川西亚高山不同演替阶段天然次生林土壤碳氮含量及酶活性特征

Characteristics of soil carbon and nitrogen contents and enzyme activities in sub-alpine secondary forests with different successional stages in Western Sichuan, China

为了解次生林自然更新演替过程中土壤碳氮含量及酶活性的变化规律,采用空间代替时间的方法,在川西亚高山米亚罗林区选取环境条件基本一致的20世纪60、70和80年代采伐迹地自然更新演替的次生林(60-NSF、70-NSF和80-NSF)和岷江冷杉(Abies faxoniana)原始林(对照,CK)为对象,研究了表层(0-20 cm)土壤碳氮含量和土壤酶活性的关系。结果表明:表层土壤有机碳(SOC)、可溶性有机碳(DOC)、轻组有机碳(LFOC)含量均随森林植被更新演替呈显著降低趋势,而全氮(TN)和可溶性有机氮(DON)含量则表现为60-NSF<80-NSF<70-NSF,但70-NSF和80-NSF间差异不显著;次生林表层土壤有机碳氮及其活性组分含量均低于CK,其中80-NSF的DOC和DON含量与CK差异不显著。次生林的β-葡萄糖苷酶(βG)、β-N-乙酰氨基葡萄糖苷酶(NAG)和多酚氧化酶(PHO)活性均显著低于CK,纤维素水解酶(CBH)和过氧化物酶(PEO)活性与CK无显著差异;天然次生林中,60-NSF的βG和CBH活性显著低于70-NSF和80-NSF;80-NSF的NAG活性显著高于60-NSF和70-NSF;4种林型之间的PEO活性无显著性差异。Pearson相关分析和冗余分析显示,土壤TN、LFOC和DOC含量与土壤酶活性显著相关,其中TN含量解释了酶活性变化的65.4%,说明土壤氮含量变化可能会影响到土壤碳的水解酶活性,同时也表明土壤微生物优先利用易分解碳和氮。因此,次生林近60年的天然更新演替引起了TN、LFOC及DOC含量的显著下降,导致表层土壤某些胞外酶(如βG、CBH和NAG)活性降低。从土壤酶活性角度看,岷江冷杉原始林比早期演替阶段的次生林(<60 a)更有利于川西亚高山高海拔森林生态系统的碳氮循环。

Aims Regeneration of sub-alpine forests have an capacity to sequester carbon and nitrogen.Our objectives were to quantify variations of soil organic carbon and nitrogen content and enzyme activities at different successional stages of natural secondary forests,and to better understand the underlying mechanisms of carbon and nitrogen sequestration in these sub-alpine forests.Methods We used the space-for-time substitution method and selected four sub-alpine forests in Miyaluo forest of Western Sichuan,China.The secondary forests were at different successional stages with natural regeneration on cutting-blanks in 1960 s(60-NSF),1970 s(70-NSF)and 1980 s(80-NSF),and the Abies faxoniana primary forest was used as control(CK).The soil samples were taken from 0 to 20 cm depths in each forest in late July,2019,and were transferred to the laboratory.Soil organic carbon(SOC),soil total nitrogen(TN),dissolved organic carbon(DOC)and nitrogen(DON),light fraction organic carbon(LFOC)content and soil enzyme activities were measured.The activities of five soil extracellular enzymes related to soil carbon and nitrogen cycling were determined to explain their relationships with soil physico-chemical properties.Important findings The contents of topsoil SOC,DOC,LFOC decreased with succession stage,whereas TN and DON were all in the order of 60-NSF<80-NSF<70-NSF,although 80-NSF and 70-NSF exhibited no significant difference.The topsoil organic carbon and nitrogen and their active fractions contents in natural secondary forests were lower than those in the primary forest,while no significant difference in DOC and DON contents was observed between 80-NSF and CK.The activities of soilβ-4-glucosidase(βG),β-4-N-acetylglucosaminidase(NAG)and polyphenol oxidase(PHO)in CK were significantly higher than those in natural secondary forests,whereas soil cellulose hydrolysis(CBH)and phenol oxidase(PEO)activities had no significant difference between secondary forests and CK.Activities ofβG and CBH in 60-NSF were significantly lower than those in 70-NSF and 80-NSF,but activities of NAG in 80-NSF were significantly higher than those in 60-NSFand 70-NSF.There was no significant difference in PEO activities among different types of forest.Both Pearson correlation analysis and redundancy analysis showed that soil enzyme activities were significantly correlated with soil TN,LFOC and DOC contents.TN content explained 65.4%of the variations in enzyme activity,which implied that change in soil nitrogen content might affect C-related hydrolytic enzyme activities(e.g.βG,CBH and NAG).Meanwhile soil microorganisms prefer to use readily decomposable carbon and nitrogen.Therefore,activities of some soil enzymes such asβG,CBH and NAG in natural secondary forests decreased due to the declines in soil TN,LFOC and DOC contents.We conclude that soil enzyme activities could be more favorable to C and N cycling in the Abies faxoniana primary forest than in the secondary forest at the early-successional stages(<60 a)in high-altitude sub-alpine forest ecosystems in Western Sichuan,China.