Semester

Summer

Date of Graduation

2020

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Division of Plant and Soil Sciences

Committee Chair

Louis M. McDonald

Committee Member

Dorothy Vesper

Committee Member

James Thompson

Committee Member

William Peterjohn

Abstract

Continued global warming and surface water brownification are two main environmental issues which have attracted attention and are related to soil organic carbon (SOC) cycling. Iron oxides differ in reducibility and thus have essential roles in regulating SOC preservation and remineralization in soil and transport of dissolved organic carbon (DOC) from soil to surface water. In the central Appalachian region, anthropogenic disturbances are increasing, which leads to major issues of soil degradation and depletion of SOC concentrations. Cropland and pasture soils are subject to intense disturbances compared to the forest soil, which may lead to differences in SOC fractions and Fe oxides, their interactions, and the export of DOC to surface water. Variable geology and climate have confounding effects on SOC fractions and dynamics. Thus, the use of pseudo-replicated studies may be informative. Thus, a single fine-scale watershed was chosen in this study, which was derived from the same parent materials and had similar climate conditions. These perspectives will provide a theoretical basis for a better understanding of SOC cycling in watersheds of differing scales. They will also aid the development of agricultural best management practices to increase soil ecological functions in mitigating global warming and surface water brownification.

SOC fractions have various stabilization mechanisms and turnover times, which change with depth and are highly influenced by land use. In this study, we used total organic carbon (TOC), particulate organic carbon (POC), mineral-associated organic carbon (MOC), and carbon management index (CMI) as indicators to compare cropland with manure application (CM) and continuous pasture (CP) to a hardwood forest (HF) at soil depths of 0-10 and 10-25cm. Land use, depth, and the interactions between them all had significant influences on TOC, POC, MOC, MOC/TOC ratio, and CMI except for the main effect of land use on TOC. CM showed significantly larger POC (12.4 g kg-1) and smaller MOC (8.36 g kg-1) at 10-25cm compared to HF and CP soils. CM soil at 10-25cm had improved soil quality and SOC lability as indicated by a significantly larger CMI value (419.2) while CP soil had decreased soil quality and SOC lability at both 0-10cm (83.7) and 10-25cm (73.6) compared to HF soil. This study implied high sensitivity of the SOC in cropland and pasture surface soils to degrade under disturbance, which implies that better management strategies are still needed to improve soil carbon quality for these agricultural systems.

The essential roles of Fe oxides in stabilizing long-term soil SOC, especially aromatic dissolved organic carbon (DOCaro), are well-established in forest soils and sediments. We chose to focus on these processes in agricultural soils in which the input and translocation of native DOC to deeper soils is impacted by management practices. We quantified SOC, Fe oxide bound SOC (Fe-bound OC), and the DOCaro sorption in a forest, a cropland, and a pasture soil at 0-10 and 10-25 cm. Significantly larger amounts of Fe oxides in the cropland soil was observed compared to the forest and pasture soils at both depths (p < 0.05). Land management practices and depth both significantly influenced proportion of the Fe-bound OC (p < 0.05). Larger maximum sorption of DOC in the cropland (315.0 mg kg-1) and pasture (395.0 mg kg-1) soils than the forest soil (96.6 mg kg-1) at 10-25 cm was found. DOCaro sorption decreased in the three soils at 0-10 cm (slope of -0.002 to -0.014 L2 mg-2 m-1) as well as the forest soil at 10-25 cm (-0.016 L2 mg-2 m-1) with increasing equilibrium DOC concentration. Conversely, the cropland and pasture soils at 10-25 cm increased (0.012 to 0.014 L2 mg-2 m-1). These results indicate that the forest, cropland, and pasture managed soils may have more complex sorption behaviors in stabilizing DOCaro and non- DOCaro than previously known.

DOC and iron (Fe) have been observed to be important contributors to surface water brownification. Additionally, the DOC quality influences water color by forming Fe-DOC complexes that provide additive effects and is influenced by dominant land use type within watersheds. However, the influence of quantity and quality of DOC on Fe and water color is poorly understood in headwater streams. The aim of this study was to investigate the effects of DOC and Fe on water color in forest (FC) and pasture (GFC) fine-scale watersheds to remove the confounding effects of climate and soil parent materials. Significant differences of DOC, Fe, and water absorbance at 420 nm (a420) between FC and GFC were found (p < 0.05). A dominant contribution to water color was from DOC (95.5 - 63.7%) with a decreasing trend when Fe increased from 0.011 to 0.258 mg L-1. There were no significant interactions between FC and GFC and Fe on either a420/DOC (p = 0.06) or specific ultraviolet absorbance at 254 nm (SUVA254) (p = 0.30). Increasing a420/DOC and SUVA254 were significantly associated with increasing Fe concentration (p < 0.01). Significant interactions were found between FC and GFC and Fe on spectral slope ratio (S ratio) (p < 0.01). The response rate of S ratio with increasing Fe per unit was 0.235 for GFC while it was -11.043 for FC. These differences indicate that land use may change the quality of DOC, influence Fe-DOC interactions, and thus affect water color.

DOC and Fe concentrations cause surface water brownification. Land-use effects on the quantity and quality of DOC are well-established in large-scale watersheds. However, there is more to understand about how soil Fe oxides are involved in DOC and Fe processes in soil and stream, especially in fine-scale catchments. We investigated DOC, SUVA254, exchangeable Fe (Feex), and amorphous Fe concentrations (Feamor) in soils as well as DOC, SUVA254, and dissolved Fe in stream water within a fine-scale forest and pasture catchment. Forest soil had a significantly larger average DOC concentration (71.7 ± 33.8 mg kg-1) and lower average SUVA254 (2.8 ± 1.0 L m-1 mg-1) than the pasture soil at 0-10 cm (DOC: 71.7 ± 33.8 mg kg-1; SUVA254: 4.2 ± 1.4 L m-1 mg-1). The pasture soil at 0-10cm had significantly larger average Feex (132.8 ± 57.6 mg kg-1) and Feamor (813.9 ± 461.2 mg kg-1) concentrations than the forest soil (Feex:120.3 ± 55.4 mg kg-1; Feamor: 303.2 ± 213.5 mg kg-1). Negative correlations between Feex and DOC in the forest soil and positive correlations between Feex, Feamor and SUVA254 in both forest and pasture soils were found (p < 0.05). The pasture headwater stream had significantly larger DOC, SUVA254, and Fe than the forest headwater stream (p

Collectively, the results indicated land use and season were important factors altering the SOC and Fe dynamics in agricultural and forest catchments. Additionally, this study indicated Fe oxides may interact with DOC differently in watersheds differing in scale, which leads to differences in the quantity and quality of DOC and dissolved Fe in these watersheds. Therefore, studies combining the effects of Fe-reducing processes are helpful in explaining the changes of DOC and Fe and thus surface water brownification at local scale. To better understand Fe-oxyhydroxides and DOC interactive processes at fine-scale local environments, the next step should be to establish the effects of hydrologic processes and DOC sources. This may help explain DOC stabilization and destabilization as well as surface water brownification.

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