Changes from pasture to a native tree plantation affect soil organic matter in a tropical soil, Panamá

Background and aims We examined changes in soil organic matter arising from conversion of a 45-year old pasture to a 10 yr. old native tree plantation in Panamá, to evaluate the effect of monoculture and mixtures. Methods We intensively sampled the soil 0–10 cm depth in the pasture in 2001 and in 22 plantation plots in 2011, ranging from 5 monocultures to 3- and 6- species treatments; samples were also taken from an undisturbed forest site. Soil analyses included organic carbon (SOC) and δ13C. Results Conversion of the pasture to tree plantation resulted in an overall loss of SOC of 0.6 kg m−2 (18%) in the top 10 cm, but neither tree species nor diversity had a significant effect. End-member δ13C values suggested that the contribution of C3 plants to SOC was increased from 26% in the pasture to 55% after 10 years of plantation and SOC turnover times were calculated to be 21–36 yr. Conclusions The magnitude of the loss in soil SOC is smaller than the increases in tree biomass (∼3 kg C m−2) and litter (∼0.3 kg C m−2) in the plantation, but still a significant part of the ecosystem C balance.