三峡库区典型农业小流域溶解态养分输移对“源—汇”景观格局的响应

Response of Dissolved Nutrient Transport to“Source”and“Sink”Landscape Pattern in Typical Agricultural Small Watersheds in the Three Gorges Reservoir Area

“源—汇”景观格局反映了流域土地利用配置与空间要素分布,导控流域能源流动与水沙过程,是影响小流域养分流失的重要因素。为探明景观格局对不同降雨类型下,农业小流域溶解态养分输移的影响,以三峡库区石盘丘小流域的传统农业型集水区与农林复合型集水区为研究对象,监测不同强度侵蚀性次降雨事件中,两个集水区出口断面处径流溶解态氮、磷浓度,利用航测数据结合最小累计阻力模型识别“源-汇”景观空间格局,引用景观空间负荷对比指数(Location-Weighted Landscape Index,LWLI)进一步分析小流域溶解态养分输移对“源—汇”景观格局的响应机制。结果表明:(1)传统农业型集水区内,“源”“汇”景观面积比为1.8︰1,以“源”景观为主,景观垂直分异性明显;农林复合型集水区内“源”“汇”景观比例约1︰1,且均衡分布,但高陡坡区域占比高、平均坡度大。(2)两种类型集水区内,溶解态氮、磷流失负荷均表现为暴雨>中雨>大雨,而不同降雨事件中,传统农业型集水区养分输出负荷及其变异系数均高于农林复合型集水区,养分输移波动性强,更易受到降雨强度变化的影响。(3)农业用地在两种集水区内均是主要的养分流失来源,但受坡度限制,其他林地可能是农林复合集水区中重要的养分迁出区域;传统农业型集水区的LWLI高达0.75,表征了该集水区的高氮、磷流失风险;农林复合型集水区LWLI为0.28,养分输出量低,这是合理的景观空间格局与农林复合经营模式共同作用的结果。“源—汇”景观格局对小流域养分流失影响显著,可通过“源—汇”景观空间负荷对比指数对小流域养分流失风险进行判别,并为“源—汇”景观格局优化及小流域面源污染防控提供依据。

【Objective】The“source”and“sink”landscape pattern reflects the land use allocation and spatial element distribution.It equally controls the energy flow and water sediment process of the watershed,which is an important factor affecting nutrient loss in small watersheds.The purpose of this research was to investigate the effect of landscape patterns on the transport of dissolved nutrients in agricultural small watersheds under different rainfall types.【Method】The traditional agricultural catchments and agroforestry catchments in the Shipanqiu watershed of the Three Gorges Reservoir Area were selected as the research objects.The concentrations of dissolved nitrogen and phosphorus in the runoff at the outlet section of the catchments were monitored during different intensities of erosive rainfall events.Also,aerial survey data combined with the minimum cumulative resistance model were used to identify the“source”and“sink”landscape spatial pattern.The location-weighted landscape index(LWLI)was used to analyze the impact of the“source”and“sink”landscape pattern on dissolved nutrient transport in the small watershed.On this basis,the response mechanism of dissolved nutrient transport in the small watershed to the“source”and“sink”landscape pattern was further clarified.【Result】The results showed that:①In the traditional agricultural catchment area,the landscape area ratio of“source”and“sink”was 1.8:1,which was dominated by“source”landscape,and the vertical differentiation of landscape was obvious.In the agroforestry composite catchments,the landscape ratio of“source”and“sink”was about 1:1,which was evenly distributed,but the high and steep slope area accounted for a high proportion and the average slope was large.②In the two catchments,the loss load of dissolved nitrogen and phosphorus was rainstorm>moderate rain>heavy rain.In different rainfall events,the nutrient output load and its coefficient of variation in the traditional agricultural catchments were higher than those in agroforestry catchments.The nutrient transport had strong volatility and was more vulnerable to the change in rainfall intensity.③Agricultural land was the main source of nutrient loss in both catchments,but due to the limitation of a slope,other woodlands may be important nutrient migration areas in the agroforestry composite catchments.The LWLI of the traditional agricultural catchment was as high as 0.75,which indicated the risk of high nitrogen and phosphorus loss in this catchment.Also,the LWLI of the agroforestry catchment area was 0.28 and with a low nutrient output.This was attributed to the joint action of reasonable landscape spatial patterns and the agroforestry management model.【Conclusion】The“source”and“sink”landscape patterns had a significant impact on nutrient loss in small watersheds.The risk of nutrient loss in small watersheds can be judged by the“source”and“sink”location-weighted landscape contrast index.Accordingly,it can provide a basis for the optimization of“source”and“sink”landscape pattern and the prevention and control of non-point source pollution in small watersheds.