Modelling soil carbon storage in grasslands through the LEAP guidelines and the GLEAM model


Managed grasslands and rangelands represent around 70 percent of the global agricultural area. The soils of these agroecosystems contain nearly 20 percent of the world’s soil organic carbon (SOC) stocks, which implies that they play a significant role in the global carbon and water.

Changes in SOC levels are relevant to the environmental performance assessment of livestock product systems, primarily due to its effects on the balance of greenhouse gas (GHG) emissions in the system, which affects climate change impacts. Moreover, SOC is an indicator of soil quality, reflecting its ability to provide ecosystem services, such as biotic production and climate change mitigation.

In the past months, LEAP has worked closely with FAO and INRAE to incorporate an additional component in the Global Livestock Environmental Assessment Model (GLEAM), a modelling framework to estimate livestock and environment interactions such as GHG emissions, nitrogen use, water, soil carbon, and biodiversity.

The LEAP guidelines for SOC assessment describe the approaches to measure and model SOC stock and  stock changes in livestock production systems. For this purpose, three modelling approaches have been recommended: empirical models (Level 1), soil models (Level 2), and ecosystem models (Level 3).

On the other end, GLEAM is a process-based model based on a Life Cycle Assessment (LCA) framework. It covers 11 main livestock commodities on a global scale and provides spatially disaggregated estimates on GHG emissions and commodity by production system.

To include estimates of changes in SOC stocks in LCA assessments, the LEAP guidelines for SOC assessment recommends using an adequate model (Level 2 or 3) to model SOC after a land management change. While for large scale soil carbon changes occurring after a land-use change, or when a study context does not allow specific data collection, model Level 1 could be used to provide a first estimate of the expected SOC change direction or amplitude.

These recommendations set the benchmark for the development of a framework to include SOC estimates into the GLEAM model. The tier-approach framework that GLEAM is following for assessing SOC and global and national scale can be summarized as follows:

  • Tier 1 approach (Level 1), preliminary global SOC estimates for business-as-usual (BAU) scenario, and best-management-practices (BMP) scenarios on global grasslands.
  • Tier 2 approach (Level 2) tailored global and national SOC estimates for BAU scenario.

The RothC soil model, which is at the core of the new GLEAM soil carbon module, has been selected as it is one of the most common Level 2 models used in livestock systems. The soil module directly links the manure module and the allocation module in the GLEAM structure. The function of the manure module is to calculate the losses of nitrogen through manure management and the rate at which excreted nitrogen is applied and deposited in feed crops’ fields and pastures. The deposited nitrogen is then converted into carbon and used as input in the soil module. Plant residues, both from above- and below-ground, are also used as input to the model, together with soil properties and climatic conditions.  The soil module calculates the C stocks, which is then used to estimate the total emissions (including sequestration) of the livestock production systems.

One of the most contentious issues with the inclusion of SOC stock changes in LCA is the temporal aspect.  In livestock production systems, a reference of one year is usually sufficient to account for temporal variations. However, when agricultural practices change, SOC levels will change accordingly. This change is usually rapid soon after the introduction of the new practice and eventually stabilizes when a new equilibrium is approximated. Since this process varies depending on climate, soil type, agricultural practice or initial SOC levels, it is challenging to give an approximate indication of a representative time interval. An approach used in other recommendations (such as IPCC) is to allocate the benefi­ts of carbon sequestration (or the burden of carbon emissions) throughout a fi­xed period of 20 years. However, carbon stocks may not reach equilibrium after 20 years. If SOC is included in LCA estimates, the LEAP guidelines suggest that time perspective should correspond to the most commonly used one for the global warming potential in LCAs, which is 100 years.

At present, changes in SOC stock of grasslands supporting livestock production have been prioritized. However, other LU and LUC directly or indirectly linked to livestock production (e.g., soybean) should be included in the evaluation, so that the sector’s impact can be estimated comprehensively. Therefore, the expected future steps in the inclusion of SOC stock changes in GLEAM are the following:

  • Tier 2 approach (Level 2), global SOC estimates for BMP grassland scenarios.
  • Tier 2 approach (Level), tailored national SOC estimates for BMP grassland scenarios (e.g., national projects).
  • Tier 2 approach (Level 2 model), global SOC estimates for BAU cropland scenarios
  • Tier 2 approach (Level 2 model), global SOC estimates for BMP cropland scenarios.