Fates of Terrigenous Dissolved Organic Carbon in the Gulf of Maine.

A significant amount of organic carbon is transported in dissolved form from soils to coastal oceans via inland water systems, bridging land and ocean carbon reservoirs. However, it has been discovered that the presence of terrigenous dissolved organic carbon (tDOC) in oceans is relatively limited. Therefore, understanding the fates of tDOC in coastal oceans is essential to account for carbon sequestration through land ecosystems and ensure accurate regional carbon budgeting. In this study, we developed a state-of-the-art modeling approach by coupling a land-to-ocean tDOC flux simulation model and a coastal tDOC tracking model to determine the potential fates of tDOC exported from three primary drainage basins in the Gulf of Maine (GoM). According to our findings, over half a year in the GoM, 56.4% of tDOC was mineralized. Biomineralization was responsible for 90% of that amount, with the remainder attributed to photomineralization. Additionally, 37% of the tDOC remained suspended in the GoM, and 6.6% was buried in the marine sediment.

Impact of long-term application of biosolids to surface water nitrogen in a working forest plantation in King County, Washington.

Forest fertilization with municipal biosolids has been shown to increase tree growth and enhance forest soils. However, there are concerns that nitrogen from the biosolids could impact surface waters through movement from subsurface flow. Here we analyzed data on soil and surface water nitrogen from a working tree plantation that has used biosolids for over three decades to see if there was evidence of N movement through the soil to surface waters. GIS (Geographic Information System) was used to map application units over time and LiDAR (Light Detection and Ranging) was used to delineate watersheds. The program is located in King County Washington with biosolids provided by the King County Wastewater Treatment program. We assembled records to determine if there is any evidence of movement of NO3- through soils or any enrichment in surface waters. While soils show evidence of NO3- enrichment following biosolids application with cumulative loading rates up to 26 Mg ha-1, this is generally limited to the 'A' soil horizon and does not increase linearly with increased biosolids loading rates. There was no indication of increased surface water NO3- concentration relative to biosolids application rates, with a small trend of decreasing water NO3- over time. Surface water NO3- concentration was not correlated with the fraction of the watershed area that had been amended with biosolids, and there was no observable increase in surface water NO3- with increased frequency of biosolids applications to the tree plantations. These results suggest that the current biosolids program is sufficiently protective of ground and surface waters. These observations suggest that biosolids application can be conducted on a large scale with multiple benefits and no discernible impact to surface waters.

Revealing the hidden carbon in forested wetland soils.

Inland wetlands are critical carbon reservoirs storing 30% of global soil organic carbon (SOC) within 6% of the land surface. However, forested regions contain SOC-rich wetlands that are not included in current maps, which we refer to as 'cryptic carbon'. Here, to demonstrate the magnitude and distribution of cryptic carbon, we measure and map SOC stocks as a function of a continuous, upland-to-wetland gradient across the Hoh River Watershed (HRW) in the Pacific Northwest of the U.S., comprising 68,145 ha. Total catchment SOC at 30 cm depth (5.0 TgC) is between estimates from global SOC maps (GSOC: 3.9 TgC; SoilGrids: 7.8 TgC). For wetland SOC, our 1 m stock estimates are substantially higher (Mean: 259 MgC ha-1; Total: 1.7 TgC) compared to current wetland-specific SOC maps derived from a combination of U.S. national datasets (Mean: 184 MgC ha-1; Total: 0.3 TgC). We show that total unmapped or cryptic carbon is 1.5 TgC and when added to current estimates, increases the estimated wetland SOC stock to 1.8 TgC or by 482%, which highlights the vast stores of SOC that are not mapped and contained in unprotected and vulnerable wetlands.