Intermittent drainage in paddy soil: ecosystem carbon budget and global warming potential

Intermittent drainage of rice fields alters soil redox potential and contributes to the reduction of CH₄ emission and thus may reduce net global warming potential (GWP) during rice cultivation. Incorporation of green biomass helps maintaining soil organic matter, but may increase CH₄ emission. We investigated net ecosystem carbon budget (NECB) and net GWP under two water management regimes—continuous flooding and intermittent drainage—having four biomass incorporation levels (0, 3, 6 and 12 Mg ha^?1). Water management and biomass incorporation level demonstrated significant (P < 0.05) interaction effect on the NECB and GWP. Intermittent drainage decreased the NECB by ca. 6–46 % than continuous flooding under same rates of cover crop biomass (CCB) incorporation. Moreover, intermittent drainage reduced seasonal CH₄–C fluxes by ca. 54–58 % and net GWP by 35–58 % compared to continuous flooding. There was also no significant reduction in rice yield because of intermittent drainage under similar CCB. This implies that incorporation of 3 Mg ha^?1 CCB and intermittent drainage could be a good option for reducing net GWP and higher grain yield.     

Influence of rice varieties,nitrogen management and planting methods on methane emission and water productivity

A field experiment was conducted during rainy seasons of 2009 and 2010 at New Delhi, India to study the influence of varieties and integrated nitrogen management (INM) on methane (CH₄) emission and water productivity under flooded transplanted (FT) and aerobic rice (AR) cultivation. The treatments included two rice (‘PB 1’ and ‘PB 1121’) varieties and eight INM practices including N control, recommended dose of N through urea, different combinations of urea with farmyard manure (FYM), green manure (GM), biofertilizer (BF) and vermicompost (VC). The results showed 91.6–92.5 % lower cumulative CH₄ emission in AR compared to FT rice. In aerobic conditions, highest cumulative CH₄ emission (6.9–7.0 kg ha^?1) was recorded with the application of 100 % N by organic sources (FYM+GM+BF+VC). Global warming potential (GWP) was significantly lower in aerobic rice (105.0–107.5 kg CO₂ ha^?1) compared to FT rice (1242.5–1447.5 kg CO₂ ha^?1). Significantly higher amount of water was used in FT rice than aerobic rice by both the rice varieties, and a water saving between 59.5 and 63 % were recorded. Under aerobic conditions, both rice varieties had a water productivity of 8.50–14.69 kg ha^?1, whereas in FT rice, it was 3.81–6.00 kg ha^?1. In FT rice, a quantity of 1529.2–1725.2 mm water and in aerobic rice 929.2–1225.2 mm water was used to produce one kg rice. Thus, there was a saving of 28.4–39.6 % total water in both the rice varieties under AR cultivation.     

Comparison of methanotrophic bacteria, methane oxidation activity, and methane emission in rice fields fertilized with anaerobically digested slurry between a fodder rice and a normal rice variety

Methane oxidation activity (MOA), methanotrophic bacteria (MOB), and CH₄ emission in a paddy field fertilized with anaerobically digested slurry were compared between two varieties: a fodder rice variety Leaf star (LS) and a normal rice variety Kinuhikari (KN). Average MOA and MOB per unit dry root were significantly higher in KN (7.1?μmol?g^?1 CH₄?g^?1 dry root h^?1 and 3.7?×?10⁷ MPN?g^?1 dry root, respectively) than in LS (4.3?μmol?g^?1 CH₄?g^?1 dry root h^?1, 2.1?×?10⁷ MPN?g^?1 dry root), although MOA in the rice root per whole plant was not significantly different since root biomass of LS (1.5?g dry root plant^?1) was significantly higher than that of KN (1.2?g dry root plant^?1). MOA in the soil ranged from 0.031 to 0.087?μmol?g^?1 CH₄?g^?1 dry soil h^?1, but there were no significant differences in both depths (0–5 and 5–15?cm) between the two rice varieties. MOA in the rhizosphere soil was significantly different between the rice varieties at flowering, but not at tillering. Methane emission in the field was lower and MOA and MOB in the rice root were higher in LS than in KN at tillering, but the reverse results were found at flowering and maturity stages. Total CH₄ emission during a growing period was not significantly different between LS (27.4?±?16.9?g CH₄?m^?2) and KN (24.0?±?19.5?g CH₄?m^?2). There was a significant negative relationship between CH₄ emission and rice root MOB (P?<?0.01) or MOA (P?<?0.05) and significant positive relationship was observed between root MOA and MOB (P?<?0.01). This study revealed that choice of rice variety might be an important environmental issue in paddy cultivation since it can influence MOA and MOB in rice root and rhizosphere soil which relate with CH₄ emission.     

Behavior of carbon dioxide in soils affected by tillage systems

Agricultural management plays an important role in the storage of carbon in soils. The behavior of soil CO₂ in an Andisol in two different tillage systems (no tillage and tillage) was studied. Soil-column incubation experiments were performed for a period of 150 days to conduct this study. Soil CO₂ flux, under no-tillage and tillage treatments, was observed to be 0.557 and 0.616 gCO₂-C m^?2 d^?1, respectively. The cumulative CO₂ flux under tillage treatment was observed to be higher than that under no-tillage treatment, but no distinct difference in the soil carbon stock was observed between both treatments. The soil CO₂ concentration under no-tillage treatment was clearly much higher than that under tillage treatment, except at a depth of 2.5 cm. Tillage decreased soil dry bulk density and enhanced soil air-filled porosity. Soil gas diffusivity, which depends on air-filled porosity, was increased by tillage at a depth of 0–15 cm, which contributed to their lower soil CO₂ concentration. CO₂ flux through the soil profile, calculated from the CO₂ concentration, decreased with depth. Compared to the deep soils, the surface soil (0–5 cm) showed greater variation in CO₂ flux. The CO₂ production at depths of 0–10 cm accounted for 62.2 and 51.7 % of the whole CO₂ production of the 0–30-cm soil profile for no-tillage and tillage treatments. CO₂ production was higher for soil under no tillage at depths of 0–10 cm, but contrary results were observed for soil at depths of 10–30 cm.     

Pattern of methane emission and water productivity under different methods of rice crop establishment

Methane (CH₄) emission and water productivity were estimated in an experiment conducted during wet (rainy) season of 2010 at the research farm of Indian Agricultural Research Institute, New Delhi, India. Treatments comprising three methods of crop establishment viz., conventional transplanting (CT), system of rice intensification (SRI) and double transplanting (DT) were laid out in randomized block design with four replications. Scented rice (Oryza sativa L) variety ‘Pusa Basmati 1401’ was transplanted in puddle field. In CT and SRI 21 and 12-day-old seedlings, respectively, were transplanted while in DT overall 45-day-old seedlings were transplanted. In CT and DT flooded conditions while in SRI saturated conditions were maintained. Results indicated that among the methods of crop establishment, CT had maximum cumulative CH₄ emission (32.33 kg ha^?1) followed by DT (29.30 kg ha^?1) and SRI (19.93 kg ha^?1). Temporal CH₄ flux fluctuated between 79.7 and 482.0 mg m^?2 day^?1 under CT; 46.0 and 315.0 mg m^?2 day^?1 in SRI and 86.7 and 467.3 mg m^?2 day^?1 in DT. Considerable temporal variations in the individual CH₄ fluxes were observed. Flux of CH₄ was generally higher in early stage of crop and peaked about 21 days after transplanting coinciding with tillering stage of crop. CH₄ flux declined gradually from 75 days after transplanting and stabilized at the harvest stage of rice in all the three methods of transplanting. Global warming potential was highest in CT (807.4 kg CO₂ ha^?1) and lowest in SRI (498.25 kg CO₂ ha^?1). However, a reverse trend was observed with carbon efficiency ratio. The water savings to the extent of six irrigations was recorded in SRI over CT. A saving of 27.4 % irrigation water and 18.5 % total water was recorded in SRI over CT while the corresponding values of DT over CT were 14.5 and 9.8 %. Water productivity of SRI (3.56 kg/ha mm) was significantly higher as compared to DT (2.87 kg/ha mm) and CT (2.61 kg/ha mm).     

Rice husk biochar application to paddy soil and its effects on soil physical properties,plant growth,and methane emission

The objective of this study was to investigate the effects of the application of rice husk biochar on selected soil physical properties, rice growth, including root extension, and methane (CH₄) emissions from paddy field soil. Three replication experiments were conducted using outdoor pot experiments utilizing commercial rice husk biochar mixed with paddy soil at a rate of 0 (control), 2, and 4 % (weight biochar/weight soil) in which the rice was cultivated for 100 days under a continuously flooded condition. The physical properties of soils were analyzed before and after the growing periods. Some parameters of rice growth and CH₄ emissions of paddy soils were monitored weekly during the experiment. Root extension was also analyzed after harvesting. The experiments showed that the application of rice husk biochar improved the physical properties of paddy soils. It led to a decrease in bulk density and an increase in saturated hydraulic conductivity, including the total pore volume as well as the available soil water content. The shoot height of rice plants was significantly higher in soil amended with 4 % biochar than that in the control soil. However, other plant growth parameters and root extension were only slightly affected by the application. It was also found that amending soil with biochar led to a reduction of the total CH₄ emissions by 45.2 and 54.9 % for an application rate of 2 and 4 %, respectively, compared with the control. Our results showed that the higher the application rate, the stronger the effect of biochar was observed. More research is still necessary for a better understanding of the underlying mechanisms.     

Effect of organic,inorganic and slow-release urea fertilisers on CH<Subscript>4</Subscript> and N<Subscript>2</Subscript>O emissions from rice paddy fields

Vietnam is one of the world’s top two rice exporting countries. However, rice cultivation is the primary source of agriculture’s greenhouse gas (GHG) emissions in Vietnam. In particular, strategies are required to reduce GHG emissions associated with the application of organic and inorganic fertilisers. The objective of this study was to assess the effects of various combinations of biochar (BIOC), compost (COMP) and slow-release urea (SRU) on methane (CH₄) and nitrous oxide (N₂O) emissions. In total, 1170 gas samples were collected from closed gas chambers in rice paddies at Thinh Long commune and Rang Dong farm in northern Vietnam between June and October 2014. The gas samples were analysed for CH₄-C and N₂O-N fluxes using gas chromatography. The application of BIOC alone resulted in the lowest CH₄ emissions (4.8–59 mg C m^?2 h^?1) and lowest N₂O emissions (0.15–0.26 µg N m^?2 h^?1). The combined application of nitrogen–phosphorus–potassium (NPK) + COMP emitted the highest CH₄ (14–72 mg C m^?2 h^?1), while ½NPK + BIOC emitted the highest N₂O (1.03 µg N m^?2 h^?1 in the TL commune), but it was the second lowest (0.495 µg N m^?2 h^?1) in the RD farm. Green urea and orange urea reduced N₂O emissions significantly (p < 0.05) compared to white urea, but no significant differences were observed with respect to CH₄ emissions. SRU fertilisers and BIOC alone measured the lowest greenhouse gas intensity, i.e. <2.5 and 3 kg CO₂ eq. kg^?1 rice grain, respectively. Based on these results, application of fertilisers in the form of BIOC and/or orange or green urea could be a viable option to reduce both CH₄ and N₂O emissions from rice paddy soils.     

The effect of floating vegetation on CH<Subscript>4</Subscript> and N<Subscript>2</Subscript>O emissions from subtropical paddy fields in China

Duckweed (Lemna minor), a floating macrophyte belonging to the Lemnaceae family, is commonly found in subtropical paddy fields. This plant rapidly takes up nutrients from water and forms dense floating mats over the water surface that may impact the biogeochemical processes and greenhouse gas production in paddy fields. In this study, we measured CH₄ and N₂O emissions from duckweed and non-duckweed plots in a subtropical paddy field in China during the period of rice growth using static chamber and gas chromatography methods. Our results showed that CH₄ emission rate ranged from 0.19 to 26.50 mg m^?2 h^?1 in the duckweed plots, and from 1.02 to 28.02 mg m^?2 h^?1 in the non-duckweed plots. The CH₄ emission peak occurred about 1 week earlier in the duckweed plots compared to the non-duckweed counterparts. The mean CH₄ emission rate in the duckweed plots (9.28 mg m^?2 h^?1) was significantly lower than that in non-duckweed plots (11.66 mg m^?2 h^?1) (p < 0.05), which might be attributed to the higher water and soil Eh in the former. N₂O emission rates varied between ?50.11 and 201.82 µg m^?2 h^?1, and between ?28.93 and 54.42 µg m^?2 h^?1 in the duckweed and non-duckweed plots, respectively. The average N₂O emission rate was significantly higher in the duckweed plots than in the non-duckweed plots (40.29 vs. 11.93 µg m^?2 h^?1) (p < 0.05). Our results suggest that the presence of duckweed will reduce CH₄ emission, but increase N₂O flux simultaneously. Taking into account the combined global warming potentials of CH₄ and N₂O, we found that growing duckweed could reduce the overall greenhouse effect of subtropical paddy fields by about 17 %.     

Effect of biochar on CH4 and N2O emission from soils vegetated with paddy

Biochar is believed to have positive impact on soil properties and plant yield. Due to the presence of C, it can also enhance CH₄ emission in paddy soils. On the other hand, ammonium sulphate can decrease CH₄ emission due to negative impacts on methanogenesis. Keeping these points in view, a pot experiment was conducted to determine the effect of biochar along with ammonium sulphate on CH₄ and N₂O emission from paddy soil. Analysis revealed that biochar treated soils released more CH₄ compared to untreated. Ammonium sulphate treated soil emitted the highest N₂O whereas biochar addition decreased its emission significantly. Further, total emission was found to be higher for CH₄ (16.9–34.7 g/m²) in comparison to N₂O (?0.05 to 0.02 g/m²) for all treatments. Biochar application has positive impact on plant variables such as panicle number and weight of panicles. This study suggests that biochar application significantly decrease N₂O emission and increase CH₄ emission possibly due to affecting the availability of organic C in the soil to microbial activity for methanogenesis. Another possibility for enhancing CH₄ emission by following biochar could be attributed to the increase in plant biomass.     

Response of the leaf photosynthetic rate to available nitrogen in erect panicle-type rice (Oryza sativa L.) cultivar,Shennong265

Increasing the yield of rice per unit area is important because of the demand from the growing human population in Asia. A group of varieties called erect panicle-type rice (EP) achieves very high yields under conditions of high nitrogen availability. Little is known, however, regarding the leaf photosynthetic capacity of EP, which may be one of the physiological causes of high yield. We analyzed the factors contributing to leaf photosynthetic rate (P_n) and leaf mesophyll anatomy of Nipponbare, Takanari, and Shennong265 (a EP type rice cultivar) varieties subjected to different nitrogen treatments. In the field experiment, P_n of Shennong265 was 33.8 μmol m^?2 s^?1 in the high-N treatment, and was higher than that of the other two cultivars because of its high leaf nitrogen content (LNC) and a large number of mesophyll cells between the small vascular bundles per unit length. In Takanari, the relatively high value of P_n (31.5 μmol m^?2 s^?1) was caused by the high stomatal conductance (g_s; .72 mol m^?2 s^?1) in the high-N treatment. In the pot experiment, the ratio of P_n/C_i to LNC, which may reflect mesophyll conductance (g_m), was 20–30% higher in Nipponbare than in Takanari or Shennong265 in the high N availability treatment. The photosynthetic performance of Shennong265 might be improved by introducing the greater ratio of P_n/C_i to LNC found in Nipponbare and greater stomatal conductance found in Takanari.     

Rice yield and soil carbon dynamics over three years of applying rice husk charcoal to an Andosol paddy field

Rice husk charcoal (RC) produced from the pyrolysis of rice husk (RH) can be one of the cost-effective biochars for use in rice-based farming systems. This study investigated changes in rice yield and soil carbon sequestration over three years of RC application to an Andosol paddy field. The treatments were RC application at 0.02, 0.2, and 2 kg m^?2 (RC0.02, RC0.2, and RC2, respectively), RH application at 0.2 kg m^?2 (RH0.2), and a control with no RC or RH application (CONT). The results showed that RC2 increased culm length by 4% and straw weight by 14% on average over the three years. These increases in plant growth coincided with a higher level of silicon uptake by the rice plants, although they did not significantly affect grain yield. The soil carbon content was progressively increased by RC2 over the three years, whereas it was not significantly affected by RC0.02 or RC0.2. A considerable amount (>72%) of the applied carbon with RC2 remained in the soil by taking account of its downward movement below the 10 cm layer of the paddy field after three consecutive years of RC application. We conclude that rice husk charcoal application to Andosol paddy fields is an effective option for increasing carbon sequestration. Furthermore, the increase in silicon uptake by rice plants suggests that rice husk charcoal can also be functioning as a silicon fertilizer.     

Elevated carbon dioxide and temperature effects on rice yield,leaf greenness,and phenological stages duration

The present field experiment was conducted during two consecutive cropping seasons in central Portugal to study the effects of simultaneous elevation of carbon dioxide concentration ([CO₂]) (550 μmol mol^?1) and air temperature (+2–3 °C) on japonica rice (Oryza sativa L. “Ariete”) yield, crop duration, and SPAD-values across the seasons compared with the open-field condition. Open-top chambers were used in the field to assess the effect of elevated air temperature alone or the combined effect of elevated air temperature and atmospheric [CO₂]. Open-field condition was assessed with randomized plots under ambient air temperature and actual atmospheric [CO₂] (average 382 μmol mol^?1). Results obtained showed that the rice “Ariete” had a moderate high yielding under open-field condition, but was susceptible to air temperature rise of +2–3 °C under controlled conditions resulting in reduction of grain yield. The combined increase of atmospheric [CO₂] with elevated air temperature compensated for the negative effect of temperature rise alone and crop yield was higher than in the open-field. SPAD-readings at reproductive stage explained by more than 60 % variation the straw dry matter, but this finding requires further studies for consolidation. It can be concluded that potential increase in air temperature may limit rice yield in the near future under Mediterranean areas where climate change scenario poses a serious threat, but long term field experiments are required.     

Evaluation of technologies for sequestering carbon in agricultural land by using organic filter underdrains

Technology for storing carbon in the subsoil of agricultural land using organic filter materials in underdrains was evaluated as an activity for sequestering CO₂ to mitigate global warming. First, the quantity of carbon remaining in wood chips and rice husks in underdrains was determined over 11 years after construction. Moreover, the quantity of CO₂ emissions from the construction of two underdrain types was calculated. Then, a survey was conducted to investigate the effect on greenhouse gas emissions of the filter material. The results indicate that greenhouse gas emissions would not increase. The quantity of storage carbon in the wood chip filter material of underdrains during their 15-year service life was estimated to be 6.76 tCO₂eq ha⁻¹. Rice husks, in contrast, were found ineffective in storing carbon. Therefore, the selection of the filter material for underdrains is important. The value of storage carbon in the wood chip filter material is similar to the amount of carbon sequestered by no-till farming, which is now being implemented as a global warming countermeasure based on soil management in agricultural land.

     

Evaluation of technologies for sequestering carbon in agricultural land by using organic filter underdrains

Technology for storing carbon in the subsoil of agricultural land using organic filter materials in underdrains was evaluated as an activity for sequestering CO₂ to mitigate global warming. First, the quantity of carbon remaining in wood chips and rice husks in underdrains was determined over 11 years after construction. Moreover, the quantity of CO₂ emissions from the construction of two underdrain types was calculated. Then, a survey was conducted to investigate the effect on greenhouse gas emissions of the filter material. The results indicate that greenhouse gas emissions would not increase. The quantity of storage carbon in the wood chip filter material of underdrains during their 15-year service life was estimated to be 6.76 tCO₂eq ha^?1. Rice husks, in contrast, were found ineffective in storing carbon. Therefore, the selection of the filter material for underdrains is important. The value of storage carbon in the wood chip filter material is similar to the amount of carbon sequestered by no-till farming, which is now being implemented as a global warming countermeasure based on soil management in agricultural land.     

Effects of rice straw,biochar and mineral fertiliser on methane (CH<Subscript>4</Subscript>) and nitrous oxide (N<Subscript>2</Subscript>O) emissions from rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) grown in a rain-fed lowland rice soil of Cambodia: a pot experiment

We studied the effects of water regimes and nutrient amendments on CH₄ and N₂O emissions in a 2 × 3 factorial, completely randomised growth chamber experiment. Treatments included continuously flooded (CF) and alternate wetting and drying (AWD), and three organic amendments: no amendment-control, rice straw (RS) and biochar (BC). Compound fertiliser was applied to all treatments. Rice was grown in columns packed with a paddy soil from Cambodia. Results revealed faster mineralisation of organic carbon (RS and BC) when applied in water-saturated conditions lasting for 2 weeks instead of flooding. This resulted in lower total CH₄ emissions in treatments under AWD than those under the CF water regime, namely 44 % in RS treatments and 29 % in BC treatments. Nitrous oxide fluxes were generally non-detectable during the experimental period except after fertilisation events, and the total N₂O–N emissions accounted for on average 1.7 % of the total applied mineral fertiliser N. Overall, the global warming potentials (GWPs) were lower in treatments under AWD than those under the CF water regime except for the control treatment with only mineral fertiliser application. Grain yields were slightly higher in treatments under AWD than the CF water regime. Hence, the yield-scaled GWP was also lower in the treatments under the AWD water regime, namely 51 % in RS, 59 % in BC and 17 % in control treatments. Control treatments had the lowest GWP, but provided the highest yield. The yield-scaled GWP under these treatments was therefore lower than under the other treatments.     

Bed planted rice–wheat rotation at differential soil moisture regimes on soil hydro-physical properties, root growth, nitrogen uptake, and system productivity

Along with most widely practiced resources conserving technology zero-tillage wheat after rice, adoption of permanent beds for rice–wheat rotation is also gaining popularity. Since relatively a new approach particularly for dry-seeded rice and permanent beds for wheat, very little information is known about permanent beds on soil properties, nutrient (N) use efficiency, and system productivity. A field experiment was carried out in a Typic Haplustept soil of New Delhi, India to study the effect of permanent beds on soil hydro-physical properties, root growth, nitrogen uptake, and system productivity of irrigated rice–wheat rotation. Results revealed that direct-seeded rice followed by wheat on permanent beds irrigated at different soil water tensions (field capacity, 20 and 40 kPa) reflected a significant variation in soil hydro-physical properties, reduced total nitrogen uptake, contribution by different plant parts, and N use efficiency compared to wheat after flooded transplanted rice system. Rice root weight density at flowering was also significantly low at 0–15 cm depth but higher at 15–60 cm depth in dry-seeded rice on beds. System productivity of rice–wheat rotation was 25–33% lower in permanent beds compared to flooded transplanted system. For wider acceptability of permanent beds as a promising resource conserving technology, system productivity needs to be improved.     

Arsenic in irrigated water,soil, and rice: perspective of the cropping seasons

Arsenic contamination of shallow groundwater and related health problems are threats for the millions in endemic regions of West Bengal. Contamination of rice grain creates the food chain pathway of mineral arsenic besides drinking water contamination. Present study concentrated on association of arsenic concentration in irrigated water, paddy field soil and rice with the cropping seasons. Irrigated ground water arsenic concentration decreased significantly (p = 0.007) from summer (median 0.42 mg l^?1) to winter (median 0.35 mg l^?1). Carried over effect created significant decrease (p = 0.03) of paddy field soil arsenic concentration from summer (median 8.35 mg kg^?1) to winter (median 6.17 mg kg^?1). Seasonal variation was observed in rice straw (p = 0.03) but not in husk (p = 0.91). Arsenic concentration decreased significantly (p = 0.05) in the rice grains collected in winter season (median 0.23 mg kg^?1) than the samples collected in the summer season (median 0.30 mg kg^?1). In conclusion, seasonal effects need to be considered in case of human health risk assessment from arsenic consumption.     

Plastic film mulching reduces net greenhouse gas emissions in a rice–rapeseed rotation field

With the aim of assessing differentiation of greenhouse gas emissions as manipulated by plastic film mulching (PFM) from paddy field from a year-round perspective, we determined net ecosystem CO₂ exchange (NEE, CO₂ flux), CH₄ and N₂O fluxes from a rice–rapeseed rotation field. PFM and non-mulching (NM) treatments were set from 2014 to 2017 (May 2014 to April 2015, May 2015 to April 2016 and May 2016 to April 2017 were set as Annual 1, Annual 2 and Annual 3, respectively) in Southwest China. Compared with NM, CH₄ emissions were increased by 60.00% (P?<?0.05), 111.54% (P?<?0.05) and 62.07% (P?<?0.05) under PFM in Annual 1, 2 and 3, respectively. Additionally, PFM delayed the peaks of CH₄ fluxes by 5–10 days during rice season. However, PFM did not affect N₂O emissions on the annual basis. PFM reduced the net carbon loss from soil during rice season while had insignificant influence on soil carbon sequestration capacity during fallow and rapeseed seasons. Overall, the mean annual net ecosystem greenhouse gas exchange among three annuals was 32.11% lower under PFM than under NM. Moreover, PFM slightly increased crop yields of both rice and rapeseed. Accordingly, PFM recommended the suitable agricultural management in the rice–rapeseed rotation field for simultaneously alleviating global warming and maintaining crop yields.

     

Rice–wheat cropping system: tillage,mulch, and nitrogen effects on soil carbon sequestration and crop productivity

Maintenance of organic carbon in soil (SOC) is critically important for sustained agricultural productivity and environmental quality. This paper presents SOC resulting from differences in tillage types and demonstrates how mulch and nitrogen (N) application can mediate the tillage functions on SOC and crop productivities. The results are derived from a 4-year field-scale study carried out in a low-land under sub-tropical hot and humid environment of Nepal. It compared eight treatment combinations, viz., tillage (no-tillage and conventional tillage), mulch (no-mulch and 12 Mg ha^?1 year^?1 of mulch), and N application (recommended versus leaf color chart method) under rice–wheat cropping system. Seasonal grain and biomass yields of these crops were recorded and at the end of the 4-year study, quantified the organic carbon stock of soil; Within 15 cm of surface soil, SOC stock (Mg C ha^?1) was statistically (p < 0.05) higher on no-tillage plots (11.2–11.8) than on conventional tillage plots (9.2–10.5). The treatment effect was more pronounced on winter wheat productivity where conventional tillage combined with straw-mulch exceled the performance of no-tillage. Clearly, no-tillage had the environmental benefit, and conventional tillage had the crop productivity benefit.     

Influence of irrigation frequency on greenhouse gases emission from a paddy soil

Water management is known to be a key factor on methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) emissions from paddy soils. A field experiment was conducted to study the effect of continuous irrigation (CI) and intermittent irrigation (II) on these emissions. Methane, CO₂, and N₂O emissions from a paddy soil were sampled weekly using a semi-static closed chamber and quantified with the photoacoustic technique from May to November 2011 in Amposta (Ebro Delta, NE Spain). Intermittent irrigation of rice paddies significantly stimulated (N₂O + N₂)–N emission, whereas no substantial N₂O emission was observed when the soil was re-wetted after the dry phase. The cumulative emission of (N₂O + N₂)–N was significantly larger from the II plots (0.73 kg N₂O–N ha^?1 season^–1, P < 0.05) than from the CI plots (?1.40 kg N₂O–N ha^?1 season^?1). Draining prior to harvesting increased N₂O emissions. Draining and flooding cycles controlled CO₂ emission. The cumulative CO₂ emission from II was 8416.35 kg CO₂ ha^?1 season^?1, significantly larger than that from CI (6045.26 kg CO₂ ha^?1 season^?1, P < 0.05). Lower CH₄ emission due to water drainage increased CO₂ emissions. The soil acted as a sink of CH₄ for both types of irrigation. Neither N₂O–N nor CH₄ emissions were affected by soil temperature. Global warming potential was the highest in II (4738.39 kg CO₂ ha^?1) and the lowest in CI (3463.41 kg CO₂ ha^?1). These findings suggest that CI can significantly mitigate the integrative greenhouse effect caused by CH₄ and N₂O from paddy fields while ensuring the highest rice yield.