Rice Paddy Ecosystems Research Articles

Depth weakens effects of long-term fertilization on dissolved organic matter chemodiversity in paddy soils.

Dissolved organic matter (DOM) is pivotal for soil biogeochemical processes, soil fertility, and ecosystem stability. While numerous studies have investigated the impact of fertilization practices on DOM content along soil profiles, variations in DOM chemodiversity and the underlying factors across soil profiles under long-term fertilization regimes remain unclear. Using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and high-throughput sequencing, this study investigated DOM composition characteristics and microbial community compositions across different soil layers (0-20, 20-40, 40-60, and 60-100cm) in paddy soil under different long-term fertilization treatments, including Control (no fertilizer), NPK (mineral NPK fertilizer), NPKHS (NPK fertilizer with half straw return), and NPKS (NPK fertilizer with full straw return). The results revealed that fertilization regimes significantly increased soil TC, TN, and NO3- contents, as well as DOM chemodiversity in the top soil layer, particularly under NPKHS and NPKS treatments. Both the DOM chemodiversity and bacterial diversity decreased with soil depth. However, below 0-20cm, DOM chemodiversity was not significantly affected by fertilization treatments. Co-occurrence network analysis further showed that microbial decomposition primarily drove the changes in DOM composition across soil profile. Overall, our study suggests that long-term NPK fertilization and straw return significantly increased DOM chemodiversity only in the top layer of paddy soil by regulating soil TC, TN, and NO3- contents. Our study provides useful information regarding the vertical molecular composition of DOM and enhances the understanding of DOM chemodiversity along soil profile in rice paddy ecosystems.

Three-year elevated carbon dioxide concentration does not enhance soil organic carbon quantity due to simultaneously facilitated carbon input and decomposition in a single rice paddy soil evidenced by natural 13C tracing.

Soil organic carbon (SOC) markedly contributes to maintaining soil nutrient cycling and mitigating climate change. Elevated carbon (C) dioxide concentration ([CO2]) is widely expected to improve crop yield and increase C storage; however, its effects on rice growth and SOC dynamics remain greatly unclear. Therefore, a three-year (2007-2009) chamber experiment with two [CO2] treatments (380 vs. 680ppm) was conducted during rice growing seasons. Ultisol soil, taken from a sugarcane (C4 plant) field on Ishigaki island, Okinawa, was used to grow rice (C3 plant). The natural 13C tracing method was utilized to measure the fraction of SOC derived from rice plant, and δ13C values and concentrations of CO2 and CH4 dissolved in soil solutions were determined. Elevated [CO2] significantly increased rice aboveground biomass (AGB) by 11.8%-28.8% and assimilated C content by 12.2%-28.3%. However, no significant differences were observed in SOC, total nitrogen (N) content, and the C/N ratios between ambient and elevated [CO2]. Elevated [CO2] induced markedly lower δ13C values in both plant and soil samples relative to ambient [CO2]. The annual fractions of plant-derived C input ranged from 5.0% to 21.2% in ambient [CO2] and from 5.6% to 21.9% in elevated [CO2] without significant differences. Elevated [CO2] stimulated marked increases in dissolved CO2 and CH4 concentrations, and δ13C values of CH4, indicating a positive priming effect of elevated [CO2] on native SOC decomposition for methanogenesis. In conclusion, elevated [CO2] did not affect SOC accumulation by simultaneously increasing C input evidenced by increased AGB, and SOC decomposition as CO2 and CH4 emissions, hence resulting in a stable SOC quantity in rice paddy ecosystems. Our study delves in the nexus between C input and soil C decomposition under elevated CO2 condition, highlighting its significance in prediction of the responses of C storage in paddy ecosystems to future climate change.

Conversion of Rice Field Ecosystems from Conventional to Ecological Farming: Effects on Pesticide Fate, Ecotoxicity and Soil Properties.

Rice is an important staple food around the world, the cultivation as sustainable agriculture and food supply are key to achieving the Sustainable Development Goals (SDGs) of 2030. In order to analyze the sustainability of the rice paddy ecosystem, a comparative study was carried out during the rice growing season between paddies with conventional agriculture (CA) and ecological agriculture (EA), integrating analysis of physico-chemical characteristics of soil and soil pore water, pesticide residues, acute toxic effects and potential ecological risk, as well as aquatic invertebrate community structure dynamics. Our study found that total carbon and nitrogen present in soil were significantly higher in CA than in EA, while opposite results were found in soil pore water, implying the improvement on soil properties in EA. Neonicotinoid pesticides (thiamethoxam and thiacloprid) were still detected in EA, although no pesticides were applied after conversing CA to EA. Additionally, toxic effects to zebrafish embryos with a peak toxicity in summer (July, LC50 = 55.26 mg soil equivalent/L) were also found in EA, which was lower than in CA. The dynamics of the aquatic invertebrate community structure were correlated with the toxicity results, with higher diversity recorded in EA. Therefore, for the purpose of ecosystem sustainability, the long-term implementation of EA is highly recommended.

Investigating drivers of free-living diazotroph activity in paddy soils across China

Microbially mediated N fixation is widespread in rice paddy ecosystems and crucial in maintaining soil fertility. However, our understanding of the factors determining the distribution of free-living diazotrophic microorganisms that perform this process in paddy fields is limited. This study investigated the spatial distribution and factors influencing presence and potential activity of free-living microorganisms capable of N2 fixation in addition to dissimilatory nitrate reduction to ammonium (DNRA), anaerobic ammonium oxidation (anammox), and denitrification in 50 paddy soils across China. Using 15N isotope tracing in laboratory incubations and microbial community analysis via metagenomics, we demonstrate that paddy soils may represent a previously underappreciated hotspot for N2 fixation with mean potential rates of 24.4 ± 17.8 nmol N g−1 h−1, 10-fold higher than DNRA (2.55 ± 0.4 nmol N g−1 h−1), and could counterbalance a portion of N2 losses through anammox and denitrification (9.24 ± 1.1 nmol N g−1 h−1). Site longitude and organic carbon (C) concentrations, as well as the diazotrophic community composition, were the dominant abiotic and biotic factors accounting for regional variations in potential N2 fixation rates. The N2 metabolic pathways predicted from the metagenome-assembled genomes (MAGs) revealed significant co-occurrence of the diazotroph marker gene nifH with denitrification-associated genes (nirS/K and nosZ) and organic C oxidation-related genes (yiaY and galM). Furthermore, enzymes involved in organic C oxidation, particularly glycoside hydrolases and glycosyltransferases, were not only phenotypically correlated with free-living N2 fixation rates but were also identified in nifH-containing MAGs, indicating the heterotrophic capabilities of diazotrophs in paddy soils. Collectively, our results underscore the substantial contribution of free-living N2 fixation to soil N fertility in paddy fields, and highlight the importance of coupling organic C oxidation with nitrate reduction to enhance N2 fixation.

Winter forage crops influence soil properties through establishing different arbuscular mycorrhizal fungi communities in paddy field

Winter planting is promising for improving the utilization rate of fallow paddy fields in southern China by establishing arbuscular mycorrhizal fungi (AMF) communities. However, the effects of different winter forage crops on AMF community construction remain unknown. The AMF community establishment of different winter planting forage crops were conducted in oat, rye, Chinese milk vetch, and ryegrass, with winter fallow as a control. The AMF colonization rate, soil AMF spore density, community structure and diversity, and soil physicochemical properties were determined. The results showed that the total nitrogen and available nitrogen in winter Chinese milk vetch were 11.11% and 16.92% higher than those in winter fallow (P < 0.05). After planting winter forage crops, the AMF spore density in winter oat, rye, Chinese milk vetch, and ryegrass soil were 127.90%, 64.37%, 59.91%, and 73.62% higher than that before planting, respectively (P < 0.05). Claroideoglomus was the dominant AMF genus in the soil of winter planting oat, rye, and ryegrass. The average membership function value of winter Chinese milk vetch was the highest, indicating that it had the best comprehensive effect on soil physicochemical properties, AMF community structure and diversity, and fresh forage yield. Winter forage crops could increase the spore pool of soil AMF and improve the soil AMF community structure and diversity. Winter Chinese milk vetch in paddy field had the best comprehensive effect on soil physicochemical properties and soil AMF community according to the comprehensive evaluation. These findings provide a theoretical basis for sustainable development and utilization of the southern rice paddy ecosystem.

Carbon budget of paddy ecosystems in China simulated by denitrification‐decomposition model

AbstractQuantifying the carbon budget in rice paddy ecosystems is fundamental to understanding the role of paddy fields as carbon sinks or sources and to mitigating global climate change. This study used a process‐based model of the denitrification‐decomposition (DNDC) model to estimate greenhouse gas (GHG) emissions and carbon sinks in the paddy ecosystem across China. The simulation results showed that the GHG emissions were 0.147 Pg C‐equivalent year−1 and that the carbon sequestration in rice plants and soils reached 0.253 Pg C year−1 in 2016, indicating high carbon sequestration potential of the Chinese paddy ecosystem. Land management had a great influence on carbon budgets, among which straw residue incorporation rate and nitrogen fertilizer applications had a stronger influence on carbon budgets, particularly for double cropping rice. These results indicate that appropriate tillage practices can reduce GHG emissions and increase carbon sequestration of rice ecosystems while ensuring rice yields.

Using Mercury Stable Isotopes to Quantify Directional Soil-Atmosphere Hg(0) Exchanges in Rice Paddy Ecosystems: Implications for Hg(0) Emissions to the Atmosphere from Land Surfaces.

Gaseous elemental mercury [Hg(0)] emissions from soils constitute a large fraction of global total Hg(0) emissions. Existing studies do not distinguish biotic- and abiotic-mediated emissions and focus only on photoreduction mediated emissions, resulting in an underestimation of soil Hg(0) emissions into the atmosphere. In this study, directional mercury (Hg) reduction pathways in paddy soils were identified using Hg isotopes. Results showed significantly different isotopic compositions of Hg(0) between those produced from photoreduction (δ202Hg = -0.80 ± 0.67‰, Δ199Hg = -0.38 ± 0.18‰), microbial reduction (δ202Hg = -2.18 ± 0.25‰, Δ199Hg = 0.29 ± 0.38‰), and abiotic dark reduction (δ202Hg = -2.31 ± 0.25‰, Δ199Hg = 0.50 ± 0.22‰). Hg(0) exchange fluxes between the atmosphere and the paddy soils were dominated by emissions, with the average flux ranging from 2.2 ± 5.7 to 16.8 ± 21.7 ng m-2 h-1 during different sampling periods. Using an isotopic signature-based ternary mixing model, we revealed that photoreduction is the most important contributor to Hg(0) emissions from paddy soils. Albeit lower, microbial and abiotic dark reduction contributed up to 36 ± 22 and 25 ± 15%, respectively, to Hg(0) emissions on the 110th day. These novel findings can help improve future estimation of soil Hg(0) emissions from rice paddy ecosystems, which involve complex biotic-, abiotic-, and photoreduction processes.

Observation and Simulation of CO2 Fluxes in Rice Paddy Ecosystems Based on the Eddy Covariance Technique

As constituents of one of the vital agricultural ecosystems, paddy fields exert significant influence on the global carbon cycle. Therefore, conducting observations and simulations of CO2 flux in rice paddy is of significant importance for gaining deeper insights into the functionality of agricultural ecosystems. This study utilized an eddy covariance system to observe and analyze the CO2 flux in a rice paddy field in Eastern China and also introduced and parameterized the Jarvis multiplicative model to predict the CO2 flux. Results indicate that throughout the observation period, the range of CO2 flux in the paddy field was −0.1 to −38.4 μmol/(m2·s), with a mean of −12.9 μmol/(m2·s). The highest CO2 flux occurred during the rice flowering period with peak photosynthetic activity and maximum CO2 absorption. Diurnal variation in CO2 flux exhibited a “U”-shaped curve, with flux reaching its peak absorption at 11:30. The CO2 flux was notably higher in the morning than in the afternoon. The nocturnal CO2 flux remained relatively stable, primarily originating from respiratory CO2 emissions. The rice canopy CO2 flux model was revised using boundary line analysis, elucidating that photosynthetically active radiation, temperature, vapor pressure deficit, phenological stage, time, and concentration are pivotal factors influencing CO2 flux. The simulation of CO2 flux using the parameterized model, compared with measured values, reveals the efficacy of the established parameter model in simulating rice CO2 flux. This study holds significant importance in comprehending the carbon cycling process within paddy ecosystems, furnishing scientific grounds for future climate change and environmental management endeavors.

Niche-specification of aerobic 2,4-dichlorophenoxyacetic acid biodegradation by tfd-carrying bacteria in the rice paddy ecosystem.

This study aimed for a better understanding of the niche specification of bacteria carrying the tfd-genes for aerobic 2,4-dichlorphenoxyacetic acid (2,4-D) degradation in the rice paddy ecosystem. To achieve this, a dedicated microcosm experiment was set up to mimic the rice paddy system, with and without 2,4-D addition, allowing spatial sampling of the different rice paddy compartments and niches, i.e., the main anaerobic bulk soil and the aerobic surface water, surface soil, root surface and rhizosphere compartments. No effect of 2,4-D on the growth and morphology of the rice plant was noted. 2,4-D removal was faster in the upper soil layers compared to the deeper layers and was more rapid after the second 2,4-D addition compared to the first. Moreover, higher relative abundances of the 2,4-D catabolic gene tfdA and of the mobile genetic elements IncP-1 and IS1071 reported to carry the tfd-genes, were observed in surface water and surface soil when 2,4-D was added. tfdA was also detected in the root surface and rhizosphere compartment but without response to 2,4-D addition. While analysis of the bacterial community composition using high-throughput 16S rRNA gene amplicon sequencing did not reveal expected tfd-carrying taxa, subtle community changes linked with 2,4-D treatment and the presence of the plant were observed. These findings suggest (i) that the surface soil and surface water are the primary and most favorable compartements/niches for tfd-mediated aerobic 2,4-D biodegradation and (ii) that the community structure in the 2,4-D treated rice paddy ecosystem is determined by a niche-dependent complex interplay between the effects of the plant and of 2,4-D.

Management and Development of Paddy Ecosystem Services in Hongthai, Vietnam by Using SWOT Analysis Method

With an increase in the world population, the global demand for tourism development is rising, promoting the increased utilization of natural ecosystem services. The agricultural ecosystem, particularly the rice paddy ecosystem, is paramount in socio-economic and tourism development for developing countries. The aim of this study is to find solutions for management and development of paddy ecosystem services towards sustainable ecotourism development in poor and vulnerable areas in developing countries. This study assessed the management and development of paddy ecosystem services in Hongthai, Vietnam via the Strengths, Weaknesses, Opportunities, and Threats (SWOT) method. The results indicate that despite the high potentialities of paddy ecosystem services, much more investigation is required for the management and development of these services for tourism development. The intricate and disjoined mountainous topography of Hongthai poses challenges to the establishment of infrastructure networks and economic advancement. This results in elevated travel expenses for tourists, with limited return on investment, thereby impeding Hongthai's ability to compete with tourist destinations in neighboring provinces. The results provide a theoretical and practical basis for the process of evaluating the effectiveness of investment programs and projects for economic and social development and ecotourism activities in Hongthai, Vietnam. Also, the SWOT results recommend and orient local people's agricultural development activities towards improving the efficiency of exploitation of available ecosystem services. The findings of this study may be useful for formulating effective management strategies at different scales.

Relative importance of aceticlastic methanogens and hydrogenotrophic methanogens on mercury methylation and methylmercury demethylation in paddy soils

The accumulation of methylmercury (MeHg) in paddy soil results from a subtle balance between inorganic mercury (e.g., HgII) methylation and MeHg demethylation. Methanogens not only act as Hg methylators but may also facilitate MeHg demethylation. However, the diverse methanogen flora (e.g., aceticlastic and hydrogenotrophic types) that exists under ambient conditions has not previously been considered. Accordingly, the roles of different types of methanogens in HgII methylation and MeHg degradation in paddy soils were studied using the Hg isotope tracing technique combined with the application of methanogen inhibitors/stimulants. It was found that the response of HgII methylation to methanogen inhibitors or stimulants was site-dependent. Specifically, aceticlastic methanogens were suggested as the potential HgII methylators at the low Hg level background site, whereas hydrogenotrophic methanogens were potentially involved in MeHg production as Hg levels increased. In contrast, both aceticlastic and hydrogenotrophic methanogens facilitated MeHg degradation across the sampling sites. Additionally, competition between hydrogenotrophic and aceticlastic methanogens was observed in Hg-polluted paddy soils, implying that net MeHg production could be alleviated by promoting aceticlastic methanogens or inhibiting hydrogenotrophic methanogens. The findings gained from this study improve the understanding of the role of methanogens in net MeHg formation and link carbon turnover to Hg biogeochemistry in rice paddy ecosystems.

The divergent effects of nitrate and ammonium application on mercury methylation, demethylation, and reduction in flooded paddy slurries

The production of methylmercury (MeHg) in flooded paddy fields determines its accumulation in rice grains; this, in turn, results in MeHg exposure risks for not only rice-eating humans but also wildlife. Nitrogen (N) fertilizers have been widely applied in rice cultivation fields to supply essential nutrients. However, the effects of N fertilizer addition on mercury (Hg) transformations are not unclear. This limits our understanding of MeHg formation in rice paddy ecosystems. In this study, we spiked three Hg tracers (200HgII, Me198Hg, and 202Hg0) in paddy slurries fertilized with urea, ammonium, and nitrate. The influences of N fertilization on Hg methylation, demethylation, and reduction and the underlying mechanisms were elucidated. The results revealed that dissimilatory nitrate reduction was the dominant process in the incubated paddy slurries. Nitrate addition inhibited HgII reduction, HgII methylation, and MeHg demethylation. Competition between nitrates and other electron acceptors (e.g., HgII, sulfate, or carbon dioxide) under dark conditions was the mechanism underlying nitrate-regulated Hg transformation. Ammonium and urea additions promoted HgII reduction, and anaerobic ammonium oxidation coupled with HgII reduction (Hgammox) was likely the reason. This work highlighted that nitrate addition not only inhibited HgII methylation but also reduced the demethylation of MeHg and therefore may generate more accumulation of MeHg in the incubated paddy slurries. Findings from this study link the biogeochemical cycling of N and Hg and provide crucial knowledge for assessing Hg risks in intermittently flooded wetland ecosystems.

Industrial and agricultural waste amendments interact with microorganism activities to enhance P availability in rice-paddy soils

Adding industrial and agricultural wastes to farmland can increase soil available phosphorus (P) pool and boost crop production, but the process affecting soil P transformation and bioavailability is still poorly understood. We studied the effects of straw (ST), biochar (BC) and Si-modified biochar (Si-BC) amendments on the available-P content and its fraction transformation in rice-paddy soils. Our results showed that these three soil amendments significantly increased the concentrations of both microbial biomass carbon (MBC) and microbial biomass-P (MBP) during the first rice season; by contrast, the effects of ST and BC application were relatively poor on acid-phosphatase (ACP) activity, which was increased by 24 % under ST and 14 % under BC. Soil total P concentrations did not differ significantly, although the concentration and percentage of each P-fraction were altered significantly among treatments. Although all three applications increase soil available-P concentration by promoting the transformation of organic-P (Po) components to inorganic-P (Pi), there are differences in the transformation efficiency of the soil P fraction between these amendments. Redundancy analysis results also showed significant clustering of soil P-fraction transformations after ST and BC treatments. Structural equation model analysis further indicated that all amendments regulated microbial processes by changing soil pH and dissolved organic carbon (DOC), thereby promoting soil P transformation and improving P efficiency. Sodium bicarbonate-extractable Po (NaHCO3-Po) contributed most to soil available-P under the different amendments. Compared to ST and Si-BC, BC application improved more soil microbial status and the transformation of soil unavailable-P into available-P, therefore the application of BC in rice fields is the most beneficial method to promote phosphorus use and production sustainability in rice. These findings helped to understand the effects of using industrial and agricultural waste (e.g. straw, biochar and Si-modified biochar) on soil P-fractions and so provided a reference for sustainable resource use and green production in rice-paddy ecosystems.

Macro-scale perspectives in conservation biological control: latitudinal differences in the effects of organic farming on insect pests and natural enemies in rice paddy ecosystems

The pest control services provided by natural enemies are important for maintaining crop productivity in environmentally friendly agriculture with reduced use of pesticides. Organic farming is generally considered to increase the abundance of natural enemies and to enhance ecosystem services, but species and guild compositions vary from region to region, and so the population levels of pests and their natural enemies can be expected to vary geographically. Here, we surveyed the abundance of rice insect pests and spiders as their natural enemies in 51 organic and 45 conventional paddy fields at 22 locations in Japan and examined latitudinal differences in the effects of farming practices on their abundance. Independently of latitudinal differences, organic farming had positive effects on the abundance and species diversity of spiders caught in nets and on Tetragnatha and lycosid spiders. It increased the abundances of the leafhopper Nephotettix cincticeps and the white-backed planthopper Sogatella furcifera, and decreased that of the small brown planthopper Laodelphax striatellus. Both spiders and insect pests showed diverse responses to the interaction between farming practice and latitude. In particular, insect pests distributed in warmer regions, such as So. furcifera and Ne. cincticeps, tended to be more abundant in the south. The ratios of spider to pest abundance, as indicators of pest suppression, were lowest in organic paddies at lower latitudes for So. furcifera and Ne. cincticeps; at higher latitudes they were higher in organic paddies than in conventional paddies for La. striatellus. These results suggest that the effectiveness of pest suppression by natural predators in organic paddy fields varies by region depending on the ecological characteristics of each organism. As such, it is necessary to devise field management strategies appropriate to each region to balance crop production and biodiversity.

Concentrations and biomagnification of multiple metals/metalloids are higher in rice than in sugarcane agroecosystems of southern China

Agricultural lands have been increasingly reported to be contaminated by metals and metalloids, leading to exposure to these toxins for humans and wildlife. Previous studies on metal/metalloid contamination have reported biomagnification of total elemental Hg (THg) and, less consistently, of Cd, in both aquatic and terrestrial ecosystems. Whether other metals or metalloids can biomagnify in food webs, especially those that produce food for humans, is uncertain. We aimed to compare metal/metalloid contamination and their biomagnification patterns between rice paddy and sugarcane agroecosystems. We collected samples at an inactive Pb-Zn mine site, and two reference sites in southwestern Guangxi, southern China. Samples were widely distributed across the food web, including soil, rice grain and leaves, sugarcane leaves, crickets, grasshoppers, spiders, and frogs. We found that Cr, Cd, Pb, THg, and Zn were the metals associated with mining activities, while As and Cu were higher at the reference sites. In soil, and in the majority of species for which there were significant differences between agroecosystems, concentrations of Cd, Cr, Cu, Pb, THg, and Zn were higher in rice, suggesting that the rice paddy ecosystem is particularly sensitive to metal contamination. When quantitatively rating the patterns of biomagnification using post-hoc multiple comparisons among species’ bioaccumulation factors (BAFs), rice paddies had stronger patterns of biomagnification (3 strong relationships [e.g., all insectivores had higher or as high BAFs compared to all other species], 7 medium relationships [e.g., one of the insectivores had the highest BAFs, but another had an intermediary value, similar to an herbivore], 6 weak [e.g., one insectivore had the highest BAFs, but the other was as low as a primary producer], and 3 no relationships [e.g., no significant differences among species in BAFs]) than sugarcane ecosystems (1 strong, 2 medium, 8 weak, and 9 no relationships; Repeated measures t-test, P-value = 0.0023) across 6 metals (Cr, Cd, Cu, Hg, Pb, Zn). The strongest biomagnification was evident for THg and Zn, whereas an intermediary level of biomagnification was found for Cd, Cr, Cu, and Pb; biodilution was observed for As and Mn. Therefore, focusing on metal/metalloid apportionment to better understand the sources of the metals at our study sites would be of utility to lower the exposure of wildlife and people. Most importantly, the local authorities should check metal concentrations in any farming product from the mining area and even the close reference site to safeguard human health.

Aquatic methylmercury is a significant subsidy for terrestrial songbirds: Evidence from the odd mass-independent fractionation of mercury isotopes

In contrast to aquatic food chains, knowledge of the origins and transfer of mercury (Hg) and methylmercury (MeHg) in terrestrial food chains is relatively limited, especially in songbirds. We collected soil, rice plants, aquatic and terrestrial invertebrates, small wild fish, and resident songbird feathers from an Hg-contaminated rice paddy ecosystem for an analysis of stable Hg isotopes to clarify the sources of Hg and its transfer in songbirds and their prey. Significant mass-dependent fractionation (MDF, δ202Hg), but no mass-independent fractionation (MIF, ∆199Hg) occurred in the trophic transfers in terrestrial food chains. Piscivorous, granivorous, and frugivorous songbirds and aquatic invertebrates were all characterized by elevated Δ199Hg values. The estimated MeHg isotopic compositions obtained using linear fitting and a binary mixing model explained both the terrestrial and aquatic origins of MeHg in the terrestrial food chains. We found that MeHg from aquatic habitats is an important subsidy for terrestrial songbirds, even those that feed mainly on seeds, fruits, or cereals. The results show that MIF of the MeHg isotope is a reliable tool to reveal MeHg sources in songbirds. Because the MeHg isotopic compositions was calculated with a binary mixing model or directly estimated from the high proportions of MeHg, compound-specific isotope analysis of Hg would be more useful for the interpretation of the Hg sources, and is highly recommended for future studies.

Impact of silicate and lime application on soil fertility and temporal changes in soil properties and carbon stocks in a temperate ecosystem

Most agricultural soils are unproductive due to soil acidity which limits crop growth and yield. To ameliorate soil acidity constraints and enhance the ecosystem services the soil renders to achieve optimum crop yields, lime, and silicate are mostly used. However, there are few studies reporting the impacts of the combined application of silicate and lime on soil properties, soil fertility and carbon stocks and stratification in long-term experiments. This study evaluated the impacts of the combined application of silicate and lime on soil fertility indicators (pH, soil organic matter (SOM), available phosphorus, exchangeable potassium, magnesium, and calcium, and available SiO2), carbon stratification and stocks in comparison to sole lime and sole silicate applied to a Fluvaquentic Eutrudepts over a 67-year period. Each treatment’s rate of application was calculated to raise the soil pH to 6.5. Incorporation of the treatments increased the soil pH by 1.30, 1.22, and 1.35 folds in the sole silicate, sole lime, and combined silicate and lime-treated soils, respectively, relative to the baseline soil pH (5.5). The combined application of silicate and lime resulted in significant decreases in C stratification and C stocks. The significant reduction in the C stocks in the silicate and lime-treated soils implies that its long-term application could potentially reduce carbon sequestration and increase the carbon footprint of rice cultivation. This information is crucial to develop sustainable soil management interventions to curtail the environmental footprint in rice paddy ecosystems. Comparatively, the combined silicate and lime treatment caused an increase in the relative change of soil fertility by 65.7% and the average annual growth rate in the soil fertility improvement by 0.0115, as compared to the untreated soil. Among the soil variables, soil pH, exchangeable calcium, and available silicate contributed most to the variation the soil fertility. The obtained results could serve as a guide in revising the lime and silicate fertilization programs instituted by the Korean Government and reviewing national soil fertility management systems in temperate paddy ecosystems.

Exploring light use efficiency models capacities in characterizing environmental impacts on paddy rice productivity

Remote sensing-driven light use efficiency (LUE) models have been widely used to calculate gross primary productivity (GPP) for various terrestrial ecosystems, but there was limited knowledge on the capacity of LUE models to evaluate the GPP in paddy rice ecosystems. In this study, at seven rice-growing sites over the Northern Hemisphere and based on six commonly used LUE models, we calibrated the parameters (i.e., maximum LUE (LUEmax) and optimum temperature) by separating the growing period into four phenological transitions and evaluated the performance of models, and investigated the impact of changes in cloud conditions and environmental factors (i.e., air temperature and vapor pressure deficit) on GPP simulations. The calibrated LUEmax corresponded closely to phenology, allowing the six LUE models to track the seasonal variations in GPP reasonably well. The sensitivity of GPP estimates to sky clearness index (CI) indicated that the TL-LUE model incorporating diffuse radiation fractions outperformed other models under cloudy conditions. Environmental stressors including along with changes in the diffuse radiation fractions synergistically affected GPP simulation, resulting in distinctly variable performances of the LUE models under different water and temperature conditions, with the TL-LUE model always performing well during the suitable rice growing season. These results demonstrate that it is crucial to consider the diffuse radiation fraction and to better represent environmental stresses under certain environmental conditions in LUE models for accurate estimation of rice GPP.

Assessment Paddy Rice Ecosystem Services on Awareness and Satisfaction Level of Local People In Hongthai, Vietnam

The paddy fields in Hongthai is considered as one of the most beautiful terraced fields in the Northeast of Vietnam. However, the efficiency of use of paddy rice ecosystem services here is still limited. The study was conducted to assess farmers’ awareness and satisfaction level of the paddy rice ecosystem services in Hongthai by using questionnaire survey. The results show that the paddy rice ecosystem provides many services to the farmers, specifically 12 non-market services, in which the majority of farmers clearly identify regulating services such as groundwater replenishment, flood prevention and soil erosion control. Also, although income from tourism is still low, most farmers in Hongthai are aware of the importance of cultural services such as tourist landscapes and culture. However, for other regulating and supporting services, farmers’ awareness is still limited. The satisfaction level of farmers in Hongthai with the paddy rice ecosystem services is still low (2.7/10). The reason is explained by the limited awareness of the ethnic people and the local government have not yet exploited and used the non-market ecosystem service values ​​to bring benefits. In the future, it is necessary to actively propagate and raise farmers' awareness about paddy rice ecosystem services and valuate non-market ecosystem services to provide a scientific basis for policy makers. The appropriate legal framework and supporting policies need to be developed for the conservation of paddy rice ecosystems through the services it provides.

Direct Canopy Uptake of Atmospheric Reactive Nitrogen: A Significant Pathway for Airborne Nitrogen Input into Rice Paddy Ecosystems.

Direct canopy uptake of atmospheric reactive nitrogen (ARN) is an important process, but the magnitude of ARN assimilation in agricultural ecosystems is unclear. We used a combination of a water-culture rice-growing system with a 15N tracer to investigate canopy uptake of ARN under field conditions. Gross uptake was quantified using the plant N partitioning approach, and the net uptake of ARN was estimated based on the system N balance. Gross ARN uptakes were 23.1 and 38.2 kg N·ha-1 during vegetative and reproductive growth, respectively. Although a certain amount of N was lost mainly from the rice canopy during the reproductive phase, the net gain of ARN was 34.3 kg N·ha-1 over the entire growing season, much higher than the present estimates of dry N deposition using an indirect inferential approach. Our results highlight the magnitude of direct canopy uptake of ARN in rice paddy ecosystems, an important N process that is rarely considered in present N budgets.