Effect Of Seasonal Freeze-thaw Cycles On Fine Root Decomposition In The Subalpine/Alpine Forests

Fine root decomposition is one of the essential elements in forest ecological process for energy flux and material cycle, and fine root decomposition can effort on global biogeochemical cycle. Most of existing researches concern fine root decomposition is conducted in temperate forest, subtropical and tropical forest, only few researches executes in cold forest, moreover, research which focuses fine root decomposition in winter is even rarer. So, current uncompleted realization about fine root decomposition would hinder global change research. Seasonal soil freeze-thaw cycles generally occur in western Sichuan subalpine/alpine forest area, and it could significantly impact on a great deal of ecological process. To realize the process of fine root decomposition in subalpine/alpine forest, and then to analyze how seasonal freeze-thaw cycles can effect on fine root decomposition, an experiment which through buried litterbag has been conducted. This research conducts an altitude gradient to simulate warming, three sites are choose at3023m(A1)、3298m(A2)、3582m(A3), which represents air temperature rising4℃, rising2℃and no change respectively. Fine roots of three predominate species in this area, namely spruce (Picea asperata), birch(Betula albo-sinensis) and fir(Abies faxoniana), have been selected as research object. After two years’experiment, we found that,(1) Fine root mass loss continually occurs during freeze-thaw season and the ratio of mass loss in each stage is close, but decomposition rate exhibits difference among each stage that OF>TS>DF. Fine root carbon concentration falls slightly from OF to DF, and rises in TS. Fine root nitrogen concentration of spruce and fir sustains soar during freeze-thaw season, and meanwhile nitrogen concentration of birch fluctuates. Fine root phosphorus concentration decreases little during freeze-thaw season. Both cellulose and lignin concentrations of fine root maintain in OF, then cellulose concentration continually increase, and lignin concentration fluctuate with slight raise. Fine root water uptake capacity enhances during freeze-thaw season, and there is a negative relationship between change in water uptake capacity and lignin. (2) In freeze-thaw season, significant divergent soil freeze-thaw regimes can be observed along altitude gradient by simulating warming. Among different simulate warming situation, fine root decomposition rate of each stages and change in concentrations of nitrogen and lignin exhibit significant difference, but change in concentrations of carbon, phosphorus and cellulose show no obvious discrepancy. Difference in fine root decomposition rate of each stages only originates from simulate warming situation, and different species contributes little to this difference. Source of difference in fine root nitrogen concentration comes from all of simulate warming situation, species and their interaction. Both simulate warming situation and species attributes to difference in fine root lignin concentration, but they do not exert interaction impact.(3) Fine root mass loss keeps on continually happen during growing season. In growing season, fine root carbon concentration proceeds raise, nitrogen concentration continually declines, phosphorus concentration exhibits a peak in middle growing season then drops down. Both cellulose and lignin concentration increase from early growing season to middle growing season, then cellulose concentration maintain at that level and lignin concentration keeps rising.(4) Fine root quality significant changes at the end of freeze-thaw season due to seasonal soil freeze-thaw cycles. In the first year, by comparing the ratio of quantity change in fine root between freeze-thaw season and growing season, carbon reducing is close in two periods; nitrogen undergoes a procedure that immobilizes in freeze-thaw season and releases in growing season; majority of phosphorus reduces in freeze-thaw season; cellulose is mainly degraded in growing season. Only the ratio of lignin change does not appear general tend, which depends on simulate warming situation.(5) Seasonal freeze-thaw cycles could exert prolonged effect on fine root decomposition even after freeze-thaw season, and this effect can be measured by comparing mass loss and quality change after same incubate time of fine roots whether endure seasonal freeze-thaw cycles. In short-term time scale, no significant difference appears in mass loss, but the effect can be detected in fine root quality change.(6) Whether considering fine root decomposition occur in freeze-thaw season could lead to obvious less mass loss and distinct quality change except in phosphorus concentration after two years’incubation. The prolonged effect of seasonal freeze-thaw cycles on fine decomposition can be observed in medium short-term time scale.(7) The result of overall fine root decomposition pattern in two years fitting single-exponent decay model is considerable well. When based on the data of fine root decomposition in freeze-thaw season to forecast further decomposition process, fitness of single-exponent decay model decreases, predicted value always bias measured value. This discrepancy indicates the regulation of fine root decomposition is different between freeze-thaw season and growing season.This paper describes fine root decomposition process in western Sichuan subalpine/alpine forest area during two years. Diverse factors existed in freeze-thaw season and growing season that leading different fine root decomposition pattern exhibits in two periods. Freeze-thaw seasonal includes three different stages (onset of freezing, OF, deep frozen stage, DF, and thawing stage, TS), all of them occupy unique mechanism influences fine root decomposition, which can exert either direct and prolonged effect. Simulating warming along altitude gradient can result in soil freeze-thaw regime change and then impact on fine root decomposition process. By comparing decomposition patterns of fine root whether enduring seasonal freeze-thaw cycles, the effect of seasonal freeze-thaw cycles on fine root decomposition can be confirmed reaching significant magnitude. This research dedicates in integrated realizing fine root decomposition process in subalpine/alpine forest, and contributes to profoundly understand the mechanisms concern fine root decomposition in winter.