Soil Properties in CRUI Land Use Project at Harvard Forest 1995-1998

Intersection points within the 5m x5m grid were used as loci for measurements of soil chemistry and nutrient processes, with samples collected at 27 to 35 of the intersection points. Soil samples were collected during the summers of 1995-7. Forest floor (Oe plus Oa layers) measurements were conducted on 15 x 15 cm forest floor samples removed from each sampling location. Mineral soil samples (5cm dia. x 15 cm depth) were obtained immediately below the forest floor samples using stainless steel soil corers equipped with plastic sleeves to hold samples intact. The 15 cm mineral soil depth represents the depth of the plow layer. Upon return to the laboratory, forest floor samples were passed through a 5.6 mm sieve to remove roots and stones. Mineral soil was air-dried, then passed through a 2.00 mm sieve. Samples for nutrient analysis were subsequently dried at 60 deg C, with subsamples dried at 105 deg C for conversion to oven-dry weight. We determined forest floor thickness in the field where the 15x15 cm forest floor samples had been removed, measuring the thickness on each of the four sides, with a mean value calculated for each sample. Soil mass, bulk density, pH, C, N, organic matter, exchangeable phosphorus, available NH4, available NO3, net mineralization, net nitrification, cation exchange capacity (CEC), and exchangeable Ca, Mg, and K were measured in forest floor and mineral soil. Bulk density values were determined after removal of roots and stones, and were uncorrected for volume occupied for root and stone volume. Soil pH was measured in a distilled water slurry (Wilde et al. 1979), with water:soil (volume:volume) ratios of 3:1 for forest floor and 1:1 for mineral soil. Soil pH was measured after samples equilibrated for 30 minutes. Soil moisture content for forest floor and the upper 15 cm of mineral soils was calculated gravimetrically (drying at 105 deg C for 24 h) during the summer of 1998. Soil moisture capacity was determined on June 16, a day after three days of nearly continuous rainfall (81.8 cm from June 13-15). Soils were visibly saturated, with pools of standing water observed within the forest. Soil C and N were measured using a Leco 600 CHN Determinator (St. Joseph, MI). Samples were ground using a Wiley Mill to homogenize forest floor samples and with a mortar and pestle to homogenize mineral soil. Soil organic matter was measured by combustion at 550 deg C for 4 h, and exchangeable P was measured using a molybdophosphate ascorbic acid technique and Lachat Autoanalyzer analysis of PO43 on 0.2 g mineral soil and 0.3 g forest floor. Cation exchange capacity was measured using unbuffered 1.0 N ammonium acetate and NH4 analysis via Lachat autoanalyzer. Exchangeable Ca, Mg, and K were determined by extraction with NH4Cl and analysis via Perkin-Elmer atomic absorption spectrometry. Net N mineralization and nitrification were measured in the field using a one-month buried bag incubation. Pairs of forest floor samples (15 x 15 cm) and mineral soil cores were removed from the field as described previously. One of each forest floor pair was placed into an air-permeable polyethylene bag, which was then inserted into a nylon mesh bag to exclude soil fauna. Similarly, one of the mineral soil cores was left in a perforated plastic core that was then placed into an air-permeable polyethylene bag, and then inserted into a nylon mesh bag. The bagged forest floor and mineral soil samples were replaced in the soil in the original place of extraction for in situ incubation. The remaining pairs of soil samples were placed into a cooler and returned to the laboratory within four hours, where they were passed through a 4.0 mm sieve and extracted for NH4+ and NO3- in 1N KCl for 48 hr. Extracts were frozen until analysis. Soil NH4+ and NO3-concentrations were determined with a Lachat Quick Chem AE Automated Ion Analyzer, using a phenate method for NH4+ analysis and a Cd reduction method for NO3-. At the end of the incubation, the incubating soil core was removed from the field and returned directly to the laboratory for N extraction and analysis. Differences in NH4+ and NO3- between the initial soil core and the incubated soil core indicated net mineralization and net nitrification. Available soil NH4+ and NO3- were determined from soil N concentrations in the initial cores. Plot locations: Plow #1 Site: Prospect Hill Tract, Compartment # III, southern end. Plow #2 Site: Prospect Hill Tract, Compartment # III, southern end. Pasture #1 Site: Prospect Hill Tract, Compartment # I, southeastern edge. Pasture #2 Site: Prospect Hill Tract, Compartment # VIII, central edge. Woodlot #1 Site: Prospect Hill Tract, Compartment # VIII, northeastern edge. Woodlot #2 Site: Prospect Hill Tract, Compartment # VII, central.