Soil carbon, nitrogen, and base cation cycling 17 years after whole-tree harvesting in a low-elevation red spruce ( Picea rubens)-balsam fir ( Abies balsamea) forested watershed in central Maine, USA

Abstract

Forest harvesting and regeneration may cause changes in soil and solution chemistry that adversely affect forest productivity and environmental quality. The objective of this study was to assess soil carbon (C), nitrogen (N), and base cation cycling 17 years following whole-tree harvesting in a low-elevation red spruce ( Picea rubens)-balsam fir ( Abies balsamea) watershed (regenerating watershed) in central Maine, USA. Results for forest floor and mineral soil nutrient concentrations and pools were compared to an adjacent non-harvested 77–85-year-old spruce-fir watershed (reference watershed) and to preharvest conditions obtained 17 years earlier. The total soil (forest floor + mineral soil) exchangeable magnesium (Mg) pool was 32% and 44% lower ( P ≤ 0.05), respectively, in the regenerating and reference watersheds compared to the preharvest condition. Nitrogen and exchangeable potassium (K) contents in the forest floor, however, were 26% and 65% lower ( P ≤ 0.05), respectively, for the preharvest condition compared to the reference watershed; no differences ( P > 0.05) occurred between the reference and regenerating watersheds. Mineral soil exchangeable calcium (Ca) and sodium (Na) concentrations and contents, effective cation exchange capacity (CEC e), and base saturation were at least twice as high ( P ≤ 0.05) in the regenerating compared to the reference watershed or preharvest conditions. Changes in the N and Mg pools indicate that atmospheric deposition may be a concern for Mg depletion and N saturation at this site. However, Ca depletion does not appear to be occurring at this site as a result of either acid deposition or whole-tree harvesting, although there are indications that the reference watershed may be in the initial stages of altered Ca cycling, which may lead to depletion. Soil solution and stream nutrients were compared between the regenerating and reference watersheds. The soil solution base cation and inorganic anion concentrations were two to four times higher ( P ≤ 0.05) in the reference compared to the regenerating watershed. In addition, bicarbonate (HCO 3 −) accounted for 63% of the cation:anion balance in soil solution of the regenerating watershed, but less than 1% in the reference watershed ( P ≤ 0.05). Soil solution Ca 2+ and Mg 2+ concentrations at 50-cm depth in the regenerating watershed showed a linear decrease ( P ≤ 0.05) 10–17 years after regeneration, whereas the stream Ca 2+ and Mg 2+ concentrations showed a linear increase ( P ≤ 0.05) during that timeframe. No consistent temporal trends ( P > 0.05) in stream Ca 2+ and Mg 2+ occurred in the reference watershed. Higher concentrations ( P ≤ 0.05) of soil air carbon dioxide (CO 2), soil solution dissolved inorganic C (DIC), and soil exchangeable Na, at or below 20 cm in the regenerating watershed suggest that these contrasting results were due to higher mineral weathering rates in the regenerating watershed. The soil elemental and exchangeable cation pools indicate that whole-tree harvesting may be a sustainable practice for at least one rotation. However, the soil solution and stream chemistry indicate that the regenerating watershed has not yet reached equilibrium for nutrient cycling.