Soil carbon & water stability: Lessons from the Eastern Himalayas
T R Shankar Raman
India’s climate strategy often looks skyward towards renewable energy targets, carbon markets, and industrial decarbonisation. But a new study from Mizoram, published in Next Sustainability (2025), suggests that some of the country’s most consequential climate decisions may lie closer to the ground—quite literally, in the soils and vegetation of the Eastern Himalayas.
The paper, “Improved land use systems in the Eastern Himalayan region of India (Mizoram): The potential for carbon storage in the ecosystem and soil carbon sequestration”, by Singson Lungmuana et al., offers one of the most comprehensive ecosystem-carbon assessments conducted in India’s hill regions. Its findings challenge conventional assumptions about plantations, agroforestry, and “green” land-use transitions—and carry urgent lessons for climate policy, forest governance, and rural livelihoods.
The carbon question in India’s hill states
The Eastern Himalayan Region is one of India’s ecological paradoxes. It is a global biodiversity hotspot, receives some of the highest rainfall in the world, and yet suffers from severe soil erosion, declining soil fertility, and chronic livelihood vulnerability. Shifting cultivation (jhum), monoculture plantations, and horticulture have all been promoted—often simultaneously—as solutions to poverty, land degradation, and deforestation.
What has been missing, until now, is a rigorous, ecosystem-level accounting of how these land-use choices actually perform in terms of carbon storage—both above ground (trees, litter, shrubs) and below ground (soil carbon down to one metre).
The Mizoram study fills this gap. Researchers compared eight land-use systems—secondary forest, bamboo, teak, rubber, oil palm, areca nut, orange orchards, and leguminous tree-bean agroforestry—measuring biomass carbon, deep soil carbon pools, and total ecosystem carbon stocks. The results are striking.
Forests still matter, but not in simplistic ways
Secondary forests stored the highest overall ecosystem carbon: about 317 Mg of carbon per hectare, combining vegetation and soil. Forests also held the highest above-ground biomass carbon, reinforcing their central role in climate mitigation. But the study goes beyond reaffirming forest superiority. It shows that not all non-forest systems are equal and that some “productive” land uses can approach forest-level carbon storage—while others dramatically underperform.
A 12-year-old bamboo plantation, for example, stored nearly as much ecosystem carbon as a 30-year-old teak plantation and came close to forest values. This finding disrupts the common perception that plantations are inherently carbon-poor substitutes for forests. At the same time, orange orchards and rubber plantations recorded the lowest ecosystem carbon stocks, with orange systems storing barely 55 per cent of the carbon found in forests.
The implication is clear: land-use change is not just about whether land is forested or cultivated, but instead, what kind of cultivation replaces forests.
Soil carbon: the invisible climate asset
Perhaps the most important contribution of the study lies underground. Unlike many earlier assessments that stop at topsoil (15–30 cm), this research measured soil carbon stocks down to one metre, capturing stable, long-term carbon pools that are critical for climate mitigation. Here, a surprising winner emerged: tree-bean agroforestry (Parkia spp.). Despite relatively low above-ground biomass, tree-bean systems recorded the highest deep soil carbon stocks—even exceeding forests in some layers.
Crucially, most of this carbon was stored in non-labile (passive) pools, meaning it is chemically stable and resistant to rapid loss through erosion or decomposition. In climate terms, this is the gold standard of sequestration. By contrast, oil palm and orange systems had a higher proportion of very labile carbon, which is easily lost when land management changes—making these systems risky from a long-term mitigation perspective.
The hidden water story beneath Mizoram’s carbon debate
Although the study is framed around carbon sequestration, its findings point to a deeper and often overlooked truth: carbon-rich soils are also water-regulating soils. Mizoram lies in one of the wettest ecological zones in the world, receiving up to 11,000 mm of rainfall annually. In such landscapes, the central environmental challenge is not rainfall scarcity but runoff, erosion, and the rapid loss of water from hill slopes. The paper shows that land-use systems which deplete soil organic carbon—particularly deep, stable carbon pools—also undermine the soil’s ability to absorb, store, and slowly release water.
Forests and legume-based agroforestry systems, especially tree-bean plantations, store large amounts of carbon in non-labile soil pools down to one metre. These pools are critical for soil aggregation and porosity, allowing rainfall to infiltrate rather than rush downhill as destructive runoff. In contrast, orange orchards and oil palm plantations show lower soil carbon stocks and higher proportions of labile carbon, making them more vulnerable to erosion under intense monsoon rains.
This matters for water security. Reduced infiltration means weaker groundwater recharge, declining spring flows, and greater sediment loads in streams—problems already reported across the Eastern Himalayan region. In this context, the study’s carbon findings double as a warning about watershed degradation driven by inappropriate land-use choices.
By highlighting deep soil carbon as a stabilising force, the research implicitly argues that climate mitigation, soil conservation, and water security in hill regions cannot be treated as separate policy domains. In Mizoram’s steep landscapes, what stores carbon also governs water.
Why legumes punch above their weight
Tree-bean agroforestry works not because it mimics forests, but because it exploits a different ecological mechanism. As a leguminous system, tree beans fix atmospheric nitrogen, stimulate microbial activity, and promote deeper root systems. These processes enhance soil aggregation and push carbon into deeper, more stable layers. The study shows strong correlations between plant height, vegetation carbon, non-labile soil carbon, and total ecosystem carbon. In simple terms: systems that invest in soil biology, not just tree trunks, deliver more durable climate benefits. This finding has major implications for India’s approach to “restoring” degraded hill landscapes.
The plantation paradox
For decades, state governments in the North-east have promoted monoculture plantations like teak, rubber, and oil palmas alternatives to shifting cultivation. These systems were expected to stabilise slopes, increase incomes, and reduce pressure on forests. The Mizoram data complicates this narrative. While teak and bamboo plantations can accumulate substantial biomass carbon over time, most monoculture systems reduce soil carbon stocks by 3–28 per cent compared to forests. Oil palm plantations, in particular, performed poorly due to shallow root systems, low litter input, and wide inter-row spacing that leaves soil exposed to erosion. In carbon terms, this creates a plantation paradox: systems that look “green” from above may be quietly losing carbon below ground, undermining both climate goals and soil health.
Recommendations: what this means for policy
The policy implications of this study are immediate and far-reaching.
First, forest conservation remains non-negotiable. No plantation or agroforestry system fully matches forests in combined vegetation and soil carbon storage. Any climate strategy that allows continued forest conversion—even for “productive” land uses—will incur long-term carbon losses.
Second, when land-use change is unavoidable, legume-based agroforestry should be prioritised, especially on degraded slopes. Tree-bean systems demonstrate that it is possible to combine livelihoods, soil restoration, and stable carbon sequestration—something monoculture plantations largely fail to do.
Third, plantation policy needs differentiation. Bamboo and teak can play a role in restoring ecosystem carbon, but only under conditions of high stand density, minimal soil disturbance, and long rotation periods. Treating all plantations as equivalent climate solutions is scientifically indefensible.
Fourth, oil palm expansion in hill regions demands urgent re-evaluation. Without strict soil-cover management, residue retention, and mixed cropping, oil palm systems risk becoming net carbon losers—especially in high-rainfall, erosion-prone landscapes like Mizoram.
Fifth, India’s climate accounting frameworks—whether under NDCs, carbon markets, or green finance taxonomies—must begin to recognise deep soil carbon. Current methodologies that focus on tree biomass alone systematically undervalue agroforestry and soil-centric systems.
A warning against carbon shortcuts
The broader lesson from Mizoram is a warning against carbon shortcuts. Fast-growing monocultures may deliver quick economic returns or visible “green cover", but climate mitigation is a long game. Systems dominated by labile carbon pools are vulnerable to disturbance, market shifts, and policy reversals. By contrast, carbon stored deep in soils—especially through biologically driven processes—offers durability, resilience, and co-benefits for water retention, fertility, and erosion control. In a warming world where extreme rainfall events are intensifying across the Eastern Himalayas, these co-benefits may matter as much as carbon itself.
The road ahead
The authors rightly caution that their study covers a limited geographical area and a single sampling season. They call for multi-season, landscape-scale assessments across the Eastern Himalayas. But even with these caveats, the evidence is strong enough to demand a shift in how India thinks about land-based climate solutions. The Mizoram study shows that climate mitigation is not just about planting more trees. It is about choosing the right systems, in the right places, for the right reasons. In the hills of North-east India, the future of carbon may depend less on forests alone—and more on what grows beneath them.