Anthropogenic effects on soil physical, chemical, and biological properties in the Missouri Ozarks
The maintenance of healthy forest ecosystems is critical to ensuring the sustainability of forest harvest activities. Forest soil health is a critical component of the knowledge required to manage a forest ecosystem; however, forest systems are understudied relative to agronomic systems with respect to soil health. The primary objective of this study was to enhance the understanding of forest soil health in the Missouri Ozarks via three research objectives: (1) quantification of the effects that clearcut and single-tree selection harvests have upon soil chemistry (2) enhance understanding of the effect forest harvest has upon the microbial populations responsible for driving nutrient cycling and (3) examine potential forest soil health indicators for their sensitivity to harvest at two post-harvest time intervals (5 and 22 years), eventually leading to the development of a forest soil health index specific to the Missouri Ozarks. To better understand changes in soil nutrient concentrations, a series of zero tension samplers (ZTS) were installed (15 and 40 cm depths) in conjunction with throughfall samplers prior to a scheduled harvest in 2012 at the Missouri Ozark Forest Ecosystem Project (MOFEP) Site in the southern Missouri Ozarks. Soil solution samples were collected after rainfall events from pre-harvest until 2015. The samples were analyzed for AlT; Br-; Ca2+; CL-; DOC (dissolved organic carbon); EC (electrical conductivity); F-; K+; Mg2+; Na+; NH4+; NO2-; NO3-; pH; PO43- SO42-; and TN (total nitrogen). Soil samples were collected at 10 cm depth intervals to a total depth of 40 cm on a yearly basis from 2011 to 2016 and analyzed for ALT; exchangeable Ca2+, K+, Mg2+ and Na+; pH (via KCl and water extraction); TN; TOC (total organic carbon); sum of bases; base saturation; Bray-1 phosphorus and effective cation exchange capacity. The data indicated the presence of a nutrient flush affecting concentrations of carbon, nitrogen, calcium, magnesium, potassium and phosphorous within soil solution beginning within 2 to 14 months after the clearcut and ceasing 17 to 31 months after the flush began. After the flush all nutrient concentrations returned to pre-harvest levels, with the exception of NO3- which remained elevated through the end of the study. Analysis of pre-harvest and yearly soil samples concentrations were either elevated or remained unchanged, with the sole exception of available phosphorus, which experienced a significant decline in concentration due to clearcut stem-only harvest. Determination of the effect of forest harvest on microbial populations was accomplished via phospholipid fatty acid (PLFA) analysis and the measurement of four soil enzyme activities: β-glucosidase (β-glu); N-acetyl-β-D-glucosaminidase (NAG); arylsulfatase (Aryl); and acid phosphomonoesterase (AcdP). The following important soil geochemical properties were also analyzed: total organic carbon (TOC); active carbon (AC); total nitrogen (TN) and potentially mineralizable nitrogen (PMN); total sulfur (TS); Bray-1 extractable phosphorus (Bray1P) and soil pH. Carbon to nitrogen and total organic carbon to active carbon ratios were also computed as an indicator of substrate quality. Samples for this analysis were taken from the top 5 centimeters of the mineral horizon to maximize the capture of biological activity. Nominal to significant increases were seen in soil carbon, nitrogen and sulfur concentrations within the clearcut stem-only treatments. Paired with shift toward more neutral pH and improved substrate quality, as measured by C:N and TOC:AC ratios appear to indicate a healthy recovering environment within the clearcut environment. The sole exception is an inferred reduction in phosphorous cycling indicated by reduced Bray-1 P concentrations and acid phosphomonoesteratase activity. Reduction in PLFA-derived fungal biomass and increases in arbuscular mycorrhizal fungi indicate changes in microbial communities linked to increased pH and better substrate quality. As a result, the fungal:bacterial and actinobacterial:fungal ratios indicate shifts in the dominant microbial communities away from fungal populations. The determination of appropriate forest soil health indicators was performed via analysis of the previously collected data, limited to the top 10 cm of the mineral soil horizons for geochemical analyses and the top 5 cm of the mineral horizons for the biological analyses. Soil physical properties were also analyzed. Samples were collected at the MOFEP site and the adjacent Long Term Soil Productivity (LTSP) site, corresponding to a time interval of 5 and 22 years post-harvest. The LTSP site consists not only of a stem-only harvest, similar to MOFEP, but also has a whole-tree harvest treatment. Specifically the following soil properties were analyzed: aggregate stability; soil bulk density; fine fraction bulk density; percentage of coarse fragments; active carbon; base saturation; Bray-1 P; exchangeable Ca2+, K+, Mg2+ and Na+; carbon to nitrogen ratio; effective cation exchange capacity; pH; potentially mineralizable nitrogen, total nitrogen; total organic carbon; total sulfur; fungal to bacterial ratio; and β-glu; NAG; Aryl; and AcdP soil enzyme activities. At five years post-harvest, the following soil properties were found to have changed due to stem-only harvest: bulk density; percent coarse fragments; base saturation; Bray-1 P; C:N ratio; effective cation exchange capacity; exchangeable Ca2+ and K+; pH; total nitrogen; total organic carbon:active carbon ratio; total sulfur; Aryl and AcdP soil enzyme activities and the fungal:bacterial ratio. At 22 years post-harvest, only the C:N ratio and acid phosphomonoesterase (reduction) varied by treatment. We suggest the following list of indicators for monitoring forest soil health as affected by harvest in the Missouri Ozarks: BS; CF%; exchangeable Ca2+ and K+; C:N; ECEC; pH; TN; TOC:AC; T; and Bray-1 P. Although fine fraction bulk density was not found to be sensitive to harvest in this study, we would recommend it be retained as part of a soil health assessment due to the strong influence it has on limiting root growth. Short-term impacts were seen in soil chemistry and microbiological indicators as a result of stem-only harvest; however, all (with the exception of phosphorus) were consistent with the healthy cycling of a forest ecosystem. Concentrations either returned to pre-harvest levels, or were slight elevated above pre-harvest levels. These changes, paired with observed microbiological shifts represents a natural response to shifts in soil chemistry and microclimate as a result of clearcutting. However, reductions in available phosphorous and acid phosphomonoesteratase activity present a potential scenario of concern for the long-term prognosis for phosphorus cycling in the area with repeated forest harvest activities.