| Forest input litter to stream is the way for material cycle and energy flow in forest-water system, also it's the important material and energy source of stream ecosystem. The variation of Water soluble components(WSC), Acid soluble components(ASC), Acid insoluble residue(AIR) and other organic components is a main process during litter decomposition while it may be influenced by environmental deviation caused by the change of water flow and water temperature in different season, but little information has been available on. Therefore, a field experiment using litterbag method was conducted in an alpine forest in Western Sichuan, China from 13 November 2013 to 13 November 2015. The foliar litter of two dominant canopy trees (cypress:Sabina saltuaria, and larch:Larix mastersiana) and two shrubs (willow:Salix paraplesia and azalea:Rhododendron lapponicum) were selected. The litterbags were placed in a headwater stream, river, riparian zone and closed canopy, and sampled in different freezing-thawing periods of winter (pre-freezing period, freezing period and thawing period), growing season and late growing season in the two years of decomposition, the dynamics of WSC, ASC and AIR content will contribute to know the ecological relationship between alpine forest and water basin.After two years litter decomposition, content of water soluble components(WSC) of four foliar litters kept a continue decreasing tendency. Because of the leaching process in water environment, water soluble components as important easy-decomposed component loss rapidly, but the variation trend and loss of WSC is different among four litter species. The loss of willow is biggest (88.29%) while larch is minimum loss species (47.61%). River and stream obviously promoted the decrease of water soluble components content of four foliar litters at the beginning of decomposition, but there is still some diversity in different periods. At the pre-freezing period in the first year of litter decomposition, the content of water soluble components of four foliar litters decreased at river, stream and riparian zone, which is lower than closed canopy during the whole winter, until growing season, the content of water soluble components in closed canopy reduced obviously. In the second year, the content of WSC reduced in winter, but in growing season, WSC increased firstly and then decreased.Water soluble components of different species show different variations in periods under the influence of water flow. During the two years of litter decomposition, the content of water soluble carbon (WC) of four kinds of foliar litters in water environment reduced obviously at the pre-freezing period in the first year. WC in river and stream is easy to flow away. In closed canopy, WC decreased in the growing season, but the change is not obvious in the second year. After two years litter decomposition, water soluble carbon of four kinds of foliar litters reduced obviously, the loss of willow is the biggest which is 95.15% and larch is minimum loss species (74.09%). The content of water soluble nitrogen (WN) reduced at the pre-freezing period in the first year, but it's increased at freezing period or thawing period and decreased obviously in growing season. In the second year, water soluble nitrogen content changed slightly. Larch reduced at the largest level in closed canopy,67.78%, azalea is the species which has the lowest content of water soluble nitrogen and decreased is the minimum as 24.58%. The water soluble phosphorus(WP) content kept a continue decreasing tendency in the first year and it's lower obviously when it's in water environment compares to closed canopy, but there was not a big change of WP in second year. Cypress, larch and willow reduced at the largest level in riparian zone. Loss is 96.47%,98.90% and 98.75%, respectively, cypress in closed canopy was the smallest level which is 72.64%.After two years of decomposition, acid soluble components (ASC) content changed a little except willow. There is a significant effect on acid soluble components from quality differences of litter substrate in different species. ASC content of willow in closed canopy reduced at the largest level as 31.40% while larch in riparian zone increased 18.90%. Acid insoluble residue (AIR) content of larch almost constant after two years of decomposition while the other foliar litters AIR content increased gradually. The increasing of AIR in cypress, larch, willow, azalea are 119.40%,5.64%,96.55% and 25.52%, respectively. Larch is the specie that contain most acid insoluble residue while its increasing is the minimum as 1.43% in riparian zone; AIR content of cypress is the minimum while its increasing is the largest. The Lignocellulose index [LCI=AIR/(ASC+AIR)] stands for relative ratio between content of ASC and AIR changed slowly, in the first year of litter decomposition, LCI of cypress and willow increased while larch and azalea only presented a small fluctuation. In the second year, LCI of four foliar litters changed slightly. In addition, the dynamics of WSC, ASC and AIR content in litter significantly correlate to average temperature, positive accumulated temperature, negative accumulated temperature, flow velocity, pH and other water characteristics during litter decomposition.In summary, litter quality, different hydrothermal environment at the pre-freezing period, freezing period and thawing period in winter of alpine forest stream, water flow and water temperature changed in growing season, all of these have significant impact on the organic components variation during litter decomposition. Compared to riparian zone and closed canopy, the content of WSC in forest stream loss faster in the early stage of decomposition while the difference becomes little later on. The ASC content of litter in stream has few differences with closed canopy, or even higher than closed canopy. Content of AIR in river and stream are higher and the differences of habitats decreased at later stage of litter decomposition. These results provide some scientific basis for ecological relationship between alpine forest and water basin, furthermore, contribute to deeply understanding the nutrient cycle and energy flow in alpine forest stream ecosystem. |
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