Effects of Simulated Atmospheric Nitrogen Deposition on Forest Ecosystems in China: An Overview

Abstract:

Human activities, such as combustion of fossil fuel, production and application of nitrogenous fertilizer, and intensive livestock production, have been accelerating the production and emission of reactive nitrogen (e.g., NH₄⁺, NO₃^-), leading to elevated nitrogen (N) deposition at regional and global scales. Human interference with N cycle has gone beyond the safe operating space for humanity. China is one of the three regions with the highest N deposition in the world. High N deposition has threatened the health and safety of terrestrial ecosystems, which should be addressed urgently during the process of ecological civilization construction. The research history on simulated N deposition in China and world was reviewed, focused on how simulated N deposition affects forest ecosystems in China, including soil acidification, plant element chemistry, plant growth and diversity, soil microbial community and enzyme activities, soil fauna, greenhouse gas emissions, ecosystem N and phosphorus cycles, soil N transformation, ecosystem N fixation, litter decomposition, and ecosystem carbon sequestration. The atmospheric N deposition has been concerned since 2000s. In 2002, the first long-term forest ecosystem N manipulative experiments were established by South China Botanical Garden (SCBG) of the Chinese Academy of Sciences, which is playing a leading role in the field of nitrogen deposition and forest ecosystems in China. In 2013, SCBG, for the first time, designed a novel experiment with canopy addition of N (CAN) vs. understory addition of N (UAN) in China. Results from N manipulative experiments across China showed that continuing high N deposition greatly altered forest structure and functioning, threatening ecosystem health, especially in the south-central China. The main results are as follows:(1) There is a fertilization effect of N deposition in temperate and boreal forests, but there seem no positive effects on plant growth in N-rich tropical forests because of N saturation. (2) Excess N deposition can lead to soil acidification and nutrient imbalance. (3) Elevated N deposition has accelerated N cycling rate and its transformation process, but depressed ecosystem N fixation rate, and altered ecosystem P availability and cycling, litter decomposition process and greenhouse gas emissions. (4) High N deposition reduced understory plants diversity and changed the structure of soil microbial community. (5) Nitrogen deposition generally simulates aboveground vegetation C sequestration across China, but there remains uncertain on belowground soil C sequestration. (6) Tropical and subtropical forest ecosystems are non-ignorable N sinks, depending on the forms and fates of added N. (7) The effects of N deposition on forest ecosystems are variable, depending on ecosystem N status, land-use history, climate, and forest types and ages. Considering that there remain uncertainties on the long-term effects of N deposition in China, it is suggested that it is necessary to continue the present studies in a longer term, and to expand a network research among field sites along climate gradients. It would be further highlighted to explore how forest ecosystems respond and acclimate to long-term N inputs, to quantify belowground C and N sequestration, and to jointly consider multiple global change factors (e.g., climate warming, CO₂ enrichment, changes in precipitation patterns), all of which are important for forest management and sustainable development in the future.