| 摘 要： |
Although watershed nutrient exports have been well studied and modelled, the dynamic changes in the contributions of individual nutrient sources to reservoirs are less certain under changing land use and climate. The intensification of agriculture has led to expansion in plantation areas and the corresponding increased use of chemical and organic fertilisers in many headwater catchments in recent decades in China. Nutrient yields are potentially substantial because of the expansion of agricultural ecosystems. Here, in combination with biogeochemical nutrient budgets, we developed a synthesis model to quantify daily nutrient yields in terms of dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP). We also identified their sources in an experimental headwater reservoir catchment based on high-frequency observations of daily precipitation with nutrient concentrations in the stream outlets from January 2020 to December 2021. Annual DIN, DON, DIP and DOP yields were 31.68 +/- 2.38, 5.98 +/- 0.41, 1.12 +/- 0.08 and 0.37 +/- 0.27 kg ha(-1) yr(-1), respectively. All nutrient yields showed significant seasonal patterns, with over 60% of the total nutrient yield obtained from May to July. Our model suggests that over 70% of the annual nutrient yields were driven by rainfall exceeding 40 mm. Approximately 70% of catchment nutrient yields were from chemical and organic fertiliser applications. Triple nitrate isotopes (delta N-15-NO3-, delta O-18-NO3- and delta O-17-NO3-) revealed that stream nitrate sources and their contributions were almost consistent with the estimation from our model. The estimates demonstrated that orchard expansion by 25% of the total area led to a 63% increase in dissolved total nitrogen and an 88% increase in dissolved total phosphorus exports. Our results reveal that diffuse sources from orchard plantations are an important yet overlooked source of nutrients to the receiving headwater reservoir budget. |
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