| Global warming has become an indisputable fact, especially obvious response at high latitude and altitudes. Litter decomposition is mainly controlled by climate, litter quality and soil organisms, and the order of their efficiencies is climate>litter quality>soil organisms. Therefore, global warming certainly will produce intense effects on litter decomposition in forest ecosystems. Root litter as an important component of forest litters, its decomposition usually provide a main source of underground nutrients and organic matter input. However, due to the root structure is complex and it is difficult when sampling, up to present, some researches about root decomposition and theirs response to global warming tend to be ignored. Therefore, a simulation waring experiment by elevation gradients (3580 m and 3037 m) be used to investigate root decomposition of different diameters (<2 mm, 2-5 mm,≥5 mm) of three dominant subalpine trees (Abies faxoniana, Picea asperata and Betula albo-sinensis) and its responses to experimental warming in the western Sichuan. The main results were as follows:1) During the two years experimental period, the seasonal dynamics of soil temperature was basically identical, but the soil temperature had significant defferences between two elevations, and these defferences depending on seasons. The average soil temperature of high altitude (3580 m) was lower than low altitude (3070 m), which were 3.06℃ and 4.53℃, respectively. Compared with high altitude, the soil temperature of low altitude were increased by 1.3℃ 2.8℃、1.7℃and 2.5℃ in the first winter, the first growing season, the second winter and the second growing season respectively.2) Species, root diameter and their interactions all exhibited significant effects on initial quality of root. A. faxoniana and P. asperata roots showed higher concentrations of carbon (C) and higher C/N, C/P, lignin/N and lignin/P ratios than B. albo-sinensis roots, but the opposite pattern was found for the nitrogen (N) and phosphorus (P) concentrations and the N/P ratio. The C and cellulose concentration and C/N, C/P, lignin/N, lignin/P ratios increased, but the concentrations of N, P and lignin and N/P, lignin/cellulose ratios decreased with increasing root diameter.3) After two years decomposition, experimental warming decreased the root mass residual rate. Root decomposition constant that lvalue ranged of 0.079-0.177, and the average value is 0.133. Low altitude usually had greater k value than high altitude. Meanwhile, the root decomposition rate also been effected by tree species, root diameters and sampling dates. Broadleaved tree species showed greater decomposition constant than coniferous species, fine root decomposition constant also was greater than medium root and coarse root. According to sampling time, root mass loss in the first year accounted for 72.70%-76.86% of two years, and the first winter accounted for 59.61%-69.98% of the first year.4) In the two-year decomposition, carbon components and nutrients have taken place different level changes for three root diameters of three tree species in two elevations. The concentration of C decreased slightly or did not change significantly with the passage of decomposition time. Simulation waring decreased roots C concentration. The N concentration of three root diameters of A. faxoniana and P. asperata increased at first and then decreased, however, B. albo-sinensis roots showed a fluctuation by decreased-increased. The differences of N concentration in two elevations greatly depended on sampling time. The concentration of P of three root diameters of three tree species all increased at first and then decreased. Nevertheless, no significant differences were observed between two elevations. Lignin concentration of A. faxoniana and P. asperata roots increased slowly during the whole period, but B. albo-sinensis roots decreased at first and then increased. Similarly, the elevation effect was dependent on sampling time. No significant changes in cellulose concentration of A. faxoniana and P. asperata roots, but the cellulose concentration of B. albo-sinensis fine root and medium root displayed a fluctuation variation. Also, the elevation effect was dependent on sampling time.5) After two-year decomposition, root nutrients (components) occurred in different degrees of release (or degradation) of three root diameters of three trees species in two elevations. The C element always has been slowly releasing during two years decomposition, and the low altitude had higher C release rate than high altitude. The N release of all root diameters of A. faxoniana and P. asperata and coarse root of B. albo-sinensis was characterized by an enrichment-release pattern, however, the fine root and medium root of B. albo-sinensis maintain releasing slowly. The N release rate varied in two elevations and related to decomposition time. The P element of all root diameters of three tree species showed an alternate mode of enrichment-release, and which also varied in two elevations depending on decomposition time. The lignin of three tree species root s showed an alternate mode of degradating-accumulating. The warming effect on lignin degradation rate had no significant effects, which were greatly dependent on sampling date. There exsited great defferences of cellulose degradation rate among tree species and root diameters, but no obious influence caused by elevation effect.In conclusion, the increase of soil temperature has promoted root decomposition for subalpine trees in the western Sichuan. Warming effect on root decomposition was dependent on tree species, root diameters and seasons.The root decompositiom rate decreased with increasing root diameter. The B. albo-sinensis root had higher decompositiom rate than that of A. faxoniana and P. asperata. The no-growing season had greater contribution to decompositiom rate than growing season. The differences of decomposition rate induced by substrates (tree species and root diameters) were greater than differences induced by soil temperature. |
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