Altered Seasonal Sensitivity of Net Ecosystem Exchange to Controls Driven by Nutrient Balances in a Semi-arid Savanna

Abstract. Semi-arid ecosystems dominate variability and trend of the terrestrial carbon sink. They are sensitive to environmental changes following anthropogenic influence, such as an altered ratio of nitrogen (N) to phosphorus (P) due to increasing N deposition. Semi-arid savannas with different vegetation compositions have complex carbon dynamics, and their responses to environmental change are not yet well understood. We analysed a long-term (2016–2022/2023) dataset of flux, biometeorological and vegetation data (satellite and ground measurements) of a manipulated semi-arid savanna to reveal how altered nutrient levels and stoichiometric balance affect the seasonal sensitivity of net ecosystem exchange (NEE) to its drivers. We used the Singular Spectrum Analysis to extract the seasonal signal of all variables and assessed the key drivers of NEE over the study period as a whole and in different seasons, using Pearson correlation and Information Theory. We found that both N and N+P addition to the ecosystem increased seasonal NEE variability, driven by greenness of the herbaceous layer. Analysing 7 years of data together, the water limitation in summer and energy limitation in winter outcompeted the fertilization effect. By investigating different phenological seasons, effects of nutrient addition on NEE-control relationships became clearer. In the summer, N+P addition led to a potential change in species composition and productivity resulting in a stronger interaction between herbaceous layer and NEE. During the transitional seasons (i.e., drydown and regreening), which determine the senescence and regreening of the herbaceous layer, we found NEE to be less sensitive towards meteorological drivers like relative humidity, radiation and air temperatures with N addition. The increasing NEE variability might become even more pronounced with N deposition and a changing climate in the future.