土地利用对流域氮素记忆效应的影响研究

"记忆效应"是一些流域实行了多年的氮污染控制实践却仍未见成效的主要原因之一.然而,目前对流域氮素记忆效应的认识还很有限.本研究利用温州市珊溪水源地13个集水区近5年的河流TN浓度逐月监测数据,运用重标极差(R/S)分析和Spearman秩相关分析方法,探讨了土地利用对流域氮素记忆效应的影响.结果表明,河流TN平均浓度与耕地(r=0.495,p=0.086)和建设用地(r=0.692,p=0.009)面积比例正相关,与林地(r=-0.604,p=0.029)面积比例显著负相关,与园地(r=-0.039,p=0.900)、未利用地(r=-0.176,p=0.566)、集水区面积(r=-0.335,p=0.263)和河网密度(r=0.148,p=0.629)的相关性不显著.13条入库支流TN浓度序列的Hurst指数变异范围为0.33~0.72,意味着其中11个集水区具有显著的氮素记忆效应(0.5Hurst指数1.0),而其余2个集水区不显著(0Hurst指数0.5).Hurst指数与耕地(r=-0.482,p=0.095)和建设用地(r=-0.311,p=0.301)面积比例均负相关,而与园地(r=0.479,p=0.098)和林地(r=0.510,p=0.075)面积比例正相关.同时,Hurst指数与集水区面积大小(r=0.118,p=0.700)和未利用地面积比例(r=0.032,p=0.917)的相关性不显著,而与河网密度负相关(r=-0.529,p=0.063).流域内"源"功能为主土地利用面积的增加和氮输移效率的提高加剧了河流氮污染,而对氮素记忆效应的作用则相反.因此,尽管增加"汇"景观面积、降低氮输移效率能缓解河流氮污染,但"汇"景观中截留的遗留氮对河流氮浓度会造成更长时间的影响.

Influence of land use on memory effect of watershed nitrogen

Memory effect (i.e. contribution of legacy sources) has been regarded as one of the major causes of increasing riverine nutrient exports observed in many regions even with pollution control for many years. However, knowledges on the factors influencing the memory effect are still limited. This study adopted the Rescaled Range (R/S) and Spearman analysis methods to address the influence of land use on watershed nitrogen (TN) memory effect based on monthly river records during 2010-2014 in 13 catchments within Shanxi Water Source Watershed in Wenzhou, China. Results indicated that both area percentages of cultivated land (C) (r=0.495, p=0.086) and developed land (D) (r=0.692, p=0.009) had positive correlations with riverine TN concentrations across the 13 catchments, while forest (F) (r=-0.604, p=0.029) area percentage showed negative correlations. Riverine TN concentrations had no significant correlations with garden (G) (r=-0.039, p=0.900) and unused land (U) (r=-0.176, p=0.566) area percentages, as well as catchment areas (r=-0.335, p=0.263) and stream length densities (r=0.148, p=0.629). Estimated Hurst Indexes by R/S analysis for monthly riverine TN concentration time series ranged 0.33~0.72 in the 13 catchments. This implied that 11 catchments had significant nitrogen memory effect (0.5 < Hurst < 1.0), while the remaining 2 catchments had no significant nitrogen memory effect (0 < Hurst < 0.5). Across the 13 catchments, both C (r=-0.482, p=0.095) and D (r=-0.311, p=0.301) had negative correlations with Hurst Indexes, while G (r=0.479, p=0.098) and F (r=0.510, p=0.075) showed positive correlations. Although both catchment areas (r=0.118, p=0.700) and U (r=-0.176, p=0.566) showed no significant correlation with Hurst Indexes, negative correlation was observed between catchment stream length densities and Hurst Indexes (r=-0.529, p=0.063). Increasing areas of land use having "source" function (e.g., C and D) and enhancing nitrogen delivery efficiency can elevate riverine TN concentrations, while they had the opposite impact on catchment nitrogen memory effect. Although riverine nitrogen pollution can be mitigated by increasing areas of sink landscapes (e.g., F and G) and decreasing nitrogen delivery efficiency within the catchment, the legacy nitrogen that was trapped and stored in the sink landscapes would impact riverine TN concentration for a longer time.