Are nitrogen and carbon related in the forest? Uncovering the love-hate relationship between nitrogen and carbon

The increase in atmospheric carbon dioxide concentration caused by human activities (including fossil fuel combustion, land use changes, etc.) has become an important scientific topic today and has attracted widespread social attention.

Both the Paris Agreement and the IPCC report point out that effectively curbing the increase of atmospheric carbon dioxide is one of the important measures to address climate change. It also emphasizes that we need to deeply understand and explore where the "carbon" in the atmosphere comes from (carbon source) and where it will go (carbon sink).

Soil: The largest carbon reservoir in terrestrial ecosystems

When it comes to absorbing carbon dioxide, the first thing that comes to mind is plants. Plant leaves absorb carbon dioxide through photosynthesis, and their respiration can release carbon dioxide.

That's right. But soil also affects atmospheric CO2 concentrations.

Soil is the largest carbon pool in terrestrial ecosystems, and about half of soil organic carbon is stored in forests. Tropical (including subtropical) forests play an important role in the global forest carbon cycle, accounting for 78% of global forest carbon emissions and 55% of carbon absorption.

This means that even small changes in forest soil carbon stocks can have significant effects on atmospheric carbon dioxide concentrations.

So, how does soil affect atmospheric carbon concentration?

In the forest, not only plants breathe, but soil also breathes. Soil respiration mainly comes from two sources: one is the anoxic respiration produced by microorganisms, and the other is the autotrophic respiration produced by plant roots. These two parts of respiration are important channels for soil carbon emissions.

Soil respiration (Image source: author)

Are nitrogen and carbon related in forests?

Today, human activities not only lead to an increase in atmospheric carbon dioxide concentration, but also an increase in atmospheric nitrogen deposition.

Since the mid-20th century, the amount of reactive nitrogen compounds released into the atmosphere by human activities has increased rapidly. These nitrogen-containing substances enter the land and water bodies through dust precipitation (dry deposition) or dissolve in rainwater (wet deposition) , affecting the soil and water environment, natural ecosystems, biodiversity, etc.

According to the simulation and assessment of global nitrogen deposition, the atmospheric nitrogen deposition rate increased by an average of 8% from 1984 to 2016.

China is one of the three regions with the most serious nitrogen deposition pollution in the world (Europe, North America and China). Although the nitrogen deposition rate in China has stabilized and started to decline through the improvement of management policies and technologies, nitrogen deposition in some areas still reaches 30-40 kg N ha-1 yr-44.

Reactive nitrogen produced by human activities over the past 60 years (Image source: Reference [7])

What effects will long-term high nitrogen deposition have on forest ecosystems or soil carbon pools? In other words, what is the relationship between nitrogen and carbon, two different elements in forests?

Nitrogen is an important nutrient element in living things and is also the basic substance for synthesizing proteins. Unlike farmland, which can obtain nitrogen and other nutrients through artificial fertilization, forests mainly obtain nitrogen through atmospheric nitrogen deposition or biological nitrogen fixation.

When forest plants and microorganisms have access to sufficient nitrogen, they can grow rapidly and accumulate biomass (carbon). Plants can also further transport carbon into the soil in the form of fallen leaves or root secretions, increasing the soil's carbon sink.

The amount of nitrogen also affects the respiration of forest organisms. Sufficient nitrogen is conducive to plant respiration and microbial decomposition, thereby releasing carbon dioxide. It can be seen that the amount of nitrogen determines the absorption and emission of carbon by the forest to a certain extent.

Nitrogen and carbon fluxes in forests (Image source: author’s own)

For living organisms, is the more nitrogen supply the better?

In fact, this is not the case. In the short term, the supply of nitrogen can solve the "food and clothing problem" to a certain extent, but the long-term and continuous supply of nitrogen will produce other "side effects."

For example, too much nitrogen entering the forest will lead to a low relative content of other nutrients in plants and soil microorganisms (nutritional imbalance), promote microbial nitrification reactions (releasing hydrogen ions) and aggravate soil acidification, increase nitrogen-containing greenhouse gas emissions, reduce biodiversity, and even affect soil and groundwater quality.

So, how do we determine whether nitrogen deposition has a positive or negative impact on forest soil carbon emissions?

The relationship between atmospheric nitrogen deposition, plant nitrogen uptake, soil nitrogen transformation and acidification (Image source: Reference [6])

On the one hand, we need to look at the status of forest nitrogen , because the nitrogen status of different forests is different.

For forests that are already "nitrogen-poor", nitrogen deposition is undoubtedly a "timely help". The growth of soil microorganisms and underground plant roots will be accelerated (increasing biomass and carbon sinks), but it will also promote soil respiration (increasing carbon emissions). For forests that are already "nitrogen-rich", the effect of nitrogen deposition may be "a drop in the ocean", with little impact on soil carbon and respiration, and may even be counterproductive and inhibit soil respiration.

On the other hand, it is necessary to assess the duration of nitrogen deposition , since nitrogen deposition into the forest is a cumulative process.

In the short term, nitrogen deposition may be beneficial to many forests and promote soil biological respiration. However, under the influence of long-term nitrogen deposition, nitrogen will gradually accumulate in plants and soil. When the nitrogen critical point is reached , it will also have a negative effect and eventually soil respiration will weaken.

It seems that although a small amount and moderate nitrogen deposition is beneficial to biological growth, it also promotes carbon emissions from soil respiration. On the contrary, although excessive nitrogen deposition inhibits biological growth, it also slows down soil carbon emissions.

Using nitrogen to regulate carbon: a 13-year experiment

Since the duration and state of nitrogen deposition will affect forest carbon emissions, let's take a look at the response of forests under extreme conditions (i.e., soil carbon emissions from nitrogen-rich forests under long-term nitrogen deposition environments)?

A 13-year simulated nitrogen deposition study conducted at the Dinghushan National Field Forest Station in Guangdong answered this question.

Field control test platform for simulating nitrogen deposition (Image source: self-made by the author)

By simulating long-term nitrogen deposition, the soil respiration of nitrogen-rich forests showed a three-stage response of "no change-decrease-no change".

In the first stage, after nitrogen deposition into the soil, the physical and chemical properties of the soil were the first to change. This was mainly reflected in the increase in the concentration of inorganic nitrogen in the soil, the acceleration of the mineralization and nitrification of soil nitrogen, and the leaching and loss of nitrogen.

In contrast, the plant community structure (diversity, richness, fine root biomass, etc.) and soil microbial community composition (bacterial and fungal biomass, etc.) did not change much. Therefore, the anoxic respiration of soil microorganisms and the autotrophic respiration of plant roots did not change significantly.

This shows that the impact of early nitrogen deposition on nitrogen-rich forests is not significant.

Effects of nitrogen deposition on soil (Image source: self-made by the author)

However, everything has its limits. After entering the second stage, plants and microorganisms began to feel "uncomfortable" .

The mineralization and nitrification rates of soil nitrogen began to weaken, indicating that the soil nitrogen supply may have exceeded the ecosystem's demand for nitrogen. Nitrogen deposition exacerbates forest soil acidification and produces a series of "side effects", including: reduced plant richness, reduced diversity, impaired photosynthetic physiological functions of plants, reduced biomass of underground fine roots of plants, weakened root respiration, and more root death.

At the same time, underground microorganisms also decreased, the microbial function of degrading carbon weakened, and microbial respiration was also inhibited.

During this stage, the total respiration rate and carbon emission rate of the soil were significantly reduced.

Forest degradation (Image source: Veer Gallery)

Surprisingly, this "side effect" of nitrogen deposition did not last forever, and the forest began to "fight back."

Entering the third stage, forest plants and microorganisms did not continue to "sit and wait for death". Under the harsh environment of long-term high nitrogen and acidification, they gradually adjusted their community composition, and some new vines and microbial groups appeared. The fittest survived and the unfit were eliminated.

At this stage, the fine root biomass of plants and soil microbial biomass no longer show large-scale reductions, and the forest soil respiration and carbon emission rates return to stability.

Although plants and microorganisms adapted and restored their respiration rates in the later stages, soil carbon emissions decreased by 6.53-9.06 Mg CO2 ha-1 throughout the experiment. It seems that the reduction in soil respiration is "beneficial" to mitigating climate change, but this is at the expense of biological growth and reduced biodiversity.

In fact, atmospheric nitrogen deposition and soil acidification are still continuing. We do not know what changes will occur in the forest in the next stage, and researchers are still exploring the answer.

Effects of simulated nitrogen deposition on forest soil respiration (Image source: References [11,12])

This 13-year simulated nitrogen deposition experiment not only tells a story about how forest nitrogen regulates carbon, but also describes how plants and microorganisms survive in adversity.

What should we do under the background of the “dual carbon” strategy?

At present, the rapid development of the country's economy and society will inevitably have a certain degree of impact on the ecological environment. How to achieve a good balance between economic development and environmental protection is one of the issues that need to be considered in the future.

The "dual carbon" strategic goal is proposed based on the inherent requirement of promoting sustainable development and is also the responsibility of building a community with a shared future for mankind. The purpose is to control the global surface temperature rise to within 2 degrees Celsius compared to the pre-industrial revolution level by the end of this century.

The world emits about 51 billion tons of greenhouse gases into the atmosphere every year. To avoid disasters caused by extreme weather caused by global warming, humans need to reduce greenhouse gas emissions into the atmosphere and achieve net zero emissions as soon as possible.

In addition to reducing anthropogenic carbon emissions, it is also necessary to reasonably control nitrogen deposition pollution caused by human activities. Although nitrogen deposition may help slow down forest soil carbon emissions in certain periods of time, we cannot ignore the side effects of long-term nitrogen deposition, especially soil acidification, slowed plant growth and reduced biodiversity, which will eventually weaken the carbon sink function of forests.

Conclusion

It is undeniable that human activities and the global environmental changes they cause (global warming, increased nitrogen deposition, drought, extreme climate, etc.) have had a series of impacts on natural ecosystems and exacerbated ecosystem degradation.

Actively advocating nature-based solutions, relying on the power of nature and ecosystem-based approaches, promoting the integrated protection and restoration of mountains, rivers, forests, fields, lakes, grasslands and deserts, improving the quality and stability of ecosystems, and strengthening actions to protect, manage and restore natural and altered ecosystems are important measures to effectively respond to social, economic and environmental challenges in the future.

Nature-based solutions (Image source: Reference [3])

References:

1. Bai Yongxiu, Luneng, Li Shuangyuan. Background, challenges, opportunities and implementation paths of the dual carbon goals. China Economic Review, 2021:10-13.

2. Ning Jing. Chinese version of global standard on nature-based solutions and typical cases of Chinese practice released. China Natural Resources News, 2021:001.

3. IUCN Global Standards for Nature-Based Solutions: A framework for the review, design and promotion of nature-based solutions: First edition.

4. Ackerman, D., Millet, DB, Chen, X. Global estimates of inorganic nitrogen deposition across four decades. Global Biogeochemical Cycles, 2019:100–107.

5. Adoption of the Paris Agreement FCCC/CP/2015/L.9/Rev.1, UNFCC, 2015.

6. Chen, C., Xiao, WY, Chen, HYH Mapping global soil acidification under N deposition. Global Change Biology, 2023:4652-4661.

7. Galloway, JN, Bleeker, A., Erisman, JW The Human Creation and Use of Reactive Nitrogen: A Global and Regional Perspective. Annual Review of Environment and Resources, 2021:255-288.

8. Harris, NL, Gibbs, DA, Baccini, A., et al. Global maps of twenty-first century forest carbon fluxes. Nature Climate Change, 2021:234–240.

9. IPCC Special Report on Climate Change and Land (eds Shukla, PR et al.), IPCC, 2019.

10. Yu, GR, Jia, YL, He, NP, et al. Stabilization of atmospheric nitrogen deposition in China over the past decade. Nature Geoscience, 2019:424–429.

11. Zheng, MH, Zhang, T., Luo, YQ, et al. Temporal patterns of soil carbon emission in tropical forests under long-term nitrogen deposition. Nature Geoscience, 2022:1002-1010.

12. Zheng, MH, Mo, JM Phased variation of soil respiration in tropical forests in response to nitrogen deposition. Nature Geoscience, 2022:965-966.

Produced by: Science Popularization China

Author: Zheng Mianhai (South China Botanical Garden, Chinese Academy of Sciences) Producer: China Science Expo

This article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

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