Soil DOC and moisture measurements along climate and black spruce productivity gradients in interior Alaska

Water-soluble organic carbon data extracted from organic and mineral soils along gradients in stand productivity and soil temperature throughout 2004. 200g of organic soil or 400g of mineral (5 cm) soil were used. Values should be corrected for oven dry moisture percent of soil (also included in data set). Nine soil cores were obtained randomly on a 20 x 20 m sampling grid at each site in May, June-July, and again in September 2004. Cores were parsed in the field into organic (Oi+Oe+Oa) and mineral soil (5 cm, A+B). All cores were immediately sealed in polyethylene bags and were kept on ice in an insulated cooler while being transported to laboratory refrigerators kept at approximately 4 degC. All organic soil samples were extracted for WSOC content within 24 hours, and mineral soils were extracted within 48 hours. Water-soluble organic C concentrations from subsets of mineral soils extracted 24 and 48 hours after collection did not significantly differ (p = 0.30, 5.3 mg C l-1). The method for extracting WSOC from the soil followed Huang and Schoenau (1996), which was modified from McGill et al. (1986). Briefly, field moist soil samples were homogenized on a tray and roots greater then 2 mm were removed. Then 20 g of organic soil or 40 g of mineral soil were gently shaken on a rotary table with 100 ml (n = 9 per horizon per date) of deionized water for 1 hour, filtered through a Whatman GF/A filter, and then passed through a Whatman 0.45 um membrane filter. Soil extracts were preserved at pH 2 using H3PO4 and refrigerated at 4 degC prior to analysis. Each field moist soil sample was subsampled to determine moisture content (gravimetrically) and WSOC was adjusted to an oven-dry basis. Bulk density and depth measurements for each soil horizon were used to relate mg WSOC kg oven dry soil-1 on an area basis at each site (g WSOC m-2). Three zero-tension lysimeters (85 x 19 cm) were installed perpendicular to slope at the organic-mineral soil interface at the end of the growing season in 2003 at the PL, PM, and PH sites. Water was harvested from these, and standing soil water was obtained where available, throughout May 2004. Three first order streams adjacent to upland sampling points were also sampled. These samples were filtered (0.45) and WSOC concentrations were compared to DI H2O extracted WSOC from complementary soil cores obtained nearby (less then 10m, n = 9). Water-soluble organic C fractions were determined in soil extracts obtained from the PL, PM, and PH sites. Three replicate samples (bulk product of 9-12 cores each) of organic and mineral soil were obtained at each site in September 2004. These soils were collected and prepared as previously described. We equilibrated 200 g of organic soil and 400 g of mineral soil (field moist) with 1 l of deionized water for approx. 12 hours. Samples were filtered through a 0.45 um polysulfanone filter and concentrations of each WSOC fraction were determined on tandem XAD8-XAD4 resin columns described by Ping et al. (2001), following Malcolm (1992). In this study WSOC was separated based on sorption to and desorption from the exchange resins and sequential acid and base treatments, into four fractions (HA, soluble C at pH = 1; FA, eluted from XAD8 resin; HiN, soluble C passing XAD4; and LMWFA, eluted from XAD4 resin). Total Ho and Hi organic matter were determined by retention on XAD8 and XAD4 resins, respectively. Total WSOC and concentrations in different fractions were determined on an OI model 700 total organic carbon analyzer (OI Analytical, College Station, Texas).