Soil solution chemistry and nutrient flux in Ozark Highland forest soils
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Soil solution chemistry and nutrient flux may differ among soils due to differences in parent material, biota, landscape position, climate, and degree of weathering. Within soils, soil solution chemistry changes seasonally (e.g., leaf senescence) or following disturbances such as forest harvests that disturb the soil, alter soil temperature, deposit slash, and induce mineralization. Highly weathered soils with inherently low nutrient supply capacity in forested ecosystems may be more vulnerable to nutrient loss via leaching and decreased soil fertility following harvest than soils with greater nutrient supply capacity. Little information is available regarding soil solution chemistry and nutrient flux in highly weathered soils of the Missouri Ozark Highlands, and such background information is essential for evaluating changes in soil chemistry that may be induced by forest harvest. Therefore, the objectives of this work were to: (1) investigate the effects of simulated temperature changes associated with forest harvest on soil solution chemistry; (2) characterize soil solution chemistry in Ozark Highland soils with different nutrient status prior to timber harvest. In order to simulate temperature changes that occur following harvest, laboratory soil column experiments were conducted in constant temperature rooms to monitor the effect of incubation temperature on contrasting nutrient status soils. Three common soil groups present at the Missouri Ozark Forest Ecosystem Project (MOFEP) in southeastern Missouri were selected for study. Each soil group represents a different class of relative nutrient status as indicated by subsoil percent base saturation (BS): low, ≤ 20 % BS; medium, 20 – 50 % BS; and high, ≥ 50 % BS. Field replicated sampling sites were identified in non-harvested stands based on available soil characterization data. Soil samples were collected from the 0 – 10 cm depth, air-dried, passed through a 2-mm sieve, and packed into polyvinyl chloride pipes to create soil columns. Columns incubated at 21, 23, and 26 °C were leached weekly with synthetic precipitation over the course of three months and leachates were analyzed for pH and electrical conductivity, and concentrations of cations (K+, Na+, NH4+, Ca2+, Mg2+, and total aluminum), anions (Cl-, NO3-, SO42-, and PO43-), dissolved organic carbon, and total nitrogen. Soil solid phase characteristics and soil microbial enzyme activities were also determined post-incubation. Overall, the effect of incubation temperature was not significant for all bulk soil chemical properties and most leachate analytes. Temperature had the most pronounced effects for some N species and soil microbial activity. Leachate NH4+concentration in columns incubated at 23°C was significantly greater than in columns incubated at 26°C. Additionally, mean β-glucosidase activity was greatest in soils incubated at 21°C and was determined to be significantly greater than the activity in the soils incubating at 26°C. To characterize soil solution in the field, soil solution chemistry and nutrient flux was monitored at MOFEP in the low and medium nutrient status soils that had not been harvested in the past 40 years. A total of 18 locations were monitored (nine locations for each soil studied); twelve of the sampling locations were located in areas scheduled for future clearcut (CC) or single-tree selection (STS) harvests. All observations in this study were made prior to harvesting. Throughfall and soil solution samples collected with zero-tension solution samplers at 15 and 40 cm depths were analyzed for the same suite of analytes measured in the column experiments. Cumulative ion flux through the system was captured using ion exchange resin samplers installed at 15 and 40 cm depths. Field data from the pre-harvest sampling period demonstrated seasonal fluctuations in pH, base cation, DOC, and TN concentrations for both soils, though few significant differences in soil solution chemistry were observed between the low and medium nutrient status soils. Cumulative Ca2+ flux was significantly greater at the 40 cm depth of the medium nutrient status soils compared to that of the low nutrient status soils. Overall, soil solution chemistry in the low and medium nutrient status soils was very similar which suggests little influence of nutrient uptake and cycling by plants. The soil column study provides some insight into soil chemical and biological changes, or lack thereof, that may be associated with elevated soil temperatures following forest harvest. Results from the field study enhances our understanding of soil solution chemistry and ion flux in Ozark Highland soils and will aid in a better understanding of timber harvest effects on nutrient cycling and loss. Ultimately, this information will assist in the development of forest management policies that ensure sustainable use of Missouri Ozark forests.