Changes in agricultural nitrogen (N) balance of OECD countries and its causes and impacts.

The Organization for Economic Co-operation and Development (OECD) developed soil surface nutrient balance and made it mandatory for member countries to report annual nutrient budgets since 1990. This study aimed to evaluate the status of nitrogen (N) management in member countries and to figure out why N surplus levels differ across countries and how they relate to other agri-environmental indicators, by analyzing the N budgets from 35 OECD countries over the last 30 years. Of the three factors determining N balance (agricultural land area, N input, and N output), agricultural land area decreased in most OECD countries, negatively affecting N balance reduction. However, OECD's average N balance highly decreased from 91 to 46 kg ha-1 over the last 30 years due to the decrease in N input through inorganic fertilizers and manure, especially in EU countries with high N input levels, while N output did not meaningfully change. In comparison, in Japan and Korea, the N balance slightly increased and they became the highest N balance country recently. A higher N balance led to lower N use efficiency and higher ammonia (NH3) and nitrous oxide (N2O) emission intensities. More densely populated countries with smaller agricultural land per capita (ranging from 0.03 to 0.47 ha capita-1) showed a higher N balance (228-80 kg ha-1), presumably due to higher N input for more agricultural production on limited land. The most densely populated countries among OECD members (Belgium, the Netherlands, Korea, and Japan) had similar N input levels. However, two EU countries had much higher N output than two Asian countries due to higher pasture production, which led to a lower N balance and higher N use efficiency. Therefore, highly populated countries with small arable land areas per capita might need multilateral efforts to alleviate agricultural N balance.

Elucidating growth and biochemical characteristics of rice seedlings under stress from chromium VI salt and nanoparticles.

Plants are usually provoked by a variety of heavy metal (HM) stressors that have adverse effects on their growth and other biochemical characterizations. Among the HMs, chromium has been considered the most toxic for both plants and animals. The present study was conducted to compare the phytotoxic effects of increasing chromium (VI) salt and nanoparticles (NPs) concentrations on various growth indexes of rice (Oryza sativa L. var. swat 1) seedlings grown in a hydroponic system. The 7-day rice seedlings were exposed to Cr (VI) salt and NPs hydroponic suspensions which were adjusted to the concentration of 0, 50, 100, 150, 200 and 250 mg/L. Both the Cr (VI) salt and NPs with lower concentrations (up to 100mg/L) exerted minimum inhibitory effects on the growth performance of rice seedlings. However, a significant decrease in shoot and root length and their fresh and dry weight was recorded at higher doses of Cr (VI) salt (200 mg/L) and NPs (250 mg/L). The stress induced by Cr (VI) salt has drastically affected the roots, whereas, Cr (VI) NPs significantly affected the shoot tissues. Photosynthetic pigments decreased significantly in a dose-dependent manner, and the reduction was more pronounced in rice seedlings exposed to Cr (VI) NPs compared to Cr (VI) salt. Cr (VI) NPs enhanced the membrane permeability in shoots and roots as compared to that of Cr (VI) salt, which resulted in higher concentration of reactive oxygen species (ROS) and increased lipid peroxidation. The activities of antioxidant enzymes superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) increased significantly in shoot/root tissue following exposure to higher doses of Cr (VI) salt (200 mg/L) and NPs stress (250 mg/L), while minor changes in CAT and APX activities were observed in root and shoot tissues after exposure to higher concentration of Cr (VI) NP. Furthermore, the increasing concentrations of Cr (VI) NPs increased the length of stomatal guard cells. Conclusively, Cr (VI) salt and NPs in higher concentrations have higher potential to damage the growth and induce oxidative stress in rice plants.

Pastures and Climate Extremes: Impacts of Cool Season Warming and Drought on the Productivity of Key Pasture Species in a Field Experiment.

Shifts in the timing, intensity and/or frequency of climate extremes, such as severe drought and heatwaves, can generate sustained shifts in ecosystem function with important ecological and economic impacts for rangelands and managed pastures. The Pastures and Climate Extremes experiment (PACE) in Southeast Australia was designed to investigate the impacts of a severe winter/spring drought (60% rainfall reduction) and, for a subset of species, a factorial combination of drought and elevated temperature (ambient +3°C) on pasture productivity. The experiment included nine common pasture and Australian rangeland species from three plant functional groups (C3 grasses, C4 grasses and legumes) planted in monoculture. Winter/spring drought resulted in productivity declines of 45% on average and up to 74% for the most affected species (Digitaria eriantha) during the 6-month treatment period, with eight of the nine species exhibiting significant yield reductions. Despite considerable variation in species' sensitivity to drought, C4 grasses were more strongly affected by this treatment than C3 grasses or legumes. Warming also had negative effects on cool-season productivity, associated at least partially with exceedance of optimum growth temperatures in spring and indirect effects on soil water content. The combination of winter/spring drought and year-round warming resulted in the greatest yield reductions. We identified responses that were either additive (Festuca), or less-than-additive (Medicago), where warming reduced the magnitude of drought effects. Results from this study highlight the sensitivity of diverse pasture species to increases in winter and spring drought severity similar to those predicted for this region, and that anticipated benefits of cool-season warming are unlikely to be realized. Overall, the substantial negative impacts on productivity suggest that future, warmer, drier climates will result in shortfalls in cool-season forage availability, with profound implications for the livestock industry and natural grazer communities.

Cover cropping and its biomass incorporation: Not enough to compensate the negative impact of plastic film mulching on global warming.

Plastic film mulching (FM) became a general practice to enhance crop productivity and its net primary production (NPP), but it can increase greenhouse gas (GHG) emissions. The proper addition of organic amendments might effectively decrease the impact of FM on global warming. To evaluate the feasibility of biomass addition on decreasing this negative influence, cover crop biomass as a green manure was incorporated with different recycling levels (0-100% of aboveground biomass) under FM and no-mulching. The net global warming potential (GWP) which integrated with soil C stock change and GHG (N2O and CH4) fluxes with CO2-equivalent was evaluated during maize cultivation. Under the same biomass incorporation, FM significantly enhanced the grain productivity and NPP of maize by 22-61 and 18-58% over no-mulching, respectively. In contrast, FM also highly increased the respired C loss, which was 11-95% higher than NPP increase, over no-mulching. Irrespective with biomass recycling ratio and mulching system, negative NECB which indicates the decrease of soil C stock was observed, mainly due to big harvest removal. FM decreased more soil C stock by 57-158% over no-mulching, but its C stock was clearly increased with increasing biomass addition. FM significantly increased total N2O and CH4 fluxes by 4-61 and 140-600% over no-mulching, respectively. Soil C stock changes mainly decided net GWP scale, but N2O and CH4 fluxes negligibly influenced. As a result, FM highly increased net GWP over no-mulching, while this net GWP was clearly decreased with increasing biomass application. However, cover cropping, and its biomass recycling was not enough to compensate the negative impact of FM on global warming. Therefore, more biomass incorporation might be essential to compensate this negative effect of FM.

A new approach to suppress methane emissions from rice cropping systems using ethephon.

Rice is the main staple food for more than half of the world's population. Yet, rice cultivation is subjected to criticism because of its important role in methane (CH4) emissions. Although several agronomic practices such as controlled irrigation and conservation tillage have been widely adopted to mitigate CH4 emissions from rice cultivation, the benefits gained by these practices are highly dependent on site-specific soil and climate conditions, and often offset by yield reduction. The use of plant growth regulating compounds having the potential to increase the crop yield and mitigate CH4 emissions may be an innovative approach to sustainable agriculture. Ethylene (C2H4), a plant growth regulator is known to have a strong inhibitory effect on methanogenesis. However, due to gaseous form and low water solubility, C2H4 has not been used to suppress methanogenesis in paddy fields. To develop C2H4 as a prospective soil amendment for reducing methane (CH4) emissions, ethephon (2-Chloroethylphosphonic acid), a precursor of C2H4 was tested. We found that ethephon reduced CH4 formation by 43%, similar to other well known methanogenic inhibitors (2-Bromoethanesulfonate, 2-Chlomoethanesulfonate, 2-Mercaptoethanesulfonate). However, ethephon rapidly hydrolyzed to C2H4 and methanogenic activity recuperated completely after C2H4 removal. To slow down the release of C2H4, ethephon was mixed with bio-degradable polymers such as cellulose acetate and applied to paddy soils. We found that compared with the control, the C2H4 release of ethephon slowed down to 90 days, and the CH4 emissions were reduced by 90%. The application of ethephon at lower concentrations did not significantly alter bacterial communities, their relative abundance, and the abundance of methanotrophs, but it significantly reduced archaeal communities and the relative abundance and expression level of methanogens in paddy soils. Results suggest that cellulose acetate-mixed ethephon has great promise to suppress CH4 emissions in rice paddies while ensuring sustainable yields.

Uncertainty of methane emissions coming from the physical volume of plant biomass inside the closed chamber was negligible during cropping period.

In rice paddy, the closed chamber method is broadly used to estimate methane (CH4) emission rate. Since rice plants can significantly affect CH4 production, oxidation and emission, rice plantation inside the chamber is standardized in IPCC guidelines. Methane emission rate is calculated using the increased concentration inside the headspace. Biomass growth might decrease the headspace volume, and thus CH4 emission rates might be overestimated. To evaluate the influence of chamber headspace decreased by rice plant development on CH4 emission rates, five Korean rice cultivars were cultivated in a typical rice paddy, and physical volume changes in rice biomass were assayed using water displacement method. The recommended acrylic closed chambers (H. 1.2 m x W. 0.6 m x L. 0.6 m) were installed, and eight rice plants were transplanted inside the chamber with the same space interval with the outside. Biomass growth significantly decreased the headspace volume of the chamber. However, this volume covered only 0.48-0.55% of the closed chamber volume at the maximum growth stage. During the whole cropping period, mean 0.24-0.28% of chamber headspace was allocated by plant biomass, and thus this level of total CH4 emissions was overestimated. However, this overestimation was much smaller than the errors coming from other investigation processes (i.e., chamber closing hour, temperature recording, inconstant flooding level, different soil environments, etc.) and rice physiological changes. In conclusion, the influence of physical biomass volume inside the closed chamber was negligible to make the error in total CH4 emission assessment in rice paddies.

Silicate fertilization improves microbial functional potentials for stress tolerance in arsenic-enriched rice cropping systems.

The host plant and its rhizosphere microbiome are similarly exposed to abiotic stresses under arsenic (As)-enriched cropping systems. Since silicon (Si) fertilization is effective in alleviating As-induced stresses in plants, and plant-microbe interactions are tightly coupled, we hypothesized that Si-fertilization would improve soil microbial functional potentials to environmental stress tolerance, which was not yet studied. With the help of high throughput metagenome, microarray and analyzing plant impacts on soil microbiome and the environment, we tested the hypothesis in two geographically different rice (i.e., Japonica and Indica) grown on As-enriched soils. Silicate fertilization in rice grown on As-enriched soils altered rhizosphere bacterial communities and increased several commensal microorganisms and their genetic potential to tolerate oxidative stress, osmotic stress, oxygen limitation, nitrogen and phosphate limitation, heat and cold shock, and radiation stress. The stress resistant microbial communities shifted with the changes in rhizosphere nutrient flows and cumulative plant impacts on the soil environment. The study highlights a thus-far unexplored behavior of Si-fertilization to improve microbial stress resilience under As-laden cropping systems and opens up a promising avenue to further study how commonalities in plant-microbe signaling in response to Si-fertilization alleviates As-induced stresses in agro-systems.

Eco-friendly yield-scaled global warming potential assists to determine the right rate of nitrogen in rice system: A systematic literature review.

Rice paddies are one of the largest greenhouse gases (GHGs) facilitators that are predominantly regulated by nitrogen (N) fertilization. Optimization of N uses based on the yield has been tried a long since, however, the improvement of the state-of-the-art technologies and the stiffness of global warming need to readjust N rate. Albeit, few individual studies started to, herein attempted as a systematic review to generalize the optimal N rate that minimizes global warming potential (GWP) concurrently provides sufficient yield in the rice system. To satisfy mounted food demand with inadequate land & less environmental impact, GHGs emissions are increasingly evaluated as yield-scaled basis. This systematic review (20 published studies consisting of 21 study sites and 190 observations) aimed to test the hypothesis that the lowest yield-scaled GWP would provide the minimum GWP of CH4 and N2O emissions from rice system at near optimal yields. Results revealed that there was a strong polynomial quadratic relationship between CH4 emissions and N rate and strong positive correlation between N2O emissions and N rate. Compared to control the low N dose emitted less (23%) CH4 whereas high N dose emitted higher (63%) CH4 emission. The highest N2O emission observed at moderated N level. In total GWP, about 96% and 4%, GHG was emitted as CH4 and N2O, respectively. The mean GWP of CH4 and N2O emissions from rice was 5758 kg CO2 eq ha-1. The least yield-scaled GWP (0.7565 (kg CO2 eq. ha-1)) was recorded at 190 kg N ha-1 that provided the near utmost yield. This dose could be a suitable dose in midseason drainage managed rice systems especially in tropical and subtropical climatic conditions. This yield-scaled GWP supports the concept of win-win for food security and environmental aspects through balancing between viable rice productivity and maintaining convincing greenhouse gases.

Unexpectedly higher soil organic carbon accumulation in the evapotranspiration cover of a coal bottom ash mixed landfill.

Monolayer barriers, which are usually known as evapotranspiration (ET) covers, have long been used as alternative final cover systems in waste landfills. Coal bottom ash was evaluated as a good alternative to soil in landfill ET cover systems to increase the bottom ash (BA) recycling ratio in the past. In a previous study, applying BA promoted plant growth characteristics and improved the soil physicochemical properties, particularly the soil organic carbon (SOC) content. In this study, we investigated the effect of BA on the SOC increase by examining the chemical and physical characteristics of ET cover systems, and we compared BA mixed and pure soils. We collected two types of soil from the landfill cover, namely, BA mixed soil (BA 35% + soil 65%) and soil alone (100%), for treatments during the 5th year after installation. Bottom ash mixed soil has four times more SOC than the pure soil at the surface soil layer, but the SOC contents significantly decreased with the soil depth in BA mixed soil, and no differences were found between BA mixed soil and pure soil below a 25 cm soil depth. In addition, there was no significant difference in the chemical composition of the SOC according to a13C NMR. However, the allophane contents were significantly higher in BA mixed soil than pure soil, which physically protects the material from organic matter decomposition. Conclusively, the higher allophane content originating from BA might act as the primary factor in the high accumulation of soil organic carbon in the BA mixed soil layer by retarding the organic matter decomposition.

Cropping With Slag to Address Soil, Environment, and Food Security.

The effective utilization of slag fertilizer in agriculture to neutralize soil acidity, improve crop productivity, mitigate greenhouse gas emissions, and stabilize heavy metals in contaminated soils turns it into a high value added product in sustainable agriculture. These effects could be due to the shift in microbial metabolism and/or modification of microbial habitats. At the system level, soil microorganisms play an integral role in virtually all ecosystem processes. There is a growing interest to reveal the underlying mechanisms of slag-microbe interactions and the contribution of soil biota to ecosystem functioning. In this perspective, we discuss the possible driving mechanisms of slag-microbe interactions in soil and how these slag-microbe interactions can affect crop yield, greenhouse gas emissions, soil carbon sequestration, and heavy metal stabilization in contaminated soils. In addition, we discuss the problems and environmental concerns in using slag in agriculture. Emphasis has been given for further research to validate the proposed mechanisms associated with slag-microbe interactions for increasing soil quality, crop productivity, and mitigating environmental consequences. While evaluating the slag amendment, effects on agriculture and environment, the potential risks, socio-economics, techno-economics, and ethics should be assessed.

Beneficial effect of compost utilization on reducing greenhouse gas emissions in a rice cultivation system through the overall management chain.

Livestock manure application can stimulate greenhouse gas (GHG) emissions, especially methane (CH4) in rice paddy. The stabilized organic matter (OM) is recommended to suppress CH4 emission without counting the additional GHG emission during the composting process. To evaluate the effect of compost utilization on the net global warming potential (GWP) of a rice cropping system, the fluxes of GHGs from composting to land application were calculated by a life cycle assessment (LCA) method. The model framework was composed of GHG fluxes from industrial activities and biogenic GHG fluxes from the composting and rice cultivation processes. Fresh manure emitted 30MgCO2-eq.ha-1, 90% and 10% of which were contributed by CH4 and nitrous oxide (N2O) fluxes, respectively, during rice cultivation. Compost utilization decreased net GWP by 25% over that of the fresh manure during the whole process. The composting process increased the GWP of the industrial processes by 35%, but the 60% reduction in CH4 emissions from the rice paddy mainly influenced the reduction of GWP during the overall process. Therefore, compost application could be a good management strategy to reduce GHG emissions from rice paddy systems.

Stimulation of methane oxidation potential and effects on vegetation growth by bottom ash addition in a landfill final evapotranspiration cover.

The landfilling of municipal solid waste is a significant source of atmospheric methane (CH4), contributing up to 20% of total anthropogenic CH4 emissions. The evapotranspiration (ET) cover system, an alternative final cover system in waste landfills, has been considered to be a promising way to mitigate CH4 emissions, as well as to prevent water infiltration using vegetation on landfill cover soils. In our previous studies, bottom ash from coal-fired power plants was selected among several industrial residues (blast furnace slag, bottom ash, construction waste, steel manufacture slag, stone powder sludge, and waste gypsum) as the best additive for ET cover systems, with the highest mechanical performance achieved for a 35% (wtwt(-1)) bottom ash content in soil. In this study, to evaluate the field applicability of bottom ash mixed soil as ET cover, four sets of lysimeters (height 1.2m×width 2m×length 6m) were constructed in 2007, and four different treatments were installed: (i) soil+bottom ash (35% wtwt(-1)) (SB); (ii) soil+compost (2% wtwt(-1), approximately corresponding to 40Mgha(-1) in arable field scale) (SC); (iii) soil+bottom ash+compost (SBC); and (iv) soil only as the control (S). The effects of bottom ash mixing in ET cover soil on CH4 oxidation potential and vegetation growth were evaluated in a pilot ET cover system in the 5th year after installation by pilot experiments using the treatments. Our results showed that soil properties were significantly improved by bottom ash mixing, resulting in higher plant growth. Bottom ash addition significantly increased the CH4 oxidation potential of the ET cover soil, mainly due to improved organic matter and available copper concentration, enhancing methanotrophic abundances in soil amended with bottom ash. Conclusively, bottom ash could be a good alternative as a soil additive in the ET cover system to improve vegetation growth and mitigate CH4 emission impact in the waste landfill system.