Dissolved soil organic carbon and nitrogen were affected by conversion of native forests to plantations in subtropical China

Abstract

To better understand the impact of converting native forests to intensively managed plantations on soil carbon (C) and nitrogen (N) dynamics in subtropical China, we examined the seasonal patterns of water-soluble organic C (WSOC) and N (WSON) concentrations in soils in Chinese chestnut (Castanea mollissima Blume) (CF) and bamboo (Phyllostachys praecox C.D. Chu & C.S. Chou) plantation forests (BF) and adjacent native evergreen broadleaf forests (NF) in Ling-long Mountain, Zhejiang Province, China. The plantations were disturbed through surface soil removal and were fertilized and/or mulched, from which economic products (such as nuts and bamboo shoots) were annually harvested. We found that WSOC and WSON had large seasonal variations and were lower in the warmer than in the colder season. Average WSOC concentrations followed the order of BF (58.6) > NF (35.1) > CF (18.1 mg C kg-1), a pattern mainly caused by mulching in BF in winter and the removal of surface soil in CF. Soil total C and N followed the order of BF > NF > CF. The extensive inorganic and organic fertilizer application in BF caused WSON concentrations to be 21 and 14 times higher than those in NF and CF, respectively. Conversion of native forests to plantations lowered soil WSOC:WSON and soil C:N ratios. The seasonal dynamics of WSOC:SOC (soil organic C) and WSON/TN ratios followed the same patterns of WSOC and WSON, respectively. The impacts of forest types on WSOC/SOC ratio, which is a measure of the quality of organic matter, were dependent on seasonal changes of management practices and/or tree growth. Nevertheless mean annual WSON/TN ratios of BF and CF were 2 and 12 times that of NF, indicating that a greater proportion of the total soil N pool became solubilized in the intensively managed plantations. We conclude that land-use conversion and associated management practices had a profound impact on WSOC, WSON, and total C and N concentrations in the studied forest soils in subtropical China.Key words: Forest management, water-soluble organic C, water-soluble organic N, WSOC/WSON ratio