The impact of continuous amending of alkaline materials on soil organic carbon(C)lability remains unknown.The typical acidic purple paddy of double-cropped rice in eastern Hunan province was selected.A paired field experiment was applied to study the labile organic C fractions and extracellular enzyme activities in topsoil at 0-15 cm depth in response to 5 years continuous singly amending alkaline commercial organic fertilizer,quicklime,and Si-Ca-Mg soil conditioner.Comparing to the CK,the content of coarse particulate organic C(cPOC),fine particulate organic C(fPOC),and light fraction organic C(LFOC)in soil increased by 57%-136%,27%-77%and 51%-60%(P＜0.05),respectively,after continuous amending alkaline materials.The ratios of soil labile organic C fractions to total organic C were increased.The content of Ca-SOC raised by 22%and 34%,respectively,after continuous amending lime and soil conditioner.The enzyme activities of β-glucosidase(βG)and N-acetyl-β-D-glucosaminidase(NAG)in soil were significantly reduced by 20%after continuous amending organic fertilizer.The soil labile organic C fractions contents were significantly correlated with soil organic C and available nutrient contents.Continuous input of alkaline materials in paddy enhanced the soil organic C level and lability,which was beneficial to soil C sequestration.The POC and LFOC fractions in soil were sensitive to the alkaline materials input,which could effectively indicate the early change in paddy soil organic C.
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