Nitrogen turnover and N2O production in incubated soils after receiving field applications of liquid manure and nitrification inhibitors

Applying abundant manure to soils can accelerate nitrogen (N) transformations and nitrous oxide (N2O) emissions. We conducted a laboratory incubation to examine the turnover of labile N in manured soils. Soils were collected from agricultural fields that had recently received spring-injected liquid dairy manure with or without admixing nitrification inhibitors. Bands and interbands of the manure plots were incubated separately. Time courses of ammonium (NH4+) and nitrate (NO3-) were used to derive and contrast zero-, first-, and second-order kinetics models. We found that nitrification rates were consistently better represented by first- order kinetics (k(1)). Furthermore, across all evaluated soils, the dependency of nitrification rate (k(1) of NH4+) on initial NH4+ concentration was modelled by Michaelis-Menten kinetics reasonably well, with an affinity (Km) of 63 mg N.kg(-1) soil (R-2 = 0.82). Compared with the manure interbands, the initially NH4-enriched bands had a much faster nitrification rate, with half-life for NH4+ of only 4 d and rapid k(1) (0.186 versus 0.011 d(-1)). Soil N substrate and k(1) exerted control on N2O production. Nitrous oxide production increased linearly with both measured NH4+ intensity (R-2 = 0.47) and modelled k(1) - NH4+ (R-2 = 0.48). Conversely, N2O production increased non-linearly with NO3- intensity (R-2 = 0.68), where high NO3 - caused a saturation plateau with a threshold of 96 mg N.kg(-1).d(-1) - beyond which no additional N2O was produced. During peak N transformations, measured N2O-N flux was 1.4% +/- 0.3% of the inorganic N undergoing nitrification. Heavily manured soils exhibited augmented N turnover that increased N2O fluxes, but inhibitors reduced these emissions by half.

Automated summary

The study shows that applying abundant manure to soils can accelerate nitrogen transformations and nitrous oxide emissions, with nitrification rates being better represented by first-order kinetics. Additionally, the dependency of nitrification rate on initial NH4+ concentration can be well modeled by Michaelis-Menten kinetics. Furthermore, soil N substrate and k(1) exert control on N2O production, with nitrous oxide production increasing linearly with NH4+ intensity and non-linearly with NO3- intensity.