By Lydia Messling
The IPCC’s recent report on climate change and land, highlighted the pressing need for changes to be made to land management practice. Land degradation and climate change threaten to reduce food production and lead to a 25% food production deficit by mid-century. Globally, soil biodiversity has been estimated to annually contribute between US$ 1.5 and 13 trillion to the value of ecosystems services. Despite this, soil biodiversity is often overlooked in policy. This neglect poses a serious threat to food security.
Producing sufficient food to feed a growing population, relies upon being able to grow healthy crops that survive until harvest season after season. To a large extent, this depends on the health of the thin 30-40cm layer of topsoil that plants grow in. Measuring the soil organic carbon (SOC) of this soil provides an indicator of the soil’s health. When soils reach low levels SOC they can tip past the point where they have any hope of being restored, resulting in devastating irreversible degradation of the land. Poor land management practice, soil erosion, and other land degradation processes can reduce soil health. Climate change and its impacts act as an additional stress factor.
Just as poorly managed soils can exacerbate climate change and reduce food security, healthy soils can have the opposite effect. Sequestering carbon in the soil can help reduce greenhouse gas emissions, as well as improving the soil’s resilience to extreme weather events and also increase crop yields.
How does climate change affect soil health and what can be done?
Through changes in average temperatures, more frequent and intense extreme events, and other factors, climate change can affect soil structure, stability, topsoil water holding capacity, nutrient availability and erosion. The IPCC’s report on land lists many interconnected processes that affect degradation processes, but climate change also directly effects salinization, and permafrost thawing, waterlogging of dry ecosystems and drying of ecosystems, and a broad group of biologically mediated processes like woody encroachment, biological invasions, pest outbreaks, together with biological soil crust destruction and increased burning.
For example, prolonged dry seasons can dry out the soil, causing the organisms in the soil to die and for nutrients to be lost. Similarly, prolonged wet seasons can inundate the soil and wash away the nutrients needed for plant growth, as well as eroding and removing the soil. One of the ways that soil can be made more resilient to climate change is by increasing the soil’s organic matter.
What are soil organic carbon (SOC) and soil organic matter (SOM)?
Soil organic matter (SOM) is divided into ‘living’ and ‘dead’ components, such as roots and microorganisms and decaying plants and animals. SOM contains all sorts of elements such as carbon, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium, and also determines how much water the soil can contain – all important elements for plant growth. SOM is quite hard to measure though, so measurements are taken of Soil Organic Carbon (SOC) instead. About 58% of the mass of organic matter exists as carbon (depending on geography), so the percentage of SOM can be calculated from the SOC measurement. Therefore, a decrease in SOC means a decrease in SOM.
Why is SOM important?
Besides providing crucial plant nutrition, SOM provides soil with its structure. This structure is the reason healthy soil doesn’t just get blown away in the wind, and how plants can spread their roots to remain stable. Soil structure is also important for being able to hold moisture whilst not waterlogging plants and retain nutrients, preventing them from being washed away completely in heavy rains. Land that has been overworked and lost much of its SOM content will have poor soil structure. Farmers may use chemicals to replace the nutrients that have been lost from a lack of SOM but will find it difficult to provide the soil structure that is needed for plants to grow and be resilient to climate impacts.
How can SOM be lost, and how can it be replaced?
Agriculturalists have already been adopting different methods to increase SOM content and to improve soil health, as loss of SOM is also related to intensive farming practices. As such, many methods exist for increasing SOM in soils, many of which are easy to deploy but are yet to see the rapid adoption. These include reducing how often soil is tilled, erosion control measures, soil mulching, maintaining ground cover, rotating crops, using different crop breeds, careful timing of grazing, and diversifying plants by including trees and shrubs amongst the crops.
All of these measures seek to increase and preserve the amount of SOM in the soils. As such, they improve fertility rates, make the soil more resilient to weather events, and secure the food supply by increasing the likelihood of a good harvest year after year. Soils also have the potential to be an important carbon sink. The 4 per 1000 initiative, launched at the Paris COP in 2015, champions increasing SOC content as a climate mitigation measure. A theoretical increase of just 0.4% of the world’s SOC would be greater than the increase in atmospheric CO2 experienced in 2015.