Azolla cover significantly decreased CH₄but not N₂O emissions from flooding rice paddy to atmosphere

Abstract

Azolla (Azolla filiculoides) is a common aquatic fern that has been used successfully as a dual crop with lowland rice. It grows rapidly and has the ability to fix N 2 for rice paddy. However, its ecological significance especially on greenhouse gases emissions remains unclear. To investigate the effect of azolla cover on methane (CH 4 ) and nitrous oxide (N 2 O) emissions from rice paddy, a pot experiment with two treatments, control (rice plant only) and azolla cover (rice plus azolla covering on the flooding water), was carried out in Tsuruoka, Yamagata, Japan, in 2016. The results showed that the rice growth parameters, like shoot height, maximum and productive tiller numbers, and plant biomass were not significantly different between the two treatments. Dual cropping of azolla with rice significantly suppressed CH 4 emissions, likely due to an increase in dissolved oxygen concentration and redox potential at the soil-water interface between flooding water and soil surface. There were significant (P < 0.05) positive correlations between CH 4 flux and night respiration (CO 2 emissions) between the two treatments. The cumulated CH 4 emissions during the growth period until 106 days after transplanting (DAT) was significantly lower at 36.2 g C m −2 from azolla cover treatment than that from control treatment pot at 55.4 g C m −2 . A prolonged nonsignificant N 2 O emission under the azolla cover treatment after the initial highest peak at 15 DAT was recorded due to denitrification of the nitrate in initial soil. No further N 2 O emissions were recorded thereafter from both treatments. Azolla cover did not affect N 2 O emissions from both treatments. ARTICLE HISTORY

Co-application of poultry-litter biochar with Azolla has synergistic effects on CH4 and N2O emissions from rice paddy soils

“…The Azolla ( A. filiculoides Lam.) species IRRI code FI 1001 ( Cheng et al., 2015 ; Kimani et al., 2018 , 2020 ) was used in this study. The primary properties of biochar and Azolla, determined as describe above, are as shown in Table 2 .…”

“…After closure, a small fan was used to mix the gas in the chamber, and a 30-mL gas sample from the chamber headspace of each experimental pot was collected at 0, 15, and 30 min with a syringe and transferred into a 19-mL pre-evacuated vial. As detailed previously ( Kimani et al., 2018 ), gas sampling was conducted between 20:00–23:00 once a week in the first 84 DAT. After this date, sampling was done every two weeks until 122 DAT, a day before rice harvesting (113 DAT).…”

“…After this date, sampling was done every two weeks until 122 DAT, a day before rice harvesting (113 DAT). All gas samples were analyzed at the Institute for Agro-Environmental Sciences, NARO using an automated analysis system for three gases of CO 2 , CH 4 , and N 2 O ( Kimani et al., 2018 ; Sudo, 2006 ). The GHG fluxes were calculated from the linear increase in gas concentrations inside the chamber per square metre per hour along with atmospheric pressure and temperature ( Cheng et al., 2008 ; Kimani et al., 2018 , 2020 ; Sudo, 2006 ).…”

“…For understanding the CH 4 and N 2 O emissions with the C and N dynamics in the soils, the concentrations of dissolved CO 2 and CH 4 and nitrate (NO 3 - -N) in soil solutions were sampled using a 10 cm long microporous polymer tube (outside diameter, 2.5 mm; inside diameter, 1.5 mm) fitted to a PVC tube (length, 50 cm; outside diameter, 2.7 mm; inside diameter, 1.0 mm) and inserted vertically into the soil between the rice plant and pot edge at a depth of 10–15 cm one day after rice transplanting, as previously described ( Kimani et al., 2018 ). The 9.5-mL soil solution sample was aspirated into a 19-mL semi-vacuum bottle fitted with a rubber stopper and a screw cap and filled with pure N 2 gas at 0.5 atm ( Kimani et al., 2018 ). The concentrations of CO 2 and CH 4 in the headspace volume were measured in the laboratory using a gas chromatograph (GC-7A, Shimadzu, Kyoto, Japan), fitted with a thermal conductivity detector (TCD), and a flame ionization detector (FID), respectively.…”

“…(1997) reported substantial CH 4 and N 2 O emissions from a rice field grown with Azolla as a cover, likely due to the exudation of Azolla root and decomposition of dead Azolla. Conversely, Kimani et al. (2018) reported that Azolla as a cover significantly decreased CH 4 emission by 34%, likely due to increased levels of DO concentrations and redox potential ( Xu et al., 2017 ), and no significant influences on N 2 O from a paddy soil planted with rice, attributed to no interferences by the Azolla cover ( Cheng et al., 2006 ).…”