Soil carbon sequestration means getting CO2 out of the atmosphere, via plants, into soils where the C is locked-up in SOM. Increasing the amount of SOC by 15% globally would be the equivalent of sequestering all the greenhouse gases emitted since the onset of the industrial revolution. Also, SOM is vital for soil health and fertility - it is the most frequently reported characteristic in long-term studies on soils – so soil carbon sequestration is truly a win-win situation: good for soils, good for the atmosphere, and all-up a highly attractive part of the solution to climate change.
The current methods for increasing SOC stocks rely on changing agricultural techniques (land management) or land-use (e.g. reforestation of agricultural land). They are reliable but have a few associated problems: firstly, changing agricultural techniques often involves using other, disagreeable methods. E.g. no-till agriculture increases aggregation of soil particles, increasing SOC and reducing erosion, but this often means using more herbicides to prevent weeds, and herbicides themselves have many associated environmental problems. Planting trees is a great way to increase SOC stocks, but as the world population grows the land needed for agriculture, industry and urban areas will increase, not decrease, making conversion of land to forests an unrealistic option.
A further problem with locking C into soils is that the current strategies are only for the short-term. If a no-till farmer sells up and the new farmer decides to plough the soil, then the previously sequestered carbon (due to no-till practices) will be lost from the soils. Subsequent logging of a reforested area will lead to loss of SOC… This is because there are numerous mechanisms by which carbon is stored in soils, and only two have been identified as offering long-term (in the order of millennia) protection for C in soils. They are mineral-association – where SOC binds to the fine minerals (clays/silt) in soils, reducing accessibility to microbes and hence increasing its retention time in soil. The other is chemical recalcitrance, which means that the chemical structure of the SOM makes it hard for microbes to metabolise, resulting in a lower turnover. Currently, scientists are still undecided as to the relative relevance of these mechanisms and the verdict is still out as to which is decisive in the long-term stabilisation of SOC.
Lastly, there is still uncertainty as to the response of SOC storage to climate change. On the one hand, higher CO2 levels may act as a plant fertiliser, increasing the input of organic matter into soils. On the other hand, higher temperatures mean higher microbial activity, increasing SOM turnover and the loss of C from soils to the atmosphere and groundwaters. Deciphering the effects of climate change on SOC stocks – in particular on the long-term storage of C in soils – will pave the way for strategies to lock C into soils for the long-term.
I’m currently doing a PhD on the turnover and storage of SOC above caves. Caves are environmental archives, used to reconstruct past climate scenarios. SOC turnover affects the chemistry of caves but the association between SOC dynamics and cave chemistry hasn’t yet been established. My research will hopefully provide us with a key to understanding the links between soil C storage and past climate change, helping us to come up with strategies to lock more C into soils for the long-term.
Sorry if this was just long and boring (for about 99% of you this will be the case, I’m guessing). You didn’t have to read it…
