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Italy is the leading rice producer in Europe and the second in the Mediterranean basin (after Egypt), with most of the production concentrated in a large paddy area between the Lombardy and Piedmont regions (northern Italy). In this area, irrigation of rice was traditionally carried out by wet seeding and continuous flooding; in the last fifteen years, this technique has been gradually replaced by dry seeding followed by a delayed flooding (DFL) or by an alternation of flooding and dry periods (FTI), which are economically more advantageous. This study presents the results of an extensive monitoring campaign designed and carried out in 2021 in a representative paddy district of the Lomellina area (Pavia, northern Italy) to assess the impact of the actual rice cropping strategies on surface water and groundwater quality, with particular attention to two widely used herbicides (MCPA and clomazone) and to nutrient losses (e.g., N, P, K). Results show that MCPA and clomazone concentrations detected in surface water and groundwater are always below the RAC (Regulatory Acceptable Concentration) values. As to nutrients, they do not show significant trends along the season in surface water and groundwater: this may be due to the fact that nutrient sources are many. Concerning the concentrations, nitrates may pose a problem for the area, especially for groundwater. However, further studies would be needed to understand to which extent rice cropping can be considered the major source of contamination for water resources., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Methane concentrations have been monitored at the Anmyeon-do Observatory, Korea, since 1999. In recent years, the methane concentration has increased, but the sources of this increase have yet to be identified. This study was designed to identify the major source contributing to the increase by using World Data Centre for Greenhouse Gases (WDCGG) data and the Greenhouse Gases Emission Presumption (GEP) method. The data were collected at Anmyeon-do between 2003 and 2009 (except 2008), and the analyses showed that the increase in methane concentration originated mainly in rice paddies around the observation point. The annual average methane concentration at Anmyeon-do was 1894 ppb, of which 100-103 ppb (5.3-5.4%) was shown to originate mainly from rice paddies. The seasonal fluctuation in methane concentration from May to October estimated by the GEP method was compared with experimental data of previous research conducted on rice paddies. The close match obtained through this comparison shows that the GEP method is effective. The difference in methane concentration was also analyzed in terms of land use and land cover. It was shown that although rice paddies account for only 14.7% of the area surveyed, they accounted for between 69 and 90% of the total increase in methane concentration. These results confirm that rice paddies are the main source of the increase in methane concentration observed at Anmyeon-do.

Apportioning the sources of metals/metalloids is a critical step toward soil quality protection and ecological restoration. The objective of this study was to identify the potential sources of contamination of As, Cd, Cr, Cu, Hg, Mn, Pb, and Zn, and determine the contribution rates of each source, to rice and sugarcane agroecosystems of southwestern Guangxi, southern China. We collected a total of 300 soil samples at a former lead-zinc mine and at two reference sites, 6 and 60 km away from the mine, sampling both agroecosystems at each site. Overall, the positive matrix factorization (PMF) receptor model revealed that in rice paddies at the mine site, mining activities had the highest contribution (60.7% of all examined metals/metalloids), followed by irrigation (25.8%), and agrochemical application (13.5%). At the close reference site, agrochemical application contributed 42.8%, followed by irrigation (22.7%), natural sources (17.4%), and mining activities (17.2%). At the far reference site, agrochemical application was predominant (40.6%), followed by irrigation (32.5%), and natural sources (26.9%). In comparison, at the mine site and the close reference site in sugarcane ecosystems, agrochemical application was predominant (50.1% and 57.4%, respectively), followed by mining activities (49.9% and 42.6%). At the far reference site, agrochemical application contributed 51.2%, followed by natural sources (48.8%). Therefore, the PMF model indicated that the optimal solution was four or three sources per site for rice paddies, but only two sources per site for sugarcane, suggesting that sources of metal/metalloid contamination were more complicated in rice paddy than in sugarcane agroecosystems., (© 2023. The Author(s), under exclusive licence to Springer Nature Switzerland AG.)

Organic farming was developed to reduce agriculture's negative impacts on the environment and enhance biodiversity for sustainable productivity in agricultural ecosystems, but the long-term effectiveness of its application in Japanese rice paddies is unclear. We sought to understand how long-term organic farming affects the abundance of animals in both the rice growth and fallow seasons, and how soil properties change. We investigated the abundance of fishes, frogs, beetles, and shellfish in the floodwater in summer, and the abundance of earthworms (mainly Enchytraeidae), arthropods (spiders and springtails), and soil properties in aerobic soils in autumn. We examined fields which had been farmed organically for 10 and 18 years in Tochigi, Japan. Fields farmed with conventional management, located close to the selected organic fields, were used as a control. All selected fields were located in a valley, which is the typical landscape of a traditional Japanese farming village, called a yatsuda in Japanese. The results showed an increase in soil organic carbon, total nitrogen, and available phosphorus in plowed soils that had been converted from conventional to organic farming both 10 and 18 years earlier. However, the abundance of various animals were not affected significantly by long-term organic rice farming, other than arthropods in the aerobic soils that had been farmed organically for 18 years. The quantity of most animals in floodwater and fallow season soil was unaffected by long-term organic rice farming in the yatsuda paddy fields, probably due to the circumstances and similar irrigation systems for both conventional and organic rice farming, as well as lighter doses of agrochemical application for conventional rice cultivation.

Methane (CH 4 ) emission is the consequence of CH 4 production and consumption performed by methanogens and methanotrophs, respectively. Fertilization is an important factor that regulates the behavior of methanogens and methanotrophs; however, the effect of manure and rice straw addition combined with inorganic fertilizers on these communities is not well understood. This study aimed to explore how manure and rice straw amendments together with inorganic fertilizers influenced the methanogenic and methanotrophic communities in a 31-year fertilized rice paddy. Manure amendment significantly increased the abundance of mcrA and pmoA genes by 61.2% and 63.3% compared with the unfertilized control, whereas inorganic NPK fertilization alone or rice straw addition did not affect their abundances. Manure and rice straw amendments greatly decreased the Shannon index and ACE index of the methanogenic communities, whereas inorganic NPK fertilization alone increased the ACE index of the methanotrophic communities compared with the unfertilized control. Methanosarcinaceae and Methylococcaceae dominated at the family level, representing 23.1-35.0% and 48.7-67.2% of the total reads, for the methanogenic and methanotrophic communities, respectively. Application of manure together with inorganic fertilizers suppressed the Methanocellales methanogens and the type I methanotrophs (Methylococcus and Methylobacter). Fertilization greatly altered the community structure of methanogens and methanotrophs, and manure addition had more apparent effects than rice straw. Moreover, total nitrogen, soil organic carbon, available phosphorus, and available potassium correlated significantly to the abundance, composition, and community structure of methanogens and methanotrophs. In conclusion, our study revealed that long-term manure amendment in combination with inorganic fertilizers significantly increased the abundance of methanogens and methanotrophs, but suppressed the type I methanotrophs in rice paddies.

Methane (CH 4 ) is an important greenhouse gas (GHG), and paddy fields are major sources of CH 4 emissions. This pot experiment was conducted to investigate the integrated effects of Azolla inoculation combined with water management and N fertilization on CH 4 emissions in a double-rice cropping system of Southern China. Results indicated that midseason aeration reduced total CH 4 emissions by 46.9%, 38.6%, and 42.4%, followed by N fertilization with 32.5%, 17.0%, and 29.5% and Azolla inoculation with 32.5%, 17.0%, and 29.5%, on average, during the early, late, and annual rice growing seasons, respectively. The CH 4 flux peaks and total CH 4 emissions observed in the late rice growing season were significantly higher than those in the early rice growing season. Additionally, CH 4 fluxes correlated negatively to soil redox potential (Eh) and dissolved oxygen (DO) concentration. Azolla inoculation and N fertilization greatly increased the rice grain yields, whereas midseason aeration had distinct effects on grain yields in both rice seasons. The highest annual rice grain yields of approximately 110 g pot -1 were obtained in the Azolla inoculation and N fertilization treatments. In terms of yield-scaled CH 4 emission, Azolla inoculation combined with midseason aeration and N fertilization generated the lowest yield-scaled CH 4 emissions both in the early and in the late rice growing seasons, as well as during the annual rice cycle. In contrast, the highest yield-scaled CH 4 emission was obtained in the treatment employed continuous flooding, without Azolla and no N application. Our results demonstrated that Azolla inoculation, midseason aeration, and N fertilization practices mitigated total CH 4 emissions by 18.5-42.4% during the annual rice cycle. We recommend that the combination of Azolla inoculation, midseason aeration, and appropriate N fertilization can achieve lower CH 4 emissions and yield-scaled CH 4 emissions in the double-rice growing system.

Safe recycling of the growing amounts of municipal sewage sludge containing toxic metals had been critically challenged with the fast urbanization. In this study, we investigated soil amendment of municipal wastewater treatment (MSS) converted biochar for its recycling in agricultural soils. In a field experiment, unpyrolyzed (USS) and pyrolyzed municipal sewage sludge (PSS) was amended at 20 t ha -1 on dry base to a rice paddy before rice plantation, with a control without amendment. Grain yield and emission of non-CO 2 potent greenhouse gases were examined as well as topsoil metal mobility and plant uptake determined throughout a rice-wheat rotation year. Compared to USS treatment, addition of PSS caused a significantly increased grain yield of rice by 35% but no change in grain yield of wheat following the rice season. No distinct difference was observed in grain concentration of major nutrients of N, P, and K between USS and PSS treatments. Compared to USS treatment, PSS treatment reduced CH 4 emissions by 91.6% from soil and by 78.5% from ecosystem during rice-growing season. Whereas, PSS treatment led to a reduction of ecosystem N 2 O emissions by 70.8% relative to USS treatment during wheat-growing season. While both USS and PSS treatments slightly but insignificantly increased soil total content of heavy metals, PSS treatment reduced CaCl 2 -extractable Cd pool by 33~40% over USS treatment. Grain contents of Cd and Pb and Cd/Zn were markedly reduced under PSS over USS, without exceeding the Chinese state guideline limit. Carbon emission intensity was considerably (by over 20%) reduced for soil and ecosystem but unchanged for wheat soil, under PSS over USS. Thus, soil amendment of pyrolyzed sewage sludge could be a measure for climate smart soil and for safe grain production in rice agriculture. It deserves further study if repeated amendment could exert sustainable impacts on soil health and food security in the paddy.

Eco-ditches (ecological ditches) not only drain water from rice paddies, but also facilitate the removal of nitrogen (N). We established an experiment with both static and flowing water in 2017 to observe N removal from rice paddy drainage by eco-ditches containing three different types of monoculture vegetation: Zizania aquatica, Canna indica L., and Pontederia cordata. Results showed that ammonia volatilization and plant uptake contributed little to N removal. Harvest of Z. aquatica from the eco-ditch during the late growing season had an appreciable effect on N removal. However, harvest of C. indica L. and P. cordata had negligible effects. During static test, the concentration of total N (TN) and ammonium N (NH 4 + -N) and the pH all decreased from the surface to the bottom of water. The concentration of nitrate N (NO 3 - -N) did not exhibit stratification. In a flowing water experiment, ditches with Z. aquatica, C. indica L., and P. cordata had the following average removal rates: TN 15.8%, 11.6%, and 27.9%; NO 3 - -N 4.2%, 8.4%, and 17.8%; NH 4 + -N 22.8%, 16.4%, and 37.5%, respectively. The removal rates of TN and NH 4 + -N decreased with the increase of water level, while that of NO 3 - -N increased significantly. Nitrogen removal rates decreased with the increase of influent TN concentration or flow rate. Nitrogen removal rate of P. cordata ditch was highly dependent on the influent TN concentration, but the flow rate was not as important due to the great drag caused by its large density. While the contrary was observed in the C. indica L. ditch. For Z. aquatica ditch, both the flow rate and the TN concentration had a strong negative correlation with the N removal rate.

Knowledge about the impacts of fresh and field-aged biochar amendments on greenhouse gas (CH 4 , N 2 O) emissions is limited. A field experiment was initiated in 2012 to study the effects of fresh and field-aged biochar additions on CH 4 and N 2 O emissions and the associated microbial activity during the entire rice-growing season in typical rice-wheat rotation system in Southeast China. CH 4 and N 2 O fluxes were monitored, and the abundance of methanogen (mcrA), methanotrophy (pmoA), ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), nitrite reductase (nirS, nirK), N 2 O reductase (nosZ), and potential soil enzyme activities related to CH 4 and N 2 O were simultaneously measured throughout different rice developmental stages. There were three treatments: control (urea without biochar), fresh BC (urea with fresh biochar added in 2015), and aged BC (urea with 3-year field-aged biochar added in 2012). Results showed that field-aged biochar significantly decreased seasonal CH 4 emissions by 16.8% in relation to the fresh biochar, though no significant differences were detected between biochars and control treatment. The structural equation model indicated that soil pH, microbial biomass carbon (MBC), pmoA, and mcrA were the main factors directly influenced by fresh and aged biochar amendments; aged biochar showed a negative effect while fresh biochar showed positive effects on CH 4 fluxes. Both fresh and field-aged biochar obviously increased AOA and AOB abundances and reduced the (nirS+nirK)/nosZ ratio during the entire rice-growing season, although no significant effects were observed on seasonal N 2 O emissions. Therefore, biochar amendment produced long-term effects on total CH 4 and N 2 O emissions through observed influences of soil pH and functional gene abundance. The figure shows how fresh and field-aged biochar differentially affected CH4 production and oxidation and N2O production and reduction through related functional gene abundances. Blue arrows indicate suppressing while pink arrows indicate promoting effect.

In recent years, rice paddies have been increasingly converted to vegetable production resulting from economic benefits and changes in demand of diets, potentially altering soil greenhouse gas (GHG) balance. Here, we implemented a parallel field experiment to simultaneously quantify the differences in emissions of CH 4 and N 2 O among rice paddy (RP) and conventional vegetable field (CV) and greenhouse vegetable field (GV), both of which have been recently converted from rice paddy in subtropical China over a full year. The results revealed that CH 4 emission was reduced dramatically by nearly 100% following the initial land management change from rice to vegetable production, with annual emissions of 720.9, 0.9, and 0.2 kg CH 4 -C ha -1 for RP, CV, and GV, respectively. This conversion however substantially increased N 2 O emissions, resulting in the transition from a minor sink of N 2 O in RP (-0.1 kg N ha -1  yr -1 ) to considerable N 2 O sources in CV (31.8 kg N ha -1  yr -1 ) and GV (52.2 kg N ha -1  yr -1 ). Furthermore, annual N 2 O emission from GV significantly exceeded that from CV due to lower soil pH and higher soil temperature facilitating N 2 O production in GV relative to CV. Land management change significantly decreased the annual total emissions of CH 4 and N 2 O from CV and GV by 19-51% as compared to RP, attributing to the reduced CH 4 emissions outweighing the increased N 2 O emissions in CV and GV. These results indicate that expansion of vegetable production at the expense of rice paddies for higher economic benefits also helps mitigate the total emissions of CH 4 and N 2 O.

Biochar soil amendment had been increasingly advocated for improving crop productivity and reducing carbon footprint in agriculture worldwide. However, the long-term benefits of biochar application with farming systems had not been thoroughly understood. This study quantified and assessed emergy, energy, and economic benefits of rice and wheat production throughout 6 rotation years following a single biochar amendment in a rice paddy from Southeastern China. Using the data from farm inventory, the quantified emergy indices included grain outputs, unit emergy value, and relative percentage of free renewable resources, environmental loading ratio, emergy yield ratio, and emergy sustainability index (ESI). The results indicated contrasting differences in these emergy values between biochar-amended and unamended production systems over the 6 years. The overall emergy efficiency of rice and wheat productions in biochar-amended system were higher by 11-28 and 15-47%, respectively, than that of unamended one of which the production being highly resource intensive. Moreover, ESI on average was 0.46 for rice and 0.63 for wheat in amended system, compared to 0.35 for rice and 0.39 for wheat in unamended one. Furthermore, over the 6 years following a single application, the ESI values showed considerable variation in the unamended system but consistently increasing in the amended system. Again, the biochar-amended system exerted significantly higher energy and economic return than the unamended one. Nonetheless, there was a tradeoff between rice and wheat in grain yield and net economic gain. Overall, biochar amendment could be a viable measure to improve the resilience of grain production while to reduce resource intensity and environment impacts in paddy soil from China.

The net global warming potential (NGWP) and net greenhouse gas intensity (NGHGI) of double-rice cropping systems are not well documented. We measured the NGWP and NGHGI including soil organic carbon (SOC) change and indirect emissions (IE) from double-crop rice fields with fertilizing systems in Southern China. These experiments with three different nitrogen (N) application rates since 2012 are as follows: 165 kgN ha -1 for early rice and 225 kgN ha -1 for late rice (N1), which was the local N application rates as the control; 135 kgN ha -1 for early rice and 180 kgN ha -1 for late rice (N2, 20 % reduction); and 105 kgN ha -1 for early rice and 135 kgN ha -1 for late rice (N3, 40 % reduction). Results showed that yields increased with the increase of N application rate, but without significant difference between N1 and N2 plots. Annual SOC sequestration rate under N1 was estimated to be 1.15 MgC ha -1  year -1 , which was higher than those under other fertilizing systems. Higher N application tended to increase CH 4 emissions during the flooded rice season and significantly increased N 2 O emissions from drained soils during the nonrice season, ranking as N1 > N2 > N3 with significant difference (P < 0.05). Two-year average IE has a huge contribution to GHG emissions mainly coming from the higher N inputs in the double-rice cropping system. Reducing N fertilizer usage can effectively decrease the NGWP and NGHGI in the double-rice cropping system, with the lowest NGHGI obtained in the N2 plot (0.99 kg CO 2 -eq kg -1 yield year -1 ). The results suggested that agricultural economic viability and GHG mitigation can be simultaneously achieved by properly reducing N fertilizer application in double-rice cropping systems.

A field experiment was conducted in a Cd-contaminated rice paddy field to evaluate the effect of inorganic and organic metal stabilizers on Cd mobility and rice uptake. A dose of inorganic stabilizer of calcium hydroxide (CH), silicon slag (SS), and wheat straw biochar (BC) was amended respectively to topsoil before rice transplanting. Rice production was managed with the same water regime and fertilization practices consistently between treatments including a control without amendment. Samples of topsoil and rice plant were collected at rice harvest to analyze the Cd mobility and uptake by rice. Without affecting rice grain yield, the stabilizers significantly decreased CaCl2-extractable Cd in a range of 44 to 75 % compared to the control, corresponding to soil pH changes under the different treatments. Accordingly, Cd concentrations both in rice tissue and in rice grain were very significantly decreased under these treatments. The decrease in rice Cd uptake was correlated to the decrease in extractable Cd, which was again correlated to soil pH change under the different treatments, indicating a prevalent role of liming effect by the amendments. While applied at a large amount in a single year, organic stabilizer of BC decreased Cd extractability by up to 43 % and Cd rice uptake by up to 61 %, being the most effective on Cd immobilization. However, the long-term effect on soil health and potential tradeoff effects with different stabilizers deserve further field monitoring studies.

Soil amendment of biochar (BSA) had been shown effective for mitigating greenhouse gas (GHG) emission and alleviating metal stress to plants and microbes in soil. It has not yet been addressed if biochar exerts synergy effects on crop production, GHG emission, and microbial activity in metal-polluted soils. In a field experiment, biochar was amended at sequential rates at 0, 10, 20, and 40 t ha(-1), respectively, in a cadmium- and lead-contaminated rice paddy from the Tai lake Plain, China, before rice cropping in 2010. Fluxes of soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) were monitored using a static chamber method during the whole rice growing season (WRGS) of 2011. BSA significantly reduced soil CaCl2 extractable pool of Cd, and DTPA extractable pool of Cd and Pb. As compared to control, soil CO2 emission under BSA was observed to have no change at 10 t ha(-1) but decreased by 16-24% at 20 and 40 t ha(-1). In a similar trend, BSA at 20 and 40 t ha(-1) increased rice yield by 25-26% and thus enhanced ecosystem CO2 sequestration by 47-55% over the control. Seasonal total N2O emission was reduced by 7.1, 30.7, and 48.6% under BSA at 10, 20, and 40 t ha(-1), respectively. Overall, a net reduction in greenhouse gas balance (NGHGB) by 53.9-62.8% and in greenhouse gas intensity (GHGI) by 14.3-28.6% was observed following BSA at 20 and 40 t ha(-1). The present study suggested a great potential of biochar to enhancing grain yield while reducing carbon emission in metal-polluted rice paddies.