Comparison of isotope methods for partitioning methane production and soil C priming effects during anaerobic decomposition of rice residue in soil

Abstract

Management of rice residue in paddies often results in higher methane (CH 4) emissions, but the carbon (C) sources contributing to higher emissions are not well characterized. We examined the relative contribution of soil and residue C sources for CH 4 production in three different soils using both 13 C-labeled or unlabeled (natural abundance) rice and maize residues. The results were compared to a recently proposed natural 13 C abundance soil C partitioning method (CNAM) (Conrad et al. 2012b). As anticipated, residue addition increased CH 4 production, with the timing and magnitude of emission varying across soils, likely due to differences in soil C and amount of electron acceptors. The CH 4 derived from residue-C, when estimated using the typical 13 C natural abundance method yielded either negative values or those exceeding 100%. Similarly, the CNAM frequently overestimated the contribution of residue C compared to the 13 C enriched residue method (ERM). The over estimation is likely due to C isotopic fractionation during the utilization of rice and maize residues in the different soils. Substantial C isotope fractionation would bias the fundamental assumption of the CNAM. The ERM produced a consistent partitioning result of 50 to 70% of CH 4 derived from residue additions. When typical minimum, average, and maximum values of natural 13 C fractionation factors were applied to the ERM the partitioning outcome was unaffected. The priming effect of residue addition on CH 4 production was regulated by electron acceptor concentrations in various soils and was positively correlated to the degree of residue decomposition. The priming effect of residue, therefore, depends both on soil and residue properties, with the ERM showing to be more accurate to account for the partitioning of CH 4 from various C pools compared to CNAM.

Large but variable methane production in anoxic freshwater sediment upon addition of allochthonous and autochthonous organic matter

“…b). The sediment seemed to be poor in inorganic electron acceptors, which could outcompete methanogenesis, because there was a very short lag phase before CH 4 production started in the sediment‐only treatment E (Ye et al ). The low CH 4 production from sediment only may consequently rather be attributed to a low availability of labile compounds than a high content of inorganic electron acceptors.…”

“…The added OC contribution to the CH 4 produced may be calculated by subtracting Eq. for the two different types of added OC (Conrad et al ; Ye et al ): …”

“…As recommended by Ye et al (), we used this method only for two added types of OC which have a comparable methanogenic potential in an anoxic sediment. Indeed, if two different types of OC have a comparable methanogenic potential, it implies that the degrading OC is of equivalent quality for methanogens, and in case of a sediment rich in electron acceptors, it indicates that they were consumed at the same speed (Ye et al ). Furthermore, the sediment matrix buffers the abiotic conditions such as pH or redox conditions and in our case, it was taken care that the same microbial inoculum was initially added.…”

“…All these factors (i.e., microbial community, the environment, and the OC quality) drive the pathways of CH 4 formation (Sugimoto and Wada ; Hornibrook et al ; Conrad et al ). Consequently, when two added types of OC have a comparable CH 4 production over time, the previous assumption that the overall C fractionation between added OC and CH 4 will follow the same value over time is likely to be warranted (Ye et al ).…”

“…At oxic conditions, several studies suggest that the decomposition of refractory sediment OC tends to be stimulated by the addition of labile OC, through an effect called “positive priming” (Guenet et al ). Studies in anoxic soils on the relative contribution of soil OC and fresh OC to CH 4 production returned contrasting results: the application of fresh OC could either enhance (Chidthaisong and Watanabe ; Lu et al ; Ye et al ) or decrease (Conrad et al ) CH 4 production derived from soil OC. Therefore, it is at present not possible to gauge the contribution of the residing lake sediment OC pool to CH 4 production following fresh OC addition, calling for studies that partition the sources of CH 4 during the anoxic decomposition of fresh OC in lake sediments.…”