Comparison of two monitoring methods for ammonia volatilization based on rice-wheat rotation system

Ammonia volatilization is an important pathway of active nitrogen (N) loss from farmlands. The accurate and effective measurement of farmland ammonia emissions is the basis for environmental assessments and policymaking. Soil ammonia volatilization is controlled by fertilizer application rates, fertilizer application methods, soil properties, and meteorological conditions, so there is a wide variability in the cumulative ammonia emissions and emission factors in different studies. There are also different methods for ammonia volatilization measurements, which further reduce the comparability of data across studies. The most widely used methods for soil ammonia volatilization measurements in China are the ventilated sponge absorption method and the intermittent airflow enclosure method. However, consistency between measurements is unclear because of substantial differences in the ventilation rates between methods, and only a few studies have compared the two methods. Based on a typical rice-wheat rotation system in the Lower Reaches of the Yangtze River, this study set up treatments with different N fertilizer application rates (N0: no N fertilizer; N1: 200 kg(N)·hm⁻² for rice and 180 kg(N)·hm⁻² for wheat; N2: 300 kg(N)·hm⁻² for rice and 270 kg(N)·hm⁻² for wheat; and N3: 400 kg(N)·hm⁻² for rice and 360 kg(N)·hm⁻² for wheat). Soil ammonia volatilization was measured continuously after fertilization during rice and wheat growth using the ventilated sponge absorption and intermittent airflow enclosure methods, and the ammonia emission factors in the study area were analyzed using literature analysis. The results showed that the cumulative ammonia emission measured by the ventilated sponge absorption method was 25%–35% lower than that measured by the intermittent airflow enclosure method under the three N application treatments (except for the N0 treatment), and the results were consistent for both rice and wheat seasons. Under fertilization treatments (N1, N2 and N3), the cumulative ammonia emissions measured by the ventilated sponge absorption method ranged from 17.36 to 43.90 kg·hm⁻² in the rice season and from 5.90 to 20.43 kg·hm⁻² in the wheat season, with emission factors ranging from 2.56% to 10.39%. The cumulative ammonia emissions measured by the intermittent airflow enclosure method ranged from 23.28 to 61.05 kg·hm⁻² in the rice season and from 14.63 to 27.73 kg·hm⁻² in the wheat season, with emission factors ranging from 7.09% to 15.01%. The cumulative ammonia emissions under the N0 treatment were higher for the ventilated sponge absorption method than for the intermittent airflow enclosure method, and the measurements of the two methods were significantly different in the rice season but not in the wheat season. The results of the literatures analysis in the study area were consistent with the monitoring results. The cumulative ammonia emissions measured by the ventilated sponge absorption method were higher than those measured by the intermittent airflow enclosure method when the N application rate was between 0 and 100 kg·hm⁻², with average emission factors of 6.18% and 12.31%, respectively. When the N application rate was between 101 and 200 kg·hm⁻², the ventilation sponge absorption method led to 25% lower emissions than the intermittent airflow enclosure method, with average emission factors of 9.46% and 12.61%, respectively. When the N application rate was between 201 and 300 kg·hm⁻², the ventilation sponge absorption method led to 5% lower emissions than the intermittent airflow enclosure method, and the average emission factors were 12.71% and 13.43%, respectively. In general, the cumulative ammonia emissions measured by the two methods were consistent; the ventilated sponge absorption method led to higher measured values than the intermittent airflow enclosure method in fields without N application or with low ammonia volatilization rates, and the ventilated sponge absorption method led to lower measurements than the intermittent airflow enclosure method in fields with N application. The measurements of the two methods can be converted using a ratio. The results of this study provide support for the estimation of regional ammonia emissions using the ventilated sponge absorption and intermittent airflow enclosure methods.