Dissolved organic carbon loss from paddy fields by drainage and roles of common ions

Abstract

Cations and anions exerted important effects on dissolved organic carbon (DOC) release and retention in arable soils. However, knowledge is limited on DOC loss from rice (Oryza sativa L.) paddies by drainage and its association with common ions under a conventional irrigation and drainage regime. This study aimed to investigate DOC dynamics in the soil-overlying water system and the synergistic behaviors of common ions (Na+, K+, Ca2+, Mg2+, Cl−, NO3− and SO42−) contributing to variations in DOC flux in drainage water at the pre-growing, vegetative and reproductive stages of rice cultivation in the Sanjiang Plain of NE China. Samples of overlying water and soil in three layers (0–20, 20–40, and 40–60 cm) were collected before drainage to evaluate variations of DOC and common ions across rice growing stages and to estimate DOC loss by drainage. Sorption experiments were conducted to determine DOC sorption capacities of surface soil (0–5 cm). Compared with the pre-growing stage, DOC concentrations in overlying water and soil significantly increased at the vegetative stage, while significant increases at the reproductive stage were merely in soil. Net movement of DOC was from soil to overlying water at the pre-growing and reproductive stages, and was close to equilibrium at the vegetative stage. Ca2+ and Mg2+ in surface soils (0–20 cm) displayed stronger capacities explaining DOC dynamics in the soil-overlying water system than the other common ions. As Ca2+ and Mg2+ in surface soils (0–20 cm) increased, DOC in the uppermost soil layers (0–20 and 20–40 cm) and overlying water increased, and DOC sorption capacities in surface soil were enhanced. DOC loss from paddy fields by drainage mainly occurred at the vegetative stage, occupying 62–83 % of the total, and much more attention ought to be paid to control drainage DOC loss during this period. In light of important roles of Ca2+ and Mg2+ in DOC dynamics and then DOC loss by drainage, appropriate measures with respect to addition of Ca2+ and Mg2+ could be taken into consideration in rice cultivation to alleviate paddy field DOC export.