Phosphorus additions promote soil nitrogen runoff in a tropical forest

Abstract: Human activities have dramatically increased the deposition of reactive nitrogen (N) globally, leading to enhanced N losses from terrestrial ecosystems through hydrological pathway. This loss of reactive N from soils is controlled by complex transformation and transport processes, which are influenced by a suite of factors, including soil water conditions, vegetation types, soil microorganisms, and soil physicochemical properties. A crucial, yet often overlooked, factor is the availability of phosphorus (P). P is a limiting nutrient for soil microbial activity and vegetation productivity in many regions worldwide, especially in tropical ecosystems. Here we established a P addition experiment (+0, +25, +50, +100 kg P ha⁻¹ yr⁻¹) in an evergreen broadleaf mixed plantation. We found that cumulative dissolved total N (DTN) exhibited a concave-shaped nonlinear response to P addition. During the wet season (July 20 to September 18, 2023), a sharp cumulative increase in the mean values of DTN runoff was observed under P additions. In contrast, the cumulative DTN flux from runoff showed minimal increase during the dry season. Furthermore, the enhanced DTN runoff under P additions were linked to the elevated inorganic N assimilatory reduction genes and SWC, and seasonal precipitation. These findings offer insights into the hydrological loss of N under different P supply conditions in tropical forests, with direct implications for projecting and managing nutrients in tropical forests in the context of global change.
Acknowledgements: This work was supported by the National Natural Science Foundation of China (32301443), the Guangdong Basic and Applied Basic Research Foundation (2022A1515110926, 2023A1515010957 and 2022B1515020014, and NSFC Sino-German Mobility Program (No. M-0749).