Abstract

During the last three decades, significant progress has been made in understanding the dynamics of dissolved organic carbon (DOC) in both terrestrial and aquatic ecosystems. However, while considerable research has focused on the concentrations and fluxes of DOC, predictive models have had limited success because
the factors that control the spatiotemporal trends of DOC under field conditions remain debatable. The overall objective of this dissertation was to use DOC to couple hydropedology (integration of pedology and hydrology to better understand soil-water interactions at multiple scales) and biogeochemistry in an attempt to understand the factors that control DOC at different spatiotemporal scales at the Shale Hills Critical Zone Observatory (CZO).

Specifically, a field scale research project was initiated to: (1) investigate the impact of combined soil and landscape features on the spatial variability of soil organic carbon (SOC) storage and soil pore water DOC concentration, (2) evaluate the influence of precipitation, discharge, and temperature on stream water DOC concentration and export, (3) investigate how soil and landscape features combined influence C and nitrogen (N) concentrations along two contrasting hillslopes (swale versus planar hillslopes), (4) assess the importance of DOC and pH in controlling metal concentrations, and (5) examine soil redox potential (Eh) dynamics along two transects (hillslope and valley floor) using automated and continuous (10-minute interval) monitoring.

Results showed that in the two north-facing slopes investigated, average soil pore water DOC concentrations were noticeably higher along the swale (range: 5 – 25 mg/L) as compared to the planar hillslope (range: 5 – 15 mg/L). Elevated soil pore water DOC at the soil-bedrock interface at the ridgetop and at the Bw-Bt horizon interface at the valley floor are consistent with transport-driven “hot spots” (solute concentrations are >20 % than in surrounding areas) of soil pore water DOC at these restrictive interfaces.  Swales appeared to be hot spots for C storage and DOC export. Clay content was the single best predictor of SOC storage, explaining > 70 % of SOC storage variability within the catchment. Stream water DOC (range: 0.6 – 28.6 mg/L) was significantly correlated to stream discharge and water temperature, reflecting combined controls of flushing (linked to discharge) and biological activity (related to temperature). Transport-driven “hot moments” (short periods during which solute concentrations are significantly greater than that during the intervening time) of stream water DOC were observed during the periods of snowmelt and late summer/early fall wet-up, which contributed to ~55% of DOC exported.

Results for objective (3) showed that along both hillslopes, SOC (0.1 to 3.0 %) and TN (0.1 to 0.2 %) exponentially decreased with increasing soil depth, with a 60 % decrease across the A - B soil horizon interface. Soil pore water DOC also exponentially decreased with soil depth for both hillslopes, while nitrate (NO₃^-, range of 0.01 to 8.7 mg/L) did not show an obvious exponential decrease within the soil profile. Soil pore water DOC (but not NO₃^-) concentrations were significantly correlated to soil pore water pH, SOC, soil TN, C:N ratio. However, soil pore water concentrations of DOC and NO₃^- were consistently elevated at restrictive soil horizon interfaces. Elevated concentrations of stream water DOC and NO₃^- (range of 0.0 to 2.4 mg/L) during snowmelt and rainfall events during the early fall period are consistent with flushing of shallow soil pore waters (high concentrations) to the stream during those times.

This study also found that soil pore water concentrations of DOC, and total Al (range of 0.01 – 0.72 mg/L), Fe (range of 0.01 – 3.86 mg/L), and Mn (range of 0.01 – 10.49 mg/L) generally decreased with increasing soil depth, while pH slightly increased with depth. This depth distribution was especially evident in the swale where soils are much thicker and thus an exponential decline with depth was observed. Regardless of landscape position and soil depth, the variability in soil pore water metal concentrations was best predicted by the combined effect of soil pore water DOC and pH (R2 = 0.76).  Stream water DOC and total metal concentrations (Al: range of 0.01 – 0.15 mg/L, Fe: range of 0.01 – 5.99 mg/L, and Mn: range of 0.01 – 3.79 mg/L) were synchronized in this catchment during the late summer/early fall wet up period such that elevated stream water concentrations of both DOC and metals (especially Fe and Mn) were observed.  These results are consistent with DOC and metals being strongly correlated in this acidic forested ecosystem. Moreover, results are consistent with the inference that DOC will significantly facilitate metal transport in catchments impacted by acid deposition.

Furthermore, the 6- months monitoring of soil Eh showed a range of -240 to +750 mV from April to October 2010. A combination of landscape position, soil depth, and season explained 72% of soil Eh variability within the catchment. Soil Eh varied with topographic position where the ridgetop site was strongly oxidized (> 400 mV), while the valley floor was in general moderately to strongly reduced (< 200 mV). Differences in soil Eh at each landscape position reflected variability due to seasonal differences in soil moisture, soil temperature, and water table levels, which combined, explained 20 to 90 % of soil Eh variation. This study demonstrated that spatiotemporal variation in soil Eh in upland forested ecosystems is best interpreted in conjunction with landscape position, soil depth, and seasonal differences in soil temperature, soil moisture, and water table level, rather than water table levels only as observed in wet/anaerobic environments.

The overall findings of this dissertation demonstrated that explicit consideration of both soil physiochemical properties and hydrological characteristics can elucidate the main factors that are consistently correlated to DOC spatiotemporal patterns. Results provided a better link between hillslope soil pore water DOC and stream water DOC.  Preferential flow pathways along restrictive soil horizon interfaces are associated with elevated soil pore water concentrations which is likely the major contributor to the elevated stream water concentrations during snowmelt and the late summer/early fall wet-up period, as groundwater and rainfall concentrations are very low.

Citation

Andrews, D.M. (2011): Coupling Dissolved Organic Carbon and Hydropedology in the Shale Hills Critical Zone Observatory. Doctor of Philosophy, Soil Science, The Pennsylvania State University, p. 233..

This Paper/Book acknowledges NSF CZO grant support.