The influence of root decomposition on N2O fluxes and N2O microbial production pathways in soil with contrasting pore characteristics

An understanding of the drivers of hotspot/hot moments of N₂O production is required to better constrain the global N₂O budget and to plan the mitigation strategies. Hot spots are areas with very high N₂O emission rates relative to the surrounding area, while hot moments are short periods of time with very high emission rates. As the decomposition of fresh organic matter is transitory in nature, it may have a strong influence on hotspot and hot moment N₂O production. Roots are well known to be hotspots for microbial activity but roots direct contribution to N₂O production and emissions in soil remain poorly understood.

In this study, we evaluated the role of root decomposition on N₂O production and emissions, as a function of soil pore size and water content. We hypothesized that (i) the greatest N₂O emissions will be observed from root decomposition in the soil dominated by large (>30 µm Ø) pores due to their high connectivity and (ii) enhanced N₂O production by denitrification will be observed due to local anaerobic conditions, generated by O₂ consumption by decomposers.

To evaluate the role of root decomposition on N₂O production we used soil microcosms cultivated with switchgrass (Panicum virgatum L. variety Cave-in-rock). From the same composite soil samples we created two soil materials with contrasting pore architectures, namely soil with prevalence of large pores (≥ 35 μm Ø) and small pores (≤ 10 μm Ø). After four months of growing in a greenhouse, plants were cut and soil microcosms with roots were incubated in the dark at room T for 21 days, at two contrasting soil moisture conditions: 40% and 70% water filled pore space (WFPS). Gas headspace samples were collected at different time points during incubation for N₂O and CO₂ concentration analysis and isotopic characterization of N₂O (δ¹⁵N^bulk, site preference (S_P), and δ¹⁸O).

The daily emissions of N₂O and CO₂ from soil microcosms with grown roots showed the same trend during the incubation period and were significantly higher compared to soil microcosms without roots (control) (p < 0.05). Microcosm with large pores soil had significantly higher N₂O flux rates compared to the microcosms with small pore soil for both soil moisture treatments (p < 0.001). The relationship between S_P  and δ¹⁸O (isotope mapping) indicated that heterotrophic bacterial denitrification strongly dominated N₂O production between day 1 to 7 of the incubation (≥ 97%) and N₂O reduction was higher during this period (40 – 60%) in soil microcosms with both pore size and moisture treatment. Later on, N₂O reduction decreased (1 – 35%) while the share of nitrification/fungal sources increased for soil microcosms with large pores.

Our results indicated that decomposing roots acted as hotspots enhancing N₂O emissions and N₂O hotspots occurring during root decomposition are strongly influenced by soil pore architecture. While differences in soil pore architecture did not cause differences in N₂O production process at the initial phase of decomposition, it might influence the relative contribution of N₂O microbial production pathways in later stage of decomposition.