米槠、杉木和马尾松凋落叶和凋落枝碳氮含量及归还特征

Concentrations and returns of carbon and nitrogen in foliar and twig litters of Castanopsis carlesii,Pinus massoniana,and Cunninghamia lanceolata

大面积亚热带常绿阔叶林转换为马尾松(Pinus massoniana)和杉木(Cunninghamia lanceolata)人工林后,土壤肥力衰退.凋落叶和凋落枝是森林地上凋落物的主要组成部分,其中部分碳(C)、氮(N)经过分解归还土壤,是土壤有机碳和养分的重要来源.因此,认识马尾松和杉木新鲜凋落叶和凋落枝C、N含量和归还特征对于揭示亚热带森林转换后土壤养分周转和肥力提升具有重要意义.选取亚热带常绿阔叶林优势树种米槠(Castanopsis carlesii)和主要造林树种马尾松、杉木为研究对象,分析其新鲜凋落叶和凋落枝产量、C和N含量及归还量的动态变化规律.结果表明:米槠、马尾松和杉木凋落叶、凋落枝产量和C、N含量及归还量均具有明显的季节动态变化,C、N含量总体上在夏季(6-8月)较高,而凋落叶和凋落枝产量和C、N归还量总体上在4-5月和8-9月较高,12月-次年1月较低.米槠、马尾松、杉木凋落叶年凋落量分别为3613、3054和2587 kg/hm^(2),凋落枝年凋落量分别为881、2135和1228 kg/hm^(2).总体上看,凋落叶C、N含量高于凋落枝,而C/N值低于凋落枝;马尾松、杉木凋落叶和凋落枝C含量高于米槠,但N含量低于米槠.3个树种凋落叶和凋落枝每月的C归还量变化范围分别为5.18-210和0-205 kg/hm^(2),凋落叶和凋落枝每月的N归还量变化范围分别为0.14-4.69和0-2.34 kg/hm^(2).总之,亚热带常绿阔叶林转换为马尾松、杉木人工林后,凋落叶和凋落枝C、N含量和归还量的动态规律发生了显著改变,马尾松和杉木凋落叶和凋落枝C含量和C/N值增加,而N含量降低.同时,与米槠相比,马尾松、杉木凋落叶和凋落枝较低的C、N归还量导致从地上凋落物输入的C和养分减少,这可能是导致亚热带马尾松和杉木人工林土壤肥力下降的重要原因.

Many broad-leaved subtropical forests have been converted to plantations(mainly Pinus massoniana and Cunninghamia lanceolata)in subtropical China,resulting in a substantial decline in soil fertility.Foliar and twig litter are the primary components of aboveground litter in forest ecosystems.Parts of carbon(C)and nitrogen(N)in foliar and twig litter can be returned to forest soils after progressive decomposition and are an important source of organic C and nutrients in forest soils.Therefore,a comprehensive understanding of C and N content and return dynamics in the foliar and twig litter of P.massoniana and C.lanceolata is of great significance for assessing soil nutrient turnover and fertility improvement after subtropical forest conversion.In this study,we collected fresh foliar and twig litter of Castanopsis carlesii(a dominant tree species in subtropical evergreen broad-leaved forests)in a natural C.carlesii forest,as well as P.massoniana and Cunninghamia lanceolata(two main planted trees in subtropical China)in their separate plantations over one year to assess the seasonal dynamics of litterfall,C and N concentrations,and return amounts,as well as their C/N ratios in fresh foliar and twig litters.The results showed that not only the foliar and twig litterfall,but also the C and N concentrations and returns showed obvious seasonal dynamics in different months for all tree species studied.They were generally higher from April to May and August to September,and lower from December to January of the following year.Specifically,C and N concentrations were generally higher in summer(June to August).Moreover,the annual foliar litterfall were 3613,3054,and 2587 kg/hm^(2) for C.carlesii,P.massoniana,and C.lanceolata,respectively,and the annual twig litterfall were 881,2135,and 1228 kg/hm^(2),respectively,for the three studied litter species.The annual litterfall was greater in C.carlesii foliar litter than in the foliar litters of P.massoniana and C.lanceolata.Moreover,both C and N concentrations were greater in foliar litter than in the twig litter of the corresponding tree species,but the C/N ratios were lower in foliar litter than in twig litter.The C concentrations in the foliar litter of C.lanceolata and P.massoniana were significantly higher than those in C.carlesii,but the N concentrations were significantly lower than those in C.carlesii foliar litter.The C return from foliar litter of the three studied tree species showed a range of 5.18–210 kg/hm^(2),and those in twig litter showed a range of 0–205 kg/hm^(2).The N return of the three studied tree species were 0.14–4.69 kg/hm^(2) for foliar litter and were 0–2.34 kg/hm^(2) for twig litter.These results suggest that C and N concentrations and C/N ratios in the litter varied greatly among litter types,tree species,and the time period.The results suggest that the conversion of subtropical evergreen broad-leaved forests(i.e.,C.carlesii)to P.massoniana and C.lanceolata plantations could lead to decreased C and N return from foliar and twig litter.Lower litter production and the associated lower nutrient returns are important biogeochemical reasons for the decline in plantation soil fertility in subtropical China.