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

Abstract

<p>An understanding of the drivers of hotspot/hot moments of N<sub>2</sub>O production is required to better constrain the global N<sub>2</sub>O budget and to plan the mitigation strategies. Hot spots are areas with very high N<sub>2</sub>O 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 N<sub>2</sub>O production. Roots are well known to be hotspots for microbial activity but roots direct contribution to N<sub>2</sub>O production and emissions in soil remain poorly understood.</p><p>In this study, we evaluated the role of root decomposition on N<sub>2</sub>O production and emissions, as a function of soil pore size and water content. We hypothesized that (i) the greatest N<sub>2</sub>O 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 N<sub>2</sub>O production by denitrification will be observed due to local anaerobic conditions, generated by O<sub>2</sub> consumption by decomposers.</p><p>To evaluate the role of root decomposition on N<sub>2</sub>O 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 N<sub>2</sub>O and CO<sub>2</sub> concentration analysis and isotopic characterization of N<sub>2</sub>O (δ<sup>15</sup>N<sup>bulk</sup>, site preference (<em>S<sub>P</sub></em>), and δ<sup>18</sup>O).</p><p>The daily emissions of N<sub>2</sub>O and CO<sub>2</sub> 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 N<sub>2</sub>O flux rates compared to the microcosms with small pore soil for both soil moisture treatments (p < 0.001). The relationship between <em>S<sub>P</sub><sub>  </sub></em>and δ<sup>18</sup>O (isotope mapping) indicated that heterotrophic bacterial denitrification strongly dominated N<sub>2</sub>O production between day 1 to 7 of the incubation (≥ 97%) and N<sub>2</sub>O reduction was higher during this period (40 – 60%) in soil microcosms with both pore size and moisture treatment. Later on, N<sub>2</sub>O reduction decreased (1 – 35%) while the share of nitrification/fungal sources increased for soil microcosms with large pores.</p><p>Our results indicated that decomposing roots acted as hotspots enhancing N<sub>2</sub>O emissions and N<sub>2</sub>O hotspots occurring during root decomposition are strongly influenced by soil pore architecture. While differences in soil pore architecture did not cause differences in N<sub>2</sub>O production process at the initial phase of decomposition, it might influence the relative contribution of N<sub>2</sub>O microbial production pathways in later stage of decomposition.</p>