Search

Submerged rice cultivation is characterized by redox fluctuations and results in the formation of paddy soils, often accompanied by soil organic carbon (SOC) accumulation. The impact of redox fluctuations and the underlying soil type on the fate of organic carbon (OC) in paddy soils are unknown. Hence, we mimicked paddy soil development in the laboratory by exposing two soil types with contrasting mineral assemblages (Alisol and Andosol) to eight anoxic-oxic cycles over 1 year. Soils regularly received 13C-labeled rice straw. As control we used a second set of samples without straw addition as well as samples under static oxic conditions with and without straw. Headspaces were analyzed for carbon dioxide and methane as well as their δ13C signatures, whereas soil solutions were analyzed for redox potential, pH, dissolved iron, and dissolved organic carbon (DOC and DO13C). At the end of the experiment, when eight redox cycles were completed, mineral-associated organic matter (MOM) was isolated by density fractionation and characterized for δ13C, non-cellulosic carbohydrates, and lignin-derived phenols. Moreover, changes in the soil's microbial community structure were measured. For both soil types, headspace data confirmed less respiration in straw-amended soils with redox fluctuation than in those under static oxic conditions. The δ13C data revealed that, irrespective of soil type, straw carbon allocation into MOM was larger in soils with redox fluctuation than in those with static oxic conditions. A net increase in MOM after the one-year incubation, however, was only observed in the respective Andosol, probably due to abundant reactive minerals capable of OC uptake. In the Alisol, straw OC most likely exchanged initial MOM. A potential for lignin accumulation in the MOM of soils incubated with straw and redox fluctuation was observed for both soil types. Lignin and carbohydrates suggest a plant origin of MOM formed under redox fluctuation. The initially similar bacterial community composition of the Alisol and Andosol changed differently under redox fluctuation. The stronger change in the Alisol indicates less protective microbial habitats. In summary, the overall turnover of straw OC in soils under redox fluctuation seems to be independent of soil type, while net accumulation of SOC as well as the evolution of the bacterial community structure may in part depend on soil type, suggesting an impact of the soil's mineral composition. [ABSTRACT FROM AUTHOR]

Two major human-made problems in rice production systems in the north of Vietnam concern the low plant-available silicon content of soils and the low biodiversity. The results of the LEGATO project suggest a change to an environmentally friendly rice production system that will help to recover biodiversity. We propose here a framework for a demonstration and dissemination model that will be exemplary for the farmers once it has been successfully realized. We advocate local-option models in different districts to demonstrate to farmers. The methods should be adapted to local and ecoregional differences in climate and land-use tradition, and they explicitly take into account soil care, organic fertilizer, manual weeding, native nectar-rich plant bunds, manual pest snail collection, hymenopteran nesting aids, and biodiversity and yield monitoring. [ABSTRACT FROM AUTHOR]

Silicon (Si) mitigates abiotic and biotic stresses for rice plants (Oryza sativa L.). Here, we test relationships between Si cycling, plant growth, and pest and fungal attacks in rice agroecosystems. We conducted a plot experiment on Si fertilization in a Southern Vietnamese paddy, where plant-available Si was inherently low. For two cropping seasons, we investigated the temporal dynamics of Si in soil solution, plant Si uptake, and the occurrence of leaf folders (Cnaphalocrocis medinalis) and rice blast caused by the fungus Magnaporthe oryzae. Silicon application increased Si concentrations in soil solutions collected in the field as expected from previous laboratory experiments. Soil solution Si concentrations were furthermore affected by Si uptake by plants and by recycling Si with rice straw ash. Silicon concentrations in rice leaves at tillering stage increased with increasing Si application. However, surprisingly, no relationship between Si in soil solution and Si concentration in straw at maturity stage was found. The occurrences of leaf folders and rice blast disease were mitigated by increased Si uptake. However, rice biomass production was not affected, probably because the biotic stress level was generally low. Our field data emphasize the importance of recycling crop residues in rice fields for the Si supply to plants, especially in regions with low Si availability. They furthermore show that under field conditions, the relationship between dissolved Si in soil solution and Si uptake by rice plants is not as straightforward as expected and thus needs to be further investigated. [ABSTRACT FROM AUTHOR]

Current climate and land-use changes affect regional and global cycles of silicon (Si), with yet uncertain consequences for ecosystems. The key role of Si in marine ecology by controlling algae growth is well recognized but research on terrestrial ecosystems neglected Si since not considered an essential plant nutrient. However, grasses and various other plants accumulate large amounts of Si, and recently it has been hypothesized that incorporation of Si as a structural plant component may substitute for the energetically more expensive biosynthesis of lignin. Herein, we provide evidence supporting this hypothesis. We demonstrate that in straw of rice ( Oryza sativa) deriving from a large geographic gradient across South-East Asia, the Si concentrations (ranging from 1.6% to 10.7%) are negatively related to the concentrations of carbon (31.3% to 42.5%) and lignin-derived phenols (32 to 102 mg/g carbon). Less lignin may explain results of previous studies that Si-rich straw decomposes faster. Hence, Si seems a significant but hardly recognized factor in organic carbon cycling through grasslands and other ecosystems dominated by Si-accumulating plants. [ABSTRACT FROM AUTHOR]

Aims and background: Release of 'non-exchangeable' potassium (K) from interlayers of illite is diffusion-controlled and has been shown to depend on the solution concentration of K and other cations (Ca, Mg, NH). Methods: We analysed changes in soil solution concentrations of K and competing cations in situ at different distances from the root surface over time and related them to the transformation of illite, as revealed by X-ray diffraction, and chemical measures of differently bound K. Results and Conclusions: Within 49 and 98 days, respectively, 6.4 and 14.4 % of the illite's total K was released upon contact with the root system. Mixed layered minerals increased from 33 (0 d) to 35 (49 d) to 40 % (98 d). Release of K from interlayers and the transformation of illite occurred at soil solution K concentrations close to the threshold of 80 μM suggested earlier. Concentrations of Ca and Mg increased with decreasing distance from the root surface, promoting the release of K. The NaBPh method supposed to determine 'non-exchangeable' K extracted only 1/3 of the total K from illite. [ABSTRACT FROM AUTHOR]

More than 50% of the world's population feeds on rice. Soils used for rice production are mostly managed under submerged conditions (paddy soils). This management, which favors carbon sequestration, potentially decouples surface from subsurface carbon cycling. The objective of this study was to elucidate the long-term rates of carbon accrual in surface and subsurface soil horizons relative to those of soils under nonpaddy management. We assessed changes in total soil organic as well as of inorganic carbon stocks along a 2000-year chronosequence of soils under paddy and adjacent nonpaddy management in the Yangtze delta, China. The initial organic carbon accumulation phase lasts much longer and is more intensive than previously assumed, e.g., by the Intergovernmental Panel on Climate Change ( IPCC). Paddy topsoils accumulated 170-178 kg organic carbon ha−1 a−1 in the first 300 years; subsoils lost 29-84 kg organic carbon ha−1 a−1 during this period of time. Subsoil carbon losses were largest during the first 50 years after land embankment and again large beyond 700 years of cultivation, due to inorganic carbonate weathering and the lack of organic carbon replenishment. Carbon losses in subsoils may therefore offset soil carbon gains or losses in the surface soils. We strongly recommend including subsoils into global carbon accounting schemes, particularly for paddy fields. [ABSTRACT FROM AUTHOR]

A modelling approach was used to extend the knowledge about the processes that affect the availability of the nutrient P and the toxic agent AsV in the rhizosphere in the presence of a strong sorbent. Based on compartment system experiments in which Zea mays was grown the following hypothesis were assumed: a) measured P concentration gradients can be explained by the mobilisation of P by the root exudate citrate, and b) measured AsV concentration gradients can be explained by the simultaneous effect of the competitive sorption of AsV and P and the competitive uptake of AsV and P. First, the feasibility of the applied description of soil chemical processes was justified. Then competitive uptake was implemented in the computer code using two different mathematical approaches. Our model calculation provided support for hypothesis a) and suggested that hypothesis b) has to be extended. The results show that the competitive uptake of AsV and P has an influence on AsV concentrations in the rhizosphere, but including competitive uptake was not sufficient to predict observed AsV concentration profiles. Recent results on plant As-metabolism like AsIII efflux and Si AsIII interaction probably have to be included in addition for simulation of measured AsV concentration profiles. [ABSTRACT FROM AUTHOR]

Soil organic matter (OM) can be stabilized against decomposition by association with minerals, by its inherent recalcitrance and by occlusion in aggregates. However, the relative contribution of these factors to OM stabilization is yet unknown. We analyzed pool size and isotopic composition (14C, 13C) of mineral-protected and recalcitrant OM in 12 subsurface horizons from 10 acidic forest soils. The results were related to properties of the mineral phase and to OM composition as revealed by CPMAS 13C-NMR and CuO oxidation. Stable OM was defined as that material which survived treatment of soils with 6 wt% sodium hypochlorite (NaOCl). Mineral-protected OM was extracted by subsequent dissolution of minerals by 10% hydrofluoric acid (HF). Organic matter resistant against NaOCl and insoluble in HF was considered as recalcitrant OM. Hypochlorite removed primarily 14C-modern OM. Of the stable organic carbon (OC), amounting to 2.4–20.6 g kg−1 soil, mineral dissolution released on average 73%. Poorly crystalline Fe and Al phases (Feo, Alo) and crystalline Fe oxides (Fed−o) explained 86% of the variability of mineral-protected OC. Atomic Cp/(Fe+Al)p ratios of 1.3–6.5 suggest that a portion of stable OM was associated with polymeric Fe and Al species. Recalcitrant OC (0.4–6.5 g kg−1 soil) contributed on average 27% to stable OC and the amount was not correlated with any mineralogical property. Recalcitrant OC had lower Δ14C and δ 13C values than mineral-protected OC and was mainly composed of aliphatic (56%) and O-alkyl (13%) C moieties. Lignin phenols were only present in small amounts in either mineral-protected or recalcitrant OM (mean 4.3 and 0.2 g kg−1 OC). The results confirm that stabilization of OM by interaction with poorly crystalline minerals and polymeric metal species is the most important mechanism for preservation of OM in these acid subsoil horizons. [ABSTRACT FROM AUTHOR]

Soil solution composition changes with time and distance from the root surface as a result of mass flow, diffusion, plant nutrient uptake and root exudation. A model system was designed, consisting of a root compartment separated from the bulk soil compartment by a nylon net (30 μm mesh size), which enabled independent measurements of the change of soil solution composition and soil water content with increasing distance from the root surface (nylon net). K+ concentration in the rhizosphere soil solution decreased during the initial growth stage (12 days after planting, DAP). Thereafter K+ accumulated with time, due to mass flow as the dominating process. The extend of K+ accumulation depended on the initial fertiliser application. As K+ concentrations in soil solution increase, not only as a result of transport exceeding uptake, but also as a result of decreasing soil water content, it is hypothesised that K concentration in soil solution is not the only trigger for the activity of K transporters in membranes, but ABA accumulation in roots induced by decreasing soil matric potentials may add to the regulation. A strong decrease of rhizosphere pH with time is observed as a result of H+ efflux from the roots in order to maintain cation-anion balance. In addition the K+ to Ca2+ ratio was altered continuously during the growing period, which has an impact on Ca2+ uptake and thus firmness of cell walls, apoplast pH, membrane integrity and activity of membrane transporters. The value of osmotic potential in the rhizosphere soil solution increased with time indicating decreasing soil water availability. Modelling approaches based on the data obtained with the system might help to fill in the time gaps caused by the low temporal resolution of soil solution sampling method. [ABSTRACT FROM AUTHOR]

Copyright of Archives of Agronomy & Soil Science is the property of Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We present a method to quantify the distribution and growth of plant roots in soil using X-ray computed tomography. The method can be applied to complex root systems and will be useful for the study of root--soil interactions. Root system architecture and associated root--soil interactions exhibit large changes over time. Nondestructive methods for the quantification of root systems and their temporal development are needed to improve our understanding of root activity in natural soils. X-ray computed tomography (X-ray CT) was used to visualize and quantify growth of a single Vicia faba L. root system during a drying period. The plant was grown under controlled conditions in a sandy soil mixture and imaged every second day. Minkowski functionals and Euclidean distance transform were used to quantify root architectural traits. We were able to image the root system with water content decreasing from 29.6 to 6.75%. Root length was slightly underestimated compared with destructive measurements. Based on repeated measurements over time it was possible to quantify the dynamics of root growth and the demography of roots along soil depth. Measurement of Euclidean distances from any point within the soil to the nearest root surface yielded a frequency distribution of travel distances for water and nutrients towards roots. Our results demonstrate that a meaningful quantitative characterization of root systems and their temporal dynamics is possible. [ABSTRACT FROM AUTHOR]

The transfer of nutrients and contaminants from bulk soil to roots and into plants depends on many plant and soil processes. The RhizoMath approach for modeling co-occurring processes in the rhizosphere, including speciation in the soil solution, is based on coupling the mathematical package MATLAB with the geochemical code PHREEQC. In addition to the built-in initialization module that performs calibration against experimental data, RhizoMath's greatest advantage is that different geochemical models (with and without charge balance) and geometries (planar and radial) are already included. Moreover, due to its graphical user interface, the tool can be applied without changing the source code or a complex input file. The model was verified using a benchmark and experimental data: (i) the initialization module was successfully applied to describe concentrations measured in soil solution samples; (ii) the theoretical problem "diffusion of K toward a single root" was used to demonstrate that the performance of applied numerical methods is comparable to other approaches; and (iii) for compartment system experiments involving more complex speciation, RhizoMath was able to describe the observed effects of citrate exudates on the simultaneous transport of arsenate and phosphate that compete for surface binding sites with each other and with other oxyanions such as citrate. [ABSTRACT FROM AUTHOR]