| Previous studies showed relatively high nitrogen and phosphorus concentration in most reservoirs in Southern China. Tangpu Reservoir in our study located in the upstream of Cao’E River. Most of the water quality indexes of the reservoir meet Grade I of environmental guideline of national quality standards for surface waters, China (GB3838-2002), except for the Total nitrogen (TN) concentration which is much higher than the national standard. Some scholars suggested that the eutrophication problem may relate to the nutrients decomposed by the mass forest litter. Although the positive effects of forest lands on soil and water conservation had been proved by plenty of researches, the effects of forest litter on reservoir water quality are still not clear. It is an important question to be answered that whether the forest litter should be cleaned for protecting water quality when the accumulated forest litter reached a huge amount.Our study designed contrast experiment of the different forest litter coverage on the typical forest lands around Tangpu Reservoir, aiming to reveal the influences of forest litter on overland runoff, soil erosion and the loss of TN and Total phosphorus (TP) as well as the combined influences of the nutrient released by forest litter on water quality. The experiment set five runoff plots with no forest litter coverage, and 30%,60%,100% and 150% of the natural forest litter coverage One replicated plot was set for each treatment for monitoring overland runoff, soil evasion and the loss amounts of nutrients (TN and TP). Additional monitoring site was set for measuring the decomposition of forest litter. Our experiment was from July,2014 to June,2015.The major results are as follows:(1) Different forest litter coverage affected runoff in plots significantly with runoff decreased with the increase of litter coverage. As the litter coverage increased from 0% to 150%, the average runoff was 78.75±23.35L,70.60±24.48L, 63.53±27.49L,54.55±23.39L,41.23±21.90L(1.28 x 103,1.15 x 103,1.03 x 103, 8.86 x 102,6.70 x 102m3·ha-1·a-1), respectively. A linear relationship of runoff(L) and forest litter coverage (g) was found:yrumoff=-320.1x+1020, R2= 0.998, P<0.01)(2) TN and TP loss amount decreased with the increase of litter coverage. TN lost though runoff was largest in no litter coverage plots which is 1627.76 mg in average(2.03kg/ha). TN lost though runoff was least in 150% litter coverage plots which are 1216.70 mg in average(1.52kg·ha-1·a-1). TP lost though runoff was largest in 30% litter coverage plots which are 229.45 mg in average(0.29kg·ha-1·a-1). TP lost though runoff was least in 150% litter coverage plots which are 166.16 mg in average(0.21kg·ha-1·a-1). The relationships between forest litter cover (g) and TN as well as TP loss amount were fitted: yTN=1669e-0.18x, (R2= 0.835,0.01<P0.05) å'Œ yTP=-9.815x2-22.28x+216.1, R2= 0.711,0.01<P<0.05)(3) Soil erosion amount in forest land decreased significantly with the increase of litter coverage while the trend slowed down as the litter coverage increased. The relationship between soil erosion amount (g) and forest litter cover (g) was shown as y (0%<x<30%)=-805.1x+390.5,and y (30%<x<150%)=-44.43x+166.83. Soil erosion amount was largest with on forest litter coverage (390.5±155.42 g) and least with 150% of natural forest litter coverage (100.65±14.35 g). Nutrient loss amount reduced significantly with the increase of the forest litter coverage as well. As the forest litter coverage increased from 0% to 150%, the amounts of TN loss caused by soil erosion were 124.96 mg,35.75 mg,34.22mg,25.97 mg,22.14 mg(0.156kg/ha,0.045,0.043,0.032,0.028kg·ha-1·a-1); The amounts of TP loss were 163.42 mg,44.64 mg,28.82 mg,45.54 mg,22.50 mg(0.204,0.056,0.036, 0.057,0.028kg·ha-1·a-1), respectively. Our experiments suggested the existence of forest litter could prevent soil erosion and soil nutrient loss caused by precipitation, and this positi e effect of the forest litter became stronger with the increase of litter coverage.(4) Forest litter decomposed 5.75 g (42.5%) from July,2014 to June,2015. TN and TP released from decomposition increased with the litter increased. As the forest litter coverage increased from 0% to 150%, the amounts of TN released from deconposition were 0 mg,2558 mg,5116 mg,8527 mg,12790 mg(0,3.2, 6.4,10.66,16kg/ha); The amounts of TP released from decomposition were 0g, 77mg,154mg,256mg,384mg(0,0.096,0.192,0.32,0.48kg·ha-1-a-1). The OLSON model simulation results suggested the half decomposition period of litter in Tangpu reservoir forest land is 1.253 years and it will take 5.417 years for 95% decomposition. According to the quarterly and annual decomposition amounts, we calculated the decomposition percentages for spring, summer, autumn and winter which are 31.8%,37.2%,19.5%,11.5%, respectively.(5) In conclusion, although the amounts of TN and TP loss released from decomposition in plots increased while the amounts of TN and TP loss caused by soil erosion decreased significantly with the increase of forest litter coverage, which would not aggravate reservoir eutrophication. These may be explained for several reasons:the decrease of runoff in plots reduced nutrients loss through runoff; a part of nutrients decomposed by litter was detained by soil and reduced nutrient losses; the existence of litter coverage reduced the direct contact of rain and soil which reduced soil erosion. The relationship between forest litter cover (g) and total TN loss amounts (mg) as well as total TP loss amounts (mg) in plots was showed as below:yrN= 1755.-0.21x, R2= 0.895,0.001<P<0.05); yrP= 327.2e-0.41x R2= 0.868,0.0<P<0.05). Total TN loss amount was largest in no litter coverage plots which are 1752.72 mg in average(2.19kg·ha·1·a-1). Total TN loss amount was least in 150% litter coverage plots which are 1238.84 mg in average(1.55kg·ha-1·a-1). Total TP loss amount was largest in no litter coverage plots which are 368.17 mg in average(0.46kg·ha-1·a-1). Total TP loss amount was least in 150% litter coverage plots which are 188.65 mg in average(0.24kg·ha-1·a-1). Our experiments indicated the important role of forest litter on preventing TN and TP loss in forest lands and the higher coverage will lead to less total TN and TP loss amounts in forest lands. As a result, our experiments showed preliminarily that the increase of forest litter did not aggravate the reservoir eutrophication but reduced total TN and TP loss amounts in forest lands dramatically. |
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