High nitrogen pollution in South China

What is the level of nitrogen pollution entering Chinese forests? Is it harmful to trees or polluting forest waters? Researchers from Forest & Landscape and China recently noted that some of the world’s highest nitrogen inputs to forest are found in the warm and humid South China. A research cooperation aims to establish the potential consequences.

The emission of nitrogen (N) to the atmosphere is increasing globally. While the emissions have decreased in Europe, they are increasing rapidly in Asia in regions with strong economic growth. A new study compiling all the available Chinese data on N deposition due to air pollution reveals that some of the world’s highest N inputs to forest are found in South-eastern China.

Every year up to 70 kg of N per hectare (or 10 times more than normal) enters forests of these regions with tropical or subtropical climate. The possible consequences of elevated N input to such warm humid forests have only been addressed in few studies world-wide.

Tropical forests are naturally nitrogen rich

The soils in the tropics are often very old and usually have less of the nutrient phosphorus (P) than younger soils in temperate climates. Growth in tropical forest is thus often limited by P whereas northern temperate forests are limited by N and mostly retain deposition N.

Researchers therefore expect tropical forests to have enough N, and inputs from pollution will provide excess N likely to be lost by leaching to ground and surface waters. The compilation of available data from China also covered N leaching and indeed showed that in many Chinese forests, N was passing through the ecosystem and ended up in forest waters. However, forests that had been cut and exploited for fuel and fodder could still retain a part of the N.

The view from a laboratory roof where sampling of precipitation is done. Nitrogen input to the forest is measured to be 50 kgN/ha/yr at this place.

Unique forest experiments

The observational studies discussed above are good in addressing the state of the environment but have limitations in identifying effects and consequences of the N pollution.

To study the effects of elevated N deposition the first field scale nitrogen input manipulation study under warm and humid climate was established in 2004 at the Dinghushan Biosphere Reserve, China in cooperation with Per Gundersen, professor at Forest & Landscape. The impact of several levels of nitrogen deposition is studied in a land use gradient from old growth natural rainforest over an ecosystem in succession to a plantation forest.

The experiment is run by a Chinese research group headed by Prof. Jiang-Ming Mo, South China Botanical Garden. It includes studies of the impact of nitrogen on tree growth and physiology, plant diversity, soil fauna, soil microbiology, N cycling, interaction with phosphorus, nitrate leaching, soil acidification, carbon sequestration and greenhouse gas exchange with results published in several international journals.

Nutrient interactions

The most interesting recent results are related to the effects of the interaction between N and P on the activity and composition of soil microorganisms. In the N rich old growth forest, P addition stimulated the soil microbes as expected, whereas in plantations on more eroded soils P had little effect. A coming cooperation project focuses on the fate of deposition N in these ecosystems using stable isotope methods.

The mentioned study of nitrogen input and leaching was published in an article called Atmospheric deposition and leaching of nitrogen in Chinese forest ecosystems in Journal of Forest Research. The most recent paper from the cooperation focuses on Effects of phosphorus addition on soil microbial biomass and community composition in three forest types in tropical China and will be published in Soil Biology and Biochemistry in 2012.


Natural forest reestablishing on steep slopes in South China.