The Effect of Tree Species Composition on Soil C and N Pools and Greenhouse Gas Fluxes in a Mediterranean Reforestation

Abstract

The shift of tree species composition from conifers to mixed stands as a silvicultural management option for substituting pure plantations can have consequences for the greenhouse gas (GHG) budgets and climate impact. In this context, the main objective of the study was to assess the effect of tree species composition on GHG fluxes, organic matter of forest floor and soil in a degraded conifers plantation in Central Italy. Field-chamber GHG fluxes, litter, and total concentrations of soil C and N, soil temperature, and soil moisture were analyzed, assessing their relationships, under mixed and pure conifer species composition during three monitoring years. Carbon and nitrogen contents were higher under mixed than pure conifer species composition, both in forest floor and mineral soil. Soil carbon dioxide under litter of mixed tree species was significantly higher than that of pure conifers (+17.5%). Methane uptake was higher in the mixed plots than in the pure ones (+12.4%), especially in summer and autumn. Nitrous oxide fluxes were characterized by very low emissions, higher under mixed tree-species than pure conifers during winter. The relevant role of seasonality was confirmed by including in the linear mixed-effects model (LMM) the seasons as an additional random effect that produced a significant interaction between the soil moisture and soil temperature, especially on soil carbon dioxide and methane fluxes. Overall, the GHG budget was driven by organic matter availability, higher under mixed species. Our findings are a first step to help the understanding of the role of tree species composition on GHG emissions in the Mediterranean forest ecosystems.