In:
Biogeosciences, Copernicus GmbH, Vol. 16, No. 18 ( 2019-09-27), p. 3665-3678
Abstract:
Abstract. Soil denitrification is the most important terrestrial process
returning reactive nitrogen to the atmosphere, but remains poorly
understood. In upland soils, denitrification occurs in hotspots of enhanced
microbial activity, even under well-aerated conditions, and causes harmful
emissions of nitric (NO) and nitrous oxide (N2O). The timing and magnitude
of such emissions are difficult to predict due to the delicate balance of
oxygen (O2) consumption and diffusion in soil. To study how spatial
distribution of hotspots affects O2 exchange and denitrification, we
embedded microbial hotspots composed of porous glass beads saturated with
growing cultures of either Agrobacterium tumefaciens (a denitrifier lacking N2O reductase) or
Paracoccus denitrificans (a “complete” denitrifier) in different architectures (random vs.
layered) in sterile sand that was adjusted to different water saturations
(30 %, 60 %, 90 %). Gas kinetics (O2, CO2, NO, N2O and
N2) were measured at high temporal resolution in batch mode. Air
connectivity, air distance and air tortuosity were determined by X-ray
tomography after the experiment. The hotspot architecture exerted strong
control on microbial growth and timing of denitrification at low and
intermediate saturations, because the separation distance between the
microbial hotspots governed local oxygen supply. Electron flow diverted to
denitrification in anoxic hotspot centers was low (2 %–7 %) but increased
markedly (17 %–27 %) at high water saturation. X-ray analysis revealed that
the air phase around most of the hotspots remained connected to the
headspace even at 90 % saturation, suggesting that the threshold response
of denitrification to soil moisture could be ascribed to increasing
tortuosity of air-filled pores and the distance from the saturated hotspots
to these air-filled pores. Our findings suggest that denitrification and its
gaseous product stoichiometry depend not only on the amount of microbial
hotspots in aerated soil, but also on their spatial distribution. We
demonstrate that combining measurements of microbial activity with
quantitative analysis of diffusion lengths using X-ray tomography provides
unprecedented insights into physical constraints regulating soil microbial
respiration in general and denitrification in particular. This paves the way
to using observable soil structural attributes to predict denitrification
and to parameterize models. Further experiments with natural soil structure,
carbon substrates and microbial communities are required to devise and
parametrize denitrification models explicit for microbial hotspots.
Type of Medium:
Online Resource
ISSN:
1726-4189
DOI:
10.5194/bg-16-3665-2019
DOI:
10.5194/bg-16-3665-2019-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2019
detail.hit.zdb_id:
2158181-2
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