Masako Kajiura

Development of a method for estimating total CH4 emission from rice paddies in Japan using the DNDC-Rice model.

Methane (CH4) is a greenhouse gas, and paddy fields are one of its main anthropogenic emission sources. To mitigate this emission based on effective management measures, CH4 emission from paddy fields must be quantified at a national scale. In Japan, country-specific emission factors have been applied since 2003 to estimate national CH4 emission from paddy fields. However, this method cannot account for the effects of weather conditions and temporal variability of nitrogen fertilizer and organic matter application rates; thus, the estimated emission is highly uncertain. To improve the accuracy of national-scale estimates, we calculated country-specific emission factors using the DeNitrification-DeComposition-Rice (DNDC-Rice) model. First, we calculated CH4 emission from 1981 to 2010 using 986 datasets that included soil properties, meteorological data, and field management data. Using the simulated site-specific emission, we calculated annual mean emission for each of Japans seven administrative regions, two water management regimes (continuous flooding and conventional mid-season drainage), and three soil drainage rates (slow, moderate, and fast). The mean emission was positively correlated with organic carbon input to the field, and we developed linear regressions for the relationships among the regions, water management regimes, and drainage rates. The regression results were within the range of published observation values for site-specific relationships between CH4 emission and organic carbon input rates. This suggests that the regressions provide a simplified method for estimating CH4 emission from Japanese paddy fields, though some modifications can further improve the estimation accuracy.

Cropland soil–plant systems control production and consumption of methane and nitrous oxide and their emissions to the atmosphere

Abstract Croplands are an important source of atmospheric methane (CH4) and nitrous oxide (N2O), both potent greenhouse gases. Reduction of cropland CH4 and N2O emissions is expected to mitigate climate change. However, large uncertainty remains in the assessment and prediction of these emissions, which prevents us from establishing appropriate mitigation options and strategies. The uncertainty is attributed mainly to the high spatiotemporal variability in emissions (e.g., emission spikes of N2O). Understanding and quantifying how hotspots of CH4 and N2O production in soil and then hot moments of their emissions occur would help reduce the uncertainty. This review focuses on soil–plant systems, particularly the rhizosphere, as possible hotspots of production and consumption of CH4 and N2O. It is well known that the rhizosphere controls CH4 emission strongly, though each process of production and consumption remains to be quantified. On the other hand, surprisingly little attention has been paid to N2O, besides the fact that plant roots strongly control nitrification and denitrification. We review the current knowledge of cropland CH4 and N2O emissions, and conclude that soil–plant interactions strongly affect cropland emissions of both gases, in which functions of plant roots affecting biogeochemical factors (e.g., availability of oxygen, labile organic carbon and inorganic nitrogen) in the rhizosphere and phenological changes are particularly important. In relation to the status of current knowledge, we discuss future research needed.

Fully automated, high-throughput instrumentation for measuring the δ13C value of methane and application of the instrumentation to rice paddy samples

RATIONALE The stable carbon isotope ratio ((13)C/(12)C or δ(13)C value) of methane (CH4) produced in methanogenic environments contains information about primary source material, CH4 production pathways, degree of oxidation, and transport. However, the availability of δ(13)C-CH4 data is severely limited because isotope analysis methods are low throughput, owing primarily to the need for manual processing steps. High-throughput, fully automated measurement is necessary to facilitate the use of the δ(13)C signature in understanding CH4 biogeochemistry. METHODS We modified a conventional continuous-flow (CF) gas chromatography/combustion/isotope ratio mass spectrometry (IRMS) instrument system by incorporating (i) automated sample injection, (ii) a newly developed temperature-control unit for preconcentration and cryofocus traps, and (iii) an automatic system for liquid-nitrogen refilling. The system, which could run unattended for 1 day, was used to obtain δ(13)C-CH4 data for CH4 samples collected from an irrigated rice paddy with an automated closed-chamber system. RESULTS Using the fully automated CF-IRMS system, we measured δ(13)C-CH4 data for 77 samples during a 21.5-h continuous run (17 min per sample) with high precision (1σ = 0.11‰, reproducibility) and moderate consumption of liquid nitrogen (11 L). Application of the system to CH4 samples obtained from the rice paddy revealed distinct seasonal and diurnal variations in δ(13)C values with the highest temporal resolution ever reported. CONCLUSIONS A fully automated, high-throughput system for the measurement of δ(13)C-CH4 values was developed and used to analyze air samples obtained from a rice paddy. Our results demonstrate the high potential of this system for obtaining δ(13)C data useful for process-level understanding of CH4 biogeochemistry with respect to spatiotemporal variation of CH4 sources and how that variation is affected by environmental and management factors.

Optimal Thermolysis Conditions for Soil Carbon Storage on Plant Residue Burning: Modeling the Trade-Off between Thermal Decomposition and Subsequent Biodegradation.

Field burning of plant biomass is a widespread practice that provides charred materials to soils. Its impact on soil C sequestration remains unclear due to the heterogeneity of burning products and difficulty in monitoring the materials biodegradation in fields. Basic information is needed on the relationship between burning conditions and the resulting quantity/quality of residue-derived C altered by thermal decomposition and biodegradation. In this study, we thermolyzed residues (rice straw and husk) at different temperatures (200-600°C) under two oxygen availability conditions and measured thermal mass loss, C compositional change by solid-state C NMR spectroscopy, and biodegradability of the thermally altered residues by laboratory aerobic incubation. A trade-off existed between thermal and microbial decomposition: when burned at higher temperatures, residues experience a greater mass loss but become more recalcitrant via carbonization. When an empirical model accounting for the observed trade-off was projected over 10 to 10 yr, we identified the threshold temperature range (330-400°C) above and below which remaining residue C is strongly reduced. This temperature range corresponded to the major loss of O-alkyl C and increase in aromatic C. The O/C molar ratios of the resultant residues decreased to 0.2 to 0.4, comparable to those of chars in fire-prone field soils reported previously. Although the negative impacts of biomass burning need to be accounted for, the observed relationship may help to assess the long-term fate of burning-derived C and to enhance soil C sequestration.

Effect of phenolic acids on the formation and stabilization of soil aggregates

ABSTRACT To examine the effects of phenolic acids, which are generated by the decomposition of cell walls in plant residues, and other constituents on the stability of soil aggregates, phenolic acids and carbohydrates were mixed into three different types of soil. After a 1-month incubation, the plot containing soil mixed with phenolic acids showed the greatest mean weight diameter of all the soils. In the treated soils, before incubation, the decline of saturated water permeability during continuous water percolation was mitigated in the plot containing soil mixed with phenolic acids compared with that in the other plots. Soil aggregates were synthesized with the addition of phenolic acids and carbohydrates using two methods (mixing and surface brushing) and were incubated for 153 days. The aggregate stability was greatest in the plots surface-brushed with phenolic acids for Andosol and gray lowland soil, whereas the aggregate stability was most stable in the plots mixed with phenolic acids for yellow soil. This difference in the effectiveness of application methods is rationalized by the densities of the active Al and Fe contents, the carbon content, and the specific surface area of the soils. The phenolic acids also affected sandy soil. In a similar experiment using a gray lowland soil, mixing a portion of p-coumaric acid into synthetic aggregates was found to shift the molecular weight distribution of substances to larger molecular weights, as determined by size exclusion chromatography of the liquid extracted from the aggregates, which was likely accompanied by an increase in aggregate stability. The effects of fungi and bacteria on soil long-term stability were not greater than those of phenolic acids. Our findings and previous results show that microorganisms aid in soil-aggregate formation during the early stages, and phenolic acids not only aid in the formation of aggregates but also strongly stabilize them.