Effects Of Conversion From Evergreen Broad-leaved Forests To Chinese Chestnut Forests On Soil Nutrient And Carbon Pools

Land use change has significant effects on the soil physical, chemical and biological properties. Converting natural forests to plantations can cause the changes vegetation characteristics and management practices, subsequently leads to a significant influence on the characteristics of soil nutrient and carbon（C） pools. Study on the effects of conversion from natural forests to plantations would have a very important significance for revealing the response of forest soils to land-use change and associated mechanisms, and promotion of sustainable management of plantations. Chinese chestnut（Castanea mollissima Blume） is one of major economic trees in China. At present, there are about 1.25 million ha of Chinese chestnut forests in China, accounting for 38% of the world’s chestnut cultivation. In the past several years, due to high economic benefits involved in the cultivation of Chinese chestnut, farmers have frequently converted natural evergreen broad-leafed forests（EBF） to Chinese chestnut plantations（CP） that are usually intensively managed. Intensive management in the CP mainly includes chemical fertilizer application, deep tillage, and removal of understory vegetation. In the present study, soils from replicated adjacent native EBF and CP with a known land-use history were sampled. Soil different N forms, P form, K form, labile carbon pools, and other basic soil properties were determined. Soil organic carbon functional compositions were determined by the ¹³C-NMR（nuclear magnetic resonance） technique. The effects of converting natural EBF to CP have been elucidated in this study. The main results obtained are as follows:（1） Conversion of EBF to CP significantly increased the NH₄⁺-N and NO₃^--N concentrations, but decreased the water soluble organic N（WSON） and microbial biomass N（MBN） concentrations in both soil surface（0-20 cm） and subsurface（20-40 cm） layers（P < 0.05）. Conversion of EBF to CP significantly increased the total N concentration for the soil surface layer（P < 0.05）, but had no significant change for the soil subsurface layer.（2） Resin-Pi, NaHCO₃-Pi, HCl-Pi and Residual-P concentrations in the soil surface layer were increased by 61.4%, 53.5%, 55.1% and 25.3%, respectively, by the conversion from EBF to CP, and those values in the soil subsurface layer were increased by 64.6%, 61.2%, 13.7% and 17.3%, respectively. Converting EBF to CP significantly decreased the NaHCO₃-Po concentration in both soil surface and subsurface layers（P < 0.05）. Converting EBF to CP increased the NaOH-Pi concentration in the soil surface layer, but decreased the soil NaOH-Po concentration（P < 0.05）.（3） Converting EBF to CP significantly increased the available K and slowly available K concentrations in both soil surface and subsurface layers（P < 0.05）. Converting EBF to CP significantly increased the total K and mineral K concentrations in the soil surface layer, but has no significant effect on these two characters in the soil subsurface layer.（4） The soil organic C stock in the soil surface（0-20 cm） and subsurface（20-40 cm） layers were decreased by 19.7% and 13.5%, respectively, by the conversion from EBF to CP. The water soluble organic C（WSOC）, hot water soluble organic C（HWSOC）, microbial biomass C（MBC） and readily oxidizable C（ROC） concentrations in the soil surface layer were decreased by 34.4%, 25.8%, 30.4% and 25.2%, respectively, by the conversion from EBF to CP, and those values in the soil subsurface layer were decreased by 38.4%, 19.8%, 34.1% and 22.2%, respectively.（5） Solid state ¹³ C NMR spectroscopy of soil samples showed that All four NMR spectra of soils in both EBF and CP included four significant resonance areas or organic carbon forms: alkyl C（0–42 ppm）, O-alkyl C（42–110 ppm）, aromatic C（110–160 ppm）, and carbonyl C（160–220 ppm）. Significant differences in the signal intensity of different carbon forms were observed regardless of the land-use type and soil layer. The soil organic C was dominated by O-alkyl C regardless of the land-use type, and followed by the alkyl C. Converting EBF to CP decreased O-alkyl C content, aromatic C content and aromaticity of soil organic matter, while increased the alkyl C content and alkyl C to O-alkyl C ratio（A/O-A）（P < 0.05）.