The effects of shallow flooding on carbon mineralization in a paddy soil: Constraints observed with the addition of straw

Abstract

Major controversy still exists regarding the effects of flooding conditions on soil carbon (C) mineralization. Two 44-day incubation experiments were conducted to explore the effects of shallow flooding (SF) on the C mineralization of paddy soil at different temperatures (constant temperatures of 15, 20 and 25 °C, and variable temperatures between 15 and 25 °C) and rice-straw addition levels (0 to 60 g kg-1 soil), respectively. The results indicated that when compared with the no-flooding (70% water-holding capacity) conditions, the SF conditions could significantly increase the total cumulative C mineralized (C44) at each temperature regime (P < 0.05). The soil bacterial community structures significantly changed with the soil moistures, and the dominant phyla under no-flooding and SF conditions are Actinobacteria and Firmicutes, respectively. When compared with the no-flooding treatments, the promoting effects of SF on the C mineralization were observed to gradually decrease with the increasing rice-straw additions to the soil. When the straw addition reached 20 g kg-1 soil, the C44 of the SF treatments was significantly lower than that of the no-flooding treatments (P < 0.05). These results indicated that the straw input levels could determine whether SF promoted the C mineralization in the soil. With the increasing straw additions to the soil, the dissolved oxygen concentration in the near-soil water gradually decreased in the SF treatments, which confirmed that the increased straw additions resulted in more active organic C, and its oxygen-consuming decomposition had resulted in an insufficient oxygen supply in the near-soil water. This may be one of the main reasons that the increasing straw input could restrict the promoting effects of SF on the C mineralization in the soil.