High emissions of reactive nitrogen (N) by human activities have increased global N deposition. The impacts of the altered N cycle have been one of the focuses in global change research in recent decades as it can have a variety of positive and negative effects. In particular, the effects of increased N availability in the biosphere on C balance and its implication for climate change mitigation has been a major research area since the 1980s. Given that most natural ecosystems are historically N-limited, increased N deposition is thought to enhance forest plant productivity and thereby to increase C sink. This has been widely considered as the environmental befit of N deposition that contributes to the mitigation of global warming. However, there is still a large debate over the magnitude of N deposition on forest C sink. The impacts of deposited N on forest ecosystems depend on its fates, i.e., its retention in biomass and soil organic matter pools versus its losses via leaching and nitrogenous gases. The fates of deposited N are also expected to differ depending on whether it is in reduced (mainly as ammonium) or oxidized (mainly as nitrate) form. Thus, the key to estimate forest C sink due to N deposition is to quantify the fate of deposited ammonium and nitrate. 

To date, many studies have examined the fate of deposited N to forest ecosystems by adding ¹⁵N tracers, but most of which were small in scale and short in time. In addition, spatial heterogeneities in the total and form of deposited N across forest biomes increase difficulties in making reliable global estimates of the contribution of N deposition to C sink. These uncertainties hamper prediction of the effects of increased N availability in the biosphere on C balance and its implication for global climate change mitigation.

To address the problems outlined above, Prof. Yunting Fang’s laboratory at the Institute of Applied Ecology, Chinese Academy of Sciences (CAS), initiated a Chinese forest ecosystem ¹⁵N tracer network. Based on the results in nine ecosystem-scale paired ¹⁵N-tracer experiments from China and four earlier studies in temperate forests in the US and Europe, Prof Fang’s laboratory and Prof Peng’s laboratory (from Peking University) have led an investigation on the fate of atmospheric N deposited ammonium and nitrate into forests and their contribution to forest C sinks. 

They showed that about 70% of ¹⁵N tracers was retained in the forests and total ecosystem ¹⁵N retention was similar between ammonium and nitrate. However, plants utilized nitrate proportionally more than ammonium (Figure 1). Based on the ¹⁵N-tracer-derived N retention in plant and soil pools and using a stoichiometric upscaling approach, they further estimated a global forest C sink due to N deposition in 2010s at 0.72 Pg C yr^-1, which was higher than previously reported estimates (Figure 2). This new estimate suggested that about 20% of the annual global terrestrial C sink over the last decade might be due to N deposition. These findings contribute to improved understanding of the effects of N deposition on C and N cycling in forest ecosystems globally and suggest that N deposition is an important contribution factor to forest C sink.

The study entitled “Retention of deposited ammonium and nitrate and its impact on the global forest carbon sinks” has been published online on February 15, 2022 in Nature Communications. 

This work was done in collaboration with the scientists from Tsinghua University, Northeast Normal University, Beijing Normal University, Nanjing University of Information Science and Technology, Henan University, Institute of Subtropical Agriculture (CAS), Xishuangbanna Tropical Botanical Garden (CAS), Institute of Botany (CAS), Institute of Tropical Forestry (CAF), LSCE ( CEA CNRS UVSQ UPSaclay ) Centre d'Etudes Orme des Merisiers (France), University of Leeds (UK), University of Copenhagen (Denmark), Wageningen University and Research (the Netherlands), University of New Hampshire (USA), Binghamton University - State University of New York (USA), University of Michigan (USA), and Kyoto University (Japan). 

This work was financially supported by the National Key Research and Development Program of China, the Key Research Program of Frontier Sciences of Chinese Academy of Sciences, the National Natural Science Foundation of China, National Research Program for Key Issues in Air Pollution Control, and K. C. Wong Education Foundation.

Fig.1 Global distribution of total N deposition in forests and locations of the 13 paired ¹⁵N-labelling experimental sites (a); Percent recoveries in plants, organic soil layer (OSL) and mineral soil (MSL) for deposited N at the 13 paired ¹⁵N-labelling sites (b); Relationship between ¹⁵N recovery for ¹⁵NH₄⁺ tracer and ¹⁵NO₃^- tracer (c) (Image by WANG Ang).

Fig.2 Comparison of atmospheric N deposition and its contribution to global forest C sink in different studies (Image by WANG Ang).