Distribution of Mineral-bonded Organic Carbon and Black Carbon in Forest Soils of Great Xing'an Mountains,China and Carbon Sequestration Potential of the Soils
【Objective】 Sequestration of carbon (CO_2) from the atmosphere into forest soils is considered to have great potential to mitigate the momentum of global climate change. However, soil carbon saturation has been observed in some soils. When the soil is saturated with stable soil organic carbon (SSOC),any new inputsoil organic carbon (SOC), which would accumulate in labile pools, would easily be decomposed by microorganisms, contributing little to the soil carbon storage. The potential of soils to sequestrate SSOC is limited by SOC stabilization. SOC is stabilized mainly through inherent biochemical recalcitrance, protection through interaction with minerals, and occlusion in aggregates. Black carbon (BC) is an example of the first mechanism, gaining stability from condensed aromatic structure, while mineral-bonded organic carbon (MOC) is one of the second. SSOC plays a vital role in soil sequestration; however, researches on SSOC in natural forests are still inadequate. In this paper, MOC and BC,two representatives of SSOC,were studied and their carbon sequestration potentials (CSP) in three types of forest soils in the Great Xing'an Mountains, Northeast China were estimated.【Method】 MOC was analyzed with a physical method and a chemical one, too. In physical analysis, soil was divided into particulate organic matter (POM, > 53 m) and mineral-bonded organic matter (MOM, ≤53 m),and organic carbon (OC) was detected in MOM (MOC_p),whereas in chemical analysis, soil was acidified with hydrofluoric acid, and OC loss (MOC_c) was measured. BC was determined with the oxidation method using potassium dichromate. CSP was calculated based on the carbon saturation capacities of silt and clay particles according to Hassink. 【Result】 Results show that MOC and SOC was remarkably correlated to each other, however, comparative analysis shows that the particle size fractionation method overestimated MOC, because it was found that not all the OCs in MOM are bonded with minerals into MOC. Actually, only 60.6 % of the OCs in MOM are. BC in the soils was 8.46 3.85 g kg~(-1) in content and declined with soil depth, however, the proportion of BC to SOC displayed a rising trend. Most of the BC (78.8%) was enclosed in MOM, and the proportion of BC in MOM increased with soil depth, meanwhile the relevances of BC with silt and clay were also reinforced, which illustrated that soil mineral plays an important role in chemical protection of BC. BC/SOC ratio in the soils was 25.4 %, which was reasonable since the forests had not been affected directly by industrial activities. Thereinto, the BC/OC ratio was 26.3% and 24.9% in POM and MOM, respectively. MOC and BC contributed to SOC by around 67.2%, which demonstrates that the soil has a high disturbance resisting capacity in the study area. The surface soils were highly carbon saturated, reaching up around 97.8% in saturation, while carbon saturation declined with soil depth, down to 21.2% in bottom soil, which illustrated that the soil deep in profile could store a considerable amount of SOC. Based on calculation, CSP of the soil deep in profile could be 1.86 times the current SOC storage, while the CSP of surface soil accounted only for about 1% of current SOC stock.【Conclusion】 The SOC storage in forest soils is huge, with SSOC taking an crucial place, and what is more, the potential of forest soils for carbon sequestration is also enormous, especially in the soil deep in profile. The current carbon saturation theories are all based on the conception of SSOC, however, the existence of BC in POM manifests that the role of POC in soil carbon sequestration needs to be reconsidered, because POC is not invariably labile as is commonly understood.