Organic fertilization integrated with water management to remediate As and Cd contamination in a paddy soil

Soil heavy metal pollution is the main risk for sustainable agriculture, especially the combination of As and Cd pollution in paddy fields which may lead to the superimposed accumulation in rice. There is an urgent need for environmental-friendly and cost-effective strategies to remediate the contamination of As and Cd in soils. In this work, a pot culture experiment was conducted in a As and Cd polluted paddy soil to explore the effects of organic fertilization (OF) and two water managements (continuous flooding, CF; intermittent irrigation, II) on the fractionation of As and Cd in soil, and the uptake of As and Cd by rice. The results showed that OF integrated with intermittent irrigation performed best in reducing the contents of As and Cd in rice grains by 58.9 % and 69.3 %, respectively, under compound pollution. The significant conversion of available As and Cd to stable species (specifically adsorbed and Fe-Mn/Al oxide bound) under OF + II were supported by the changes in an array of soil attributes such as pH, Eh, soluble Fe and dissolved organic carbon (DOC). Intermittent irrigation was more conducive to the accumulation of As outside the roots, and Fe-plaque prevented As uptake by roots and the translocation to shoots. While more accumulation of Fe-plaque along with Cd on root surface induced by continuous flooding is helpful for depressed assimilation of Cd by rice. Considering the combined contamination of As and Cd polluted in paddy soils, a management approach was proposed based on intermittent irrigation and application of organic fertilizer at the rate of 0.1 % (~ 2.3 t/ha) in two phases (two weeks before planting or drainage). Organic fertilization will hold great promise in restoring polluted soils and maintaining soil health via suppressing the lability of heavy metals and providing nutrients.

Mitigating the accumulation of arsenic and cadmium in rice grain: A quantitative review of the role of water management

Arsenic exposure through rice consumption is a growing concern. Compared to Continuous Flooding (CF), irrigation practices that dry the soil at least once during the growing season [referred to here as Alternate Wetting and Drying (AWD)] can decrease As accumulation in grain; however, this can simultaneously increase grain Cd to potentially unsafe levels. We modelled grain As and Cd from field studies comparing AWD and CF to identify optimal AWD practices to minimize the accumulation of As and Cd in grain. The severity of soil drying during AWD drying event(s), quantified as soil water potential (SWP), was the main factor leading to a reduction in grain total As and inorganic As, compared to CF. However, lower SWP levels were necessary to decrease grain inorganic As, compared to total As. Therefore, if the goal is to decrease grain inorganic As, the soil needs to be dried further than it would for decreasing total As alone. The main factor driving grain Cd accumulation was when AWD was practiced during the season. Higher grain Cd levels were observed when AWD occurred during the early reproductive stage. Further, higher Cd levels were observed when AWD spanned multiple rice growth stages, compared to one stage. If Cd levels are concerning, the minimum trade-off between total As and Cd accumulation in rice grain occurred when AWD was implemented at a SWP of -47 kPa during one stage other than the early reproductive. While these results are not meant to be comprehensive of all the interactions affecting the As and Cd dynamics in rice systems, they can be used as a first guide for implementing AWD practices with the goal of minimizing the accumulation of As and Cd in rice grain.

Extended methane mitigation capacity of a mid-season drainage beyond the rice growing season: a case in Spain

Rice cultivation is a major source of methane (CH₄) emissions. Intermittent irrigation systems in rice cultivation, such as the mid-season drainage (MSD), are effective strategies to mitigate CH₄ emissions during the growing season, though the reduction rates are variable and dependent on the crop context. Aeration periods induce alteration of soil CH₄ dynamics that can be prolonged after flooding recovery. However, whether these changes persist beyond the growing season remains underexplored. A field experiment was conducted in Spain to study the effect of MSD implemented during the rice growing season on greenhouse gas (GHG) emissions in relation to the standard permanently flooded water management (PFL). Specifically, the study aimed at (1) assessing the CH₄ mitigation capacity of MSD in the studied area and (2) testing the hypothesis that the mitigating effect of MSD can be extended into the following winter flooded fallow season. Year-round GHG sampling was conducted, seasonal and annual cumulative emissions of CH₄ and N₂O as well as the global warming potential were calculated, and grain yield was measured. MSD reduced growing season CH₄ emissions by ca. 80% without yield penalties. During the flooded fallow season, MSD reduced CH₄ emissions by ca. 60%, despite both fields being permanently flooded. The novelty of our observations lies in the amplified mitigation capacity of MSD by extending the CH₄ mitigation effect to the following flooded winter fallow season. This finding becomes especially relevant in rice systems with flooded winter fallow season given the large contribution of this season to the annual CH₄ emissions.

Relationship between soil solution electrochemical changes and methane and nitrous oxide emissions in different rice irrigation management systems

Rice (Oryza sativa L.) intermittent irrigation is a potential strategy to mitigate methane (CH₄) and nitrous oxide (N₂O) emissions, but the effects of dry-wetting intervals on soil electrochemical changes and plant characteristics should be considered. This study was conducted in a greenhouse evaluating CH₄ and N₂O fluxes in rice under five different irrigation management practices (continuous irrigation (CI), intermittent irrigation with flooding resumption in saturated soil condition (SSI) and soil moisture at field capacity (FCI), saturated soil and irrigation resumption with soil moisture bellow field capacity (FCS), and soil at field capacity (FCD)) and its relation to plant development and global warming potential (GWP). Soil electrochemical conditions and CH₄ and N₂O emissions were expressively affected by irrigation management. The CI system presented the greatest CH₄ flux (20.14 g m^-2) and GWP (462.7 g m^-2 eq. CO₂), whereas intermittent irrigation expressively reduced CH₄ emissions. Overall, the N₂O flux was low (bellow 20 μg m^-2 h^-1) even with N application, with greater emissions occurring at the FCD treatment at the beginning of the rice season. Soil moisture at field capacity had no CH₄ flux but presented greater GWP (271 g m^-2 eq. CO₂) than intermittent irrigation systems due to N₂O flux while compromising rice plant development. The best soil moisture condition to initiate a flooding cycle during intermittent irrigation is at saturated soil conditions.

Arsenic mitigation in rice grain loading via alternative irrigation by proposed water management practices

Over the past three decades, the occurrence of high concentrations of arsenic (As) in drinking-water and its subsequent poisoning in rice has been recognized as a major public-health concern globally, especially in Ganga Delta Plain with more than 80 million peoples in serious As exposure far beyond than its allowable limit. An extensive field study was conducted for consecutive four years viz. 2013 to 2016, introducing a process of intermittent irrigation pattern comparing to the conventional practice of rice cultivation in India. The practice provides a combination of aerobic and anaerobic irrigation resulting better rice productivity with lesser arsenic mobility and accumulation in rice grains. This present research finding clearly points out to the marked reduction of arsenic load from average 1.6 mg/kg to 0.5 mg/kg in rice grain, much closer to FAO/WHO prescribed safe limit and in the continuous practice of proposed agricultural strategy resulting in a gradual decrease of 15% bioavailable arsenic in each year. Total productivity (in kg/hectare) also increased by 540 kg/year in boro and 340 kg/year in amon subsequently achieving the prescribed safe limit of As in grain.

Ultra-structure alteration via enhanced silicon uptake in arsenic stressed rice cultivars under intermittent irrigation practices in Bengal delta basin

The study implements a periodical intermittent water cycle during rice cultivation providing insight potential in minimizing soil bio-available arsenic. Soil As concentrations were 34 ± 0.49 and 72.03 ± 0.54 mg kg-1 As respectively in two selected fields with rice cultivars gosai and satabdi, in comparison to 42.26 ± 0.37 and 83.69 ± 0.48 mg kg-1 in continuously flooded field soil, determined through ICP-MS. The study found higher translocation of silicon from soil to rice plant parts under intermittent irrigation having pH range of 7.6-9.4 and greater availability of soil organic content that in turn release more labile silicon from soil to aqueous phase for plant accumulation. This increased uptake of silicon strengthens rice shoots, nodes and leaf xylem-phloem integrity compared to conventional continuously flooded rice cultivation approach, suppressing the arsenic translocation, as observed under FE-SEM real-time imaging. Fresh plants were analysed for bioaccumulation and translocation factors of arsenic and silicon to justify the enhanced silicon uptake under proposed practice. Plant stress regulator enzymes viz. malondialdehyde (MDA), total protein, superoxide dismutase (SOD), guaiacol peroxidase (GPX) and ascorbate peroxidase (APX) from both conditions and found to be better in intermittent method over conventional practice with higher productivity.

Prediction of future methane emission from irrigated rice paddies in central Thailand under different water management practices

There is concern about positive feedbacks between climate change and methane (CH4) emission from rice paddies. However, appropriate water management may mitigate the problem. We tested this hypothesis at six field sites in central Thailand, where the irrigated area is rapidly increasing. We used DNDC-Rice, a process-based biogeochemistry model adjusted based on rice growth data at each site to simulate CH4 emission from a rice-rice double cropping system from 2001 to 2060. Future climate change scenarios consisting of four representative concentration pathways (RCPs) and seven global climate models were generated by statistical downscaling. We then simulated CH4 emission in three water management practices: continuous flooding (CF), single aeration (SA), and multiple aeration (MA). The adjusted model reproduced the observed rice yield and CH4 emission well at each site. The simulated CH4 emissions in CF from 2051 to 2060 were 5.3 to 7.8%, 9.6 to 16.0%, 7.3 to 18.0%, and 13.6 to 19.0% higher than those from 2001 to 2010 in RCPs 2.6, 4.5, 6.0, and 8.5, respectively, at the six sites. Regionally, SA and MA mitigated CH4 emission by 21.9 to 22.9% and 53.5 to 55.2%, respectively, relative to CF among the four RCPs. These mitigation potentials by SA and MA were comparable to those from 2001 to 2010. Our results indicate that climate change in the next several decades will not attenuate the quantitative effect of water management practices on mitigating CH4 emission from irrigated rice paddies in central Thailand.