Temporal Dynamics of Soluble Carbon Pools in Surface Rice Soils under Alternating Oxic-anoxic Conditions in the Eastern Himalayan Terai Agro-ecological Region

Abstract

Flooding and drainage impose on lowland rice soils a distinctive alternation of anoxic and oxic conditions that is widely believed to govern the residence time of soil organic carbon. Yet the short-term, within-season expression of this control on surface soil has rarely been examined under controlled conditions that isolate texture and organic matter supply from the confounding variability of the field. A pot experiment was conducted with surface (0-20 cm) soils of three textural classes: loam, sandy clay loam and sandy loam drawn from rice croplands of the Terai agro-ecological region of West Bengal, India. Eighteen pots were arranged in a two-factor scheme combining soil texture (three levels) with the presence or absence of farmyard manure applied at a rate equivalent to 10 t ha-1, and a rice crop (cv. Bongabandhu) was raised to maturity under the flooding and drainage regime characteristic of lowland rice culture. The soils of every pot were sampled at weekly intervals over eight weeks, a period spanning an initial aerobic, drained phase (weeks 1-2) and a subsequent anaerobic, flooded phase (weeks 3-8), and the water-soluble and hot-water-soluble carbon fractions were determined on each occasion. Water-soluble carbon fluctuated within a comparatively narrow band of about 52-58 mg kg-1 of total organic carbon. In contrast, HWSC showed a sharper decline on the transition to anoxia, especially in the unamended pots. Total soluble carbon, ranging between roughly 105 and 115 mg kg-1 of total organic carbon, was raised by manure addition chiefly in the early, aerobic weeks and the amended and unamended pots converged as the season advanced. A repeated-measures analysis of variance detected a significant effect of week (p = 0.002) and a significant week-by-texture interaction (p = 0.006), but no significant main effect of texture and no significant net difference between the aerobic and anaerobic phase means (p = 0.96). These findings indicate that the soluble carbon pool of surface rice soil is a genuinely dynamic but texturally buffered fraction, whose short-term, texture-dependent trajectory rather than its season-long average carries the imprint of redox alternation and of fresh organic matter addition.