R.J. Stevens

Evidence of carbon stimulated N transformations in grassland soil after slurry application

Abstract High nitrification rates which convert ammonium (NH4+) to the mobile ions NO2− and NO3− are of high ecological significance because they increase the potential for N losses via leaching and denitrification. Nitrification can be performed by chemoautotrophic or heterotrophic organisms and heterotrophic nitrifiers can oxidise either mineral (NH4+) or organic N. Selective nitrification inhibitors and 15N tracer studies have been used in an attempt to separate heterotrophic and autotrophic nitrification. In a laboratory study we determined the effect of cattle slurry on the oxidation of mineral NH4+-N and organic-N by labelling the NH4+ or NO3− pools separately or both together with 15N. The size and enrichment of the mineral N pools were determined at intervals. To calculate gross N transformation rates a 15N tracing model was developed. This model consists of the three N-pools NH4+, NO3− and organic N. Sub-models for decomposition of degradable carbon in the soil and the slurry were added to the model and linked to the N transformation rates. The model was set up in the software ModelMaker which contains non-linear optimization routines to determine model parameters. The application of cattle slurry increased the rate of nitrifcation by a factor of 20 compared with the control. The size and enrichment of the mineral N pools provided evidence that nitrification was due to the conversion of NH4+ to NO3− and not the conversion of organic N to NO3−. There was evidence that slurry-enhanced oxidation of NH4+ to NO3− was due to a combination of autotrophic and heterotrophic transformations. Slurry application increased the mineralisation rate by approximately a factor of two compared with the control and the rate of immobilisation of NH4+ by approximately a factor of three.

The nitrification inhibitor DMPP had no effect on denitrifying enzyme activity

Abstract Denitrifying enzyme activity (DEA) and flux rates of nitrous oxide (N2O) and dinitrogen (N2) were studied in DEA assays on soils treated with 3,4-dimethylpyrazole phosphate (DMPP). Nitrous oxide and N2 fluxes were quantified by 15N gas-flux method with no additional enzymatic inhibitors, thus, overcoming problems associated with the use of chloramphenicol and acetylene. The nitrification inhibitor DMPP did not affect DEA or the measured gas emissions even when applied in concentrations 14 times higher than the recommended concentration.

Resolution of the 15N balance enigma

The enigma of soil nitrogen balance sheets has been discussed for over 40 years. Many reasons have been considered for the incomplete recovery of 15N applied to soils, including sampling uncertainty, gaseous N losses from plants, and entrapment of soil gases. The entrapment of soil gases has been well documented for rice paddy and marshy soils but little or no work appears to have been done to determine entrapment in drained pasture soils. In this study 15N-labelled nitrate was applied to a soil core in a gas-tight glovebox. Water was applied, inducing drainage, which was immediately collected. Dinitrogen and N2O were determined in the flux through the soil surface, and in the gases released into the glovebox as a result of irrigation or physical destruction of the core. Other components of the N balance were also measured, including soil inorganic-N and organic-N. Quantitative recovery of the applied 15N was achieved when the experiment was terminated 484 h after the 15N-labelled material was applied. Nearly 23% of the 15N was recovered in the glovebox atmosphere as N2 and N2O due to diffusion from the base of the soil core, convective flow after irrigation, and destructive soil sampling. This 15N would normally be unaccounted for using the sampling methodology typically employed in 15N recovery experiments.

N2O and N2 gas fluxes, soil gas pressures, and ebullition events following irrigation of 15NO3–-labelled subsoils

We examined the fate of N2O following the addition of labelled nitrate and subsequent irrigation. Repacked silt loam soil columns, 1 m deep, were wetted up and instrumented with pressure transducers, soil profile gas samplers, and time domain reflectometry rods. Combined substrates (glucose- and 15N-enriched nitrate) were injected at 0.45 m depth. N2O, N2, and NO were monitored in the soil profile and headspaces. When soil profile N2O gas concentrations became elevated, an irrigation event was applied. Immediately prior to the irrigation event, confined drainage (no drainage outlet) and unconfined drainage (lateral drainage outlet at 0.9 m depth) treatments were implemented. Soil profile gas pressures increased following irrigation with pressure changes at 0.375 m chronologically linked to increased pressure pulses in the headspace. Irrigation contributed to decreases in N2O gas concentrations in the soil profile. N2O and N2 displaced in drainage from the unconfined treatment represented 0.01 and 2.3% of the gas in the soil profile immediately prior to irrigation, respectively. Following irrigation, soil gas pressures increased to a maximum of 11.8 kPa at 0.825 m soil depth in the confined drainage treatment but only reached 4.3 kPa at the same depth in the unconfined drainage treatment. It is suggested that ebullition events could possibly contribute to the increased and variable fluxes of N2O, commonly observed, immediately following rainfall or irrigation.