The Effect of Bulk Density and Soil Water Content on N2O Emission from Agriculture Soil.

[Objective] Many studies state non-tillage caused higher N₂O emission, but which factors, changed by tillage, control N₂O emission was not so clear and need deeper study. So soil core incubation under aerobic environment was conducted to study the combined effect of bulk density (BD), soil water content (WC), and nutrient management (NM) on N₂O fluxes from agriculture soil.

[Method] Soil was collected from fertilizer (F) and manure+fertilizer (MF) plots of Shin-Hidaka in Hokkaido, Japan. Sieved soil of 0-2mm was packed into plastic rings, adjusted BD (0.45g/cm³, 0.66g/cm³) and WC (0.45g/g, 0.35g/g) manually to achieve a range of water filled pore space (WFPS) from 29% to 83%. Then put rings into Mason jars, which were filled with compressed air and air + 10%C₂H₂ (v/v) in headspace and sealed tightly. Incubation was conducted at 15℃ for 10 days. Gas samples were taken from headspace of jars at 1, 2, 3, 4, 6, 8, 10days respectively for N₂O, NO and CO₂ concentrations analysis. After each gas sampling, replacing the headspace air again. Soil chemical properties were analyzed just after water adjustment and after incubation. Denitrifying enzyme activity (DEA) was measured after incubation and was calculated as N₂O in headspace between 2 and 4 h under N₂ atmosphere. Net ammonification and net nitrification (mg N/kg dwt) are estimated as the difference of NH₄⁺ or NO₃^-before and after incubation respectively. Net mineralization is estimated as the sum of net ammonification and net nitrification. Microbial biomass nitrogen (MBN) was measured by chloroform fumigation-extraction method before and after incubation.

[Result and Discussion]

C₂H₂ had no significant effect on net mineralization or MBN. However there were differences in net ammonification and nitrification rates between air and C₂H₂ treatments because nitrification was inhibited by C₂H₂. Net ammonification of C₂H₂ treatment was significantly greater than that of air treatment. On the other hand, net nitrification of C₂H₂ treatment was smaller than that of air treatment, and ranged from -0.1 to -18 mg/kg/10 days. Net ammonification of C₂H₂treatment was greater in MF plots than in F plot, and not affected by BD or WC.

Cumulative CO₂ emission of C₂H₂ treatment (165 – 259 mgC/kg) was significantly higher than air treatment (121 – 162 mgC/kg) (p<0.001). CO₂emission tended to be higher in MF plots than in F plot, but there was no clear effect of BD and WC.

There was no significant difference in cumulative N₂O emission between air and C₂H₂ treatments. However, NM (p<0.001), WC (p<0.001) and BD (p<0.001) had significant effect on cumulative N₂O emission of both air treatment and C₂H₂ treatment. Most of time cumulative N₂O emission of MF soil was higher than F soil with same WC and BD (p<0.001), because NO₃^- (p<0.001) and MBN (p<0.001) of MF soil were larger than F soil before incubation, providing enough reactants for dentrification and nitrification. High WC and high BD promoted N₂O emission, and cumulative N₂O emission of 45%-WC with high BD was extremely high compared to the other treatment. Based on the results of the N₂O-N/NO-N ratio, N₂O production of 45%-WC with high BD treatments was mainly caused by denitirification, while N₂O was produced mainly by nitrification in the other treatments (45%-WC with low BD, 35%-WC with high BD, and 35%-WC with low BD). There was negative correlation between cumulative N₂O production and net nitrification for each headspace gas treatment, which will also indicate the NO₃-N reduction by denitrification resulted in high N₂O production. WFPS of 45%-WC with high BD treatment with greater DEA was around 80% which provide appropriate environment for denitrification. N₂O emission of 45%-WC with high BD under C₂H₂ treatment was higher than air treatment. N₂O emission/[N₂O + N₂ emission] of 45%-WC with high BD were estimated as 0.4 in F plot and 0.8 in MF plot, and this might be attributed to the inhibition of N₂O reductase by high NO₃⁻in MF plot.

[Conclusion] CO₂ emission tended to be higher in MF plot than in F plot, but there was no clear effect of BD and WC. On the other hand, N₂O emission was controlled by WC and BD. High WC and high BD promoted N₂O emission because of the contribution of denitrification. Most of time, cumulative N₂O emission of MF plot was higher than F plot with more NO₃^-and MBN.

Keyword: nitrogen dioxide (N₂O), bulk density, soil water content