Effects Of Plant Debris Input On Elements Migration In Subalpine Coniferous Forest Streams

The headwaters stream is an important link between adjacent forests and riverbanks.It is not only a significant location for the deposition,storage,processing and transportation of organic matter,but also the key to maintaining the structure,function,productivity and biodiversity of downstream ecosystems.The plant debris inputted by the forest ecosystem is the most important supply and interference for the material and energy in the aquatic environment of the stream.It will not only change the shape of the streams,intercept the sediment,provide the habitat,but also breakdown rapidly and release organic matter in the water environment.It plays an important role in stream productivity,material circulation,stream habitats and aquatic food webs,and affecting the ecological safety of downstream aquatic environment and even the material cycle pattern and ecological function of the entire ecosystem.However,the reported studies are rarely about the influence of plant debris on the process of material cycle and energy flow in forest streams.The subalpine forest streams in the upper reaches of the Minjiang River are typical headwater streams,which have significant functions in regulating regional climate,water conservation and conservation of biodiversity.Thus,the experiment was conducted in the subalpine forest streams in the upper reaches of Minjiang River.In the study area,two streams with the same basic characteristics were trenched from the same site in a long-term stable stream,one of which was equipped with an intercepting net above stream and the source to eliminate litter input,and the other one was not treated as control.At the same time,in the middle and lower reaches of the two streams,the same parts of the same log intercepted similar characteristics are placed as the wood residues in the stream.The experiment set up three representative bed material streams,the controlled experiment was conducted to compare the storage dynamic changes of nutrient elements in water and sediment during the stream flow from upstream to downstream with exclusion of litter and woody debris,and the effects of litter input and woody debris on element migration process in subalpine forest streams were discussed.The results suggested that:1.The average total input of plant debris in riparian forests was 521.39 g/m~2,and the peak input occurred in June.The average total C storage of plant debris was 168.63 g/m~2,and peaked in June and October;The average toatal N storage was 1474.25 g/m~2,peaked in June,September and October;The average toatal P storage was 572.14 mg/m~2,peaked in June and July.2.After the non-woody debris input into the stream of fine sand substrate,it reduced the retention of total C,increasd the output of toatal N,reduced the retention of total P and made it exported downstream.In the water,it increased the retention of C,reduced the retention of N and P.In the sediment,it increased retention of C,increased output of N,reduced retention of P and made it exported downstream.After the non-woody debris input into the stream of clay and silt substrate,it increased the rentention of total C,reduced the output of total N,reduced the output of P and retained it in the stream.In the water,it increased the retention of C,reduced the retention of Nand P.In the sediment,it increased the retention of C,reduced the output of N,reduced the output of P and retained in the stream.After the non-woody debris input into the stream of gravel substrate,it reduced the retention of total C and exported it to downstreams,increased the output of total N,reduced the ouput of total P.In the water,it increased the retention of C,N and P.In the sediment,it increased the output of C and N,reduced the output of P.3.When woody debris existed in streams with fine sand substrate,it reduced the retention of total C and P and exported them to downstream,reduced the output of total N.In the water,it reduced the retention of C and N and exported them to downstream,reduced the retention of P.In the sediment,it reduced the retention of C and P and exported them to downstreams,reduced the retention of N.When woody debris existed in streams with clay and silt substrate,it reduced the retention of total C,reduced the output of total N,reduced the output of total P and retained it in the stream.In the water,it reduced the retention of C,N and P.In the sediment,it reduced the retention of C,reduced the output of N,reduced the output of P and retained it in the stream.When woody debris existed in streams with gravel substrate,it increased the output of total C,N and P.In the water,it reduced the retention of C,increased the retention of N and P.In the sediment,it increased the output of C,N and P.4.When non-woody debris and woody debris co-operate in streams with fine sand substrate,they reduced the output of total C,N and P and retained them in the stream.In the water,they increased the retention of C,reduced the output of N and retained it in the stream,reduced the retention of P.In the sediment,they reduced the output of C,N and P and retained them in the stream.When non-woody debris and woody debris co-operate in streams with clay and silt substrate,they reduced the retention of total C and P and exported them to downstreams,increased the output of total N.In the water,they increased the retention of C,N and P.In the sediment,they reduced the retention of C and P and exported them to downstream,increased the output of N.When non-woody debris and woody debris co-operate in streams with gravel substrate,they reduced the output of total C,reduced the output of N and P and retained them in the stream.In the water,they increased the retention of C,reduced the retention of N and P.In the sediment,they reduced the output of C,N and P and retained them in the streams.In conclusion,due to the shallow water environment and the barren streambed substrate of the subalpine forest stream,the allochthonous input of non-woody debris and woody debris will have a critical influence to the stream,and creat complex,near-bed flow environments and change the migration process of nutrient elements in streams.This experiment provides theoretical support for understanding the effect and impact of plant debris on the material migration and cycle in subalpine forest streams.