Alternating Wet-Dry Cycles Rather than Sulfate Fertilization Control Pathways of Methanogenesis and Methane Turnover in Rice Straw-Amended Paddy Soil.

Alternate wet-drying (AWD) and sulfate fertilization have been considered as effective methods for lowering CH4 emissions from paddy soils. However, there is a clear knowledge gap between field studies that focus on the quantification of emissions and laboratory studies that investigate mechanisms. To elucidate mechanisms of CH4 production and oxidation under field conditions, rice was planted in straw-amended mesocosms with or without sulfate fertilization under continuously flooded conditions (FL) or two wet-dry cycles. CO2 and CH4 concentrations in soil air and their natural C isotope compositions were measured at stem elongation, booting, and flowering stages. CH4 concentration reached 51 mg C L-1 at the flowering stage under FL, while it decreased to 0.04 mg C L-1 under AWD. Relative 13C enrichment in CH4 and depletion in CO2 under AWD indicated CH4 oxidation. Ample organic substrate supply may have reduced competition between sulfate-reducing bacteria and methanogenic archaea, and therefore, it explains the absence of a decrease in CH4 concentrations in sulfate treatments. 13C enrichment in CO2 over time (6 and 7‰ with and without sulfate fertilizers, respectively) under FL indicates continuous contribution of hydrogenotrophic methanogenesis to CH4 production with ongoing rice growth. Overall, AWD could more efficiently reduce CH4 production than sulfate fertilization in rice straw-amended paddy soils.

Redox Dependence of Thioarsenate Occurrence in Paddy Soils and the Rice Rhizosphere.

In flooded paddy soils, inorganic and methylated thioarsenates contribute substantially to arsenic speciation besides the much-better-investigated oxyarsenic species, and thioarsenate uptake into rice plants has recently been shown. To better understand their fate when soil redox conditions change, that is, from flooding to drainage to reflooding, batch incubations and unplanted microcosm experiments were conducted with two paddy soils covering redox potentials from EH -260 to +200 mV. Further, occurrence of thioarsenates in the oxygenated rice rhizosphere was investigated using planted rhizobox experiments. Soil flooding resulted in rapid formation of inorganic thioarsenates with a dominance of trithioarsenate. Maximum thiolation of inorganic oxyarsenic species was 57% at EH -130 mV and oxidation caused nearly complete dethiolation. Only monothioarsenate formed again upon reflooding and was the major inorganic thioarsenate detected in the rhizosphere. Maximum thiolation of mono- and dimethylated oxyarsenates was about 70% and 100%, respectively, below EH 0 mV. Dithiolated species dominated over monothiolated species below EH -100 mV. Among all thioarsenates, dimethylated monothioarsenate showed the least transformation upon prolonged oxidation. It also was the major thiolated arsenic species in the rhizosphere with concentrations comparable to its precursor dimethylated oxyarsenate, which is especially critical since dimethylated monothioarsenate is highly carcinogenic.

An integrated, multisensor system for the continuous monitoring of water dynamics in rice fields under different irrigation regimes.

The cultivation of rice, one of the most important staple crops worldwide, has very high water requirements. A variety of irrigation practices are applied, whose pros and cons, both in terms of water productivity and of their effects on the environment, are not completely understood yet. The continuous monitoring of irrigation and rainfall inputs, as well as of soil water dynamics, is a very important factor in the analysis of these practices. At the same time, however, it represents a challenging and costly task because of the complexity of the processes involved, of the difference in nature and magnitude of the driving variables and of the high variety of field conditions. In this paper, we present the prototype of an integrated, multisensor system for the continuous monitoring of water dynamics in rice fields under different irrigation regimes. The system consists of the following: (1) flow measurement devices for the monitoring of irrigation supply and tailwater drainage; (2) piezometers for groundwater level monitoring; (3) level gauges for monitoring the flooding depth; (4) multilevel tensiometers and moisture sensor clusters to monitor soil water status; (5) eddy covariance station for the estimation of evapotranspiration fluxes and (6) wireless transmission devices and software interface for data transfer, storage and control from remote computer. The system is modular and it is replicable in different field conditions. It was successfully applied over a 2-year period in three experimental plots in Northern Italy, each one with a different water management strategy. In the paper, we present information concerning the different instruments selected, their interconnections and their integration in a common remote control scheme. We also provide considerations and figures on the material and labour costs of the installation and management of the system.

Microbial communities in paddy soils: differences in abundance and functionality between rhizosphere and pore water, the influence of different soil organic carbon, sulfate fertilization and cultivation time, and contribution to arsenic mobility and speciation.

Abiotic factors and rhizosphere microbial populations influence arsenic accumulation in rice grains. Although mineral and organic surfaces are keystones in element cycling, localization of specific microbial reactions in the root/soil/pore water system is still unclear. Here, we tested if original unplanted soil, rhizosphere soil and pore water represented distinct ecological microniches for arsenic-, sulfur- and iron-cycling microorganisms and compared the influence of relevant factors such as soil type, sulfate fertilization and cultivation time. In rice open-air-mesocosms with two paddy soils (2.0% and 4.7% organic carbon), Illumina 16S rRNA gene sequencing demonstrated minor effects of cultivation time and sulfate fertilization that decreased Archaea-driven microbial networks and incremented sulfate-reducing and sulfur-oxidizing bacteria. Different compartments, characterized by different bacterial and archaeal compositions, had the strongest effect, with higher microbial abundances, bacterial biodiversity and interconnections in the rhizosphere vs pore water. Within each compartment, a significant soil type effect was observed. Higher percentage contributions of rhizosphere dissimilatory arsenate- and iron-reducing, arsenite-oxidizing, and, surprisingly, dissimilatory sulfate-reducing bacteria, as well as pore water iron-oxidizing bacteria in the lower organic carbon soil, supported previous chemistry-based interpretations of a more active S-cycling, a higher percentage of thioarsenates and lower arsenic mobility by sorption to mixed Fe(II)Fe(III)-minerals in this soil.

Different farming and water regimes in Italian rice fields affect arbuscular mycorrhizal fungal soil communities.

Arbuscular mycorrhizal fungi (AMF) comprise one of the main components of soil microbiota in most agroecosystems. These obligate mutualistic symbionts colonize the roots of most plants, including crop plants. Many papers have indicated that different crop management practices could affect AMF communities and their root colonization. However, there is little knowledge available on the influence of conventional and low-input agriculture on root colonization and AMF molecular diversity in rice fields. Two different agroecosystems (continuous conventional high-input rice monocropping and organic farming with a five-year crop rotation) and two different water management regimes have been considered in this study. Both morphological and molecular analyses were performed. The soil mycorrhizal potential, estimated using clover trap cultures, was high and similar in the two agroecosystems. The diversity of the AMF community in the soil, calculated by means of PCR-RFLP (polymerase chain reaction-restriction fragment length polymorphism) and 18S rDNA sequencing on clover trap cultures roots, was higher for the organic cultivation. The rice roots cultivated in the conventional agrosystem or under permanent flooding showed no AMF colonization, while the rice plants grown under the organic agriculture system showed typical mycorrhization patterns. Considered together, our data suggest that a high-input cropping system and conventional flooding depress AMF colonization in rice roots and that organic managements could help maintain a higher diversity of AMF communities in soil.

Cadmium transfer in contaminated soil-rice systems: Insights from solid-state speciation analysis and stable isotope fractionation.

Initial Cadmium (Cd) isotope fractionation studies in cereals ascribed the retention of Cd and its light isotopes to the binding of Cd to sulfur (S). To better understand the relation of Cd binding to S and Cd isotope fractionation in soils and plants, we combined isotope and XAS speciation analyses in soil-rice systems that were rich in Cd and S. The systems included distinct water management (flooded vs. non-flooded) and rice accessions with (excluder) and without (non-excluder) functional membrane transporter OsHMA3 that transports Cd into root vacuoles. Initially, 13% of Cd in the soil was bound to S. Through soil flooding, the proportion of Cd bound to S increased to 100%. Soil flooding enriched the rice plants towards heavy isotopes (δ114/110Cd = -0.37 to -0.39%) compared to the plants that grew on non-flooded soils (δ114/110Cd = -0.45 to -0.56%) suggesting that preferentially light Cd isotopes precipitated into Cd sulfides. Isotope compositions in CaCl2 root extracts indicated that the root surface contributed to the isotope shift between soil and plant during soil flooding. In rice roots, Cd was fully bound to S in all treatments. The roots in the excluder rice strongly retained Cd and its lights isotopes while heavy isotopes were transported to the shoots (Δ114/110Cdshoot-root 0.16-0.19‰). The non-excluder rice accumulated Cd in shoots and the apparent difference in isotope composition between roots and shoots was smaller than that of the excluder rice (Δ114/110Cdshoot-root -0.02 to 0.08‰). We ascribe the retention of light Cd isotopes in the roots of the excluder rice to the membrane transport of Cd by OsHMA3 and/or chelating Cd-S complexes in the vacuole. Cd-S was the major binding form in flooded soils and rice roots and partly contributed to the immobilization of Cd and its light isotopes in soil-rice systems.

Mycotoxins and Related Fungi in Italian Paddy Rice During the Growing Season and Storage.

Mycotoxigenic fungi and relative mycotoxins contamination were monitored in Italian paddy rice samples both in field during the growing season and the first five months of storage. Three experimental fields, nine rice varieties and three sowing densities were considered; then, different lots of paddy rice were stored in warehouses at different temperature regimes. Fusarium spp. and Aspergillus spp. were found to be the fungi most likely to produce mycotoxins throughout the growing season. In particular, A. flavus and A. niger were found only rarely both in field and in post-harvest, while A. versicolor was always present although in low concentrations. Penicillium spp. strains were isolated sporadically and were found to be irrelevant in Italian rice fungal contamination. Sterigmatocystin (STC) was the main mycotoxin found in Italian rice, while aflatoxin (AFB1), deoxynivalenol (DON) and ochratoxin A (OTA) were rarely detected. Contamination generally increased from post-flowering to ripening; considering rice varieties, significant differences (p ≤ 0.01) were found in fungal contamination and STC production; no differences were observed between sowing densities. During storage, an increase in STC content was observed in higher temperature regimes, while all the other considered mycotoxins remained unchanged. These results indicated that contamination by STC, an emerging mycotoxin not legislatively regulated by the European Union, can be relevant in rice.

Efficacy of Azoxystrobin on Mycotoxins and Related Fungi in Italian Paddy Rice.

The efficacy of azoxystrobin was evaluated in the presence of mycotoxigenic fungi and relative mycotoxins in Italian paddy rice during the growing season in the field. Three experimental fields were considered and the applied experimental design was a strip plot with three replicates; rice samples were collected at four different growing stages. The efficacy of the fungicide treatment on rice fungal population was demonstrated with around 20% less total fungal incidence in sprayed samples compared to untreated ones; the same decrease was noted also in Fusarium spp. species but not in Aspergillus versicolor. Of the mycotoxins considered, ochratoxin A (OTA) and aflatoxins (AFBs) were never detected, deoxynivalenol (DON) was found in 46% of samples at levels always lower than 100 µg/kg, while sterigmatocystin (STC) occurred in all the paddy rice samples collected after flowering, with a maximum value of 15.5 µg/kg. Treatment with azoxystrobin was not effective in reducing DON contamination, but it had an important and significant effect on STC content, showing a decrease of 67% in the sprayed samples.

Sterigmatocystin Occurrence in Paddy and Processed Rice Produced in Italy in the Years 2014-2015 and Distribution in Milled Rice Fractions.

The occurrence of sterigmatocystin (STC) in paddy and processed rice samples produced in Italy was surveyed. After extraction and purification, STC was analysed using HPLC-MS/MS. STC was detected in all paddy rice samples (n = 49), in the range 0.29-15.85 µg·kg-1. As regards processed rice, a widespread contamination was found in brown and parboiled rice. All the brown rice samples were contaminated between 0.12 and 1.32 µg·kg-1; for parboiled rice, the incidence was 90.9% and the maximum level was 1.09 µg·kg-1. The contamination in white rice was significantly lower (p < 0.01). The STC distribution in different rice fractions, obtained by the de-hulling and polishing processes, was evaluated. After de-hulling, the STC percentage remaining in brown rice was in the range 21.2%-30.8%. The polishing process, from brown to white rice, caused another remarkable decrease of contamination; the STC remaining in white rice was 2.2%-8.3% of the amount found in paddy rice.

Total As and As Speciation in Italian Rice as Related to Producing Areas and Paddy Soils Properties.

Rice and rice-based foodstuffs are important pathways for inorganic As dietary intake. This work shows a detailed picture of As content and speciation in Italian rice, which contributes to more than one-half of the European production, and addresses the role of soil chemistry and agronomic management on As concentration in rice grain, in view of ameliorative strategies. The mean total As content in Italian white rice was 155 ± 65 µg kg-1 with significant differences among producing areas, while the mean inorganic As was 102 ± 26 µg kg-1, largely below the E.U. limit of 200 µg kg-1 for white rice, although part of the production would not be suitable for baby food production, which requires less than 100 µg kg-1 of inorganic As. The differences in As content and speciation in rice among the studied areas resulted from the complex interactions of soil, plant, and anthropic factors. Among others, Si nutrition seemed to play a key role in regulating As transfer from soil to plant.