Research progress on nitrogen balance potentials in terrestrial ecosystems

Terrestrial ecosystems dominated by soils and plants (soil-plant systems) occupy only 28% of the Earth’s surface while they contain up to 75% species and 86% biomass on the Earth, identifying as priority areas for ecological protection. The Earth is now outside of the planetary nitrogen (N) boundary for humanity, thus understanding spatiotemporal patterns, dominant drivers and ecological effects of the N input-output balance potentials in terrestrial ecosystems is critical for sustainable N management. Terrestrial N balance potentials typically include positive balance, zero balance, and negative balance, which are susceptible to two aspects: (1) key N-cycling processes, such as atmospheric N deposition, anthropogenic N fertilization, natural N fixation, rock N weathering, leaching N losses, gaseous N losses, N losses by plant-based products/wastes, microbial N metabolism, plant N metabolism, and N return from plant to soil; (2) a wide range of natural and anthropogenic drivers, such as climatic conditions, plant traits, soil properties, and other factors (e.g., humans, animals and microbes).Currently, four paradigms of N balance potentials have been developed, including N surplus paradigm (1970s–), N use efficiency paradigm (1980s–), microbial N metabolism paradigm (1990s–), and stable N isotope paradigm (2000s–). The N surplus paradigm analyzes the difference between the amount of N entering a system (inputs) and the amount of N leaving the system (outputs). The N use efficiency paradigm explores the proportion of the N recovered (uses) by an organism or a system relative to the N supplied (sources) from soils, fertilizers, and so on. The microbial N metabolism paradigm considers the ratio of microbial N mineralization (catabolism) to microbial N assimilation (anabolism), denoted as microbial N metabolism ratio. The stable N isotope paradigm is based on Rayleigh fractionation theory, which describes isotopic enrichment or depletion as material moves between reservoirs in an equilibrium process. Over a half century, these paradigms are used to understand N balance potentials in terrestrial ecosystems.However, several limitations and challenges remain: (ⅰ) despite its greater promise, stable N isotope paradigm should extend theoretical consideration and empirical evidence; (ⅱ) the integration of observation-statistics-theory-model offers an optimistic way forward, which can help bridge the gaps among empirical, theoretical, and modeling studies on N balance potentials; (ⅲ) given that current knowledge is fragmentary, multi-dimensional explorations will provide a more comprehensive understanding for dominant drivers and ecological effects of terrestrial N balance potentials. Therefore, future studies will benefit from considering emerging progress and interdisciplinary collaboration in the fields of N isotope techniques, theoretical ecology, and information ecology.