The addition of N fertilizer in agricultural soils increases both the amount of mineral N present and the potential for nitrous oxide (N₂O) emission to the atmosphere (Granli and Bokman, 1994). The N₂O emissions from soils are caused principally by processes of microbial nitrification and denitrification. The key factors affecting N₂O emissions from agricultural soils are soil water-filled pore space, temperature and concentration of mineral nitrogen in the soil (Dobbie and Smith, 2001). The link between N₂O emission and amount N-fertilizer applied has given rise the concept of the emission factor (EF), where EF is the amount of N₂O-N emitted expressed as a fraction (or a percentage) of the N applied (Dobbie and Smith, 2003). The aim of this study was (1) to quantify the N-losses as N₂O from arable soils depending on water-filled pore space (WFPS) and (2) to determine the processes responsible for the N₂O emission from soils.

A microcosm study was conducted with arable loess soil (Halpic Luvisol, Low Saxony, Germany) fertilized with NH₄NO₃ (20 mg N per 1 kg of soil). The soil was incubated during two weeks at 15^oC and under varying moisture levels (50, 70 and 80% of WFPS). The rate of N₂O and CO₂ emissions were measured in 4 hours intervals with an automated gas chromatographic system equipped with a ⁶³Ni electron-capture detector. Analysis of soil extracts (0.5M K₂SO₄) were carried out before, 4 times during incubation and at the end of experiment. Soluble organic C, NO₃^--N and NH₄⁺-N concentrations were measured colorimetrically by TRAACS 800 auto-analyser. C and N of microbial biomass were calculated as a difference between C and N contents in 0.5 M K₂SO₄ extracts from soils before and after fumigation.

It was found that N₂O emission from soils at 50 and 70% WFPS was very low and varied between 0 and 12 μg N₂O-N·m^-2·h^-1. The rate of N₂O emission from wet soils (80% WFPS) was significantly higher, i.e. 400-600 μg N₂O-N·m^-2·h^-1 at the beginning of incubation period and 80-100 μg N₂O-N·m^-2·h^-1 at the end of experiment. The total N-losses has been found to depend on the moisture level and amounted 0.52-0.78, 0.74-1.34, and 44.1-74.1 mg N₂O-N·m^-2 for the whole experiment at 50%, 70% and 80% WFPS, respectively.

The results obtained showed that the WFPS was a major controller of mineral nitrogen concentration in the soil (total, NO₃^--N and NH₄⁺-N) and in the soil microbial biomass during the experiment. The amount of NH₄⁺-N in the soil declined rapidly over the incubation period and at the end of experiment, the soil NH₄⁺-N concentration was 3.3%, 6.7% and 13.3% from its initial level at 50%, 70% and 80% of WFPS, respectively. On contrary, soil NO₃^--N concentration increased over the incubation period and averaged 112-114% from its initial level when WFPS values were 50-70% and 105% from the initial level when WFPS value was 80%. The end concentration of total mineral nitrogen in the soil was about equal to the initial level at 80% WFPS and 7-9% higher at 50-70% WFPS. The amount of nitrogen immobilized in the soil microbial biomass decreased over the incubation period which was caused mainly by the decrease of NH₄⁺-N concentration in soil microbial biomass. Immobilized N losses comprised 22-30% depending on moisture level and obviously, the increase of total mineral nitrogen concentration in the soil during experiment at 50 and 70% WFPS arose from mineralization of nitrogen in the soil microbial biomass.Therefore, our investigations allow to conclude that water-filled pore space is a key factor affecting the level of N-N₂O losses from soils, their emission factor and processes responsible for N₂O emission from soils. Mineral nitrogen of fertilizer and nitrogen immobilized in the soil microbial biomass are main sources of N-N₂O emitted from arable soil.

This study was supported by German Academic Exchange Program, Deutsche Forschungsgemeinschaft and Russian Foundation for Basic Researches.

References:

Dobbie K.E. and Smith K.A. 2001. The effect of temperature, water-filled pore space and land use on N₂O emission from an imperfectly drained gleysoil. European Journal of soil Science, 52, 667-673.

Dobbie K.E. and Smith K.A., 2003. Nitrous oxide emission factors for agricultural soils in Great Britain: the impact of soil water-filled pore space and other controlling variables. Global change Biology, 9, 204-218.

Granli T. and Bokman J.C. 1994. Nitrous oxide from agriculture. Norwegian Journal of Agricultural Science, 12, 1-128.