Changes in soil pore structure generated by the root systems of maize, sorghum and switchgrass affect in situ N2O emissions and bacterial denitrification

Abstract

Due to the heterogeneous nature of soil pore structure, processes such as nitrification and denitrification can occur simultaneously at microscopic levels, making prediction of small-scale nitrous oxide (N2O) emissions in the field notoriously difficult. We assessed N2O+N2 emissions from soils under maize (Zea mays L.), switchgrass (Panicum virgatum L.), and energy sorghum (Sorghum bicolor L.), three potential bioenergy crops in order to identify the importance of different N2O sources to microsite production, and relate N2O source differences to crop-associated differences in pore structure formation. The combination of isotopic surveys of N2O in the field during one growing season and X-ray computed tomography (CT) enabled us to link results from isotopic mappings to soil structural properties. Further, our methodology allowed us to evaluate the potential for in situ N2O suppression by biological nitrification inhibition (BNI) in energy sorghum. Our results demonstrated that the fraction of N2O originating from bacterial denitrification and reduction of N2O to N2 is largely determined by the volume of particulate organic matter occluded within the soil matrix and the anaerobic soil volume. Bacterial denitrification was greater in switchgrass than in the annual crops, related to changes in pore structure caused by the coarse root system. This led to high N-loses through N2 emissions in the switchgrass system throughout the season a novel finding given the lack of data in the literature for total denitrification. Isotopic mapping indicated no differences in N2O-fluxes or their source processes between maize and energy sorghum that could be associated with the release of BNI by the investigated sorghum variety. The results of this research show how differences in soil pore structures among cropping systems can determine both N2O production via denitrification and total denitrification N losses in situ.

The distribution of particulate organic matter in the heterogeneous soil matrix - balancing between aerobic respiration and denitrification

“…However, our observations underline the major importance of denitrification for the total N 2 O emissions from soil, particularly under near saturated conditions when N 2 O production rates were high and almost all N 2 O is produced by denitrification (Fig. S2b), corroborating previous findings (Gao et al 2023; Liang et al 2021; Lucas, Gil, et al 2023; Ostrom et al 2021). In addition, the sustained N 2 O emissions observed even after 10 days of incubation support the notion, that hotpots of N 2 O can persist for several weeks under static conditions (Christensen et al 1990).…”

“…3). This shows that in soils with an even distribution of hotspots (oxygen demand), the reason for heterogeneity in denitrification is the variation of oxygen supply at the microscale (Lucas, Gil, et al 2023; Rohe et al 2021)). However, our observations underline the major importance of denitrification for the total N 2 O emissions from soil, particularly under near saturated conditions when N 2 O production rates were high and almost all N 2 O is produced by denitrification (Fig.…”

“…The extent of denitrification in soils is a result of balancing between O 2 demand and supply (G P Robertson 2023; Rohe et al 2021). The O 2 supply, on the one side, is controlled by the soil structure through the regulation of diffusion distances and the establishment of potential anaerobic volumes (Du et al 2023; Lucas, Gil, et al 2023; Rohe et al 2021, Fig. 3).…”

“…For this, we computed linear regressions of the anaerobic volume resulting from varying distance thresholds of 0.06 mm - 5 mm with the N 2 O+N 2 fluxes and minimized the p -value. We also used the distance maps to calculate the distance of POM to air-filled macropores within the subsamples similar to Ortega-Ramírez et al 2023 and Lucas, Gil, et al 2023. The same sensitivity analysis as for the anaerobic soil volume was conducted to estimate the distance threshold.…”

“…also relatively fresh, but dead root residues cut during sampling. To further elucidate, if the distribution of fresh roots is different from the one of POM, we segmented roots from the total POM as written in Lucas, Gil, et al 2023. In short, all POM particles, which were connected to the outer boundary of the image and were larger than 10.000 voxels, i.e.…”