Understory species regulate litter decomposition and accumulation of C and N in forest soils: A long-term dual-isotope experiment

We investigated the effect of understory vegetation on soil C and N dynamics in two Pinus ponderosa plantations under contrasting edaphic and climatic conditions, where understory is often removed to reduce fire risk and competition for water and nutrients. A dual-isotope (13C and 15N) experiment was used to trace C and N from various litter mixtures (i.e., pine and understory litter, as well as their mixture, with and without isotope label replicated in a completely randomized design at each site) and into multiple soil depths and physical fractions following 10 years of decomposition. The presence of understory shrubs increased decomposition and accumulation of C and N from P. ponderosa litter in forest soils. Patterns of C and N accumulation varied with both treatment (litter composition) and environmental conditions (site), but the general response was similar in both plantations. Understory removal favored accumulation of undecomposed residues (light fraction), while addition of understory litter induced aggregate formation and accumulation of litter-derived C and N in occluded and mineral fractions. After 10 years of decomposition, most of litter biomass was lost (70-89%) and litter-derived C and N represented less than 1% of the original soil pool, but the presence of understory vegetation increased accumulation of litter C and N into occluded soil fractions. Despite large C and N losses the presence of understory vegetation increased the long-term productivity of forest soils, enhancing turnover, stabilizing organic matter and conserving N.