Use of laser spectroscopy to evaluate the influence of soil storage on N2O emission

Accurate measurements of nitrous oxide (N₂O) fluxes from soils are necessary to understand dynamic changes in soil nitrogen cycles. Laboratory incubation experiments provide a controlled condition to measure these N₂O fluxes. Before incubation experiments, soils are often stored at certain conditions to minimize the microbial activities. However, the effect of soil storage on N₂O emission has been poorly studied. A laboratory incubation experiment was conducted using disturbed soils to study the storage effect. The soil was sieved to 2mm and the following four treatments were tested: fresh undisturbed (FU), fresh sieved (FS), fridge stored at 4ºC (ST), and stored at room temperature after drying (PI). After soil samples were brought to 60% water-filled pore space (WFPS), ¹⁵N labelled urea (1 At%) was applied at the rate of 50 mg N kg^-1 soil and the soil was incubated at room temperature (23 ºC). The N₂O fluxes were measured for 7 weeks using off-axis integrated cavity output spectroscopy (OA-ICOS, Los Gatos Research, California, USA). Cumulative N₂O fluxes and Keeling plot intercepts (δ¹⁵N source) were calculated. The results showed that soil storage has a significant effect on N₂O emission. Over the 7-week period, ST produced the highest cumulative N₂O emissions (2.70 µg N g^-1 soil) as well as the largest amount of N derived from fertiliser (Ndff) (1.4 µg N g^-1 soil). FU produced the lowest cumulative N₂O emissions (1.0 µg N g^-1 soil) but the largest amount of N derived from soil (Ndfs) (0.6 µg N g^-1 soil). The daily N₂O fluxes of FS and FU declined rapidly after the peak emissions, but the fluxes of PI and ST fluctuated after the peaks. These results indicate that soil storage affects microbial processes and therefore N₂O emissions. Our results suggest using fresh soil to avoid storage effects. If this is not possible the effect of soil storage should be considered before the experiment.