Previous research has suggested that the phylogenetic structure and metabolic function of the soil microbial community may be a good indicator of overall soil health and recovery following disturbance events. A major factor controlling microbial communities in soil is the physicochemical soil environment, which can be substantially altered following a disturbance. A common source of forest soil disturbance is timber harvesting, particularly in the Pacific Northwest, USA, where a significant amount of forested land is managed. Although acute effects of disturbance are readily measurable, we lack extensive data on the long-term effects of harvest-related disturbances that could help to inform more sustainable harvest techniques. Our goal was to investigate the effect of harvest-related disturbances, such as degree of organic matter removal and compaction, on soil processes related to the soil biological community and overall soil health. We studied six Long-term Site Productivity (LTSP) experiments across the Pacific Northwest where treatments were installed between 15 and 27 years prior to sampling. We measured soil properties such as texture, pH, available water-holding capacity, total C and total N, and potassium-permanganate oxidizable carbon (POX-C). A 28-day incubation was used to measure soil respiration, N-mineralization, and nitrification rates.
Results/Conclusions
POX-C was only found to be significantly different between two separate treatments at two of the six sites. A positive correlation between rates of N-mineralization and soil respiration was anticipated and observed. However, significant differences between treatments in these measurements were rare. At one site, nitrification was significantly lower in the most severe harvest treatment (OM2; whole tree harvesting with forest floor removal) compared to intermediately severe (OM1; whole tree harvesting), and at another site nitrification was significantly lower in OM2 compared to least severe (OM0; bole-only harvesting). Our hypothesis that site (geographic location) would explain more soil parameter variation than harvest severity was supported. However, the hypothesis that some of the measured soil parameters would consistently differ between the severe OM2 treatment and the unharvested reference, irrespective of site, was not supported. If further sampling supports our observation of minimal long-term impact of organic matter removal and compaction on soil health, resources could be allocated to investigate different factors which may be pertinent to overall forest ecosystem function, furthering our understanding of the long-term response to harvest disturbance.