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A balancing act: nutrient enrichment and soil carbon storage

Logo https://crowtherlab.pageflow.io/a-balancing-act-nutrient-enrichment-and-soil-carbon-storage

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The Earth’s soil is crucial for regulating the carbon cycle and climate; it stores the vast majority of terrestrial carbon and approximately twice as much carbon as the atmosphere. The amount of carbon is defined by the balance between the carbon uptake by plants and the release through, for example, the respiration by soil organisms.

As temperatures rise, microbial activity in the soil increases, leading to more respiration and accelerated loss of carbon into the atmosphere. This effect can accelerate climate change by up to 17 percent. However, scientists think that these losses could potentially be offset by global soil nutrient enrichment, frequently used in agriculture, for example, that could stimulate plant growth around the world.   

Understanding how the human production of fossil fuels and farming fertilizers affects the fragile balance between carbon uptake and release is crucial to understanding the rate of climate change.
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Over the past century, fossil fuels and farming fertilizers have increased the amount of nitrogen, phosphorus and potassium (‘nutrients’) in soils, not only in agricultural areas, but all across the globe. It is generally expected that these increased levels of nutrients should stimulate plant growth through ‘nutrient enrichment’, and therefore enhance carbon capture from the atmosphere. However, until recently, there was no scientific evidence to support this, as the effects of nutrient enrichment on the soil carbon balance across the globe had not been previously evaluated.  

In collaboration with the Nutrient Network, a unique project that brings together ecologists worldwide, the Crowther Lab analyzed the changes in soil carbon storage. For this, we compared control plots with plots that were enriched with nutrients to see if more carbon had accumulated over a 2 to 4-year period. In a second step, we used big-data modelling approaches to understand the particular environments that experienced gains or losses in carbon under nutrient enrichment

This approach will not only indicate which ecosystems are able to store more carbon under nutrient enrichment and which lose carbon, but also helps us improve Earth system models that allow us to predict the rate of climate change.  
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As expected, our research shows that, in many parts of the world, plant growth in grasslands is stimulated by soil nutrient enrichment.  At the same time, it’s likely that decomposition slows down as well, leading to the build-up of carbon in the soil. However, our findings reveal two important considerations:  

1. Nutrient mix: In isolation, adding nitrogen and phosphorus had a minimal impact on the amount of carbon stored in the soil. Only in combination with potassium did the soil carbon potential increase significantly.  

2. Region specific: This positive effect only happens in dry and sandy regions, where there is less rain and vegetation. In high-latitude regions like Northern Scandinavia, Siberia, Alaska or Canada the opposite effect holds true as nutrient enrichment actually leads to the loss of carbon from those soils.
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The global patterns in these effects put all of the findings into context. Given that the vast majority of the Earth’s carbon is stored in the soil in these high-latitude regions, these losses suggest that nutrient enrichment may in fact enhance the release of soil carbon into the atmosphere. That means that anthropogenic nutrient enrichment can reinforce the feedback loop of soil carbon loss and accelerated climate change. Yearly, this accounts for 1.5 Gigatons of extra carbon emitted into the atmosphere.  

Our study provides important mechanistic insights into the sensitivity of carbon stocks to nutrient enrichment. With this knowledge Earth systems models can be improved, enabling us to better predict the dynamics of climate change and the impact of negative feedback loops.
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Want to learn more? Read the full paper "Sensitivity of global soil carbon stocks to combined nutrient enrichment" published in Ecology Letters here: 

https://onlinelibrary.wiley.com/doi/abs/10.1111/ele.13258

Want to learn more about how soil carbon loss can accelerate climate change? Read the full paper "Quantifying global soil carbon losses in response to warming" published in Nature here: 

https://www.nature.com/articles/nature20150
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