Progressive freeze-thaw redistributes water, solute and CO2 emissions across soil layers – The role of soil particle size

Abstract

Progressive freeze-thaw redistributes water and solute among soil layers, and thus perturbs soil carbon mineralization. This potentially introduces variations when differently-sized soil aggregates experience progressive thawing and detachment. In this study, a Mollisol was dry-sieved into three particle sizes (<0.125, 0.125–0.5 and 0.5–1 mm). Each was refilled into a soil column, rewetted to saturation, frozen at −15 °C for 12 h, and then thawed at ambient temperature. During thawing, the soil columns were peeled by layers from the outermost to the inner core into six layers (T1 ∼ T6), to simulate soil detachment while progressive thawing. Our results show that: 1) The outermost layer T1, thawed earlier and thus peeled off first, had the highest water content (102.1–151.2 %) but the lowest electrical conductivity (EC, 63.2–68.3 μS·cm−1). On the contrary, the inner core T6, thawed the last, was driest with merely 24.1–28.4 % water content yet higher EC of 61.0–87.0 μS·cm−1. Such polarized distributions were most pronounced among the soil layers composed of fine aggregates. 2) The CO2 emission rates from the T1 were in general lower than that from the T6 (on average 56.1–65.6 vs. 70.0–84.8 μg CO2-C·g soil −1 7d −1). The soil columns composed of fine aggregates released significantly less CO2 than the coarser aggregates. Overall, the convection of free water during freezing and the diffusion of ice-insoluble materials from freezing front to unfrozen zones, collectively resulted in the polarized patterns of soil water, solute and CO2 emissions. This challenges the conventional investigations based on complete freeze-thaw, highlighting the necessity to distinguish inter-layer heterogeneity during progressive thawing. The varying responses of differently-sized aggregates call for systematic investigations on how progressively thawing and detachment affect slope-scale carbon biogeochemical processes.