X‐ray computed tomography to predict soil N₂O production via bacterial denitrification and N₂O emission in contrasting bioenergy cropping systems

Abstract

While renewable biofuels can reduce negative effects of fossil fuel energy consumption, the magnitude of their benefits depends on the magnitude of N2O emissions. High variability of N2O emissions overpowers efforts to curb uncertainties in estimating N2O fluxes from biofuel systems. In this study, we explored (a) N2O production via bacterial denitrification and (b) N2O emissions from soils under several contrasting bioenergy cropping systems, with specific focus on explaining N2O variations by accounting for soil pore characteristics. Intact soil samples were collected after 9 years of implementing five biofuel systems: continuous corn with and without winter cover crop, monoculture switchgrass, poplars, and early‐successional vegetation. After incubation, N2O emissions were measured and bacterial denitrification was determined based on the site‐preference method. Soil pore characteristics were quantified using X‐ray computed microtomography. Three bioenergy systems with low plant diversity, that is, corn and switchgrass systems, had low porosities, low organic carbon contents, and large volumes of poorly aerated soil. In these systems, greater volumes of poorly aerated soil were associated with greater bacterial denitrification, which in turn was associated with greater N2O emissions (R2 = 0.52, p < 0.05). However, the two systems with high plant diversity, that is, poplars and early‐successional vegetation, over the 9 years of implementation had developed higher porosities and organic carbon contents. In these systems, volumes of poorly aerated soil were positively associated with N2O emissions without a concomitant increase in bacterial denitrification. Our results suggest that changes in soil pore architecture generated by long‐term implementation of contrasting bioenergy systems may affect the pathways of N2O production, thus, change associations between N2O emissions and other soil properties. Plant diversity appears as one of the factors determining which microscale soil characteristics will influence the amounts of N2O emitted into the atmosphere and, thus, which can be used as effective empirical predictors.

What can we learn from N2O isotope data? – Analytics, processes and modelling

“…Although these simple parameterization methods provide the first step in constraining emission processes, recent results highlight the importance of also considering other drivers in these models, such as pH regulation of nitrification 77 and N 2 O reduction, microbial biomass and land use history, and substrate mobilization and availability 36,78 . The N 2 O emissions during “hot moments” and from “hot spots” in the environment are also increasingly recognized as playing a major role in annual and regional N 2 O budgets, but their controls are particularly challenging to understand in their full complexity, and thus difficult to model 79–82 . Using natural abundance and isotope labelling approaches to gain a mechanistic understanding of the response of N 2 O transformation pathways to the most important drivers will be key to improving models and allowing predictions of the N 2 O budget in heterogeneous environments, in particular in the context of a changing climate.…”

“…In addition, the bulk isotopic composition of the N precursor might not be representative of the actually utilized N substrate pool. Particularly in soils where the soil matrix can be markedly heterogeneous and “hotspots” of denitrification can occur in isolated anoxic soil microsites, 82 NO 3 − near and in the reactive zones may be strongly enriched in 15 N compared with the bulk soil 41,91 and may also be derived from various soil N pools including organic and mineral N 92 . Similarly, in the case of nitrate consumption at strong redox gradients in the ocean and in lakes, most denitrifying activity is localized where the δ 15 N value of the nitrate pool has already been elevated.…”

Denitrification in soil as a function of oxygen supply and demand at the microscale

“…Either it is measured locally via oxygen sensors with needle-type microsensors (Sexstone et al, 1985;Højberg et al, 1994;Elberling et al, 2011) or with foils (Keiluweit et al, 2018;Elberling et al, 2011), which requires to average or to extrapolate measured O 2 saturation for the entire soil volume. Or it is estimated for the entire sample volume from pore distances in X-ray CT images of soil structure assuming that there is a direct relationship between pore distances and anaerobiosis (Kravchenko et al, 2018;Rabot et al, 2015).…”

“…Completeness of denitrification is another important controlling factor that modulates the relationship between oxygen availability and N 2 O emissions (Morley et al, 2014) which has previously been neglected in similar incubation studies (Rabot et al, 2015;Porre et al, 2016;Kravchenko et al, 2018) due to methodological challenges imposed by measuring N 2 emissions from soil (Groffman et al, 2006).…”

Physical constraints for respiration in microbial hotspots in soil and their importance for denitrification

“…in an intact soil column (Rabot et al, 2015). In agricultural soil with different crop rotations, N 2 O emissions were shown to correlate positively with the volume fraction of soil with macropore 430 distances larger than 180 µm, used as an ad-hoc definition for poorly aerated soil (Kravchenko et al, 2018). In a mesocosm study on microstructural drivers for local redox conditions, none of the investigated soil pore metrics derived from X-ray CT data (excluding those examined here) correlated with redox kinetics during a wetting/drying cycle (Wanzek et al, 2018).…”