Separating autotrophic and heterotrophic soil CO₂ effluxes in afforested peatlands

Peatlands are a significant global carbon (C) store, which can be compromised by drainage and afforestation. Quantifying the rate of C loss from peat soils under forestry is challenging, as soil CO2 efflux includes both CO2 produced from heterotrophic peat decomposition and CO2 produced by tree roots and associated fungal networks (autotrophic respiration). We experimentally terminated autotrophic belowground respiration in replicated forest plots by cutting through all living tree roots (trenching), and measured soil surface CO2 flux, litter input, litter decay rate and soil temperature and moisture over two years. Annual peat decomposition (heterotrophic CO2 flux) was 115 ± 16 g C m−2 y−1, representing c. 40 % of total soil respiration. Decomposition of needle litter is accelerated in the presence of an active rhizosphere, indicating a priming effects by labile C inputs from roots. This suggests that our estimates of peat mineralization in our trenched plots are conservative, and underestimate overall rates of peat C loss. Considering also input of litter from trees, our results indicate that the soils in these 30 year-old drained and afforested peatlands are a net sink for C, since substantially more C enters the soil as organic matter, than is decomposed heterotrophically. However, the C balance for these soils should be taken over the lifespan of the trees, in order to determine if the soils under these drained and afforested peatlands are a sustained sink of C, or become a net source over longer periods of forestry.

Forest Soil Profile Inversion and Mixing Change the Vertical Stratification of Soil CO2 Concentration without Altering Soil Surface CO2 Flux

In order to gain more detailed knowledge of the CO2 concentration gradient in forest soil profiles and to better understand the factors that control CO2 concentration along forest soil profiles, we examined the soil surface CO2 flux, soil properties and soil profile CO2 concentration in upright (CK), inverted and mixed soil columns with a depth of 60 cm in two subtropical forests in China from May 2008 to December 2009. The results showed that: (1) The SOC (soil organic carbon), TN (total N) and microbial biomass were higher in the deeper layers in the inverted soil column, which was consistent with an increase in CO2 concentration in the deeper soil layer. Furthermore, the biogeochemical properties were homogenous among soil layers in the mixed soil column. (2) CO2 concentration in the soil profile increased with depth in CK while soil column inversion significantly intensified this vertical stratification as the most active layer (surface soil) was now at the bottom. The stratification of CO2 concentration along the soil profile in the mixed soil column was similar to that in CK but it was not intensified after soil was mixed. (3) The soil surface CO2 flux did not significantly change after the soil column was inverted. The surface CO2 flux rate of the mixed soil column was higher compared to that of the inverted soil column but was not significantly different from CK. Our results indicated that the profile soil CO2 production was jointly controlled by soil properties related to CO2 production (e.g., SOC content and soil microbial biomass) and those related to gas diffusion (e.g., soil bulk density and gas molecular weight), but the soil surface CO2 flux was mainly determined by soil surface temperature and may be affected by the intensity of soil disturbance.