Forests play key roles in terrestrial carbon sinks, climate regulation, and water conservation. Nitrogen (N) is a major limiting factor for forest productivity, and nitrate (NO3-) is one of the bioavailable N species. Over the past decades, elevated emissions of nitrogen oxides (NOx) from industrial and agricultural activities, together with elevated wet and dry NO3- deposition, have significantly altered atmospheric NO3- inputs, biological N uptake and turnover, gaseous N emissions, and hydrological N leaching in forests. These changes could arouse N saturation, soil acidification, and vegetation degradation, thereby weakening forest ecosystem function and threatening downstream water quality. However, mechanisms underlying the accumulation and retention of atmospheric NO3- in forest soils under varying N pollution and climatic conditions remain poorly understood, hindering accurate assessments of forest NO3- budgets.

Forests in the Beijing-Tianjin-Hebei (BTH) region of northern China are under high atmospheric N pollution and a semi-arid climate. With approximately 40% of forested land receiving N deposition above 50 kg N ha–1 yr–1, both soil nitrification and NO3- concentrations have shown high sensitivity to elevated N deposition. Thereby, understanding how atmospheric NO3- accumulates and influences microbial production is essential for evaluating forest N retention capacity under anthropogenic N loading.

Recently, Xueyan Liu’s group investigated oxygen isotope anomalies of soil NO3- (Δ17O) across forests in the BTH region, spanning gradients of atmospheric N deposition (30–70 kg N ha–1 yr–1) and precipitation (350–760 mm), and also integrated existing Δ17O datasets from temperate forests in North America and East Asia (Figure 1). They aimed to explore the atmospheric contribution and accumulation mechanisms of soil NO3-. Key findings include: (1) despite the semi-arid climate, soil nitrification was not significantly influenced by increased precipitation but was primarily driven by atmospheric N pollution, and (2) elevated N pollution stimulated nitrification, diluting the Δ17O signature and lowering the proportional contribution of atmospheric NO3-. Nevertheless, both the accumulation of atmospheric NO3- and nitrification rates in soils increased with the concentrations and fluxes of NO3- in precipitation (Figure 1).

Figure 1. Atmospheric N pollution drove soil nitrate production and accumulation in temperate forests under a semi-arid climate（Image by IGCAS）

These results suggest that long-term anthropogenic N pollution facilitates the accumulation of both atmospheric and microbially derived NO3- in forest soils under semi-arid conditions, ultimately leading to a NO3- supply that exceeds biological demand. This enrichment may increase the risk of NO3- leaching during extreme precipitation events or in the non-growing season. The study enhances our understanding of forest N accumulation processes and offers key insights into how climate change and human disturbance shape forest N cycling and related ecological risks.

This work was recently published in Geophysical Research Letters, entitled “Atmospheric N deposition as a key driver of soil nitrate production and accumulation in temperate forests under semi-arid climate” (DOI: 10.1029/2025GL117029).

The research was supported by the National Natural Science Foundation of China (No. 42125301).

Contact：

LIU Xueyan 

Institute of Geochemistry, Chinese Academy of Sciences

Email: liuxueyan@mail.gyig.ac.cn

(By Prof.LIU Xueyan’s group)