[Commentary] Soil organic carbon as an indicator of health of agroecosystems requires qualification

Organic rich soil deposits in the Kole Wetlands of Kerala.

  • Soil Organic Carbon (SOC) as an indicator for land and soil degradation, is becoming central to climate change mitigation attempts.
  • While SOC is a useful measure, it doesn’t fully capture the health of agroecosystems.
  • The SOC indicator needs to be read in conjunction with microbial, biophysical, and biochemical properties of the soil.
  • The views in this commentary are that of the authors.

The year 2022 saw a lot of frenzy around soil in the lead up to the United Nations Climate Change Conference (COP27) and the World Soil Day. The Coalition of Action for Soil Health (CA4SH), a multi-stakeholder organisation to improve global soil health, brought focus to soil at COP27, by organising and co-hosting the BOOST nature positive production and soil health event at the first-ever Food Systems Pavilion at an UN Climate Change Conference. This was followed by releasing a draft Soil Health Resolution by CA4SH and a series of decisions around soil health by the Koronivia Joint Work on Agriculture, established in 2017 to advance discussions on agriculture in the United Nations Framework Convention on Climate Change (UNFCCC). In India, the ISHA foundation, a volunteer-run, spiritual foundation, launched a global campaign ‘Save the Soil’.

The 2022 World Soil Day’s theme, Soils: Where Food Begins, is widely seen as a symbolic culmination of the yearlong effort to place ‘soil’ at the centre of global food and environmental security debates.

Soil Organic Carbon taking centre stage: a cautionary note

The UN climate conference of 2022 also witnessed calls for integrating soil health and specifically soil organic carbon (SOC) in the Nationally Determined Contributions (NDCs). This gains significance, in the background that several IPCC reports have demonstrated the potential of land-based mitigations in the NDCs under the Paris Agreement. SOC pool is one of the most important carbon stocks on the earth which contains approximately twice as much carbon as in the atmosphere. Soils store over 1550 Pg of soil organic carbon (SOC) in the land based ecosystems and are key in the global carbon (C) budget. SOC stock is also proposed as a globally relevant indicator within the monitoring framework for land and soil degradation in the UN Sustainable Development Goals (SDGs).

Among the land-based mitigation measures, often referred to as ‘nature-based solutions’ agriculture and more specifically, SOC management on croplands and grasslands are estimated to have the greatest mitigation potential at USD20 tCO2-eq-1. The ‘4 per 1000’ movement initiated by the French Government in the Paris climate change conference in 2015 emphasises SOC restoration of agricultural soils, as a means for achieving food, environmental and economic security.

Image shows a farm surrounded by trees
Wetland paddy systems of Kerala are considered valuable land use systems, contributing to the ecological health of the state. Photo from MSSRF.

India with 162 million hectares (Mha) of arable land and 8 Mha of permanent crop land which is about 12% and 6% of the respective global figures, has a huge potential for integrating SOC in its national mitigation strategies. This gains relevance, amidst reports of 0.2 to 4% decline in SOC concentration in the predominantly agrarian states of Punjab, Haryana, Uttar Pradesh, Madhya Pradesh, West Bengal and Tamil Nadu. There are already schemes and suggestions for potential strategies for SOC enhancement in India. Soil nutrition and soil management for food security is also attracting R&D and capital expenditure from private sector in India.

In this context, it is important to investigate the relevance of making SOC, as a central indicator in soil health measurements. The underpinnings for ‘centrality’ of this measure comes from recognition of the role of SOC in facilitating ecosystem services. Thus, SOC also gets into debates around ‘payment for ecosystem services’ through a ‘soil-based carbon economy’. Consequently, certain land use management and agricultural practices get promoted as strategies for improving SOC content. However, globally, there is no common framework for measuring ecosystem services of soils. Moreover, different land uses, and farming practices affects the soils in various ways than just influencing the SOC.

Tillage practices, input and labour intensiveness, and intensity of intercultural operations have multiple pathways of impacting on the physical, biological, and chemical properties of the soil, which in turn impacts the overall ecological equilibrium and functionality of soils, which cannot be captured by a single indicator like SOC. Hence, it’s important to have a systematic approach that is broad based on multiplicity of competing and complementary indicators while assessing soil health of agroecosystems. This is further corroborated, through an empirical study on comparative analysis of SOC among alternate land uses, with an aim to establish their ecological viability, in Wayand district of Kerala.

Ecological health of agroecosystems: evidence from land use study

Wetland paddy systems of Kerala are considered valuable land use systems, contributing to ecological health of the state. Soil scientists and ecologists consider wetland paddy to have higher carbon sequestration potential, given the strong aggregate stability of paddy soils. The state has special legislation restricting conversion of wetland paddy to other land uses, despite which large scale conversion of wetland paddy is rampant in the state of Kerala. This is largely attributed to the ecology-economy tradeoffs in conservation strategy, with economics of paddy cultivation paling in front of competing land uses viz. banana and betelnut.

Given this background, a study was undertaken in Wayand district by M.S.Swaminathan Research Foundation with funding from SANDEE, to understand the carbon sequestration potential of wetland paddy and its competing land uses. The study revealed useful insights that need to be accounted for while using SOC as a key or sole measure of soil health of agroecosystems. Soils from banana and betelnut plots, which were relatively highly chemical input intensive systems, reported higher values of SOC compared to wetland paddy. The average SOC measure across the land uses was in the range of 0.81 to 0.90 %, with banana reporting the highest value. Almost 60% of samples across all land uses reported a high SOC percentage.

Soil microbial load is a relatively stable measure of soil health. Soil microbe species diversity and activity are responsible for various ecosystem services. The higher the microbial load in the soil, higher is the aerobic activity and better the soil health and SOC. The results from the soil microbial load analysis were in sharp contrast to the SOC measures. Highest microbial activity was reported in paddy lands followed by betelnut. At 10-7 dilution, the number of bacterial colonies ranges from 31 to 47 in paddy, 20 to 39 in betelnut and 6 to 20 in banana. Fungi colonies were also highest in paddy, followed by betelnut and banana.

The catch in the empirical evidence  

These were puzzling results, that could be answered only if the overall management practices on the land use systems were also accounted for while interpreting data on SOC, in conjuncture with the microbial load. Betelnut is the least disturbed land use and reports high biomass application. The per acre manure addition in banana is in the range of 1 to 8 tonnes, comprising of a mixture of cattle, pig, poultry, and bone meal. The banana suckers are also left to decompose and ploughed back to the field. In contrast, just cattle manure and green leaves are added to paddy lands.

On the flip side, banana has the highest use of synthetic fertiliser and pesticide, compared to wetland paddy or betelnut. The fertilizer application was three to seven times higher, and pesticide application was two to three times higher than in betelnut and wetland paddy respectively. Further analysis, incorporating synthetic input use, established the adverse effect of pesticide use on SOC. The excessive use of synthetic inputs has fallouts in terms of affecting conservation of soil biota, which are key determinants of ecological health of agroecosystems. The results of the soil microbial load analysis across land uses must be read along with this.

The reason banana plots reported the lowest number of microbial colonies despite high SOC values, is attributable to the high level of pesticide use.  Hence, from an ecological perspective, though banana reports high SOC, unlike paddy, banana, is not conducive for sustenance of soil biota. Soil biota plays a major role in supporting key ecological functions of agroecosystems. This has implications not just in terms of beneficial biodiversity loss, but also in terms of long-term food security, through productivity decline attributable to poor soil health.

The results of the empirical study point to the risk of myopic interpretation of ecological health of agroecosystems, solely based on SOC measures. The SOC indicator needs to be read in conjunction with microbial, biophysical, and biochemical properties of the soil, which are key to determining health of agroecosystems. Otherwise, nations run the risk of promoting agroecosystems and farming practices, that are both organic and inorganic external input intensive, with the ill effects of excessive use of inorganic inputs negating the SOC enhancing potential of the organic inputs, in the long run. These intensive agroecosystems, if promoted as carbon sinks, as part of the mitigation strategies can lead to defeating the very idea of land-based solutions to SOC sequestration. The results of the analysis are also in line with arguments put forward by several soil scientists and ecologists, pushing for evolving a comprehensive framework for measuring soil health.


Banner image: Organic rich soil deposits in the Kole Wetlands of Kerala. Photo by Manoj Karingamadathil/Wikimedia Commons.

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