Methane (CH4) and nitrous oxide (N2O) are both considered greenhouse gases which can be lost from agricultural fields. In general, flooded agricultural fields display a higher methane loss while upland fields display a higher nitrous oxide loss.
New rice cultural management practices such as furrow-irrigated rice (row rice) and alternate wetting and drying (AWD) may result in less methane with more nitrous oxide emissions, since they are not flooded as long as traditional rice production.
However, more research is needed to evaluate the overall global warming potential from these systems compared to traditionally produced rice.
A three-year research study funded by the LouisianaRice Research Board was conducted from 2018-2020 to evaluate methane and nitrous oxide emissions from different cultural management practices. The three cultural practices evaluated were conventional delayed flood, AWD and row rice.
Phosphorus and potassium fertilizers were applied at planting at the rate of 67 kg P2O5 and K2O /ha. Nitrogen (N) was applied pre-flood at a rate of 168 kg N/ha one day before permanent flood establishment or the initial irrigation event in the case of row rice.
Greenhouse gas samples were collected twice a week when the field was flooded until rice was harvested from static chambers installed in each cultural practice.
Gas samples were analyzed for methane and nitrous oxide using gas chromatography. Methane emissions from the three cultural management practices are shown in Figure 1.
The highest methane emissions were detected in delayed-flood rice and ranged from 90 to 127 kilograms of methane per hectare per season (kg CH4/ha/season)*.
The lowest methane emissions were from the row rice system which ranged from 20 to 22 kg CH4 /ha/season. The methane emissions in AWD ranged from 21-36 kg CH4/ha/season.
The differences in methane emissions were highly related to flooding duration.
Nitrous oxide emissions were highest in row rice system (Figure 2) which ranged from 1,989 to 2,826 grams of nitrous oxide per hectare per season (g N2O/ha/season). The lowest nitrous oxide emissions were observed in the delayed flood system which ranged from 111 to 297 g N2O /ha/season. The nitrous oxide emissions from AWD ranged from 206 to 1,439 g N2O /ha/season.
Even though higher emission rates of nitrous oxide were observed in row rice, the total global warming potential from this practice was lower than the delayed flood system (Figure 3).
The global warming potential was calculated in mass of CO2 equivalent over a 100-year time horizon. The radioactive potentials relative to CO2 of 28 and 265 were used for CH4 and N2O, respectively (IPCC, 2013).
Results from this research suggests that the reduced flood time in both row rice and AWD reduces the global warming potential impact compared to traditional flooded rice production.
* Note: 1 kilogram is 2.2 pounds, 454 grams per pound and a hectare is approximately 2.5 acres.
Dr. Manoch Kongchum is an assistant professor, research, at the LSU AgCenter H. Rouse Caffey Rice Research and Extension Center. He may be reached at email@example.com.