased watershed nitrogen export, suggesting that these two phenomena may be coupled. Here we examine stream nitrate dynamics in this watershed acid remediation experiment for indicators of nitrogen saturation in the terrestrial and aquatic ecosystems. Post-treatment, the (positive) slope of the relationship between nitrate concentration and discharge increased by a median of 82% (p = 0.004). This resulted in greater flushing of nitrate during storm events, a key indicator of early stage nitrogen saturation. Hysteretic behavior of the concentration-discharge relationship indicated that the mobilization of soil nitrate pools was responsible for this increased flushing. In contrast to this evidence for nitrogen saturation in the terrestrial ecosystem, we found that nitrogen uptake increased, post-treatment, in the aquatic ecosystem, substantially attenuating growing-season nitrate flux by up to 71.1% (p = 0.025). These results suggest that, as forests slowly recover from acid precipitation, terrestrial, and aquatic ecosystem nitrogen balance may be substantially altered.

Reductions in acid precipitation across North America and Europe have been linked to substantial declines of soil organic carbon (SOC) stocks in temperate forests, but the mechanisms underlying these declines remain poorly understood. As forests recover from acid precipitation, soil pH and calcium fertility are both expected to increase, and these changes in soil chemistry may drive altered SOC dynamics. Here, we performed a year-long pot experiment on acid-impacted soils to test the independent and interactive effects of increased soil pH and Ca fertility on SOC solubility, microbial activity and sugar maple (Acer saccharum) sapling growth. We found that microbial respiration and SOC solubility was strongly stimulated by increased soil pH, but only in the presence of plants. In planted pots, a soil pH increase of 0.76 units increased soil respiration by 19% in the organic soil horizon and 38% in the mineral soil horizon, whereas in unplanted pots, soil pH had no effect on microbial respiration. While increased soil pH enhanced plant-mediated heterotrophic respiration, it had no effect on plant growth. By contrast, soil Ca enrichment increased the relative growth rate of plants by 22%, but had no impact on microbial respiration. Our results suggest that, in terms of ecosystem carbon balance, losses of SOC due to increasing soil pH may offset potential gains in primary productivity due to enhanced Ca fertility as ecosystems recover from acid precipitation.