Lied in large amounts (up to 12 t ha21 annually) to maintain soil fertility [5?]. Methane production is partitioned mainly between these three types of organic matter. Knowledge of partitioning is important for improving process-based modeling of CH4 emission from rice fields [8,9], which is the basis for predicting methane flux and assessing the impact of agricultural management and global change. Quantification of carbon partitioning can in principle be achieved by pulse-labeling of rice plant with 13CO2 or 14CO4 [10?2]. Recently, free-air CO2 enrichment (FACE) using 13Cdepleted CO2 was used for determining the contribution of ROC to production of CO2 and CH4 in rice field soil [13]. However, pulse-labeling only assesses the immediate contribution of root exudates, while the contribution of sloughed-off dead root cells cannot be fully accounted for [13?6]. Since FACE INCB039110 site experiments apply elevated CO2 concentrations, photoassimilation of CO2 may be enhanced and thus increase the contribution of plants and soilorganic Cucurbitacin I chemical information matter to carbon flux [17?9]. Furthermore, most studies of carbon flux partitioning in rice fields have been done without application of straw, so that full partitioning of the origin of carbon flux into SOM, ROC and RS was not possible [4]. However, application of RS should be taken into account, since RS may not only be used as substrate for CH4 production, but might also enhance CH4 production from other carbon sources [20,21]. The partitioning of the CH4 production from different sources of organic carbon (SOM, ROC, RS) can be achieved, if these have different isotopic signatures. However, a major difficulty during partitioning the sources of CH4 is caused by the carbon isotopic fractionation during the conversion of organic matter to CH4, which is typically 10?0 [22]. Nevertheless, the relative contribution of acetoclastic versus hydrogenotrophic methanogenesis to CH4 production has been determined successfully in environments such as rice field soil [23] and lake sediments [24], after the isotopic fractionation factors in both methanogenic pathways were determined. The d13C values of CH4 from the two pathways are substantially different, since the isotopic fractionation factors of the two pathways are largely different [22,24,25]. Analogously, it is possible to partition the sources of CH4 if the d13C of CH4 derived from each carbon source in the rice field soil is known. Normally, 18325633 the CH4 derived from SOM, ROC and RS has similar d13C values, since all the organic matter has eventually been derived from rice plant material [23,26]. However, thisSources of Methane Production in Rice Fieldsproblem may be solved by cultivation of rice in soil amended with 13 C-labeled RS. The aim of this study was to determine the partitioning of the carbon flux involved in methanogenic degradation of carbon sources by determining the d13C of CH4 derived from ROC. We therefore prepared rice microcosms with two treatments of 13Clabeled RS, both having the same amount of RS (5 g kg21 soil, equals about 5 t ha21) but different content of 13C. We determined the produced CH4 and CO2 by collecting soil cores and incubating samples anoxically [27].Materials and Methods Planted and unplanted rice microcosmsSoil samples were provided by the Italian Rice Research Institute in Vercelli. Soil was taken from a drained paddy field in spring 2009 and was air dried and stored at room temperature. The soil was sieved (,2 mm) prior to use. The characterist.Lied in large amounts (up to 12 t ha21 annually) to maintain soil fertility [5?]. Methane production is partitioned mainly between these three types of organic matter. Knowledge of partitioning is important for improving process-based modeling of CH4 emission from rice fields [8,9], which is the basis for predicting methane flux and assessing the impact of agricultural management and global change. Quantification of carbon partitioning can in principle be achieved by pulse-labeling of rice plant with 13CO2 or 14CO4 [10?2]. Recently, free-air CO2 enrichment (FACE) using 13Cdepleted CO2 was used for determining the contribution of ROC to production of CO2 and CH4 in rice field soil [13]. However, pulse-labeling only assesses the immediate contribution of root exudates, while the contribution of sloughed-off dead root cells cannot be fully accounted for [13?6]. Since FACE experiments apply elevated CO2 concentrations, photoassimilation of CO2 may be enhanced and thus increase the contribution of plants and soilorganic matter to carbon flux [17?9]. Furthermore, most studies of carbon flux partitioning in rice fields have been done without application of straw, so that full partitioning of the origin of carbon flux into SOM, ROC and RS was not possible [4]. However, application of RS should be taken into account, since RS may not only be used as substrate for CH4 production, but might also enhance CH4 production from other carbon sources [20,21]. The partitioning of the CH4 production from different sources of organic carbon (SOM, ROC, RS) can be achieved, if these have different isotopic signatures. However, a major difficulty during partitioning the sources of CH4 is caused by the carbon isotopic fractionation during the conversion of organic matter to CH4, which is typically 10?0 [22]. Nevertheless, the relative contribution of acetoclastic versus hydrogenotrophic methanogenesis to CH4 production has been determined successfully in environments such as rice field soil [23] and lake sediments [24], after the isotopic fractionation factors in both methanogenic pathways were determined. The d13C values of CH4 from the two pathways are substantially different, since the isotopic fractionation factors of the two pathways are largely different [22,24,25]. Analogously, it is possible to partition the sources of CH4 if the d13C of CH4 derived from each carbon source in the rice field soil is known. Normally, 18325633 the CH4 derived from SOM, ROC and RS has similar d13C values, since all the organic matter has eventually been derived from rice plant material [23,26]. However, thisSources of Methane Production in Rice Fieldsproblem may be solved by cultivation of rice in soil amended with 13 C-labeled RS. The aim of this study was to determine the partitioning of the carbon flux involved in methanogenic degradation of carbon sources by determining the d13C of CH4 derived from ROC. We therefore prepared rice microcosms with two treatments of 13Clabeled RS, both having the same amount of RS (5 g kg21 soil, equals about 5 t ha21) but different content of 13C. We determined the produced CH4 and CO2 by collecting soil cores and incubating samples anoxically [27].Materials and Methods Planted and unplanted rice microcosmsSoil samples were provided by the Italian Rice Research Institute in Vercelli. Soil was taken from a drained paddy field in spring 2009 and was air dried and stored at room temperature. The soil was sieved (,2 mm) prior to use. The characterist.
