Saturday, 15 July 2006

Effects of Anthropogenic Disturbance on Riparian Hydric Soils in Urbanizing Coastal Landscapes.

Sean W. Donohue1, Mark H. Stolt1, Arthur J. Gold2, and Peter Groffman3. (1) Univ of Rhode Island, Room 105 , Coastal Inst, 1 Greenhouse Rd, Kingston, RI 02881, (2) Univ of Rhode Island, Room 105 Coastal Inst, 1 Greenhouse Rd, Kingston, RI 02881, (3) Inst Of Ecosystem Studies, Box AB, 65 Sharon Turnpike, Millbrook, NY 12545-0129

Hydric riparian soils in coastal landscapes are capable of reducing groundwater nitrate pollution to coastal waters. Thus, coastal watersheds rely upon hydric soils of the coastal riparian zone to serve as “nitrate sinks”. The nitrate sink function is driven by microbial denitrification, which requires the presence of labile carbon, saturation, and a reducing soil environment. Anthropogenic activities have significantly altered many coastal riparian zones. The effects of such disturbance and the addition of human transported materials (HTM) on soil morphology, water table fluctuation, and reducing conditions remains poorly understood. The objective of this study was to investigate the effects of anthropogenic activities on riparian coastal soils and landscapes relative to the conditions necessary for the soil to function in the denitrification capacity. Eleven disturbed coastal riparian sites and four reference sites were investigated in Rhode Island, USA. Based on analysis of historic aerial photography, initial additions of HTM at seven of the disturbed sites occurred between 1939 and 1976. Deposition at four of the sites predated 1939. Auger transects were completed to characterize soil morphology across disturbed and reference coastal riparian landscapes. At representative locations pits were opened and the soils were described and sampled. Soil organic carbon (SOC), carbon lability, and bulk density were determined. Water table levels were monitored twice a month. Reducing conditions were evaluated using Indicators of Reduction in Soils (IRIS) Tubes. HTM deposits ranged from 53 to 270 cm in thickness. These materials were derived from dredged subaqueous soils and off-site soil materials with and without artifacts. Textures were predominantly sandy. Adjacent to open water, HTM were eroded and the reworked materials deposited as slopewash or local alluvium in adjacent areas. Soil development in HTM was limited to the formation of redoximorphic features and darkening of HTM by humification of grass roots in developing A horizons. Forms of SOC in disturbed riparian sites included roots at the soil surface and organic-rich lenses and masses of silty dredged subaqueous soils. Roots, FOM, and horizon carbon were common forms of SOC at reference sites. In sites with considerable HTM deposits, water table levels rose as much as 2 meters above the original buried soil surface. Water table levels remained above the buried soil surface throughout all or most of the growing season at certain sites; in some cases developing hydric soil conditions. IRIS tube morphology and the formation of redoximorphic features within the HTM indicated that reducing conditions can exist above buried soil surfaces. The water table, IRIS tube, and soil morphology results suggest that riparian zones affected by anthropogenic activities may continue to provide soil functions such as denitrification.

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