Carbon Stock
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Increase grassland carbon stored below ground
Data collection in progress
Kakheti Steppes
Georgia
Problem: Combined pressures in this landscape such as overgrazing, increasing temperatures and less rainfall leading to erosion and desertification, are contributing to poor soil health, and lower grassland plant cover and biomass. Grassland plants take carbon in the air and sequester it in the soil, so the fewer plants, the less carbon sequestration is occurring in this grassland system.
Action: SABUKO is working with farmers and shepherds to increase their participation in rotational grazing, which means that portions of the pastures will be left ungrazed at any given time to rest and regain biomass (vegetation cover) and reduce erosion.
Indicator: If sites show an increase the amount of carbon stored below ground over time, it will indicate a recovery of this function within this habitat.
Methods
In order measure below-ground carbon, SABUKO collected samples from 52 grazed grassland locations across a range of 16 sites with different soil profiles and grass cover conditions. Covering varying conditions allows for SABUKO to understand the distribution of organic carbon within the project area. At each sampling point, multiple samples were taken to capture samples at various depths, or soil horizons, because each horizon has distinct physical and chemical properties. Sampling includes digging to expose multiple soil horizons, identifying and labelling each horizon, collecting and labelling samples from each horizon, and assessing the surface litter and grass plant density above the sampling point, as an indicator of soil fertility (see photos below). This methodology follows established guidelines from the WRB and FAO (Blum et al., 2017; IUSS Working Group WRB, 2018). This process resulted in a total of 329 samples from 52 plots, which were sent to a laboratory for analysis.
An example of a soil profile from a semi-desert plot. Photo: Nika Marsagishvili.
An example of a soil profile with a relatively high organic carbon stock. Photo: Nika Marsagishvili.
Sample bags show soil (left) and above-ground litter (right). These are carefully labelled by plot name and horizon and provided to the lab, which will return the amount of carbon in each sample. Photo: Nika Marsagishvili.
To calculate Soil Organic Carbon (SOC) per hectare, soil samples were analysed for carbon content using laboratory methods like dry combustion. The SOC content was measured per unit weight of soil, and the soil’s bulk density was also determined. SOC was then scaled to a per-area basis by integrating carbon values over the sampled soil depth and applying the formula that includes bulk density and depth. This resulted in SOC per hectare, which represents the total organic carbon stored in a hectare of land, as typically shown in graphs for such studies.
After this baseline data was collected, a subset of these plots will be set aside for rotational grazing (treatment plots), while other plots will continue to be grazed with overstocking rates (control plots). SABUKO will revisit these soil analysis in future years to see how carbon storage differs in plots with sustainable grazing, versus plots with overgrazing.
Nikoloz Marsagishvili collecting soil samples across various grassland sites to measure soil organic carbon, providing critical baseline data to assess how grazing practices can enhance carbon sequestration and improve soil health. Photo: Nikoloz Marsagishvili.
Results
The baseline data presented here (Fig. 1) show a summary of the soil organic carbon per hectare for each of the 16 sites sampled.
This baseline data highlights a range of SOC conditions, including areas with low SOC and declining soil fertility, as well as areas with high SOC and therefore high carbon storage. This baseline data will allow SABUKO to target areas where restoration is a high priority. Implementing rotational grazing and revisiting these variables in several years will provide insights into how carbon storage can be altered with the implementation of sustainable rotational grazing practices.
Figure 1: Boxplots showing average carbon stock in kg per hectare measured at each site.
References
Blum, W. E. H., Schad, P., & Nortcliff, S. (2017). Essentials of Soil Science: Soil formation, functions, use and classification (World Reference Base, WRB). Gebr. Borntraeger Science Publishers.
IUSS Working Group WRB. (2018). World Reference Base for Soil Resources 2015, update 2015. International soil classification system for naming soils and creating legends for soil maps. World Soil Resources Reports No 106. FAO, Rome.
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