Decomposition Of Rice Straw and Soil Organic Matter In Organic Soils: Priming Effect and Microbial Dynamics.

Straw incorporation is a common practice to increase soil fertility in rice fields, but it also may increase soil organic matter (SOM) decomposition and the production of greenhouse gas methane (CH₄).  To quantify the priming effect of straw addition, we anaerobically incubated a peat soil from rice paddies with ¹³C-enriched rice straw for 3 months and measured δ¹³C values in DOC, CO₂, and CH₄ that produced during the incubation.  Stable-isotope probing PLFA analysis was also conducted to assess the changes in soil microbial community structure and composition.  Rice straw contributed to <5% of the total DOC, yet its addition increased DOC production from SOM by up to 233% resulting in a positive priming effect.  Similarly, straw addition increased total CO₂ production, 43-67% of which was from the decomposition of SOM.  Priming effect of straw addition on CO₂ production from SOM maximized on day 2, but the effect diminished gradually and was negative on day 60 and afterwards.  Methanogenesis was mostly inhibited during the whole course of incubation, primarily due to the dominance of iron reduction.  Straw amendment enhanced CH₄ production after a lag period of 10 days and contributed to 52-63% of the total production, with the remaining attributed to the priming effect.  Straw addition significantly increased total PLFA during the first week of incubation, suggesting the increases in microbial biomass.  More ¹³C was incorporated into branched-chain PLFAs (gram-positive bacteria) than straight, mono-unsaturated PLFAs (gram-negative bacteria).  Principal component analysis revealed clear successions of microbial consortia during the decomposition.  Our result suggested that microbial biomass may play an important role in the processes that lead to the timing and magnitude of priming effects.