Effect of Stand Density on Soil Organic Carbon Storage and Extracellular Enzymes Activity of Larch Plantation in Northeast China

Soil is the largest carbon (C) pool in terrestrial ecosystems. A small change of soil organic carbon (SOC) storage may have a substantial effect on the CO2 concentration in the atmosphere, potentially leading to global climate change. Forest stand density has been reported to influence SOC storage, yet the effects are often inconsistent. In order to reveal the mechanisms of effect of stand density on SOC storage, larch plantations with three different stand densities (which were 2000, 3300 and 4400 trees per hectare) were chosen. Soil properties were measured in three soil layers which are: 0–20 cm, 20–40 cm and 40–60 cm. An incubation experiment with 14C-labeled cellulose addition was subsequently conducted to study the decomposition of SOC and cellulose, as well as the enzymes activity involved in C and nutrients cycle. The results showed that SOC storage increased with increasing stand density in larch plantations, which was due to the higher C stored in heavy fraction instead of light fraction in higher density. The decomposition of added cellulose decreased with increasing stand density in each soil layer, as well as the cumulative soil derived CO2 emission rate. The activity of enzymes involved in C-cycle and C- and nitrogen (N)-cycle remained unaffected by stand density in the 0–20 cm and 20–40 cm layers. The enzyme activity involved in the phosphorus (P)-cycle did not change corresponding to the stand density in each soil layer. Enzymes involved in the N-cycle showed the highest activity in the middle stand density in 0–20 cm, but no difference was observed among different densities in the subsurface layer except for tyr in the 40–60 cm layer, which showed the lowest activity in high stand density. Cellulose addition stimulated the extracellular enzymes activity involved in the C-cycle and P-cycle in the 0–20 cm layer, and the stimulation declined with increasing stand density. However, significant stimulation of cellulose addition to C-cycle involved enzymes activity was not found in the subsurface layer. We aim to reveal the mechanism of effects of stand density of larch plantations on SOC storage by focusing on the cellulose and SOC decomposition and the corresponding extracellular enzymes activity. In the plots of higher stand density, larch plantations may lead to a weaker C output and stronger C input, which leads to the higher SOC storage.