Effects of Forest Succession on Exchangeable Cation Concentrations and Nitrogen Mineralization Rates in Soils Following Logging of Eastern Hemlock Forest, Whately, Massachusetts

Ecological forest successions associated with invasive species and human disturbance may alter biogeochemical cycles within New England forests. Spread of the invasive insect hemlock woolly adelgid (Adelges tsugae) to eastern North America is causing mortality of the eastern hemlock (Tsuga canadensis), prompting salvage logging. Regrowth by deciduous hardwood trees is often observed. To evaluate whether changes in nutrient cycling could be altered by forest succession, we investigated exchangeable cation chemistry and nitrogen mineralization rates for soil in a mature, eastern hemlock forest and in a juvenile black birch (Betula lenta) forest in western MA. Eastern hemlock on this property was selectively logged 20 years ago, with black birch regrowth succeeding hemlock. We measured soil pH, exchangeable acidity (Al³⁺ and H⁺), exchangeable base cations (Ca²⁺, Mg²⁺, Na⁺, and K⁺), and nitrogen mineralization rates of organic and mineral horizons for 7 incubation periods between May 2011 - July 2012. We also measured the cation exchange capacity and nitrogen mineralization rates of soils from May - July 2012 (2 incubations) in a mature deciduous forest composed primarily of black birch. At each field site, 7 soil cores were collected. Soil horizons (organic and mineral) were separated and homogenized, and 3 replicates of each composite sample were analyzed for soil geochemistry. Organic soils within the juvenile black birch plot (BB) exhibit a low pH (4.3) similar to hemlock organic soils (HEM, pH=4.2). Surprisingly, exchangeable Al³⁺—the dominant cation in both plots—is significantly greater in organic soils at BB than at HEM (p<.001), and base saturation is less at BB (29%) than at HEM (46%, p<0.001) due to less Ca²⁺. There are no significant differences in the exchangeable cation chemistry of the mineral horizons at both sites, suggesting that the acidity difference of organic matter is not due to different soil mineralogy. In comparison, organic soil at the mature black birch site (MBB) is less acidic (pH=4.8) than BB, and it has higher base saturation (59%, p<0.001). The dominant exchangeable cation at MBB is Ca²⁺, and exchangeable Al³⁺ is significantly less. These results suggest that acidity of hemlock soils increases after logging, despite hemlocks being succeeded by deciduous trees associated with more basic soils. We hypothesize that clear cutting reduces the supply of base cations to soil from throughfall and litter decomposition, enabling more Al³⁺ to occupy cation exchange sites. No significant differences in nitrogen mineralization rates were observed between organic soils at BB and HEM during the 2011 growing season; nitrification rates were ~1% at both sites. However, nitrogen mineralization was greater at HEM (p<0.05) during the 2012 growing season due to more NH₄⁺. Additionally, nitrogen mineralization rates at MBB were greater (p<0.01) than both BB and HEM, mostly due to much higher nitrification at MBB (41%). The organic composition of what was previously hemlock soil appears to exhibit a strong control on nitrogen cycling beneath a juvenile deciduous successional stage.