流域是陆地—水生生态系统的基本单元。流域水碳氮循环作为主要的物质循环，为生态系统结构完整和服务供应提供了重要支撑。生态系统服务是连接生态 系统和人类福祉的桥梁，服务的能力与流域水碳氮过程密切相关。此外，在水利 工程、农业灌溉等一系列人类活动的影响下，流域生态系统正在发生着变化，急 需开展人为活动下的流域水碳氮模拟。为此，本研究选取汾河流域为研究区，建 立了流域的水文生物地球化学模型 SWAT-DayCent，分析了流域水循环的时空变化，探究了碳储量的时空异质性和土壤固碳潜力的差异，揭示了流域及农田的氮 素损失特性，构造了综合生态系统服务指数并且评估了跨流域调水工程和农田灌 溉对水碳氮及生态系统服务的影响。研究主要结论如下：

（1）在流域水循环方面，2010-2020 期间，除土壤含水量年均减少 21.5mm， 其他水循环指标的变化速率很小并且最大水量都出现在 2020 年。蒸散量、产水 量和地表径流与降水的变化趋势保持一致。水循环指标值在流域内南高北低，异 常高值主要出现在 9、16、18 和 23 子流域。

（2）在流域碳储量中，植被净初级生产力（NPP）和土壤有机碳密度在 11 年间均微小减少。植被 NPP 减少的区域为流域面积的 66.5%，是增加区域面积的 2 倍。进一步的空间分析表明 NPP 的全局空间自相关表现为高值聚类。冷点主要 分布在流域上游和下游，热点主要分布在中部盆地。13 种土壤的有机碳密度中， 山地草甸土和棕壤的碳密度水平较高，冲积土的碳密度最小。高有机碳密度土壤 集中分布在汾河中上游，沿河流呈东北西南向带状分布。考虑土壤的面积后得到 流域总体碳储量为 47.22 Tg，固碳潜力为-0.153 Tg。现有状态下汾河流域表层土 壤为碳源区。

（3）在流域氮素损失中，有机氮和硝态氮进入河流的年平均输移量分别为 11910 吨和 9885 吨，硝态氮 90%通过土壤侧向流进入河流。在年内变化中，有 机氮流失量在 8 月达到峰值，而硝态氮流失量在 6 月最大。空间上，汾河流域总 体氮损失呈南高北低。农田氮损失负荷量相比于流域平均值更高，除了有机氮流 失。其中，玉米氮损失量约为农田损失量的 60%-80%。农田氮损失的年内峰值均 是在种植玉米期间达到的。空间上，除了土壤氧化亚氮，农田通过其他途径的氮 损失达到峰值的区域和流域呈现出一致性。降水量和施氮量作为农田氮素损失的 影响因素，对于农田、玉米和小麦硝态氮损失表现出显著影响。

（4）在人为活动对水碳氮影响方面，跨流域调水工程对溶解态的硝态氮流 失影响较大。净初级生产力和总氮的变化与水库月度放水表现出相似性。空间上， 在 23 子流域，有机氮大幅降低（86%），而通过地表径流和地下水硝态氮流失则 达到增长峰值。对于农田灌溉，其对产水量和硝态氮通过地下水流失的影响较大。 在年内变化中，在灌溉量减小情景下，净初级生产力的变化率在 3-4 月由正转为 负。总氮在灌溉量减少的情景下，出现双峰值，分别在 3 月和 9 月。水碳氮指标 在时间上的比较分析表明，土壤氧化亚氮和水循环指标表现为协同关系，但与产 水量却存在着差异性。空间上，碳储量和总氮的变化率均在流域南部较高，存在 着空间上的协同性。

（5）在人为活动对生态系统服务影响方面，跨流域调水工程影响下综合生 态系统服务指数（TES）基本为正值，工程对流域有积极的生态影响。年内变化 中，TES 在 6 月和 11 月最高。空间上，TES 的波动幅度较小，除 16、18 和 23 子流域，均不超过 1。对于农田灌溉，灌溉量的增加有利于生态系统服务的供应， 而灌溉量的减少则产生不利影响，且随着灌溉量的不断减少，这种影响会加强。 年内变化中，TES 的较大变化发生在 3 月和 11 月。空间上，随着灌溉量的减少， TES 最低值出现的区域由北部转移到了南部。

Watershed is the basic unit of terrestrial and aquatic ecosystem. In the watershed, the water, carbon, and nitrogen cycles, as the main material cycle, provide important support for the integrity of ecosystem structure and supply of ecosystem services. Ecosystem service is a bridge between ecosystem and human well-being, and the service capacity is closely related to water, carbon, and nitrogen processes in watershed. In addition, under the influence of a series of anthropogenic activities such as water conservancy projects and agricultural irrigation, the watershed ecosystems are changing. It is urgent to carry out simulation of water, carbon, and nitrogen of watershed under anthropogenic activities. Therefore, this study established the hydro-biogeochemical model SWAT-DayCent of the Fenhe River Basin, analyzed the spatio-temporal variation of water cycle, explored the spatio-temporal heterogeneity of carbon stocks and the potential of soil carbon sequestration, revealed the characteristics of nitrogen loss in the basin and farmland, constructed a comprehensive ecosystem service index, and evaluated the effects of inter-basin water transfer (IBWT) projects and farmland irrigation on water, carbon, nitrogen, and ecosystem services. The main conclusions are as follows:

(1) In terms of water cycle in the basin, during 2010-2020, except that soil water content decreased significantly at an average annual rate of 21.5mm, the change rates of the other water cycle indexes were very small and the maximum water volume appeared in 2020. The variation trends of evapotranspiration, water yield and surface runoff were consistent with that of precipitation. Areas with high water cycle index values were concentrated in the south areas of the basin and the abnormally high values mainly appeared in sub-basins 9, 16, 18 and 23.

(2) In terms of carbon storage in the basin, the net primary productivity (NPP) of vegetation and soil organic carbon density decreased slightly during the 11 years. The vegetation NPP decreased in 66.5% of the basin area, which was twice as much as the increase. Further spatial analysis showed that the global spatial autocorrelation of NPP showed high value clustering. Cold spots were mainly distributed in the upper and lower reaches of the basin, while hot spots were mainly distributed in the central basin. Among the thirteen soils, the carbon densities of mountain meadow soil and brown soil were higher, while that of alluvial soil was the least. Soil with high organic carbon density was distributed in the middle and upper reaches of the Fenhe River. After considering the soil area, the total carbon storage of the basin was 47.22 Tg and the carbon sequestration potential was -0.153 Tg. At present, the surface soil of the Fen River Basin was the carbon source region.

(3) In terms of nitrogen loss in the basin, the amounts of average annual transport of organic nitrogen and nitrate nitrogen into river were 11,910 tons and 9,885 tons, respectively. 90% of the nitrate nitrogen entered the river through soil lateral flow. In the annual variation, organic nitrogen loss reached the peak in August, while nitrate nitrogen loss reached its peak in June. Spatially, the overall nitrogen loss in the Fenhe River Basin was high in the south and low in the north. The farmland nitrogen loss load was higher than the watershed average, except for organic nitrogen loss. The nitrogen loss of corn was about 60%-80% of that of farmland. The annual peak of nitrogen loss was reached during corn planting. Spatially, except for nitrous oxide, the areas where farmland nitrogen loss through other ways peaked was consistent with the basin. Precipitation and nitrogen application rate, as factors affecting nitrogen loss, had significant effects on nitrate loss in farmland, corn, and wheat.

(4) In terms of the influence of anthropogenic activities on water, carbon, and nitrogen, the IBWT projects had a great impact on nitrate loss in dissolved state. The changes of net primary productivity and total nitrogen were like those of monthly discharge of reservoirs. Spatially, in sub-basin 23, organic nitrogen decreased significantly (86%), while nitrate loss through surface runoff and groundwater increased at a peak. For farmland irrigation, it had great influence on water yield and nitrate loss through groundwater. In the annual variation, the change rate of NPP changed from positive to negative in March to April under the scenario of reduced irrigation volume. Under the scenarios of reduced irrigation amount, the total nitrogen showed double peaks in March and September, respectively. The analysis of water, carbon, and nitrogen indexes in time showed that there was a synergistic relationship between nitrous oxide and water cycle indexes, but there was a difference between nitrous oxide and water yield. Spatially, the change rates of carbon storage and total nitrogen were high in the southern areas of the basin, which means that there was a spatial synergy between carbon storage and total nitrogen.

(5) In terms of the influence of anthropogenic activities on ecosystem services, the total ecosystem service (TES) index was basically positive under the influence of IBWT projects, and the projects had a positive ecological impact on watershed ecosystem. In the annual variation, TES was highest in June and November. Spatially, except for subbasins 16, 18 and 23, the fluctuation range of TES was small and was no more than 1. For farmland irrigation, an increase in irrigation was beneficial to the provision of ecosystem services, while a decrease in irrigation had an adverse effect, and this effect was strengthened as the amount of irrigation decreased. In the annual variation, the great changes in TES occurred in March and November. Spatially, with the decrease of irrigation amount, the areas with the lowest TES value shifted from the north to the south.