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

Abstract

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.