From salt to carbon sinks
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Across large stretches of western and southern India in parts of Maharashtra and Gujarat, black soils that once supported thriving sugarcane fields are slowly failing. Farmers added more water to secure crops, but over time the water had nowhere to go. Groundwater rose, salts collected near plant roots, and fields turned patchy and unproductive. In some areas, yields that once touched 200 tonnes per hectare fell sharply, pushing farmers into loss.
Yet beneath these degraded fields, an unlikely solution is at work. Buried quietly below the soil surface, networks of perforated pipes, known as subsurface drainage systems, are doing what surface interventions could not. By steadily removing excess saline water from below the root zone, they are transforming the physical, chemical, and biological health of soils.
New research by Raj Mukhopadhyay et al. titled, ‘Sub-surface drainage: A win-win technology for achieving carbon neutrality and land amelioration in salt-affected Vertisols of India’ now shows that this transformation goes far beyond yield recovery. When operated over several years, these drains are also rebuilding soil carbon stocks and creating a powerful, if largely unrecognised, climate mitigation opportunity.
The study focuses on two villages in Kolhapur district of Maharashtra, Kavathesar and Shedshal, where subsurface drainage systems were installed in waterlogged saline Vertisols and operated for three and five years, respectively. Before drainage, both sites suffered from shallow saline groundwater, poor soil structure, and high electrical conductivity values that severely restricted crop growth. Shedshal, in particular, represented an extreme case, with soil salinity levels exceeding thresholds at which most crops fail to survive.
How underground drainage lowers salts and resets soil health
The logic of subsurface drainage is straightforward. A network of underground lateral pipes, laid at depths of around 1.1 to 1.3 metres and spaced roughly 20 metres apart, intercepts saline groundwater and channels it away from fields. This lowers the water table, prevents capillary rise of salts, and allows fresh irrigation or rainfall to leach salts deeper into the soil profile, where they are captured and removed by the drainage system. What is less straightforward and far more consequential is how this physical intervention sets off a cascade of changes in soil processes.
Within just a few years of continuous operation, the most visible change is the sharp reduction in soil salinity. At Kavathesar, surface soil salinity fell from strongly saline levels to near-safe thresholds for crop growth. At Shedshal, where salinity was far higher to begin with, the decline was slower but still substantial. Across the full soil profile down to 1.2 metres, salts were consistently lower in drained fields than in adjacent undrained controls. This alone explains much of the observed recovery in crop performance, but it is only part of the story.
When salt retreats, carbon and soil life return
As salinity declined, soil organic carbon began to rise. In the drained plots, farmers returned to cultivating sugarcane, a crop that produces large amounts of above- and below-ground biomass. Crop residues, root turnover, and the widespread practice of green manuring with sunn hemp added fresh organic matter to soils that had long been biologically inactive. In Shedshal, where initial organic carbon levels were extremely low, surface soil carbon more than doubled after five years of drainage. Even at depth, carbon stocks increased, indicating that improvements were not confined to the plough layer.
This accumulation of soil carbon translated into measurable gains in carbon sequestration potential. When calculated across the full 1.2-metre soil profile, the drained fields at Shedshal were sequestering several tonnes of carbon per hectare per year, far higher than at Kavathesar, where drainage had been in place for a shorter period. Importantly, the study shows that carbon sequestration had not yet reached saturation, suggesting that these soils could continue absorbing carbon if drainage and crop management are sustained.
Biological activity followed a similar trajectory. Enzymes such as dehydrogenase and alkaline phosphatase, indicators of microbial respiration and nutrient cycling, were markedly higher in drained soils. In Shedshal, dehydrogenase activity in surface soils increased more than fourfold compared to undrained fields. This resurgence of microbial life reflects a soil environment that is no longer hostile: lower salt stress, better aeration, and a steady supply of organic substrates. Such biological recovery is critical, because it underpins nutrient availability, aggregate formation, and long-term soil resilience.
Physical properties of the soil improved as well. Bulk density declined, particularly in the surface layers, indicating reduced compaction and better structure. Saturated hydraulic conductivity increased, meaning that water could move more freely through the soil matrix. These changes are especially significant in Vertisols, whose high clay content and shrink-swell behaviour often impede drainage. By replacing sodium with calcium on soil exchange sites and encouraging aggregation through organic inputs, subsurface drainage gradually reverses the structural damage caused by prolonged waterlogging and salinity.
Yields bounce back after years of decline
To integrate these diverse improvements into a single measure of soil health, the researchers developed a soil quality index based on key indicators such as salinity, organic carbon, bulk density, and nutrient availability. The results are striking. At Shedshal, the soil quality index of the surface layer increased more than threefold after five years of drainage. Kavathesar also showed substantial gains, though less dramatic due to its lower initial degradation and shorter drainage history. In both cases, better soil quality was strongly and positively correlated with sugarcane yield.
The yield response itself underscores the scale of transformation. At Kavathesar, average sugarcane yields rose from around 36 tonnes per hectare before drainage to more than 117 tonnes per hectare after three years. At Shedshal, where cultivation had nearly collapsed, yields increased from just over 8 tonnes per hectare to more than 55 tonnes per hectare after five years. These gains represent not incremental improvements, but a fundamental reversal of land degradation.
How farmers made drainage work together
Equally important is how these drainage systems were implemented and managed. In Maharashtra, subsurface drainage has largely emerged as an autonomous, community-driven adaptation. Farmers formed groups, worked with cooperative sugar mills, and pooled resources to install and maintain the systems. This collective approach reduced individual risk and facilitated knowledge exchange, while aligning incentives across stakeholders who depend on sustained sugarcane production. Rather than a top-down engineering fix, drainage became embedded in local institutional arrangements.
The broader implications of these findings extend well beyond Kolhapur district. India has nearly three million hectares of saline soils, a significant share of which lies in Vertisol regions prone to waterlogging. To date, only a small fraction has been reclaimed using subsurface drainage. Yet the evidence suggests that, when implemented at scale and combined with appropriate crop and nutrient management, drainage can deliver multiple benefits simultaneously: restoring land productivity, increasing farm incomes, rebuilding soil carbon, and contributing to climate mitigation.
From a policy perspective, this positions subsurface drainage as more than an agronomic intervention. It becomes a tool for achieving land degradation neutrality, supporting national commitments under the Sustainable Development Goals, and advancing climate objectives through enhanced soil carbon sequestration. The study also highlights important trade-offs. Nutrient losses—particularly nitrogen and phosphorus—can occur through drainage water, underscoring the need for integrated nutrient management and monitoring.
Perhaps the most compelling insight is that degraded soils are not irreversibly lost. Even in heavily salt-affected Vertisols, biological and chemical functions can rebound when hydrological constraints are addressed. The pipes buried beneath these fields are invisible to most observers, but their effects ripple upward—through roots, microbes, crops, and livelihoods. In a country grappling simultaneously with land degradation, climate change, and rural distress, such quiet transformations may prove indispensable.
Citations:
Raj Mukhopadhyay, Ram Kishor Fagodiya, Kailash Prajapat, Bhaskar Narjary, Satyendra Kumar, Ranjay K. Singh, Devendra Singh Bundela, Arijit Barman, Sub-surface drainage: A win-win technology for achieving carbon neutrality and land amelioration in salt-affected Vertisols of India, Geoderma Regional, Volume 35, 2023, e00708, ISSN 2352-0094, https://doi.org/10.1016/j.geodrs.2023.e00708.