Microsite differences in nitrogen transformations and soil enzyme activity after partial tree harvest in a mixed deciduous forest

Background/Question/Methods

The goal of this project is to determine how selective logging impacted soil properties and how this impact varied at the microsite level following this disturbance.  Partial harvesting, or selective logging, was conducted within an approximately 5 acre stand within the Sewickley Heights Borough Park in July 2011.  Approximately 75% of the trees were removed from the site.  Soil samples were collected approximately one month prior and one month following logging both within the logging area and within an adjacent control stand that was undisturbed.  Prior to logging, soils were collected in two distinct microsites, next to tree bases and between tree bases.  Following logging, soils were collected at these same microsites and also at sites that contained bare soil, sites that had remaining vegetation and at the base of cut tree stumps.  Samples were analyzed for basic cations, organic matter, nitrogen mineralization and nitrification and enzyme activity. 

Results/Conclusions

Overall, there were no significant differences in nitrogen transformations or organic matter between sites prior to logging.  Organic matter ranged from 11.1% at the base of trees to 10.1% between tree bases.  Extractable NH₄ ranged from 84.5 mg NH₄/kg soil at the base of trees to 65.4 mg NH₄/kg soil between tree bases.  Extractable NO₃ showed an opposite, but insignificant, pattern, ranging from 61.8 mg NO₃/kg soil between tree bases to 53.0 mg NO₃/kg soil at the base of trees.  Following logging there were no significant differences in soil variables in the overall comparison between the logged and control stands.  However, there were significant differences in extractable NH4 and nitrogen mineralization rate among microsites within the logged stand.  Extractable NH4 was significantly greater at microsites with extant vegetation (42.5 mg NH₄/kg soil) and at the base of remaining trees (36.7 mg NH₄/kg soil) compared to cut stumps (20.9 mg NH₄/kg soil) and microsites with bare soil (22.1 mg NH₄/kg soil).  Similarly, nitrogen mineralization rate was significantly greater at the base of remaining trees (73.7 mg NH₄/kg soil/30 days) compared to cut stump microsites (27.9 mg NH₄/kg soil/30 days).