Part of my project is to investigate the influence of tree species on the composition of soil organic matter in the soil. On the one hand, tree species influence soil properties, for example the soil becomes more acid when coniferous trees (the ones with the needles) replace deciduous trees (also called broad-leafed trees). On the other hand, the fact that their needles and leaves have different properties can potentially change the way carbon is stored in the soil.
On the page about carbon sequestration you can find more information about the different theories concerning the role of the properties of plant material. One of these theories has been around for a long time but has been rejected by many researchers in the last years. It says that microorganisms do not want to eat the organic matter which requires a lot of energy to decompose. Most of the carbon stored in the soil is contained in organic matter and if this organic matter is not decomposed, we could think that this is beneficial for storing carbon in the long term because less decomposition means at the same time less respiration from soil organisms, thus less CO2 leaving the soil. Based on this, we could say that planting plants with low quality litter (meaning that decomposition requires more energy) is the way to go. The plant material would stay undecomposed in the upper layer of the soil and we could increase the amount of carbon put into the soil while decreasing the CO2 emission from the soil.
However, according to current research, this would only be a very short term solution. The undecomposed organic matter is less stable in the soil than organic matter which is hidden in aggregates or sticks to soil minerals. This so-called mineral associated organic matter can persist in the soil for longer and is not as sensible to external stress. Recent studies suggest that this organic matter mainly comes from microbial necromass (Lavallee et al., 2020). This means that microbes in the soil first eat the organic matter, die and their remains are incorporated into the soil matrix. The more litter the microbes eat, the more microbial residues can be added to the mineral associated organic matter. As this organic matter fraction is more stable, we could assume that in the long term, the carbon content in the soil increases.
Thus, it would actually be better to provide microbes with nutrients they can decompose faster (Cotrufo et al. 2013). Nevertheless, I am questioning myself how we can now ensure that we increase the total organic carbon stored in the soil? If we improve litter quality, the microbial activity would probably increase and more mineral associated organic matter can built up. But this organic matter pool can be full when no surfaces are left where the organic matter can stick on to. Consequently, the increase in stable organic matter in the soil is limited and we might have to rely on the more labile pool if the carbon increase should continue. Unfortunately, addition of nutrients to the soil can boost microbial activity in a way that they start to decompose the previously stabilized material as well (Sokol et al., 2019).
root vs leaves
How much of this organic matter is in the end created however also depends on soil properties. Organic matter needs a surface to stick onto. If there are no surfaces left anymore, the capacity of the soil to increase its carbon content might be decreased and there could be an increase in CO2 emissions again.
Furthermore, the microbes sometimes can still access the organic matter associated to minerals. This organic matter is often more nutritious and can potentially boost microbial activity, leading to higher CO2 emissions. High microbial activity and diversity, I would think, can increase the potential of soil organisms to decompose the organic matter which is not as easily accessible.
While I think that litter quality plays an important role, this might not necessarily be the case and soil properties and microbial community are more relevant. Some soils have a higher carbon uptake capacity than other soils and the high heterogeneity in a soil can lead to inaccessibility of organic matter, regardless its quality. Furthermore, it depends on the microbial community which organic matter it decomposes better. Soil properties and communities might also interact as for example microorganisms are influenced by soil pH and ecosystem engineers such as earthworms can shape the structure of the soil.