Disentangling the responses of dissolved organic carbon and nitrogen concentrations to overlapping drivers in a northeastern United States forested watershed

Abstract

The concurrent reduction in acid deposition and increase in precipitation impact stream solute dynamics in complex ways that make predictions of future water quality difficult. To understand how changes in acid deposition and precipitation have influenced dissolved organic carbon (DOC) and nitrogen (N) loading to streams, we investigated trends from 1991 to 2018 in stream concentrations (DOC, ~3,800 measurements), dissolved organic nitrogen (DON, ~1,160 measurements), and dissolved inorganic N (DIN, ~2,130 measurements) in a forested watershed in Vermont, USA. Our analysis included concentration-discharge (C-Q) relationships and Seasonal Mann-Kendall tests on long-term, flow-adjusted concentrations. To understand whether hydrologic flushing and changes in acid deposition influenced long-term patterns by liberating DOC and dissolved N from watershed soils, we measured their concentrations in the leachate of 108 topsoil cores of 5 cm diameter that we flushed with solutions simulating high and low acid deposition during four different seasons. Our results indicate that DOC and DON often co-varied in both the long-term stream dataset and the soil core experiment. Additionally, leachate from winter soil cores produced especially high concentrations of all three solutes. This seasonal signal was consistent with C-Q relation showing that organic materials (e.g., DOC and DON), which accumulate during winter, are flushed into streams during spring snowmelt. Acid deposition had opposite effects on DOC and DON compared to DIN in the soil core experiment. Low acid deposition solutions, which mimic present day precipitation, produced the highest DOC and DON leachate concentrations. Conversely, high acid deposition solutions generally produced the highest DIN leachate concentrations. These results are consistent with the increasing trend in stream DOC concentrations and generally decreasing trend in stream DIN we observed in the long-term data. These results suggest that the impact of acid deposition on the liberation of soil carbon (C) and N differed for DOC and DON vs. DIN, and these impacts were reflected in long-term stream chemistry patterns. As watersheds continue to recover from acid deposition, stream C:N ratios will likely continue to increase, with important consequences for stream metabolism and biogeochemical processes.

Illuminating the “Invisible”: Substantial Deep Respiration and Lateral Export of Dissolved Carbon From Beneath Soil

“…These changes may generate more flushing events that elevate stream DOC concentrations (Sebestyen et al., 2009). DOC export is primarily driven by hydrological processes, as exhibited by the co‐occurrence of DOC and discharge peaks, the flushing CQ pattern (Figure 4), and the net export rates of DOC (Figure 8) (Ruckhaus et al., 2023). DIC production and export may similarly increase, although the extent of increase may be smaller, as DIC production and export rates remained relatively consistent between the 2 years simulated here, with only slightly higher DIC export in the wetter water year 2017 (Figure 7).…”

Seasonal and interannual dissolved organic carbon transport process dynamics in a subarctic headwater catchment revealed by high-resolution measurements

“…3a), albeit still positive, indicating a relatively lower degree of transport limitation compared with other seasons (Gómez-Gener et al, 2021). This can be attributed to the limited availability of sources during the snowmelt season (Ruckhaus et al, 2023;Shogren et al, 2021). The theory of less source availability during snowmelt is further supported by fact that the regression between snowmelt DOC load yield and event wa- Additionally, DOC load yield vs. event water yield separated by yield only featuring events with event water yield < 50 mm (i.e.…”

Catchment concentration–discharge relationships across temporal scales: A review

“…Across timescales, C–Q relationships encode dynamics of C and Q that may be independent of each other or may emerge from the direct dependence of C on Q. Meteorological processes influence C–Q patterns observed at the catchment outlet by changing the timing, amount, and phase of precipitation and atmospheric chemical inputs entering the catchment (e.g., Murray et al, 2022; Ruckhaus et al, 2023). Thus, meteorological drivers can influence catchment C–Q responses through direct regulation of hydrological inputs (e.g., precipitation amount, intensity, duration, and phase), material inputs (e.g., quantity and chemistry of wet and dry deposition), or through C–Q dynamics that emerge because of enhanced connectivity of catchment source areas (Floriancic et al, 2018).…”