Excessive nitrogen exported to water bodies affects the balance of ecosystem and poses a threat to human health. Although the concept of water purification service helps quantify nitrogen export, the impact of river transport remains unclear. This study focused on nitrogen as a pollutant by utilizing the Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) model to assess nitrogen export in the Dongting Lake Basin, taking into account both the processes of sub-basin nitrogen export and river transport. Additionally, the monthly variations within the year were further explored, together with the identification of the priority area of returning cropland to forest land. The results showed that in 2021, the total nitrogen load outside the Dongting Lake was 11.34 × 10⁸ kg·year⁻¹, with the sub-basins retaining a total of 9.02 × 10⁸ kg·year⁻¹, accounting for 79.57 % of the total nitrogen load. Notably, the total nitrogen retention by the river made up 16.87 % of the total nitrogen retention, up to 1.83 × 10⁸ kg·year⁻¹. In view of monthly variation, water purification service based on the entire process was higher in winter and late autumn, and lower in summer and early autumn. The priority areas for ecological project were mainly distributed around the Dongting Lake, achieving an improvement of 58.15 % in water purification service with approximately 7.02 % of the total returnable cropland. This study proposed a new approach of water purification service assessment through integrating river transport processes and seasonal dynamics, aiming to assess watershed nitrogen export risk more accurately.

中文翻译：

---

整合河流运输过程和季节性动态，以评估流域氮输出风险


过量的氮输出到水体会影响生态系统的平衡，并对人类健康构成威胁。尽管净水服务的概念有助于量化氮气输出，但河流运输的影响仍不清楚。本研究重点研究氮作为污染物，利用生态系统服务和权衡综合评估 （InVEST） 模型评估洞庭湖流域氮输出，同时考虑子流域氮输出和河流运输过程。此外，还进一步探讨了一年内的月度变化，并确定了退耕还林的优先区域。结果表明：2021 年洞庭湖外总氮负荷为 11.34 × 10 ⁸ 千克·年 ⁻¹ ，子流域共留存 9.02 × ⁸ 10 千克·年 ⁻¹ ，占总氮负荷的 79.57 %。值得注意的是，河流的总氮滞留量占总氮滞留量的 16.87 %，在 10 ⁸ kg·year ⁻¹ ×高达 1.83 %。鉴于月度变化，基于全过程的净水服务在冬季和深秋较高，在夏季和初秋较低。生态工程的优先区域主要分布在洞庭湖周围，净水服务提高了 58.15%，约占总可还田面积的 7.02%。本研究通过整合河流运输过程和季节动态，提出了一种新的净水服务评价方法，旨在更准确地评估流域氮输出风险。