林型转化对土壤碳氮稳定性的影响

Effect of Forest Conversion on Soil Carbon and Nitrogen Stability

于2020年8月份,选取安徽省旌德县国营蔡家桥林场中的马尾松(Pinus massoniana)次生林、湿地松(Pinus elliottii)人工林、杉木(Cunninghamia lanceolata)人工林为研究对象,选择立地条件相似的林分,每个林型内随机设置3个20 m×20 m独立的重复样地(共9个样地);在每个样地的土壤剖面内采用多点混合取样法,在土层(h)0<h≤10 cm、10 cm<h≤30 cm、30 cm<h≤50 cm处采集混合土样9个(共计81个样品)。利用同位素质谱仪联用的元素分析仪(EA-IRMS)系统测定土壤样品的稳定碳同位素原子分数(δ(^(13)C))、稳定氮同位素原子分数(δ(^(15)N));应用专业仪器及规范方法测定土壤含水率、pH、电导率、有机碳、全氮、全磷、有效氮、可溶性有机碳、微生物生物量碳、微生物生物量氮。采用双因素方差分析法、杜凯氏真实显著性差异检验法(Tukey’s HSD)、普通最小二乘回归分析法、皮尔逊(Pearson)相关分析法、逐步多元回归法,分析次生林转化为人工林后(林型转化)对土壤碳氮稳定性的综合影响。结果表明:次生林转化为人工林后,土壤稳定碳同位素原子分数(δ(^(13)C))和土壤有机碳质量分数无显著变化,土壤稳定碳同位素原子分数随土层下降而升高;土壤稳定碳同位素原子分数与土壤微生物生物量碳、土壤可溶性有机碳、土壤有机碳呈显著负相关,且土壤有机碳、土壤微生物生物量碳、土壤含水率、土壤pH对土壤稳定碳同位素原子分数变异性的累计贡献率达45.06%。马尾松次生林的土壤有机碳周转,高于湿地松人工林、杉木人工林;林型转化后,土壤稳定氮同位素原子分数(δ(^(15)N))显著升高,土壤稳定氮同位素原子分数,与碳氮质量分数比(w(C)∶w(N))、土壤微生物生物量氮、土壤有效铵态氮、土壤全氮、土壤有效硝态氮呈显著负相关,土壤全氮、土壤含水率、土壤全磷、土壤微生物生物量氮、土壤有效硝态氮、碳氮质量分数比(w(C)∶w(N))对土壤稳定氮同位素原子分数变异性的累计贡献率达57.80%。综合研究结果表明,林型转化后,不同的植被类型、林分结构、管理措施等,会导致土壤碳、氮的流失,从而降低森林生态系统土壤有机质的稳定性。

We selected Pinus massoniana secondary forest(PM-SF),Pinus elliottii plantation(PE-P)and Cunninghamia lanceolata plantation(CL-P)with similar site conditions in state-owned Caijiaqiao Forest Farm in Jingde County,Anhui Province.Three 20 m×20 m independent repeated subplots were selected randomly in each forest type(a total of 9 plots),and nine soil samples were collected at soil layers of(h)0^(13)C and stable nitrogen isotope atomic fraction(δ(^(15)N))of soil samples were determined by element analyzer-isotope mass spectrometer(EA-IRMS),and soil water content(SWC),pH,electrical conductivity(EC),soil organic carbon(SOC),total nitrogen(TN),total phosphorus(TP),available nitrogen,dissolved organic carbon(DOC),microbial biomass carbon(MBC)and microbial biomass nitrogen(MBN)were determined by professional instruments and standard methods.Two-way analysis of variance,Tukey’s range test,ordinary least squares regression analysis,Pearson correlation analysis and Multiple stepwise regression analysis were used to analyze the comprehensive effects after secondary forest was converted to plantation(forest conversion)on soil carbon and nitrogen stability.The results showed that there was no significant change in soilδ(13C)abundance and SOC content after secondary forest was converted to plantation,but soilδ(13C)atomic fraction increased with the decrease of soil depth.Soilδ(13C)was negatively correlated with environmental factors such as MBC,DOC,SOC and so on,and the cumulative contribution rate of SOC,MBC,SWC,pH to the variability of soil stable carbon isotope atomic fraction was 45.06%.The turnover of soil organic carbon in secondary forest was higher than plantation.The abundance of soilδ(15N)increased significantly,and there was a significant negative correlation between soilδ(15N)and mass fraction ratio of carbon to nitrogen(w(C)∶w(N)),MBN,available ammonium nitrogen NH_(4)^(+)-N,TN,available nitrate nitrogen(NO_(3)^(-)-N)and other factors after forest conversion.The cumulative contribution rate of TN,SWC,TP,MBN,NO_(3)^(-)-N and w(C)∶w(N)to the variability of soil stable nitrogen isotope atomic fraction was 57.80%.Therefore,different vegetation types,stand structure and management measures after forest conversion would lead to the loss of soil carbon and nitrogen,thus reducing the stability of soil organic matter in forest ecosystem.