Abstract
Most studies on soil N2O emissions have focused either on the quantifying of agricultural N2O fluxes or on the effect of environmental factors on N2O emissions. However, very limited information is available on how land-use will affect N2O production, and nitrifiers involved in N2O emissions in agricultural soil ecosystems. Therefore, this study aimed at evaluating the relative importance of nitrification and denitrification to N2O emissions from different land-use soils and identifying the potential underlying microbial mechanisms. A 15N-tracing experiment was conducted under controlled laboratory conditions on four agricultural soils collected from different land-use. We measured N2O fluxes, nitrate (), and ammonium () concentration and 15N2O, 15, and 15 enrichment during the incubation. Quantitative PCR was used to quantify ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). Our results showed that nitrification was the main contributor to N2O production in soils from sugarcane, dairy pasture and cereal cropping systems, while denitrification played a major role in N2O production in the vegetable soil under the experimental conditions. Nitrification contributed to 96.7% of the N2O emissions in sugarcane soil followed by 71.3% in the cereal cropping soil and 70.9% in the dairy pasture soil, while only around 20.0% of N2O was produced from nitrification in vegetable soil. The proportion of nitrified nitrogen as N2O (PN2O-value) varied across different soils, with the highest PN2O-value (0.26‰) found in the cereal cropping soil, which was around 10 times higher than that in other three systems. AOA were the abundant ammonia oxidizers, and were significantly correlated to N2O emitted from nitrification in the sugarcane soil, while AOB were significantly correlated with N2O emitted from nitrification in the cereal cropping soil. Our findings suggested that soil type and land-use might have strongly affected the relative contribution of nitrification and denitrification to N2O production from agricultural soils.
Highlights
Ammonium-based fertilizers are extensively used in agricultural practices to meet the food demand for the increasing human population, which has resulted in an increase in atmosphericN2O concentrations (Galloway et al, 2008; Davidson, 2009)
In the vegetable soil, the 15N enrichment of the N2O pool (Figure 2D) was close to the 15N abundance of the 15NO−3 at day 7, indicating that denitrification was the predominant pathway of N2O emission and was determined to be responsible for 76.3% of N2O production (Table 2)
Under the experimental aerobic microcosm conditions, nitrification was the main contributor of N2O emissions in acidic sugarcane, dairy pasture and cereal cropping soils
Summary
Ammonium-based fertilizers are extensively used in agricultural practices to meet the food demand for the increasing human population, which has resulted in an increase in atmosphericN2O concentrations (Galloway et al, 2008; Davidson, 2009). Natural and anthropogenic N2O sources are primarily dominated by emissions from soil ecosystems, comprising approximately 65% of the total N2O emissions (IPCC, 2007). In Australia, agriculture is the second largest greenhouse gas (GHG) source, accounting for 16% of total GHG emissions, 19%. The emission of N2O is the result of multiple biological pathways, such as nitrification (autotrophic and heterotrophic), denitrification, dissimilatory nitrate reduction to ammonium (DNRA), nitrifer denitrification, and non-biological chemodenitrification (Wrage et al, 2001; Butterbach-Bahl et al., 2013; Hu et al, 2015a; Zhang et al, 2015), but is dominated by nitrification and denitrification (Davidson et al., 1986; Stevens et al, 1997; Hu et al, 2015a). Stable isotope enrichment approaches have been developed to identify N2O sources following the application of 15N-labeled fertilizers in short-term experiments, through the measurement of 15N enrichment in
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