Rota Wagai

• Ph.D, University of Maine
• Senior Researcher at National Agriculture and Food Research Organization

Driven by shifts in environmental conditions and changes in plant residue quality, converting lowland paddy fields to upland fields substantially alters soil organic matter (SOM) pools. However, the impact of residue quality on SOM dynamics in converted upland soils remains unclear. Here, we evaluated residue-derived carbon (C) accumulation pattern...

Miscanthus × giganteus (Mxg), due to its large biomass production, is a promising biomass crop for carbon (C) and nitrogen (N) sequestration and climate change mitigation. However, the relationship between the Mxg cultivation period and its effects on soil C and N stocks remains unclear due to limited information on belowground biomass and its dist...

Soil particles in plant rooting zones are largely clustered to form porous structural units called aggregates where highly diverse microorganisms inhabit and drive biogeochemical cycling. The complete extraction of microbial cells and DNA from soil is a substantial task as certain microorganisms exhibit strong adhesion to soil surfaces and/or inhab...

Multiple global change drivers have caused a large carbon (C) debt in our soils. To remedy this debt, understanding the role of microorganisms in soil C cycling is crucial to tackle the C soil loss. Microbial carbon use efficiency (CUE) is a parameter that captures the formation of microbially-derived soil organic matter (SOM). While it is known th...

The storage of soil organic matter (SOM) is essential for maintaining and improving soil fertility. To obtain basic information about the status of SOM in paddy fields in Nepal under various ecological settings, we investigated the amount and turnover rate of stored carbon (C) in fractionated SOM in the surface layer. Soil samples from the top 15 c...

An important control on long-term soil organic carbon (SOC) storage is the adsorption of SOC by short-range-ordered (SRO) minerals. SRO are commonly quantified by measuring oxalate-extractable metals (Mox = Alox + ½ Feox), which many studies have shown to be positively correlated with SOC. It remains uncertain if this organo-mineral relationship is...

Soil particles in plant rooting zone are largely clustered to form complex porous structural unit called aggregates where highly diverse microbes coexist and drive biogeochemical cycling. The complete extraction of microbial cells and DNA from soil is a substantial task as certain microbes exhibit strong adhesion to soil surfaces and/or inhabit dee...

The growing number of studies suggests that the prediction of soil organic carbon (SOC) storage and persistence can be improved by accounting for variations in soil reactive metal phases. Nevertheless, the dataset on reactive metals is often limited at a national scale. In Japan, the phosphate adsorption coefficient (PAC) has been measured for agri...

Although microbial inoculation may be effective for sustainable crop production, detrimental aspects have been argued because of the potential of inoculated microorganisms to behave as invaders and negatively affect the microbial ecosystem. We herein compared the impact of rhizobial inoculation on the soil bacterial community with that of agricultu...

Background and aims Phosphatase secretion by roots is an important phosphorus (P) acquisition strategy for plants growing under soil P deficiency. Four different classes of phosphatases degrade specific fractions of soil organic P (phosphomonoesterase, PME degrading labile monoester P; pyrophosphatase, PyP for pyrophosphate; phytase, PhT for phytat...

Climate change and water scarcity threaten the sustainability of rice production systems. Alternate wetting and drying (AWD) is a promising option to reduce methane (CH4) emission from irrigated paddy fields. However, its effect on rice yield remains to be clarified. Organic amendment can increase rice yield but may also increase CH4 emission. We t...

Background and aims Phosphatase secretion by fine roots is an important phosphorus (P) acquisition strategy for plants growing under soil P deficiency. Four different classes of phosphatases degrade specific fractions of soil organic P respectively (phosphomonoesterase, PME degrading labile monoester P; pyrophosphatase, PyP for pyrophosphate; phyta...

Soil organic C (SOC) stock in tropical forests can be reduced significantly after logging, not only due to lowered above ground biomass (AGB) and consequentially reduced organic matter (OM) input, but also consumption on sequestrated soil C protected by OM-mineral association. In this study, we studied the loss of mineral-associated OM after loggin...

Background and aims Research has focused on behavior of particulate organic matter (POM) and mineral-associated organic matter (MAOM) in acidic soils, but little attention has been given to the effects of tree species and vertical distribution of these components. With the ultimate aim of preserving soil organic matter, this study clarifies POM and...

Rice is the main staple food for more than half of the world’s population, but rice cultivation is a significant source of atmospheric methane (CH4). Alternate wetting and drying (AWD) reduces CH4 emission from paddy field, but the effect on rice yield remains unclear. Organic soil amendment increases grain yield but simultaneously increases CH4 em...

Rice is the main staple food for more than half of the world’s population, but rice cultivation is a significant source of atmospheric methane (CH4). Alternate wetting and drying (AWD) reduces CH4 emission from paddy field, but the effect on rice yield remains unclear. Organic soil amendment increases grain yield but simultaneously increases CH4 em...

Microbial–mineral interfacial reactions are crucial in the development and functioning of soil. We present basic mechanisms by which microorganisms and the polymers they secrete attach to mineral surfaces, followed by discussion on three fundamental consequences of these reactions: weathering (including secondary mineral formation), element cycling...

Highlights: · Above and-below ground carbon stock as well as short-range-ordered Fe oxides in soil are significantly lower in forests undergo severe selective logging after a decadal time · In-situ measurement reveals that top-30 cm soils drop to a reductive regime after a removal of more than half of the pristine-level AGB · Forest degradation may...

Purpose Fine roots are an important source of soil organic matter (SOM); however, it is unclear whether the cation exchange capacity (CEC) of leaf-derived and root-derived organic matter is comparable. We previously found that in surface soils of Cryptomeria japonica stands with low acid buffering capacity (ABC), fine-root biomass and total carbon...

Significant linear correlation between organic carbon (OC) and reactive aluminum and iron (metal) are often observed across a wide range of acidic soils including Andisols, and often explained by the formation of stable organo-metal complexes. However, the chemical variation and the long-term stability of the complexes remain unclear. We thus compa...

Organic matter (OM) can be protected from abiotic and biotic breakdown via its association with iron (Fe) and aluminum (Al) in both terrestrial and aquatic ecosystems. We hypothesized that the natural variation in sediment redox and pH regime govern how the two metals interact with OM in near-surface mineral sediments of 40 kettle holes of varying...

Understanding the controls on the amount and persistence of soil organic carbon (C) is essential for predicting its sensitivity to global change. The response may depend on whether C is unprotected, isolated within aggregates, or protected from decomposition by mineral associations. Here, we present a global synthesis of the relative influence of e...

Background Benefits to agricultural yield improvement, soil degradation prevention, and climate mitigation are central to the synergies of soil organic carbon (SOC) build-up. However, the contributions of small-scale farmers, the main target of recent agricultural and rural development policies, to SOC enhancement are understudied. Here, we present...

Strong control of iron (Fe) and aluminum (Al) phases on soil phosphorus (P) dynamics is well established. How organic and inorganic P forms are associated with Fe and Al phases remains poorly understood, and sequential density fractionation thus allows one to assess the soil continuum from organic-rich particles (< 1.8 g cm⁻³) to organo-mineral par...

Identifying the soil properties that control soil organic carbon (SOC) storage is a critical step to monitor and manage SOC at regional and global scales. While clay content has been commonly used to explain SOC variation and to model SOC dynamics, recent studies point to the significant control of extractable metal phases such as pyrophosphate ext...

Global significance of iron (Fe) and aluminum (Al) for the storage of organic matter (OM) in soils and surface sediments is increasingly recognized. Yet specific metal phases involved or the mechanism behind metal–OM correlations frequently shown across soils remain unclear. We identified the allocation of major metal phases and OM to density fract...

Global significance of iron (Fe) and aluminum (Al) for the storage of organic matter (OM) in soils and surface sediments is increasingly recognized. Yet specific metal phases involved or the mechanism behind metal-OM correlations frequently shown across soils remain unclear. We identified density fraction locations of major metal phases and OM usin...

Ecosystem property and function can be considerably altered by changes in agricultural land use. Despite the expansion of abandoned fields in temperate humid areas worldwide, little is known about how ecosystem C stock and its partitioning change with secondary succession, especially under the invasion of non-native, fast-growing plants. Using a pa...

Radiocarbon is a critical constraint on our estimates of the timescales of soil carbon cycling that can aid in identifying mechanisms of carbon stabilization and destabilization and improve the forecast of soil carbon response to management or environmental change. Despite the wealth of soil radiocarbon data that have been reported over the past 75...

Potentially significant effects of soil fauna on plant litter decomposition are difficult to quantify by the commonly used litter bag approach due to physical constraints of meshed bag and inevitable loss of litter fragments through the mesh. We thus tested the applicability of two bag-free approaches, physical fractionation and micromorphological...

Drought is a major risk in global agriculture. Building-up soil organic carbon (SOC) enhances soil fertility and efficient use of rainwater, which can increase drought tolerance in food production. SOC management demonstrates its benefit at various locations and is a promising means to achieve food security and climate mitigation at once. However,...

Organic matter (OM) in surface soil is largely present within porous mineral-dominant clusters called aggregates. Recent studies have shown the localization of OM within large macroaggregate structure based on synchrotron X-ray micro-computed tomography (μCT) coupled with a vapor-phase, osmium (Os)-staining pretreatment. Here we developed a new app...

Aims We aimed to compare uptake and litter flux of silicon (Si) across tropical tree species and sites on Mt. Kinabalu, Borneo. Methods Si flux components were measured at eight plots in tropical forests at four altitudes (700–3,100 m above sea level) on two types of parent materials (acidic sedimentary/granite rock and ultramafic igneous rock, pa...

Radiocarbon is a critical constraint on our estimates of the timescales of soil carbon cycling that can aid in identifying mechanisms of carbon stabilization and destabilization, and improve forecast of soil carbon response to management or environmental change. Despite the wealth of soil radiocarbon data that has been reported over the past 75 yea...

Radiocesium (RCs) is selectively adsorbed on interlayer sites of weathered micaceous minerals, which can reduce the mobility of RCs in soil. Therefore, soils developed from mica-deficient materials (e.g. serpentine soils) may have a higher risk of soil-to-plant transfer of RCs. Soils were collected from three serpentine soil profiles; Udepts in Oey...

Examination of easily soluble organic matter over a growing season in a rice paddy ecosystem under elevated CO 2 : searching for labile organic nitrogen in soil. Soil nitrogen (N) plays a crucial role for plant productivity and rice paddy field ecosystems are characterised by a low N use efficiency. While the nature of plant-available N in soil rem...

Ferromanganese minerals are widely distributed in subseafloor sediments and on the seafloor in oceanic abyssal plains. Assessing their input, formation and preservation is important for understanding the global marine manganese cycle and associated trace elements. However, the extent of ferromanganese minerals buried in subseafloor sediments remain...

Soil organic matter (SOM) turnover increasingly is conceptualized as a tension between accessibility to microorganisms and protection from decomposition via physical and chemical association with minerals in emerging soil biogeochemical theory. Yet, these components are missing from the original mathematical models of belowground carbon dynamics an...

Mechanisms of protecting soil carbon (C) are still poorly understood despite growing needs to predict and manage the changes in soil C or organic matter (OM) under anticipated climate change. A fundamental question is how the submicron-scale interaction between OM and soil minerals, especially poorly-crystalline phases, affects soil physical aggreg...

Interaction of organic matter (OM) with soil mineral components plays a critical role in biophysical organization (aggregate structure) as well as in biogeochemical cycling of major elements. Of the mineral components, poorly-crystalline phases rich in iron (Fe) and aluminum (Al) are highly reactive and thus contribute to both OM stabilization and...

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 tu...

Rice paddy fields, producing a major staple food to support growing world populations, represent a major source of greenhouse gases (GHGs) from agricultural ecosystems. The GHG emissions, mainly as CH4 and N2O from paddy ecosystems, are highly sensitive to both environmental and management factors. Yet the identification of specific factors, a fund...

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 trop...

How elevation of atmospheric CO2 concentrations may impact soil organic matter dynamics in paddy fields?

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...

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...

Probing the Future: Response of Soil Organic Carbon and Nitrogen dynamics to elevated CO2 in a paddy field

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...

Fundamental questions remain about the role of bacterivorous protists in regulating soil carbon (C) and nitrogen (N) cycling, including the ways in which protists interact with physical and chemical factors to influence soil decomposer responses to increased temperature. Amoebae in particular deserve attention given their status as one of the most...

Key message Borneo’s tropical heath (kerangas) forest has limited soil nutrient availability, and high variation in aboveground structure and fine-root biomass. This variation depends on altitude and soil nitrogen availability. Abstract To elucidate the biotic and abiotic factors affecting the variation in fine-root biomass (FRB, 4–N and NO3–N), tr...

Organic matter (OM) bound to soil mineral particles (higher-density particles) tends to be more stabilized, enriched in 13C and 15N, and has a lower C:N ratio. Yet how these variations in OM chemistry are linked to the nature of organo-mineral assemblage remains poorly understood, especially in allophanic soils where high amounts of OM are stabiliz...

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 material's biodegradation in fields. Basic information is needed on the relationship between burning conditions and the res...

Soils cover most of the Earth’s terrestrial surface and have an indispensable function in the global cycling of nutrients. A key component of soils is the assemblage of organisms present, members of which are responsible for carrying out many small scale processes that underlie environmentally important functions. One of the most important processe...

Indirect emission of nitrous oxide (N2O) through groundwater and surface drainage is commonly estimated based on the apparent positive relationship between the concentrations of nitrate ([NO3 (-)]) and dissolved N2O ([N2O]) for regional and national assessments. Field observations of the ratio of [N2O]-[NO3 (-)], however, rarely follow such relatio...

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...

The 14C contents and the carbon (C) pool sizes for Rothamsted Carbon (RothC) model pools and soil fractions determined by physical separation and subsequent chemical oxidation were directly compared for an agricultural topsoil (Dystric Cambisol) from a long‐term experimental field in central Japan. The RothC model was run from 1975 to 2009 and the...

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 substra...

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 forOM. We therefore investigated nitrogen associations w...

Soil organic matter is important carbon reservoir managing CO2 concentration in the atmosphere. However, the mechanism of soil carbon stabilization is little known. Radiocarbon data from soil organic matter and soil respiration provide powerful constrains for determining carbon dynamics (Trumbore 2000). Here, the 14C data from six sites in Japanese...

Interactions of plant- and microbially-derived organic matter with mineral phases exert significant controls on the stabilization of organic matter (OM) as well as other biogeochemical processes in soil. Density fractionation techniques have been successful in distinguishing soil organo-mineral particles of different degrees of microbial alteration...

The radiocarbon ((14)C) of total carbon (TC) in atmospheric fine particles was measured at 6 h or 12 h intervals at two sites, 50 and 100 km downwind from Tokyo, Japan (Kisai and Maebashi) in summer 2007. The percent modern carbon (pMC) showed clear diurnal variations with minimums in the daytime. The mean pMC values at Maebashi were 28 ± 7 in the...

Climate and parent material strongly control vegetation structure and function, yet their control over the belowground microbial community is poorly understood. We assessed variation in microbial lipid profiles in undisturbed forest soils (organic and surface mineral horizons) along an altitudinal gradient (700, 1,700, and 2,700m a.s.l. mean annual...

Volcanic-ash soils are often darker and hold significantly greater amounts of organic matter (OM) than non-volcanic soils presumably because inorganic constituents unique to such soil (e.g., poorly-crystalline minerals and dissolved aluminum) have high capacity to stabilize OM. It has been shown that carbon (C) in Japanese volcanic-ash soils can be...

Sequential density fractionation separated soil particles into “light” predominantly mineral-free organic matter vs. increasingly “heavy” organo-mineral particles in four soils of widely differing mineralogy. With increasing particle density C concentration decreased, implying that the soil organic matter (OM) accumulations were thinner. With thinn...

Organic matter (OM) in soils often associates intimately with the surfaces of fine-grained minerals. We used two measures of OM–mineral associations, based on either the energetics of N2 gas sorption (C-constant in BET equation) or changes in mineral specific surface area (SSA) upon removal of OM, that provide complementary information on organic c...

Abstract Density separations show great promise in elucidating the progression of organic matter decomposition and mineral association in soils. We review the literature on these separations, with a focus on the low-density material released by sonication, the so-called “occluded”, “aggregate-protected” or mineral-associated low-density fraction (m...

The contribution of soil microbial residues to stable carbon pools may be of particular importance in the tropics where carbon residence times are short and any available carbon is rapidly utilized. In this study we investigated the vertical distribution of microbially-derived amino sugars in two tropical forests on contrasting meta-sedimentary and...

Soil organic matter is an important carbon reservoir managing CO2 concentration in the atmosphere. However, the mechanism of soil carbon stabilization is little known. To explore the rate of C cycling in a temperate forest soil in Japan, at one of AsiaFlux monitoring sites, we sequentially density fractioned at 1.0, 1.6, 1.8, 2.1 and 2.4 g cm-3 on...

Density fractionation in combination with sonication is an effective approach isolating the soil organic matter (SOM) pools that differ in turnover rate and underlying stabilization mechanisms. For instance, low-density fraction (LF) is expected to have higher turnover rate and sensitivity to climate change than high-density fraction (HF). We exami...

We investigated tree species effects on the soil microbial community in the tropical montane forest on Mt. Kinabalu, in Malaysian Borneo. We investigated microbial composition (lipid profile) and soil physicochemical parameters (pH, moisture, total C, N and phenolics concentration) in top 5-cm soils underneath two conifers (Dacrycarpus imbricatus a...

Physically- and biochemically-distinct fractions of soil organic matter (SOM) can be separated by density to yield: (i) low-density plant detritus fraction easily separable from soil minerals (f-LF), (ii) low-density materials strongly associated with minerals (m-LF), and (iii) high-density fraction (HF) rich in microbially-processed organic matter...