| Ecosystem carbon cycling and soil organic carbon stability has been a hot spot and scientific frontier in ecological research due to increasing atmospheric CO2concentration and global change. Forest rebuilding on degraded shrubs was an important practice pathway of ecological construction and forestry development in hilly red soil of southern China during the past two decades. We selected wild shrubs and neighboring Masson pine(Pinus massoniana) plantations of19year-old to study ecosystem carbon storage in different slope positions, soil carbon stock, soil aggregate components, organic carbon concentration and its stability in soil aggregates in order to assess the ecological effect of forest rebuilding on steep slope land in subtropics of China. Major conclusions were summarized as follows:(1) Ecosystem carbon storage was52.85and111.31t/hm2in shrubs and Masson pine plantation, with11.2%and59.5%of the total stock stored in plants, respectively, and showed an increasing tendency from upper, middle positions to lower position. Organic carbon concentration and stock in0-75cm soil were insignificant different between two ecosystems, but both decreased with increasing soil depth. Soil bulk density was lower in forest plantation than in shrubs, and soil stone percentage negatively correlated with soil organic carbon concentration and stock, both which indicated that forest rebuilding would be helpful to improve soil physical structure, and stone percentage could affect soil carbon accumulation.2) The ratio of>5mm aggregate to bulk soil at0-15cm depth increased more than3%due to conversion from wild shrubs to forest plantation in all three slope positions. At15-30cm depth, the ratio of>5mm aggregate to bulk soil decreased in upper slope position, and increased in middle and lower slope positions with the conversion. At30-45cm, the rations of>5mm and2-5mm aggregates decreased in upper and middle slope positions, and increased in lower slope position with the conversion. In contrast, the ratio of<0.25mm aggregate to bulk soil generally showed an opposite tendency due to conversion. Other aggregates did not show significant change with forest rebuilding. 3) For at0-15cm depth, organic carbon concentration was higher in forest plantation than in shrubs in1-2mm,0.5-1mm,0.25-0.5mm aggregates in upper slope position, in0.25-0.5mm aggregate in middle slope position and0.5-1mm,0.25-0.5mm and<0.25mm aggregates in lower slope position, and other aggregates were insignificant differences between two ecosystems. For at15-30cm and30-45cm depths, organic carbon concentrations in various aggregates were also insignificant differences between two ecosystems. Organic carbon concentrations in various aggregates decreased with increasing soil depth in both ecosystems.4) The average organic carbon mineralization rate in various soil aggregates was higher in shrubs than in forest plantation. The accumulative amount of CO2emission within74days was1722mg/kg and1143mg/kg in shrubs and forest plantation, respectively. The organic carbon mineralization rate was lowest in>5mm aggregate, followed by2-5mm and1-2mm aggregates, and highest in0.5-1mm,0.25-0.5mm and<0.25mm aggregates.In conclusion, forest rebuilding would be helpful to increase ecosystem carbon storage directly due to the accumulation of stand biomass, with average carbon sink of3.08t/(hm2a), and to improve soil physical structure and carbon stability in various soil aggregates. Therefore, we suggest that conversion from wild shrubs to forest plantation could be a efficient practice to improve ecosystem carbon storage in hilly red soil region, but the potential effect of slope position on ecosystem carbon sink need be paid attention. |
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