Ecosystem N:P stoichiometric ratios determine the catchment surface water N:P ratio through subsurface hydrological processes

An imbalanced surface water N:P (nitrogen:phosphorus) stoichiometric ratio degrades aquatic ecological and environmental functions. Although it is known that the N and P balance and hydrological processes in terrestrial ecosystems affect the surface water N:P ratio, how these factors regulate surface water N:P ratios remains unclear. Here, the N:P ratios in the nutrient budgets, soil, overland flow, groundwater, and surface water in two comparable tea and forest ecosystems and catchments were observed during the 2015-2017 period to link the hydrological processes with the surface water N:P ratio. The results suggested that although the N:P ratio of the nutrients released into the soil-water system was 1.84 times greater in the tea ecosystem than in the forest ecosystem, the soil N:P ratio (5 vs. 10 mol mol(-1)) was lower in the tea ecosystem, probably due to soil N and P decoupling processes associated with tea field establishment. The overland flow N:P ratios of the tea and forest ecosystems (30 and 28 mol mol(-1), respectively) were similar and were mainly affected by surface hydrological processes. The groundwater N:P ratios were higher in the tea ecosystem than in the forest ecosystem (245 vs. 86 mol mol(-1)), indicating more intense soil N leaching than P leaching in the tea ecosystem. An ecosystem releases excessive N through the groundwater process, which contributes to the stability of the soil N:P ratio. It is estimated that the groundwater N:P ratio contributed more than the overland flow N:P ratio to the surface water N:P ratio (31.5-58.6% vs. 0.1-0.7%), which highlights the importance of the groundwater N:P ratio and subsurface hydrological processes for regulating the surface water N:P ratio in catchments. Therefore, to reduce the environmental risk posed by an unbalanced surface water N:P ratio in tea-growing regions, NP fertilizer application must be improved, and NP leaching should be minimized.