Soil organic matter is important for maintaining a healthy environment for plants and soil micro-organisms. Its main constituent, soil organic carbon, plays a vital role in removing CO2 from the atmosphere.
Importance of Soil Organic Carbon
The carbon cycle is a fundamental part of life on earth. ‘Soil organic carbon’ (SOC) – the amount of carbon stored in the soil is a component of soil organic matter – plant and animal materials in the soil that are in various stages of decay.
Soil organic carbon is the basis of soil fertility. It releases nutrients for plant growth, promotes the structure, biological and physical health of soil, and is a buffer against harmful substances.
Soil organic carbon is part of the natural carbon cycle, and the world’s soils holds around twice the amount of carbon that is found in the atmosphere and in vegetation. Organic material is manufactured by plants using carbon dioxide from the air and water. Plants (and animals, as part of the food chain), die and return to the soil where they are decomposed and recycled. Minerals are released into the soil and carbon dioxide is released into the atmosphere.
Soil organic carbon accounts for less than 5% on average of the mass of upper soil layers, and diminishes with depth. According to the CSIRO, in rain-forests or good soils, soil organic carbon can be greater than 10%, while in poorer or heavily exploited soils, levels are likely to be less than 1%.
While the agricultural sector has the ability to impact the carbon cycle on a large scale, often through the release of carbon, farmers have a vested interest in retaining and increasing soil organic carbon for individual fields because soil and yield tend to improve when the soil organiccarbon level increases. Higher soil organic carbon promotes soil structure or tilth meaning there is greater physical stability. This improves soil aeration (oxygen in the soil) and water drainage and retention, and reduces the risk of erosion and nutrient leaching. Soil organic carbon is also important to chemical composition and biological productivity, including fertility and nutrient holding capacity of a field. As carbon stores in the soil increase, carbon is “sequestered”, and the risk of loss of other nutrients through erosion and leaching is reduced. An increase in soil organic carbon typically results in a more stable carbon cycle and enhanced overall agricultural productivity, while physical disturbances of the soil can lead to a net loss of carbon into the surrounding environment due to formation of carbon dioxide (CO2).
Soil Organic Carbon as a Component of Soil Organic Matter
The term SOM is used to describe the organic constituents in soil in various stages of decomposition such as tissues from dead plants and animals, materials less than 2mm in size, and soil organisms. SOM turnover plays a crucial role in soil ecosystem functioning and global warming. SOM is critical for the stabilisation of soil structure, retention and release of plant nutrients and maintenance of water-holding capacity, thus making it a key indicator not only for agricultural productivity, but also environmental resilience. The decomposition of SOM further releases mineral nutrients, thereby making them available for plant growth, while better plant growth and higher productivity contribute to ensuring food security.
SOC and Biodiversity
Soil biodiversity reflects the mix of living organisms in the soil. These organisms interact with one another, as well as with plants and small animals, forming a web of biological activity. On the one hand, soil biodiversity contributes greatly to the formation of SOM from organic litter, thereby contributing to the enhancement of SOC content. On the other hand, the amount and quality of SOM (and consequently SOC) determines the number and activity of soil biota that interact with plant roots. Therefore, the soil microbial community structure is influenced largely by the quality and quantity of SOC and to a lesser extent by plant diversity.
Increasing organic C in croplands
Cropland soils generally store less SOC than grassland because cropland has greater disturbance from cultivation, a lack of organic manure being returned to the system, has a winter fallow period and, as a consequence, has less root and shoot material returned to the soil. Changes in SOM/SOC are not linear and reach a new equilibrium over time (Figure 1). In other words, accumulation of SOM/SOC is finite. Some examples of management options to increase organic carbon C in croplands are:
- Cover crops/rotations
Crop rotations can include cover crops, perennial grasses and legumes that maximise soil C inputs and maintain a high proportion of active C.
- Straw and manure incorporation
Straw incorporation increases SOC, as organic matter is directly inputted back into the soil. Figure 1 shows that for 4t straw incorporated over 20 years, a 7-17% increase in SOC (top 15cm only) has been observed (depending on whether reduced tillage was also applied). Manure inputs will also build SOC stocks, particularly farmyard manure
- Reduced/minimum tillage
The concept of reduced tillage is that aggregates are disrupted less, leading to reduced SOC loss. However, while SOC levels in the top 30cm are increased, there is increasing evidence that ploughing may simply redistribute SOC over a greater depth profile.
Increasing organic carbon in grassland
Soil quality in grasslands could be improved by achieving a ‘right’ balance between C and N inputs to soils. A combination of agricultural practices that promote the formation of stable soil aggregates will improve soil quality and sustainability. Some management options include:
- In permanent grasslands (>5yrs) a key step is to improve either organic or inorganic fertiliser management. A first step would be to combine liming treatments with either organic and/or inorganic nutrient fertilisation (N, P, K, Mg, etc.). In terms of temporary sown grasslands and renovation via ploughing, a key step is to increase the time between re-seeding to at least five years, as this will contribute to an organic matter build-up though reduced tillage events.
- Increasing the abundance of legume species in some grass swards can improve sequestration and forage quality, and reduce inorganic N inputs. In combination with legumes, a more diverse vegetation cover (>4 species) can make grasslands more resilient in terms of climate change, and may provide both a better forage quality and organic matter input.
- A third step is to reduce frequency of use of heavy machinery, which could cause high soil compaction and thus ‘reduce’ pore space available in the soil matrix, which is necessary to transport and accumulate extra C (via soil climate, macro fauna, earthworms, microbes, etc.). Animal grazing ispreferable to silage/hay production, due to the nutrient recycling of animals and the reduction in work (25-40% of ingested herbage is returned to the pasture in excreta).
- Finally, the development of pasture management plans, perhaps around a five- to seven-year cycle, where a combination of different practices (liming, nutrients, grazing, reseeding) guarantee balanced applications of C and N to soils under moderate (soil) disturbance (avoid high animal stock densities and intensive mowing). A soil monitoring programme including analyses of soil C and N content, soil bulk density and pH should be put in place and run every two to three years.
Article Written By:
Prof. S. B. Dahiphale
Assistant Professor of Soil Science &Agril.Chemistry
College of Agriculture, Newasa Maharashtra, India
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