Reducing the environmental impact of rice production in subtropical India by minimising reactive nitrogen loss.

The future of reactive nitrogen (N) for subtropical lowland rice to be characterised under diverse N-management to develop adequate sustainable practices. It is a challenge to increase the efficiency of N use in lowland rice, as N can be lost in various ways, e.g., through nitrous oxide (N2O) or dinitrogen (N2) emissions, ammonia (NH3) volatilization and nitrate (NO3-) leaching. A field study was carried out in the subsequent wet (2021) and dry (2022) seasons to assess the impacts of different N management strategies on yield, N use efficiency and different N losses in a double-cropped rice system. Seven different N-management practices including application of chemical fertilisers, liquid organic fertiliser, nitrification inhibitors, organic nutrient management and integrated nutrient management (INM) were studied. The application of soil test-based neem-coated urea (NCU) during the wet season resulted in the highest economic yield, while integrated nutrient management showed the highest economic yield during the dry season. Total N losses by volatilization of NH3, N2O loss and leaching were 0.06-4.73, 0.32-2.14 and 0.25-1.93 kg ha-1, corresponding to 0.06-5.84%, 0.11-2.20% and 0.09-1.81% of total applied N, respectively. The total N-uptake in grain and straw was highest in INM (87-89% over control) followed by the soil test-based NCU (77-82% over control). In comparison, recovery efficiency of N was maximum from application of NCU + dicyandiamide during both the seasons. The N footprint of paddy rice ranged 0.46-2.01 kg N-eq. t-1 during both seasons under various N management. Ammonia volatilization was the process responsible for the largest N loss, followed by N2O emissions, and NO3- leaching in these subtropical lowland rice fields. After ranking the different N management practices on a scale of 1-7, soil test-based NCU was considered the best N management approach in the wet year 2021, while INM scored the best in the dry year 2022.

Soil quality assessment of lowland rice soil of eastern India: Implications of rice husk biochar application.

The role of biochar in improving the soil properties of problem soils is well known, but its long term impact on lowland rice soil is not well recognized. The soil quality indicators of biochar applied lowland rice soil are not widely reported. We developed soil quality index (SQI) of a biochar applied lowland rice soil based on 17 soil properties (indicators). Field experimentation consisted of six treatments such as 0.5, 1, 2, 4, 8 and 10 t ha-1 of rice husk derived biochar (RHB) along with control. An overall SQI was calculated encompassing the indicators using multivariate statistics (principal component analysis) and non-linear scoring functions after generation of minimum data set (MDS). Sequential application of RHB improved the SQI by 4.85% and 16.02% with application of 0.5 t ha-1 and 10 t ha-1 RHB, respectively, over the recommended dose of fertilizer (control). PCA-screening revealed that total organic carbon (Ctot), zinc (Zn), pH and bulk density (BD) were the main soil quality indicators for MDS with 27.79%, 26.61%, 23.67% and 14.47% contributions, respectively. Apart from Ctot, Zn is one of the major contributors to SQI and RHB application can potentially be an effective agronomic practice to improve Zn status in lowland rice soil. The overall SQI was significantly influenced by RHB application even at 0.5 t ha-1. The present study highlights that application of RHB improves the soil quality even in fertile, well managed, lowland rice soil.

Multi-criteria assessment to screen climate smart rice establishment techniques in coastal rice production system of India.

Introduction: Conventional rice production techniques are less economical and more vulnerable to sustainable utilization of farm resources as well as significantly contributed GHGs to atmosphere. Methods: In order to assess the best rice production system for coastal areas, six rice production techniques were evaluated, including SRI-AWD (system of rice intensification with alternate wetting and drying (AWD)), DSR-CF (direct seeded rice with continuous flooding (CF)), DSR-AWD (direct seeded rice with AWD), TPR-CF (transplanted rice with CF), TPR-AWD (transplanted rice with AWD), and FPR-CF (farmer practice with CF). The performance of these technologies was assessed using indicators such as rice productivity, energy balance, GWP (global warming potential), soil health indicators, and profitability. Finally, using these indicators, a climate smartness index (CSI) was calculated. Results and discussion: Rice grown with SRI-AWD method had 54.8 % higher CSI over FPR-CF, and also give 24.5 to 28.3% higher CSI for DSR and TPR as well. There evaluations based on the climate smartness index can provide cleaner and more sustainable rice production and can be used as guiding principle for policy makers.

Metal(loid)s (As, Hg, Se, Pb and Cd) in paddy soil: Bioavailability and potential risk to human health.

Rice is one of the principal staple foods, essential for safeguarding the global food and nutritional security, but due to different natural and anthropogenic sources, it also acts as one of the biggest reservoirs of potentially toxic metal(loids) like As, Hg, Se, Pb and Cd. This review summarizes mobilization, translocation and speciation mechanism of these metal(loids) in soil-plant continuum as well as available cost-effective remediation measures and future research needs to eliminate the long-term risk to human health. High concentrations of these elements not only cause toxicity problems in plants, but also in animals that consume them and gradual deposition of these elements leads to the risk of bioaccumulation. The extensive occurrence of contaminated rice grains globally poses substantial public health risk and merits immediate action. People living in hotspots of contamination are exposed to higher health risks, however, rice import/export among different countries make the problem of global concern. Accumulation of As, Hg, Se, Pb and Cd in rice grains can be reduced by reducing their bioavailability, and controlling their uptake by rice plants. The contaminated soils can be reclaimed by phytoremediation, bioremediation, chemical amendments and mechanical measures; however these methods are either too expensive and/or too slow. Integration of innovative agronomic practices like crop establishment methods and improved irrigation and nutrient management practices are important steps to help mitigate the accumulation in soil as well as plant parts. Adoption of transgenic techniques for development of rice cultivars with low accumulation in edible plant parts could be a realistic option that would permit rice cultivation in soils with high bioavailability of these metal(loid)s.

Cyanobiont diversity in six Azolla spp. and relation to Azolla-nutrient profiling.

MAIN CONCLUSION: Illumina-Miseq®-based cyanobiont diversity and biomass were analyzed in six Azolla spp. Results revealed that 93-98% of total operational taxonomic units (OTUs) belong to Nostacaceae followed by Cylindrospermopsis with about 1-6% OTUs. The taxonomy of Azolla-cyanobiont is a long-term debate within the scientific community. Morphological and biochemical-based reports indicated the presence of Anabaena, Nostoc and/or Trichormus azollae as abundant Azolla-cyanobionts, however, molecular data did not support the abundance of Anabaena and/or Nostoc. To understand furthermore, the cyanobiont diversity in six species of Azolla (A. microphylla, A. mexicana, A. filiculoides, A. caroliniana, A. pinnata and A. rubra) was analyzed based on 16S rRNA Illumina-MiSeq sequencing. Additionally, biomass and nutrient profiling of Azolla spp. were analyzed and correlated with cyanobiont diversity. Illumina-MiSeq data revealed that 99.6-99.9% of total operational taxonomic units (OTUs) belonged to Nostocophycideae (class), Nostocales (order) and Nostacaceae (family). At genus level, the unassigned affiliation (93.4-97.9%) under Nostacaceae family was abundant followed by Cylindrospermopsis OTUs (1.1-6.0%). Interestingly, A. pinnata harboured maximum Cylindrospermopsis OTUs and also recorded higher biomass (40.67 g m-2 day-1), whereas crude protein (25.9%) and antioxidants (76.9%) were recorded to be higher in A. microphylla. Biplot analysis revealed that A. pinnata and its cyanobiont abundance were positively correlated with neutral and acid detergent fibers. Overall, the present findings deepened the understanding about cyanobiont in Azolla and its relations with Azolla nutrient profiling.

Effects of water deficit stress on agronomic and physiological responses of rice and greenhouse gas emission from rice soil under elevated atmospheric CO2.

Rice is the foremost staple food in the world, safeguarding the global food and nutritional security. Rise in atmospheric carbon dioxide (CO2) and water deficits are threatening global rice productivity and sustainability. Under real field conditions these climatic factors often interact with each other resulting in impacts that are remarkably different compared to individual factor exposure. Rice soils exposed to drought and elevated CO2 (eCO2) alters the biomass, diversity and activity of soil microorganisms affecting greenhouse gas (GHG) emission dynamics. In this review we have discussed the impacts of eCO2 and water deficit on agronomic, biochemical and physiological responses of rice and GHGs emissions from rice soils. Drought usually results in oxidative stress due to stomatal closure, dry weight reduction, formation of reactive oxygen species, decrease in relative water content and increase in electrolyte leakage at almost all growth and developmental phases of rice. Elevated atmospheric CO2 concentration reduces the negative effects of drought by improving plant water relations, reducing stomatal opening, decreasing transpiration, increasing canopy photosynthesis, shortening crop growth period and increasing the antioxidant metabolite activities in rice. Increased scientific understanding of the effects of drought and eCO2 on rice agronomy, physiology and GHG emission dynamics of rice soil is essential for devising adaptation options. Integration of novel agronomic practices viz., crop establishment methods and alternate cropping systems with improved water and nutrient management are important steps to help rice farmers cope with drought and eCO2. The review summarizes future research needs for ensuring sustained global food security under future warmer, drier and high CO2 conditions.

Effect of fly ash application on soil microbial response and heavy metal accumulation in soil and rice plant.

Fly ash (FA), a byproduct of coal combustion in thermal power plants, has been considered as a problematic solid waste and its safe disposal is a cause of concern. Several studies proposed that FA can be used as a soil additive; however its effect on microbial response, soil enzymatic activities and heavy metal accumulation in soil and grain of rice (cv. Naveen) to fly ash (FA) application was studied in a pot experiment during dry season 2011 in an Inceptisol. Fly ash was applied at a rate of zero per cent (FS), five per cent (FA5), ten per cent (FA10), twenty per cent (FA20), 40 per cent (FA40) and 100 per cent (FA100) on soil volume basis with nitrogen (N), phosphorus (P) and potassium (K) (40:20:20mg N:P:Kkg(-1) soil) with six replications. Heavy metals contents in soil and plant parts were analysed after harvest of crop. On the other hand, microbial population and soil enzymatic activities were analysed at panicle initiation stage (PI, 65 days after transplanting) of rice. There was no significant change in the concentration of zinc (Zn), iron (Fe), copper (Cu), manganese (Mn), cadmium (Cd) and chromium (Cr) with application of fly ash up to FA10. However, at FA100 there was significant increase of all metals concentration in soil than other treatments. Microorganisms differed in their response to the rate of FA application. Population of both fungi and actinomycetes decreased with the application of fly ash, while aerobic heterotrophic bacterial population did not change significantly up to FA40. On the other hand, total microbial activity measured in terms of Fluorescein diacetate (FDA) assay, and denitrifiers showed an increased trend up to FA40. However, activities of both alkaline and acid phosphatase were decreased with the application of FA. Application of FA at lower levels (ten to twenty per cent on soil volume basis) in soil enhanced micronutrients content, microbial activities and crop yield.

Effect of fly ash deposition on photosynthesis, growth and yield of rice.

An experiment was conducted to assess the effect of fly ash deposition without and with (0.25, 0.50, 1.0 and 1.5 g m(-2 )day(-1)) foliar dusting on the photosynthesis, stomatal conductance, transpiration, leaf temperature, albedo and productivity of rice. Dusting of 0.5 g m(-2 )day(-1) fly ash and above significantly reduced the photosynthesis, stomatal conductance, transpiration and albedo. Panicle initiation and flowering stages were more influenced by the fly ash deposition as compared to active tillering stage. At higher rates of fly ash deposition, all growth and yield parameters were significantly influenced due to increased heat load and reduced intercellular CO2 concentration. A significant reduction of 12.3, 15.7 and 20.2 % in grain yield was recorded over control when fly ash was dusted at 0.5, 1.0 and 1.5 g m(-2 )day(-1), respectively.

Fly ash addition affects microbial biomass and carbon mineralization in agricultural soils.

The microbial biomass carbon (MBC) and carbon mineralization of fly ash (FA) amended soil at (0 %, 1.25 %, 2.5 %, 5 %, 10 % and 20 % FA; v/v) was investigated under laboratory conditions for 120 days at 60 % soil water-holding capacity and 25 ± 1°C temperature. The results demonstrated that soil respiration and microbial activities were not suppressed up to 2.5 % FA amendment and these activities decreased significantly at 10 % and 20 % FA treatment with respect to control. Application of 10 % and 20 % FA treated soils showed a decreasing trend of soil MBC with time; and the decrease was significant throughout the period of incubation. The study concluded that application of FA up to 2.5 % can thus be safely used without affecting the soil biological activity and thereby improve nutrient cycling in agricultural soils.

Post-flood nitrogen and basal phosphorus management affects survival, metabolic changes and anti-oxidant enzyme activities of submerged rice (Oryza sativa).

Flooding is one of the major harmful abiotic stresses in the low lying areas of Asia and crop losses due to submergence are considerably high. Along with plant breeding techniques, agronomic management options in general and nutrient management in particular should be taken into consideration. Response of Sub 1 and non-Sub1 cultivars of rice to post-flood nitrogen (N) management under variable flood water was compared at maximum tillering stage. Submergence tolerance on survival, leaf senescence, metabolic changes, and anti-oxidant enzymatic activities were evaluated. Sub1 cultivars proved their superiority over IR-20 in terms of significantly higher survival, anti-oxidant enzymes and lower metabolic changes. Turbid water resulted in lower survival because of poor light transmission, chlorophyll retention and silt deposition. Basal phosphorus reduced the elongation, senescence and ethylene accumulation. Post-flood foliar spray of urea substantially increased the chlorophyll, soluble sugars and extenuated ethylene accumulation resulting in significantly higher survival. These nutrient management options can provide opportunities for better survival and productivity even under turbid water, helping farmers to cope with the existing problems in flood-prone areas.

Microbial biomass and carbon mineralization in agricultural soils as affected by pesticide addition.

A laboratory study was conducted with four pesticides, viz. a fungicide (carbendazim), two insecticides (chlorpyrifos and cartap hydrochloride) and an herbicide (pretilachlor) applied to a sandy clay loam soil at a field rate to determine their effect on microbial biomass carbon (MBC) and carbon mineralization (C(min)). The MBC content of soil increased with time up to 30 days in cartap hydrochloride as well as chlorpyrifos treated soil. Thereafter, it decreased and reached close to the initial level by 90th day. However, in carbendazim treated soil, the MBC showed a decreasing trend up to 45 days and subsequently increased up to 90 days. In pretilachlor treated soil, MBC increased through the first 15 days, and thereafter decreased to the initial level. Application of carbendazim, chlorpyrifos and cartap hydrochloride decreased C(min) for the first 30 days and then increased afterwards, while pretilachlor treated soil showed an increasing trend.

Neem (Azadirachta indica) seed kernel powder retards urease and nitrification activities in different soils at contrasting moisture and temperature regimes.

A laboratory experiment was conducted to examine the potentiality of a natural resource neem (Azadirachta indica) seed kernel powder (NSKP) to reduce the urease and nitrification activities in different soils (viz., normal, acid, and sodic) at contrasting moisture (1:1 soil to water and field capacity) and temperature regimes (10 degrees C and 37 degrees C). Results have revealed that application of NSKP with urea did not exhibit any urease inhibitory property in normal and sodic soils, but in acid soil it had maintained higher concentration of urea than the urea alone treated samples for two weeks after application. At 37 degrees C and under field capacity moisture level, urea hydrolysis was more rapid than at 10 degrees C and under waterlogged (1:1) conditions. The NSKP has showed variable effects (4-28%) to inhibit nitrification during 7-21 days after application, depending upon the soil types, temperature and moisture regimes. The nitrification activity was significantly low in acid soil followed by normal and sodic soils. The present study suggests that NSKP has the potential to retard the urease activity in acid soil, and nitrification in all the soils, and thus it may be used along with urea for the better use of applied -N.