Mustard catch crop enhances denitrification in shallow groundwater beneath a spring barley field

Abstract

Over-winter green cover crops have been reported to increase dissolved organic carbon (DOC) concentrations in groundwater, which can be used as an energy source for denitrifiers. This study investigates the impact of a mustard catch crop on in situ denitrification and nitrous oxide (N 2 O) emissions from an aquifer overlain by arable land. Denitrification rates and N 2 O-N/(N 2 O-N+N 2-N) mole fractions were measured in situ with a push-pull method in shallow groundwater under a spring barley system in experimental plots with an without a mustard cover crop. The results suggest that a mustard cover crop could substantially enhance reduction of groundwater nitrate (NO 3-N) via denitrification without significantly increasing N 2 O emissions. Mean total denitrification (TDN) rates below mustard cover crop and no cover crop were 7.61 and 0.002 µg kg-1 d-1 , respectively. Estimated N 2 O-N/(N 2 O-N+N 2-N) ratios, being 0.001 and 1.0 below mustard cover crop and no cover crop respectively, indicate that denitrification below mustard cover crop reduces N 2 O to N 2 , unlike the plot with no cover crop. The observed enhanced denitrification under the mustard cover crop may result from the higher groundwater DOC under mustard cover crop (1.53 mg L-1) than no cover crop (0.90 mg L-1) being added by the root exudates and root masses of mustard. This study gives insights into the missing piece in agricultural nitrogen (N) balance and groundwater derived N 2 O emissions under arable land and thus helps minimise the uncertainty in agricultural N and N 2 ON balances.

Effect N-vanggewassen na snijmaïs op zandgrond op de nitraatuitspoeling : Verslag van vierjarig veldonderzoek op zuidelijk zandgrond te Vredepeel

“…De beoordeling voor de Nitraatrichtlijn vindt echter plaats op basis van de nitraatconcentratie in het grondwater. Er zijn weinig metingen gedaan in het grondwater (Schröder et al, 2013;Premrov et al, 2012), terwijl wintervanggewassen ook een effect kunnen hebben op de hydrologie (Gabriel et al, 2012;Basche et al, 2016) en op de denitrificatie in de bodem (Jahangir et al, 2014). Zo heeft de maatregel geen effect in bodems met hoge grondwaterstanden, waar door denitrificatie de nitraatconcentraties in grondwater heel laag zijn.…”

“…Met name bij grasachtige vanggewassen en klavers is het een substantiële hoeveelheid die kan oplopen tot 20-30 kg N per ha (Schröder, 1997). Andere mechanismen die een rol zouden kunnen spelen zijn een hogere denitrificatie in de bodem (Heinen, 2006;Jahangir et al, 2014) en daardoor meer verlies van stikstof naar de lucht in plaats van naar het grondwater en/of optreden van N-immobilisatie in de bodem door de teelt van vanggewassen (Vos & Van der Putten, 2001;Vogeler et al, 2022).…”

Combining stable isotopes with contamination indicators: A method for improved investigation of nitrate sources and dynamics in aquifers with mixed nitrogen inputs

“…However, these correlations were also weak, which could simply indicate that point sources are not major NO 3 sources. Alternatively, the lowering of NO 3 concentration might be due to denitrification enhanced by higher DOC levels (Jahangir et al, 2014) and sustained by a higher density of point sources. Nonetheless, all wells with more than ten unsewered houses within 300 m radius were located in site O, where the % tillage was lower and % grassland was higher (Table SM-2).…”