Determining the effects of tillage and nitrogen sources on soil N2O emission

Soil tillage and fertilization can affect the abundance of nitrifying and denitrifying microbial communities known to regulate N2O losses from agricultural soils. We assessed the effects of mineral and organic N sources on short-term N2O emissions from a Rhodic Nitisol under contrasting soil disturbance. The experiment followed a split-plot design with two soil tillage systems as main plots (tilled soil: TS, and no-till soil: NTS) and five fertilization treatments as subplots, where 140 kg N ha(-1) were applied either as urea (UR), raw swine slurry (RS), anaerobically digested swine slurry (ADS), or composted swine slurry (CS). A treatment without N fertilization was used as control (CTR). N2O emissions were determined by using static chambers and correlated with soil (0-0.1 m) temperature, water-filled pore space (WFPS), dissolved organic carbon (DOC), ammonium (NH4+-N), nitrate (NO3--N) and the abundance of specific nitrification and denitrification biomarker genes [ammonium monooxygenase (amoA), nitrate- (narG), nitrite- (nirS), nitric oxide- (qnorB) and nitrous oxide reductases (nosZ)]. Denitrification was the main source of N2O as assessed by increased narG/16S rDNA and narG/nosZ ratios regardless of soil tillage. N2O emissions were augmented in the NTS (30 to 200% higher than TS) where higher soil WFPS (0.6-0.7 cm(3) cm(-3)) favored incomplete denitrification. The application of ADS in the NTS decreased denitrification rates and cumulative N2O emission by 47% in comparison with RS (2.9 and 5.6 kg N2O-N ha(-1), respectively). Interestingly, N2O emission from the NTS receiving CS (4.7 kg N2O-N ha(-1)) was also promoted by the proliferation of heterotrophic nitrifying bacteria communities. Overal, swine slurry treatment can help mitigate N2O emission from agricultural soils, particularly in regions where this source of fertilizer is abundant and readily available.