Monsoon Moisture, Not Heat, Drives Carbon Loss From India’s Sal Forest Floors

Beneath the canopy of India’s Eastern Ghats, the forest floor is quietly exhaling. A new field study of tropical sal forests in Odisha’s Kandhamal district has measured, season by season, how much carbon dioxide rises from the soil and what controls it — and the answer upends a common assumption. It is not heat that governs the release of carbon from these soils, but water. Across three distinct sal forest types, soil respiration peaked during the drenching monsoon months and collapsed in the dry winter, even though soil temperatures were far higher in summer. The findings, published in Discover Forests, carry real weight for climate models and forest policy in a region where sal forests anchor both biodiversity and rural livelihoods.

Soil respiration — the flux of carbon dioxide from the soil surface to the atmosphere — is one of the largest single pathways by which carbon leaves terrestrial ecosystems. It is the combined breath of two underground worlds: autotrophic respiration from living plant roots and their rhizosphere partners, and heterotrophic respiration from microbes and soil fauna decomposing dead organic matter. Because forest soils hold more carbon than the atmosphere and all vegetation combined, even modest shifts in this efflux can tip an ecosystem between acting as a carbon sink and a carbon source. That is precisely why the researchers, Madhab Chandra Behera and Uttam Kumar Sahoo, spent months measuring it in situ across nine sites spanning three reserve forests.

The study focused on three moist deciduous forest types shaped by the dominance of sal (Shorea robusta): pure sal forest, sal-dominated mixed forest, and mixed deciduous forest without sal. The team established 72 tree subplots across 7.2 hectares, quantified stand structure, litter production and fine-root turnover, and graded each site on a disturbance index that accounted for proximity to settlements, grazing, fire, fuelwood removal and the proportion of tree basal area felled. Soil respiration itself was measured with the soda-lime absorption technique: sealed chambers containing oven-dried sodium hydroxide granules were left on the forest floor for 24 hours, and the weight gain of the granules — corrected with a blank chamber and a stoichiometric factor — yielded the daily carbon dioxide efflux.

To separate root respiration from microbial respiration, the researchers used the trenching method. Beside each measurement point they dug trenches to 45 centimetres, severing root ingrowth, then let the soil settle for four weeks before measuring again. The difference between total respiration in untrenched soil and respiration in root-free soil gave the heterotrophic component. This partitioning matters because the two components respond differently to climate and disturbance: root respiration tracks the vitality and carbon allocation of living trees, while microbial respiration tracks the decomposition of soil organic matter — the process that permanently releases carbon that may have been stored for decades.

The seasonal signal was unmistakable. Soil respiration climbed to its annual maximum between July and September, coinciding with the southwest monsoon that delivers roughly 80 percent of the region’s 152 to 160 centimetres of annual rainfall, and fell to its minimum in the dry winter months. In the sal-dominated mixed forest, daily efflux peaked at 2.98 grams per square centimetre per day in September, then dropped to just 0.16 grams in February. The explanation lies in the physiology of the rhizosphere: monsoon moisture stimulates root growth and the exudation of sugars, amino acids and organic acids from roots, which in turn fuels rhizosphere microbes, while also improving nutrient diffusion through the soil. In the hot, dry pre-monsoon months, by contrast, desiccated soil constrains both microbial enzymes and root metabolism, so rising temperature alone cannot lift respiration.

Forest composition left its own fingerprint. Mixed forest without sal showed the highest mean respiration at 1.14 grams per square centimetre per day, followed by the sal-dominated mixed forest at 1.03, with pure sal forest trailing at 0.66. Annual carbon efflux ranged from 28.80 megagrams of carbon per hectare per year in pure sal forest to 49.83 in the sal-free mixed stand. Across all types, root respiration contributed slightly more than half of the total — 52.37 percent on average — meaning that living roots, not just decaying matter, dominate the carbon exit route from these soils.

Perhaps the most striking result concerns how long carbon stays locked in the ground. By dividing the soil organic carbon stock by annual heterotrophic respiration, the team calculated the mean residence time of carbon in the surface 15 centimetres. Pure sal forest held the most soil organic carbon — 2.11 percent, with a stock of 42.70 megagrams per hectare — and the longest residence time, 37.08 years. The mixed forest without sal held barely a fifth as much carbon, at 20.16 megagrams per hectare, and turned it over in just 10.84 years. In other words, the mixed forests respire more and store less, cycling carbon rapidly through a labile pool, while pure sal stands accumulate chemically resistant or physically protected organic matter that resists decomposition.

Statistical analysis sharpened the picture. Soil respiration correlated positively with soil moisture (r = 0.790) and tree density (r = 0.789), and negatively with soil temperature (r = −0.813) — a counterintuitive negative temperature relationship that the authors attribute to seasonal co-variation: the hottest months are also the driest, so moisture, not warmth, is the binding constraint. Principal component analysis placed soil moisture, tree density, basal area and litter production together on the first axis, which explained 62.50 percent of total variance, while soil temperature and the disturbance index loaded on the second. Multiple regression models built on soil temperature, moisture and tree density explained up to 78.5 percent of respiration variation in individual forest types, with soil moisture consistently carrying the strongest coefficient. The authors are careful to note that with only nine independent sites, the regression is explanatory rather than predictive, and that correlation and ordination cannot by themselves establish causation.

The disturbance story is subtler than the title of the study might suggest. Disturbance itself showed only weak, non-significant direct correlations with respiration, and its influence appears to be indirect — mediated through canopy openness, altered microclimate and shifts in organic matter inputs, as reflected in its loading on the second principal component alongside temperature. What disturbance and composition clearly do change is carbon turnover: faster cycling in diverse, more open mixed stands, slower and more stabilising cycling under closed sal canopy. For managers in the Eastern Ghats, the practical implication is that preserving structurally diverse, minimally disturbed sal stands protects the region’s largest soil carbon reservoirs, while monsoon-dependent moisture regimes — increasingly vulnerable to climate variability — set the tempo of carbon loss. As warming intensifies, the fate of these forests may hinge less on how hot they get and more on whether the rains on which their underground carbon economy depends remain reliable.

Subject of Research: Soil respiration and carbon turnover in disturbed tropical sal forests of the Eastern Ghats, India

Article Title: Impact of disturbance on spatio-temporal dynamics of carbon dioxide efflux on forest floor in tropical sal forests of the Eastern Ghats

Article References: Behera, M. C., & Sahoo, U. K. (2026). Impact of disturbance on spatio-temporal dynamics of carbon dioxide efflux on forest floor in tropical sal forests of the Eastern Ghats. Discover Forests, 2(1), Article 76. https://doi.org/10.1007/s44415-026-00139-z

Image Credits: AI Generated

DOI: 10.1007/s44415-026-00139-z

Keywords: soil respiration, carbon dioxide efflux, sal forests, Eastern Ghats, soil organic carbon, monsoon, forest disturbance, autotrophic respiration, heterotrophic respiration, carbon turnover, Shorea robusta, tropical forests