Tree basal area and conifer abundance predict soil carbon stocks and concentrations in an actively managed forest of northern New Hampshire, USA

Although northern temperate forests account for a substantial portion of global soil carbon stocks, the relationship between soil carbon and management remains unclear. As the active management of forests and associated forest product industries is often cited as a tactic to offset global greenhouse gas emissions, quantifying the effect of management activities on soil carbon pools is paramount to mitigating future climate change. Hence, our goal was to identify commonalities among the spatial variability, potential drivers, and total soil carbon stocks and concentrations at two locations with different management histories in an actively managed northern U.S. mixed wood forest. We measured the carbon pools of two soil profiles, separated into three layers, within 98 0.04-hectare plots in each of the two management areas. We scaled soil and aboveground carbon to estimate total carbon stocks in each area and examined plot-level soil carbon data in relation to landscape and vegetation factors to evaluate potential drivers of soil carbon. Soil carbon represented approximately 40% of the total carbon in both areas. Total soil carbon was similar between the two areas, but the vertical distribution of carbon differed, with more mineral soil carbon in the area with greater coniferous basal area and fewer harvests in the last 80 years. Soil carbon was moderately variable at small spatial scales (<10 m) and showed little or no spatial structure at the scale of hundreds of meters. Aboveground basal area and the proportion of coniferous vegetation were positively related to soil carbon, matching our expectations. Our best models to predict variation in soil carbon among plots also included elevation, composite topographic index, and pH, but not all relationships matched theoretical expectations. Topography and elevation influenced soil horizon depths and bulk density, and therefore had greater importance for total soil carbon than for carbon concentrations. Although many questions remain, the management activities in northern temperate forests appear to be a slow-moving driver of the relatively large but stable soil carbon pool – especially through management influences on site productivity and tree species composition.