Rota Wagai

Linking temperature sensitivity of soil organic matter decomposition to its molecular structure, accessibility, and microbial physiology

Temperature sensitivity of soil organic matter (SOM) decomposition may have a significant impact on global warming. Enzyme-kinetic hypothesis suggests that decomposition of low-quality substrate (recalcitrant molecular structure) requires higher activation energy and thus has greater temperature sensitivity than that of high-quality, labile substrate. Supporting evidence, however, relies largely on indirect indices of substrate quality. Furthermore, the enzyme-substrate reactions that drive decomposition may be regulated by microbial physiology and/or constrained by protective effects of soil mineral matrix. We thus tested the kinetic hypothesis by directly assessing the carbon molecular structure of low-density fraction (LF) which represents readily accessible, mineral-free SOM pool. Using five mineral soil samples of contrasting SOM concentrations, we conducted 30-days incubations (15, 25, and 35xa0°C) to measure microbial respiration and quantified easily soluble C as well as microbial biomass C pools before and after the incubations. Carbon structure of LFs (<1.6 and 1.6-1.8xa0gxa0cm(-3) ) and bulk soil was measured by solid-state (13) C-NMR. Decomposition Q10 was significantly correlated with the abundance of aromatic plus alkyl-C relative to O-alkyl-C groups in LFs but not in bulk soil fraction or with the indirect C quality indices based on microbial respiration or biomass. The warming did not significantly change the concentration of biomass C or the three types of soluble C despite two- to three-fold increase in respiration. Thus, enhanced microbial maintenance respiration (reduced C-use efficiency) especially in the soils rich in recalcitrant LF might lead to the apparent equilibrium between SOM solubilization and microbial C uptake. Our results showed physical fractionation coupled with direct assessment of molecular structure as an effective approach and supported the enzyme-kinetic interpretation of widely observed C quality-temperature relationship for short-term decomposition. Factors controlling long-term decomposition Q10 are more complex due to protective effect of mineral matrix and thus remain as a central question.

Association of organic matter with iron and aluminum across a range of soils determined via selective dissolution techniques coupled with dissolved nitrogen analysis

Strong correlations of soil total organic carbon (OC) with iron and aluminum phases reported frequently make it important to quantify these organic matter (OM) associations, but selective extractants sometimes contain OC. Soil nitrogen is often predominantly organic and might serve as a proxy for OM. We therefore investigated nitrogen associations with Fe and Al using several selective extractants that use reductive, complexation, and alkaline approaches. Total dissolved nitrogen (TDN) correlated strongly with extracted Fe and Al across seventeen samples, including highly- and weakly-weathered soils, iron-rich ultrabasic soils, podzolic, and volcanic soils. Typically a quarter to a third of total soil nitrogen was dissolved by the various extractions, though higher fractions (up to 60%) were found in spodic-horizon and volcanic surface-horizon samples. Similar proportions were found for OC, using three OC-free extractants, indicating that TDN provides a useful surrogate for assessing OM partitioning via extractants that contain OC. Use of TDN:metal ratios in extractant solutions allows estimation of extracted OM that could have been sorptively associated with metal oxide/hydroxides and poorly-crystalline aluminosilicates. These ratios were often high in extractions targeted at these adsorbents, and imply that usually most of the extracted TDN consists instead of organo–metal complexes. The dynamics of these complexes may have stronger control on accumulation/remobilization of soil OM than those of metal oxyhydroxides and poorly-crystalline aluminosilicates.

Beyond clay: towards an improved set of variables for predicting soil organic matter content

Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO2 to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron- and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to better reflect the role of soil physicochemical properties in SOM cycling.

Distinctive organic matter pools among particle-size fractions detected by solid-state 13C-NMR, δ13C and δ15N analyses only after strong dispersion in an allophanic Andisol

Abstract We previously showed the first clear evidence of aggregate hierarchy in an Andisol by comparing the particle-size fractions released upon different levels of dispersion energy up to the maximum dispersion – sonication at 5 kJ mL−1 following sodium saturation. While smaller particles (< 2 μm) appeared to act as major binding agents, the variation in organic matter (OM) chemistry among the size fractions remains unstudied. Here, we focused on comparing the carbon structure and carbon and nitrogen stable isotope ratio (δ13C and δ15N) among the particle-size fractions isolated by limited dispersion (mechanical shaking) and by the maximum dispersion treatments for the allophanic Andisol previously examined. Both solid-state carbon-13 nuclear magnetic resonance (13C NMR)and stable isotope ratios showed clear differences among the size fractions after the maximum dispersion but not after the limited dispersion. From 2–53 μm toward the < 0.2 μm fraction, we observed a progressive decline in the proportion of aromatic-C and an increase in that of O-alkyl-C. Similarly, the enrichment of 13C and 15N toward the smaller particle size fractions was observed after the maximum dispersion. While δ15 N had progressive enrichment from 3.6‰ (53–4000 μm) to 6.4‰ (< 0.2 μm fraction), δ13C showed a 2.5–3.0‰ enrichment from the 53–4000 μm fraction (−24.0‰) to 2–53 μm fraction and remained largely constant between 0.2–2 μm and < 0.2 μm fractions. The sonication-induced redistribution and chemical alternation of OM appeared to be minor due to the small pool sizes of low-density materials (e.g., plant litter) and microbial biomass. The emergence of the size-dependent changes in C chemistry after the maximum dispersion was consistent with the Andisol aggregate hierarchy model we previously proposed. The observed difference in C/N ratio and isotopic ratios as well as C composition implies that the OM present in the sonication-resistant particles of < `2 μm sizes (that account for roughly 70% of total C and N) are highly recycled by and/or largely originated from soil microbes. The applicability of current findings to other samples (e.g., non-allophanic Andisols) should be examined to establish the unique role of OM-enriched, micron to submicron particles in the aggregate hierarchy of Andisols.

Global distribution of clay-size minerals on land surface for biogeochemical and climatological studies

Clay-size minerals play important roles in terrestrial biogeochemistry and atmospheric physics, but their data have been only partially compiled at global scale. We present a global dataset of clay-size minerals in the topsoil and subsoil at different spatial resolutions. The data of soil clay and its mineralogical composition were gathered through a literature survey and aggregated by soil orders of the Soil Taxonomy for each of the ten groups: gibbsite, kaolinite, illite/mica, smectite, vermiculite, chlorite, iron oxide, quartz, non-crystalline, and others. Using a global soil map, a global dataset of soil clay-size mineral distribution was developed at resolutions of 2 to 2° grid cells. The data uncertainty associated with data variability and assumption was evaluated using a Monte Carlo method, and validity of the clay-size mineral distribution obtained in this study was examined by comparing with other datasets. The global soil clay data offer spatially explicit studies on terrestrial biogeochemical cycles, dust emission to the atmosphere, and other interdisciplinary earth sciences.

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.

Soil temperature and moisture-based estimation of rates of soil aggregate formation by the endogeic earthworm Eisenia japonica (Michaelsen, 1892)

Ecological importance of earthworm via aggregate production has been well studied in Europe, but much less is known for Asian species. Assessing the effects of temperature and moisture on the soil aggregate formation by earthworms is a logical step towards the quantification of earthworm’s function in ecosystem. Here, we estimated soil temperature and moisture-based rate of aggregate formation by the earthworm Eisenia japonica (Michaelsen, 1892), which is distributed widely in Japan and South Korea. Based on the data obtained from 1-week laboratory incubations, we developed a model describing the aggregate formation rate by earthworm as a function of body mass, soil temperature, and soil moisture. We then applied the model to a field mesocosm experiment. While the aggregate production rates predicted by the model were slightly underestimated, the estimated values showed significant positive correlation with the measured field data (Pu2009<u20090.001). The empirical model developed here was therefore applicable to the field condition studied, implying that our approach would help to quantify and predict the ecological function of earthworm.

Characteristics of phosphorus fractions in the soils derived from sedimentary and serpentinite rocks in lowland tropical rain forests, Borneo

ABSTRACT Soil organic phosphorus (P) is an important P source for biota especially in P-limited forests. Organic P has various chemical formations which differ in bioavailability and these organic P can be degraded by phosphatase enzymes. Here, we report soil P fractions inferred from solution 31P-NMR spectroscopy and soil phosphatase activities of two tropical rain forests on contrasting parent materials; sedimentary and ultramafic igneous (serpentinite) rocks. Compared to the sedimentary soilsu3000and previous studies, P fractions of the serpentinite soils have distinctly high proportions of pyrophosphate and scyllo-inositol hexakisphosphate (scyllo-IP6). The accumulation of pyrophosphate and scyllo-IP6 may be related to strong sorptive capacity of iron oxides present in the serpentinite soils, which implies a consequent low P availability in the serpentinite soils. Mean value of soil phosphatase activities was higher in the serpentinite soils than in the sedimentary soils, suggesting that biota in these serpentinite forests depend more on soil organic P as a P source.