| Land use/cover change (LUCC) is widely recognized as one of the most important driving forces of global carbon cycles. Tropical-subtropical soils contain at least one third of the global soil organic carbon storage and annual biosphere-atmosphere CO2 exchange. The tropical-subtropical soil respiration (Rs, soil CO2 efflux) is the major carbon flux from pedosphere into atmosphere. Assessing the impact of land-use changes on Rs is of vital significance to understand the interactions between belowground metabolism and regional carbon budgets. The Rs consisted overwhelmingly of those from root (autotrophic) and microbial (heterotrophic) components. Partitioning Rs into rhizospheric (RR) and heterotrophic (RH) components is essential for understanding and modelling belowground carbon balance.Using "space for time" method, the influence of land use/cover change (LUCC) from natural forest to secondary forest, plantations, orchard and sloping tillage on Rs and it components were evaluated in mid-subtropical mountainous area in South China. In this study, the monthly in situ Rs was examined between 09:00 and 12:00 hours over a 3-year period within the representative land-use sequence. Results showed that the RS exhibited a distinct seasonal pattern, and it was dominantly controlled by the soil temperature. After the land-use conversion, the apparent temperature sensitivity of Rs (Q10) was increased from 2.10 in natural forest to 2.71 in sloping tillage land except for an abnormal decrease to 1.66 in citrus orchard. Contrarily, the annual Rs was reduced by 32% following the conversion of natural forest to secondary forest,46-48% to plantations,63% to citrus orchard and 50% to sloping tillage land, with the average reduction of 48%. Such reduction of annual Rs could be explained by the decrease of topsoil organic carbon and light-fraction organic carbon storages, live biomass of fine root (<2 mm) and annual litter input, which indirectly/directly correlated with plant productivity. Our results suggest that substrate availability (e.g., soil organic carbon and nutrients) and soil carbon input (e.g., fine root turnover and litterfall) through plant productivity may drive the Rs both in natural and managed ecosystems following strong disturbance events.Besides, a root-exclusion approach was employed to partition Rs within a representative land-use sequence. Results showed that both RH and RR varied seasonally, and were predominately controlled by temperature as well as by the interaction between temperature and the phenology of photosynthate input. The apparent Q10 values of RR were totally higher than those of RH for various land covers, and both tended to increase following the land-use changes only except for an abnormal decrease in orchard. The contribution of RR to Rs decreased with the land-use changes resulted from the larger reduction of RR (41-91%) than RH (13-43%). The fine root biomass was remarkably declined following the land-use changes, which was considered to be the major driving force for the substantial decrease of RR. The RH also decreased with the land-use conversion, but levelled out in the sloping tillage. Both soil substrate supply from above-and underground-inputs (litterfall and fine root turnover) and from soil resources (litter layer mass, SOC and LFOC) were marked reduced, which could account for the decrease of RH after the land-use changes. Our results suggest that the RH and RR showed great potential to respond equally to substrate availability (e.g., recently photosynthate input and labile carbon) through plant productivity following the land-use changes. Besides, the RH may also be controlled by nutrient availability (e.g., soil inorganic nitrogen content) through chemical fertilizing in intensive human-disturbed land covers.
|
PDF Full Text Request