Controls on soil organic matter stability and composition of neutral-to-alkaline topsoil and subsoil across Indo-Gangetic plains

Abstract

Elusive controls over soil organic carbon (SOC) in neutral-to-alkaline soils limit long-term carbon dynamic predictions across vast agricultural areas such as Indo-Gangetic Plain (IGP). We studied the causes of low SOC content in neutral-to-alkaline topsoil and subsoil under tropical to subtropical climates by identifying factors controlling soil organic matter (SOM) fractions, SOC pools, and SOM molecular composition. We investigated topsoil and subsoil from six paired forest and agricultural fields within 12 sites across IGP, using SOM fractionation, 196-day soil incubation, and pyrolysis-GC/MS. A three-pool kinetic model estimated labile, intermediate, and stable SOC pools from respiration curves, with non-hydrolyzable SOC as the stable pool. Measured soil properties include pH, exchangeable cations, CEC, inorganic/organic C, texture, oxalate-extractable Al/Fe (active Al/Fe), and dithionite-extractable Al/Fe. SOC turnover over decades is regulated by the intermediate SOC pool, which is closely associated with mineral-associated SOM. Active Al/Fe, rather than exchangeable Ca²⁺, clay, or agricultural activity, controlled the stability of intermediate pool in both topsoil and subsoil. In topsoil, agricultural activity reduced light fraction carbon by 38 % and intermediate pool size by 34 %, but had no significant effect in subsoil or on more stable fractions/pools. Active Al/Fe had a stronger effect on stabilizing carbon in less carbon-saturated subsoil. SOM degradation was intense (e.g., high abundance of N-containing compounds) and was attributed to low active Al/Fe levels under drier conditions, and to elevated microbial activity driven by neutral-to-alkaline soil pH. Cultivation had a non-significant influence on SOM composition. In conclusion, the low SOC in neutral-to-alkaline IGP soils is primarily due to drying and elevated pH, which limits active Al/Fe formation, thereby reducing SOC stability and intensifying SOM degradation. Cultivation further exacerbates SOC loss in topsoil. This highlights the need for understanding and targeted management of these SOM regulating factors to enhance ecological and agricultural sustainability in neutral-to-alkaline soils.