| The occurrence of freeze-thaw cycles (FTCs) could alter litter quality during winter and influence latter decomposition, but few studies have addressed this topic. Moreover, in subalpine and alpine regions, the severe climate with low temperature seriously limits the processes of material cycling including litter decomposition in winter and early spring. A field experiment using the litterbag method was conducted on the two-year decomposition of three dominant species, spruce (Picea asperata), fir (Abies faxoniana) and birch (Betula albosinensis) along an elevational gradient (2,700 m, A1; 3,000 m, A2; 3,300 m, A3 and 3,600 m, A4) in Southwestern China. Moreover, the loss of mass and release of bioelement in litter were measured monthly and simultaneously in the process of litter decomposition to well understand the effects of seasonal freeze-thaw events on the process of the forest ecosystem. The results would indicate that recurrent soil freeze-thaw cycles (FTCs) during the winter enhance litter decomposability during the subsequent growing season, which is important to understand the ecological linkages between wintertime and growing season in cool-temperate and other high-altitude regions. And we found that,1) The investigated decomposition time was 371 days from 6 November 2008 to 12 November 2009. More frequencies of FTCs were detected at the upper elevations (15 times at A3 and 32 times at A4 comparing with 11 times at A1 and 7 times at A2) during the winter. The onset of temperatures below 0℃occurred at the upper elevation was earlier than lower elevation. The temperature was higher at the upper elevation in the winter.2) During the two-year decomposition,43.3-45.9% of spruce,49.8-52.0% of birch and 46.3-48.6% of fir were lost along the elevation gradient. During one-year decomposition,38.0-40.5% of spruce,41.1-43.6% of fir and 42.1-48.0% of birch masses were lost depending on altitude. Over the freeze-thaw season, a large proportion (> 28%) of mass losses occurred depending on altitudes. Moreover, mass loss was higher at the upper elevation for the fir and lower for the other species during the seasonal freeze-thaw season. During the early growing season, the highest mass loss occurred at the upper elevation, and the coefficient of determination (r2) of the number of FTCs on decomposition rates during this stage were 0.43 for the spruce,0.73 for the fir and 0.55 for the birch litter. These results indicated that physical destruction and hydraulic leaching during freeze-thaw season have relative lower contributions to litter decomposition directly, which significantly destroyed the structure of recalcitrant components and changed litter quality, subsequently increased substrate availability of decomposers during the subsequent growing season. Thereby, there must be a rapid decomposition stage just after freeze-thaw season with the increases of temperature and moisture.3) Over the freeze-thaw season,27.6%(A1),19.6%(A2),22.3%(A3) and 16.9%(A4) of the total Carbon (C) were released from the spruce litter; 25.6%(A1), 30.5%(A2),24.3%(A3) and 25.5%(A4) of the total C were released from the fir litter; and 21.9%(A1),24.0%(A2),29.2%(A3) and 20.1%(A4) of the total C were released from the birch litter.4) The total release of the nitrogen (N) from the spruce litter varied with the altitude and was 43.3%(A1),46.6%(A2),48.2%(A3) and 52.4%(A4); 54.7%(A1), 49.0%(A2),50.8%(A3) and 47.9%(A4) of the N released from the fir litter; and 35.6%(A1),48.2%(A2),48.3%(A3) and 38.3%(A4) of the N were released from the birch litter. As for the phosphor (P) of these three leaf litter,49.1-53.3%(the spruce), 36.2-42.3%(the fir) and 24.9-34.0%(the birch) of the P were released from each species litter depending on altitudes. And 38.0-57.2%(the spruce),32.2-38.0%(the fir) and 33.8-34.4%(the birch) of the potassium (K) were released from each species litter depending on altitudes. Releases of these nutrients were of ecological significance on the plant growth in the subalpine forest during the subsequent growing season.5) Significant litter decomposition rate showed the following order among different stages as OF> DF> TS. The decomposition during all stages altered the litter quality and in turn influenced the following decomposition. Our results demonstrate that litter decomposition at different altitudes exhibits respectively unique processes during different winter stages. Over the freeze-thaw season, percentage of the total losses of lignin and cellulose ranged from 2.8-16.4% and 26.6-42.6% depending on the altitude among the three species. Among different altitudes, lignin loss was generally the higher at the upper elevation (A3 and A4), whilst the less at the lower elevation (Al and A2) in all three species. The higher FTCs occurred at the upper elevations (15 times at A3 and 32 times at A4), the increased FTCs contributed to the lignin and cellulose loss during the freeze-thaw season immediately, which improve the decomposability of the litter during the subsequent growing season. During the decomposition process, the concentrations of C, N, P, K, Ca, Mg and cellulose of the decomposing litter decreased in the first year, while going to a steady stage in the second year. Moreover, the lignin concentration of the decomposing litter increased through the first year decomposition process and steady during the second year.6) Seasonal freeze-thaw cycles significantly influenced the litter quality during the winter decomposition. The concentrations of C, N, P, K Ca, Mg and cellulose in litter decreased after the first year decomposition, and these concentrations stay steadily. And litter with lower C:N and lignin:N ratios did not decompose more statistically rapidly after the winter decomposition.7) The decomposition rate was significantly higher at first year than the second year. And the loss of mass and the release of C, N, P, K, Ca, Mg, lignin and cellulose occupied a large proportion in the second freeze-thaw season of the second year decomposition. We use the stepwise regression analysis to find that Y=0.1021+ 0.0008[C]+0.0713(log [FTCs])-0.0062[Lignin]-0.0046[Cellulose]+0.0102(log [precipitation]).We found that,1) Litter decomposition in subalpine/alpine forest experience four key stage, the onset of soil freezing (OF), the deeply frozen (DF), the thawing (TS) stages and the growing season during a year; 2) Losses of mass, bio-element and lignin as well as cellulose occurred at the seasonal freeze-thaw stage, which occupied a very large proportion of the whole year decomposition; 3) Inconsistent with the general view, litter with lower C:N and lignin:N ratios did not decompose more rapidly after winter decomposition.These findings are very important to understand the ecological linkages between wintertime and growing season in cool-temperate and other high-altitude regions. |
PDF Full Text Request