Nitric oxide flux from soil during the growing season of wheat by continuous measurements of the NO soil-atmosphere concentration gradient: A processstudy

The surface flux of nitric oxide from a wheat field was investigated from 2 3 March to 29 May 1997 in the Kerzersmoos, Switzerland. A plot fertilised w ith 19 kg N ha(-)1 in cattle slurry and 40 kg N ha(-1) in mineral NH4NO3 fe rtiliser and a plot receiving no nitrogen containing fertiliser were compar ed. The flux was calculated based on hourly measurements of the NO soil-atm osphere concentration gradient using the one-dimensional soil diffusion mod el of Galbally and Johansson (1989). The soil bulk diffusion coefficient wa s determined from measurements of the Rn-222 surface flux and the activity gradient between 10 cm depth and the surface. It ranged between 79% and 0.3 % of the NO diffusion coefficient in air and was parameterised by air fille d soil pore space. The indirectly determined NO flux agreed well with stand ard flux measurements using dynamic chambers. The largest NO emission was f ound following fertiliser application and irrigation. The emission occurred in pulses, which lasted for 4 days up to 3 weeks coinciding with elevated soil ammonium concentrations. Nitric oxide emission in 5 days following app lication of cattle slurry were 31 g NO-N ha(-)1 and 5 g NO-N ha(-1) from th e non-fertilised plot, respectively. Nitric oxide emission in 15 days follo wing application of NH4NO3 was 95 g NO-N ha(-1) and 10 g NO-N ha(-1) from t he non-fertilised plot, respectively. NO emission in 4 days following irrig ation on 21 April were 36 g N ha(-1) from the fertilised and 39 g N ha(-1) from the non-fertilised plot. The daily NO emission before and after fertil iser and irrigation pulses was between 0.3 and 0.7 g NO-N ha(-1) d(-1). NO production and NO uptake of the soil was measured regularly. No systematic influence of management or climate on NO uptake was found. NO production wa s strongly stimulated by fertiliser input and soil moisture content. The si mulation of NO production could be reproduced using a nitrification algorit hm (Riedo et al., 1998) driven by soil temperature, moisture and ammonium c oncentration. A NO production rate constant of 1.1.10(-3) h(-1) at 15 degre es C was derived from a linear regression between nitrification and NO prod uction. Introducing the parameterisation of NO production into the model of Galbally and Johansson (1989) the duration and the strength of the NO emis sion pulses could be reproduced and the total NO emission during the experi ment was approximated within a factor of two.