Non-paddy Soils Research Articles

Mitigating soil nitrification and greenhouse gas emissions in non-paddy cropping systems by micro-molar hydrogen peroxide

Non-paddy cropping systems play a significant role in food production. However, excessive nitrogen loss from non-paddy soils through nitrate leaching and ammonia volatilization poses a significant challenge to environmental sustainability. In this study, microcosm and field-scale experiments were conducted to explore the potential for using hydrogen peroxide (H2O2) to mitigate nitrogen loss and greenhouse gas emissions, aiming at filling gaps in knowledge regarding the underlying biochemical mechanisms. The results show that input of micromolar H2O2 from either artificial addition or natural rainwater into soils in the presence of magnetite (Fe3O4) could trigger Fenton-like reaction, which inhibited microbially mediated nitrification of soil-borne ammonium but did not affect the growth of the test crop plant (water spinach). In the absence of Fe3O4, input of rainwater-borne H2O2 into non-paddy soils caused reduction in the emissions of nitrous oxide (N2O) and carbon dioxide (CO2). There was a trend showing that the degree of reduction in N2O and CO2 fluxes increased with increasing concentration of rainwater-borne H2O2. It was likely that microbially mediated reduction of iron oxides took place during rainfall events, providing Fe(II) that is needed for reaction with rainwater-borne H2O2, triggering Fenton-like reaction to inhibit the soil microbes that mediate production of N2O and CO2 in the soils. The findings obtained from this study have implications for developing strategies to manage soil‑nitrogen to minimize its environmental impacts.

Long-term paddy use influences response of methane production, arsenic mobility and speciation to future higher temperatures

Anaerobic microbial metabolisms make flooded paddy soils a major source of the greenhouse gas methane (CH4) and mobilize toxic arsenic (As), threatening rice production and consumption. Increasing temperatures due to climate change enhance these microbially mediated processes, increasing their related threats. Chronosequence studies show that long-term paddy use (“age”) changes soil properties and redox biogeochemistry through soil organic carbon (SOC) accumulation, its association to amorphous iron (Fe) phases, and increased microbial activity. Using paddy and non-paddy soils from a chronosequence as proxies of soil development and incubating them at different temperatures, we show that paddy soil age influences the response of paddies to changes in temperature. Older paddies showed up to a 6-fold higher CH4 production with increasing temperature, compared to a 2-fold increase in young ones. Contrarily, changes in As mobility were higher in non-paddies and young paddies due to a lack of Fe-SOC-sorption sites. Temperature increased the formation of phytotoxic methylated As in all paddies, posing a risk for rice production. Mitigation strategies for future maintenance, abandonment, or management of paddy soils should include the consideration that history of use shapes the soils' biogeochemistry and microbiology and can influence the response of paddy soils to future temperature increases.

Fertility capability classification (FCC) of soils of a lower Brahmaputra valley area of Assam, India

Fertility capability classification (FCC) is a system of classification which uses pedological data of soils and coverts it into capability classes based on major fertility constraints portrayed by the soils. The present study was aimed to classify the soils major landforms of a lower Brahmaputra valley region of Assam, India in to FCC classes, to suggest specific management practices in order to overcome the fertility constraints and improve the crop productivity.The major strata types used were found to be loamy top soil, 'L' and clayey top soil 'C'. The sub-strata type found were loamy sub soil, 'L' and clayey sub soils, 'C'. The major condition modifiers or the major fertility constraints were found to be Al toxicity 'a’ and 'a-', high leaching potential, 'e', low nutrient reserves 'k' and 'g' waterlogging. The paddy soils of alluvial plains were classified into La-eg and Lg+a-e. The tea growing soils of younger alluvial plains were classified into Car+e. The non-paddy soils of alluvial plains were categorized as Ca-gke. The soils of uplands and inselberg were categorized into LCae class. The study revealed that FCC classification can successfully bring out the soil fertility constraints and can be very much helpful in soil fertility management for sustainable crop productions.

Redox-driven changes in organic C stabilization and Fe mineral transformations in temperate hydromorphic soils

Paddy soils experience long-term redox alternations affecting the interactions between the biogeochemical cycling of iron (Fe) and carbon (C). Differences in particle aggregation and soil organic matter (SOM) turnover are likely to both affect and be affected by the trajectory of Fe mineral evolution/crystallinity with redox fluctuations. We hypothesized that the legacy effects of redox cycling under paddy management affects particle aggregation, the distribution and mineralogy of Fe (hydr)oxides between particle-size fractions, and the interaction with SOM stabilization. Moreover, we expected underlying processes to be different in paddy eluvial and illuvial horizons, particularly due to the different inputs and redox conditions these horizons experience. To test these hypotheses, we evaluated the distribution of Fe species and organic C between different aggregate and particle-size fractions in topsoil (eluvial) and subsoil (illuvial) horizons of soils under long-term paddy and non-paddy management in NW Italy, as well as mineralogical changes in Fe phases by Fe K-edge Extended X-ray Adsorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopy. Our findings indicate that although paddy topsoils are depleted in hydrous Fe oxides with respect to non-paddy soils, they can stabilize important amounts of C through mineral associations, particularly with finer particle-size fractions rich in less crystalline Fe phases. We also show that redox cycling can influence microaggregate stability and consequently the distribution of Fe phases and OC between intra and inter-microaggregate fractions. On the other hand, illuvial horizons under paddy management were enriched in short-range ordered hydrous Fe oxides and this contributed to enhanced microaggregate formation and C stabilization with respect to non-paddy subsoils.

Forms and distribution pattern of soil Fe (Iron) and Mn (Manganese) oxides due to long-term rice cultivation in fars Province Southern Iran

ABSTRACT To assess the effect of long-term rice cultivation and aquic conditions on the physicochemical properties and different forms of iron and manganese and also to make a comparison with non-paddy and adjoining uncultivated soils in calcareous soils of southern Iran. The soil samples were taken from the paddy and non-paddy soils with similar parent materials and landforms. The results showed that in the paddy fields, the amounts of available (extracted by DTPA), total (extracted by nitric acid), and poorly crystalline iron and manganese oxides (extracted by ammonium oxalate) were higher than those in the non-paddy soils and show the highest amount in the surface horizons of paddy fields. In general, the paddy land use increases the iron and manganese oxides with poorly crystalline form, Feo/Fed and Mno/Mnd ratios by 1.84, 1.57, 2 and 3.2, respectively. It also reduces the amount of pedogenic iron and manganese oxides and their crystalline forms relative to the adjacent uncultivated soils. Finally, a significant positive relationship was found between the amounts of poorly crystalline iron and manganese oxides and total form and the amount of organic matter and clay in the soils under study. Also, by changing the land use from the non-paddy to paddy field, the amount of smectite increased and, in contrast, the amounts of illite and chlorite decreased. Therefore, it seems that the significant amount of organic matter introduced and their burial caused by the plowing operation and, on the other hand, their slow decomposition due to the inactivation of aerobic activities in the reduction conditions is among the factors affecting the variations of chemical properties in paddy soils and, generally, wet land soils.

Geochemical variability in the soils of Bangladesh as affected by sources of irrigation water and inundation land types

Paddy soils in Bangladesh experience extensive irrigation with groundwater and surface water, both having variable geochemical constituents. The soils also have topological variations across the landscape. To understand the geochemical variability in the soils as affected by the different sources of irrigation water and the topographical variability, cultivation zones of paddy soils irrigated with both groundwater (n = 904) and surface water (n = 281) across Bangladesh were sampled and analyzed for a suit of seventeen geochemical elements. This study also assessed the extent and distribution of arsenic and the other geochemical elements in the paddy soils (n = 1209) as well as in a set of neighboring non-paddy soils (n = 235) within the different inundation land types (highland, medium highland-1, medium highland-2, medium lowland, lowland and very lowland) of Bangladesh. The mean concentrations of aluminum (26,000 mg/kg), cobalt (13 mg/kg), copper (32 mg/kg), iron (28,250 mg/kg), lead (18 mg/kg), magnesium (8050 mg/kg), molybdenum (1.02 mg/kg), nickel (41 mg/kg), potassium (4870 mg/kg), sodium (750 mg/kg) and zinc (70 mg/kg) in the surface water-irrigated paddy soils were found to be significantly (0.001 ≥ p ≤ 0.05) higher compared to the concentrations in the soils irrigated with groundwater (23,400; 12; 28; 25,650; 17; 7000; 0.96; 36; 4350; 600; and 62 mg/kg, respectively). Therefore, surface water used for paddy irrigation could increase the inputs of a number of toxic elements in the paddy soils having potential risk of crop contamination. Arsenic in the paddy and non-paddy soils varied significantly (F = 24.74, p < 0.001 and F = 3.42, p < 0.01, respectively) within the inundation land types, the very lowland (9.95 and 6.72 mg/kg, respectively) and lowland (8.33 and 5.20 mg/kg, respectively) having the highest mean arsenic concentrations and the medium highland-1 (5.27 and 5.17 mg/kg, respectively) having the lowest. The concentrations of the other geochemical elements analyzed were also observed to be higher, in general, in the soils of very lowland and lowland. Since the low-level lands are predominantly used for paddy cultivation, higher concentrations of various toxic elements, particularly arsenic, in such soils pose an increased risk of rice toxicity in Bangladesh. The results of this study present an inimitable geochemical database for the surface soils across Bangladesh which can be used in any future studies on the geomorphologically variable agricultural and non-agricultural Bangladeshi soils, providing a basis for environmental pollution assessment and sustainable mitigation approaches.

Redox-driven changes in water-dispersible colloids and their role in carbon cycling in hydromorphic soils

Redox fluctuations in hydromorphic soils can influence ecosystem functions by altering the cycling of organic carbon (OC) and other elements in both the aqueous and solid phases throughout the soil profile. Most studies focusing on the mobility of dissolved OC in rice paddy soils have often disregarded the contribution of colloidal particles. We provide a detailed chemical and size-related characterization of water-dispersible soil colloids and their depth distribution in two soil profiles under long-term temperate paddy and non-paddy management, by asymmetric flow field-flow fractionation. Anoxic conditions enhanced colloid dispersion with a preferential release of the finer colloid fractions (5 and 4-fold increase in the < 30 nm and 30–240 nm fractions, respectively). The mobility of OC-rich, iron (Fe) and aluminium (hydr)oxide and aluminosilicate colloids along the soil profile was probably responsible for their depletion in the topsoil and a corresponding accumulation in the deeper illuvial horizons compared to the non-paddy soil. However, the release, percolation and subsequent reoxidation of Fe2+ was also shown to be a plausible mechanism leading to the formation of fine colloids in the subsoil. Redox-driven changes in colloid distribution were also linked to the differences in OC and pedogenetic Fe stocks in these two agro-ecosystems.

Characterization and Classification of Paddy and Associated Non Paddy Soils of Upper Brahmaputra Valley of Assam

An investigation was done to characterize and classify paddy and associated nonpaddy soils of the Upper Brahmaputra Valley of Assam. In this study,three districts viz., Sivasagar, Jorhat, and Golaghat in the Brahmaputra valley of Assam were selected. These districts are located in the Upper Brahmaputra Valley Zone (UBV) of Assam. Combined, they cover an approximate area of 9021 sq. Km or about 55.7 per cent of UBV. Six soil profiles, three each from mono-cropped paddy and, associated nonpaddy areas were collected from the districts mentioned. These samples were collected Horizon wise from each soil profile. Each soil sample was dried and passed through a 2 mm sieve. The samples from each pedon were evaluated for morphological and Physicochemical characteristics,based on morphological and physicochemical properties. Thesoils were classified as Aquic Dystric Eutrudepts (P1), Dystric Eutrudepts (NP1, NP2, P3, NP3) and Dystric Fluventic Eutrudepts (P2).

Diverse biological sources of core and intact polar isoprenoid GDGTs in terrace soils from southwest of China: Implications for their use as environmental proxies

Archaea are important participants in soil biogeochemical cycles. Their membrane lipids are usually composed of isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs), which are ubiquitous in the environments and widely applied as biomarkers for archaeal biogeochemical processes and palaeothermometry. However, the biological sources of iGDGTs and environmental factors controlling their distribution in soils are elusive. We examined the archaeal lipid composition and community structure along an altitudinal soil transect in the Hani terrace wetlands in Southwest of China by combining organic geochemical and high-throughput sequencing approaches. The results show that monoglycosyl -iGDGTs (1G-iGDGTs) are the dominating intact polar lipids (IPLs) and possible precursor of core-iGDGTs preserved in soils. iGDGTs from Hani soils may come from several archaeal lineages based on their significant positive correlations in both core lipids and IPLs. Thaumarchaeota Group I.1b most likely contribute to crenarchaeol and its regio-isomer in non-paddy soils, while Bathyarchaeota, Methanobacteria and Methanomicrobia may predominantly contribute to GDGT-0 in water-saturated paddy soils. Our results indicate that the paleotemperature proxy TEX86 in soils can be affected by changes in archaeal community structure in addition to environmental variables such as soil water content and pH, but not independently by temperature. In addition, the lower TEX86 in humid soils may be partly attributed to the insufficient peak separation of iGDGTs with cyclopentyl moieties, which contain abnormally high abundances of shoulder peaks that may be contributed by anaerobic Archaea. This study sheds light on the increasing role of microbial community structure in affecting the fidelity of TEX86 in terrestrial environment, which should be taken into consideration when applying it to paleo-soil temperature studies.

An Assessment of Organic Carbon Fractions in Paddy and Associated Non-paddy Soils of Upper Brahmaputra Valley of Assam

An investigation was carried out to study the soil organic carbon fractions in paddy and associated non-paddy soils of Assam, India. Three districts viz. Sivasagar, Jorhat and Golaghat in the Brahmaputra valley of Assam were selected for this study. Soil samples were collected from six profiles, three each from mono-cropped paddy and associated non-paddy areas were collected. Horizon -wise, soil samples were analyzed for organic carbon fractions. Organic carbon, Humus C, Humin C, Humic acid C and Fulvic acid C were all analyzed from the samples. Organic carbon content in soil varied from 0.90 -7.90g kg⁻¹. Surface horizons of paddy soils contained lower amounts of humus C (0.255-0.330 per cent) and fulvic acid C (CFA) (0.135 – 0.180 per cent) and higher amounts of humin carbon (0.210 -0.475 per cent) and humic acid C (CHA) (0.120-0.150 per cent) as compared to that of non-paddy soils (0.300-0.435 and 0.195-0.300, 0.180-0.490 and 0.105-0.135 per cent respectively). It was concluded that the ratio of (CHA+CFA)/CTOC indicated less humification on the surface horizons of paddy soils. This is a direct result of prolonged submergence which led to humic acid carbon becoming less oxidized or humified.

Prediction of total silicon concentrations in French soils using pedotransfer functions from mid-infrared spectrum and pedological attributes

Silicon (Si) is the second most abundant element of the Earth's crust, and its terrestrial cycle depends on soil, vegetation, and human activities. The spatial extent of terrestrial Si perturbation is poorly documented since maps of Si concentration in soils are rare. In addition, Si content is rarely measured in non-paddy soil databases. Here we demonstrate that pedotransfer functions based on either pedological attributes (particle size fraction, pH, organic carbon, cation exchange capacity, calcium carbonate and parent material) or mid infrared spectra (MIRS) can be used to accurately predict total Si concentration. In this research, we utilised a unique dataset from the French monitoring network of soil quality (RMQS - Réseau de Mesures de la Qualité des Sols) database. Pedotransfer functions were built using a regression tree model on a subset of the data for which total Si concentration was measured. To compare the relative performance of the models obtained for the two different sources of data, a suite of performance indicators were calculated. Our results showed that PTF based on MIR spectra produces highly accurate and precise estimates of the total Si concentration for French soils. The pedological PTF is less accurate, but still provides a good estimation of the Si concentration. The pedological PTF provides an alternative method when only basic soil data are available, and an approximate estimation of Si concentrations is sufficient. These PTFs can be readily applied at the European scale except on a few soil groups not represented in France.

Contrasting evolution of iron phase composition in soils exposed to redox fluctuations

Ferric iron (FeIII) solid phases serve many functions in soils and sediments, which include providing sorption sites for soil organic matter, nutrients, and pollutants. The reactivity of Fe solid phases depends on the mineral structure, including the overall crystallinity. In redox-active soils and sediments, repeated reductive dissolution with subsequent exposure to aqueous ferrous iron (Fe2+) and oxidative re-precipitation can alter Fe phase crystallinity and reactivity. However, the trajectory of Fe mineral transformation under redox fluctuations is unclear and has been reported to result in both increases and decreases in Fe phase crystallinity. Several factors such as water budget, organic matter input, redox dynamics as well as the initial Fe phase composition might play a role. The objective of our study was to examine if Fe minerals in soils that differ in porosity-dependent water leaching rate and initial Fe phase crystallinity, demonstrate distinct mineral transformations when subjected to redox fluctuations. We sampled paired plots of two soil types under similar management but with different water leaching rates and contrasting Fe oxide crystallinity — an Alisol rich in crystalline Fe phases and an Andosol rich in short-range-ordered (SRO) Fe phases. The two soils were either exposed to several decades of redox fluctuations during rice paddy cultivation (paddy) or to predominantly oxic conditions in neighboring vegetable gardens (non-paddy). Paddy soils are uniquely suited for this type of study because they are regularly submerged and develop regular redox fluctuations. We also incubated the non-paddy soils in the laboratory for one year through eight anoxic/oxic cycles and monitored the aqueous soil geochemistry. Mössbauer spectroscopy was then used to evaluate Fe mineral speciation in field soils (paddy and non-paddy) and laboratory incubations. In the field soils, we found that redox fluctuation had contrasting effects on Fe oxide crystallinity, with crystallinity being lower in the Alisol paddy soil and higher in the Andosol paddy soil than in their corresponding non-paddy controls. In the laboratory incubation experiment, Eh, pH and dissolved Fe2+ responded as anticipated, with elevated Fe2+ concentrations during the anoxic periods as well as low Eh and high pH. Mössbauer measurements suggest the fluctuating redox incubation was beginning to alter Fe oxide crystallinity along the same trajectory as observed in the field, but the changes were within the range of fitting errors. We propose that reductive dissolution of crystalline Fe oxides prevails in the soil rich in crystalline Fe oxides (Alisol) and that re-precipitation as SRO Fe oxides is favored by constrained leaching, which leads to the observed decrease in Fe oxide crystallinity. In the soil rich in SRO Fe phases (Andosol), preferential reductive dissolution of SRO Fe oxides coupled with stronger leaching of dissolved Fe2+ causes the observed relative increase in crystallinity of the remaining Fe oxides. The observed increase in Fe oxide crystallinity may further be a result of Fe(II)-catalyzed re-crystallization of SRO Fe oxides. These findings indicate that, besides other factors, the Fe mineral composition of the initial soil or sediment as well as the leaching rate likely influence the trajectory of Fe oxide evolution under alternating redox-conditions.

Effects of human activities on pedogenesis and iron dynamics in paddy soils developed on Quaternary red clays

A paddy soil chronosequence consisting of four profiles derived from Quaternary red clay (QRC) in Southern China with cultivation history from 0 to 300 years was studied to understand the underlying mechanisms and processes controlling the centennial scale Fe evolution during anthropedogenesis. We evaluated the chronosequencial changes in depth distribution of Fe oxides and Fe isotopic compositions. Results showed that paddy soil development on QRC over a centennial time scale caused increasing profile differentiation of Fe oxides and measurable Fe isotope fractionation. Total Fe and oxide bound Fe as well as the maximum Fe illuviation depth decreased as paddy soils age, leading to significant differences in morphological properties between paddy and non-paddy soils with the same parent material. Selective extractions showed that the weakly-bound, oxide-bound and silicate bound Fe corresponded to 4–18%, 49–81%, and 11–40% of the total Fe, respectively, and these proportions varied with both time and depth due to the redox-related Fe transformation and redistribution. δ56Fe values in the studied paddy soil chronosequence ranged from 0.05‰ to 0.26‰ and exhibited a strong negative correlation with the logarithm of total Fe concentrations, suggesting mass-dependent Fe isotope fractionation occurred as a result of the preferential removal of lighter Fe isotopes during Fe leaching loss under the predominant reducing conditions. However, the Fe isotopic ratio of a specific paddy soil horizon was a result of a complex interaction of different processes, which were summarized and interpreted in our proposed conceptual model of Fe evolution in terraced paddy soils. Further investigations of Fe isotope signatures in the soil-plant-water system with a combination of laboratory simulation and prediction of Fe isotope fractionation under different pedogenic processes are needed to evaluate the relative contribution of multiple processes to Fe isotope fractionation during anthropedogenesis of paddy soils.

Nitrogen sequestration under long-term paddy management in soils developed on contrasting parent material

Long-term paddy management promotes nitrogen (N) sequestration, but it is unknown to what extent the properties of the parent soil modify the management-induced N sequestration in peptide-bound amino acids (AA-N). We hypothesized that paddy management effects on the storage of AA-N relate to the mineral assembly. Hence, we determined contents and chirality of peptide-bound amino acids in paddy soils developed on contrasting parent material (Vertisols, Andosols, Alisols in Indonesia, Alisols in China, and Gleysol/Fluvisol in Italy). Adjacent non-paddy soils served as references. Selected samples were pre-extracted with dithionite–citrate–bicarbonate (DCB) to better understand the role of reactive oxide phases in AA-N storage, origin, and composition. The results showed that topsoil N and AA-N stocks were significantly larger in paddy-managed Andosols and Chinese Alisols than in their non-paddy counterparts. In other soils, however, paddy management did not cause higher proportions of N and AA-N, possibly because N fixing intercrops masked the paddy management effects on N sequestration processes. Among the different soils developed on contrasting parent material, AA-N stocks were largest in Andosols, followed by Alisols and Fluvisols, and lowest in Vertisols. The N storage in amino acid forms went along with elevated d-contents of bacteria-derived alanine and glutamic acid, as well as with increasing stocks of DCB-extractable Fe, Mn, and Al. Other d-amino acids, likely formed by racemization processes, did not vary systematically between paddy and non-paddy managed soils. Our data suggest that the presence of oxides increase the N sequestration in peptide-bound amino acids after microbial N transformations.

Factors controlling cadmium and lead activities in different parent material-derived soils from the Pearl River Basin

Labile metals in agricultural soils are available to crops and thus pose a great health risk for human beings. Therefore, factors influencing heavy metal activity are of interest to researchers. In this study, a total of 142 soil samples representing 5 typical parent materials in the Pearl River Basin (PRB), China were collected to investigate factors impacting the distribution of labile Cd and Pb in the soils. The results showed that the labile fractions accounted for 0.03%–14.7% for Cd and 0.01%–0.39% for Pb of the total metals, and the labile fractions were linearly correlated to their corresponding total contents. The step regression analyses suggested that the key factors impacting labile Cd and Pb varied in different parent material soils. Pb activity was highly sensitive to pH in alkaline limestone soils. The quartz sand remained in granite-produced soils enhanced Cd activity. And dissolved organic matter (DOM) compositions considerably influenced Cd and Pb activities in sand shale, diluvium, and alluvium soils. Land use impacts heavy metal activities. The labile Cd and Pb in paddy soils were higher than those in non-paddy soils, although total metals in the soils were comparable. It could be ascribed to the long-term equilibrium of metals between the solution and solid phases of the paddy soils. The results provide a theoretical basis for preliminary prediction of heavy metal activity and provide a technical support for heavy metal activity management and pollution control based on soil parent materials.

Comparison of pedogenic properties of some paddy and nonpaddy soils of southern Iran

Comparison of pedogenic properties of some paddy and nonpaddy soils of southern Iran

Arsenic in Bangladeshi soils related to physiographic region, paddy management, and mirco- and macro-elemental status

While the impact of arsenic in irrigated agriculture has become a major environmental concern in Bangladesh, to date there is still a limited understanding of arsenic in Bangladeshi paddy soils at a landscape level. A soil survey was conducted across ten different physiographic regions of Bangladesh, which encompassed six types of geomorphology (Bil, Brahmaputra floodplain, Ganges floodplain, Meghna floodplain, Karatoya-Bangali floodplain and Pleistocene terrace). A total of 1209 paddy soils and 235 matched non-paddy soils were collected. The source of irrigation water (groundwater and surface water) was also recorded. The concentrations of arsenic and sixteen other elements were determined in the soil samples. The concentration of arsenic was higher in paddy soils compared to non-paddy soils, with soils irrigated with groundwater being higher in arsenic than those irrigated with surface water. There was a clear difference between the Holocene floodplains and the Pleistocene terraces, with Holocene floodplain soils being higher in arsenic and other elements. The results suggest that arsenic is most likely associated with less well weathered/leached soils, suggesting it is either due to the geological newness of Holocene sediments or differences between the sources of sediments, which gives rise to the arsenic problems in Bangladeshi soils.

Subsoil 14C Dynamics in Different Types of Tropical and Subtropical Soils under Different Crop Management

AbstractRadiocarbon has been applied as a tracer to study carbon dynamics in different types of tropical soils, under paddy and non-paddy management on Java, Indonesia. The 14C concentrations were measured in samples of total organic carbon as well as in alkali-soluble humic acids, insoluble humin, and plant remains (roots, seeds, leaves) obtained from three sites with Andosols, Alisols, and Vertisols at different altitudes. In addition, the abundance and distribution of plant macrofossils in the soil column and organic δ13C values were determined. The results obtained so far are compared with those from a chronosequence of Cambisols in China. They indicate the input of fresh plant materials into the subsoil directly via roots and/or by soil cracks and bioturbation of aboveground litter. The total organic C and 14C concentrations show the usual decrease with increasing depth in paddy and non-paddy soils, reflecting the influence of direct input as well as the downward redistribution of organic material as particulates or dissolved organic carbon (DOC) depending on soil type (pedogenesis, plough-pan formation, drying cracks), crop type and management, and climatic factors. A disturbance of the Andosol around 0.4 m depth and a change in profile properties around 0.63 m in the Alisol limit the general conclusions.

Black carbon and black nitrogen storage under long-term paddy and non-paddy management in major reference soil groups

Crop-residue burning and frequent paddy-soil flooding can lead to a substantial accumulation of black carbon (BC), thus contributing to long-term C sequestration. There is evidence that the turnover of BC in soils also depends on the soil mineral assembly. We studied the effects of paddy and non-paddy soil management and different major reference soil groups on BC storage. We hypothesized that overall BC storage in soil relates to paddy management and the abundance of reactive mineral phases such as Fe and Al oxides, and clay-sized minerals. Parallel to BC, black nitrogen (BN) should accumulate in soil.Paddy and non-paddy soils were sampled in three different climate zones (tropical, subtropical, and temperate). The soil profiles comprised six replicates of Andosols, Alisols and Vertisols from Java (Indonesia), and Alisols and Cambisols from China, as well as one Fluvisol and Gleysol from Northern Italy.Samples were taken by horizon down to >1m depth and analyzed for soil organic carbon (SOC) and BC. The latter was analyzed by oxidation to benzene polycarboxylic acids. Abundance of BN (as aromatic N) was estimated by X-ray photoelectron spectroscopic analyses of selected topsoil horizons.In topsoils BC vs. SOC accumulation was affected by management for Andosol, Alisols in China, and Vertisols. However, both flooding and crop-residue management seemed to control this. BC contents relative to SOC also differed between the reference soil groups, independent of management (p<0.0001), yet were surprisingly constant within replicates. We conclude that BC co-accumulated with SOC in all soils. However, the overall storage of BC (1m depth) was affected by a combination of soil group and management. Vertisols contained the largest BC stocks (17–19tha−1 in non-paddy and paddy fields), followed by Andosols and Alisols (6–10tBCha−1 under paddy management; 3–8tha−1 under non-paddy management). The Gleysol and Fluvisol had the smallest BC stocks, independent of soil use (3–4tha−1).Aromatic N proportions increased to >50% of total N after combustion of rice straw. However, aromatic N was barely, or not detectable in soil, and there was no correlation to BC. We conclude that burned crop residues were not a major source for aromatic N in soil. BC and aromatic N showed no distinct relations to soil properties, such as the abundance of clay-sized minerals, and Al and Fe oxides. Differences in BC stocks between the soils were most pronounced in the subsoils, likely caused by physical processes, such as swelling and shrinking of clays and/or translocation by leaching. Climate and regional soil-adjusted management also affected BC accumulation, but this first snapshot indicates that global BC maps may be linked to global soil maps.

Transformation of clay-sized minerals in soils exposed to prolonged regular alternation of redox conditions

The direction of the transformation of Fe-bearing minerals under harshly changing redox conditions is still under debate. Some studies suggest preferential accumulation of weakly crystalline Fe oxides while other studies showed that repeated redox cycles favour the presence of crystalline phases. Since characterized by distinct redox cycles, paddy soils are ideal for studying redox-related transformations of Fe oxides and Fe-bearing clay minerals. We analysed changes in the Fe mineral assemblage upon long-term reduction–oxidation cycles along a chronosequence of 100, 700, and 2000-year-old paddy soils developed on comparable parent material relative to two non-irrigated counterparts.The clay mineral assemblage, as indicated by X-ray diffraction, showed no significant differences between paddy and non-paddy soils as well as no changes with time under paddy management.Speciation of Fe mineral phases in top- and subsoil was accomplished by Mössbauer spectroscopy at room temperature and 4.2K, in combination with X-ray diffraction and selective chemical extractions (dithionite–citrate–bicarbonate, NH4 oxalate). We found ferrous and ferric Fe in silicates, nano- to micro-crystalline goethite and haematite, as well as a ferrihydrite-type phase. In topsoils, prolonged paddy cultivation resulted in general loss of silicate- and oxide-bound Fe in the topsoils and the remaining Fe oxide assemblage became more and more dominated by organic-rich weakly crystalline phases. Total contents of Fe oxides in subsoils increased due precipitation of Fe leached from the topsoils. Micro-crystalline goethite was the predominating Fe oxide phase in the upper subsoil, likely due to Fe2+-catalysed solid-state transformation of freshly precipitated hydrous Fe oxide phases. Lepidocrocite formed in the deeper subsoil, probably promoted by less CO2 in the soil water. Thus, paddy cultivation causes different Fe mineral weathering, forming, and transforming environments, with the depth differentiation of these environments becoming increasingly distinct with time under paddy management, i.e., the number of redox cycles the soils underwent.