Management matters: testing a mitigation strategy for nitrous oxide emissions using legumes on intensively managed grassland

Abstract

Abstract. Replacing fertiliser nitrogen with biologically fixed nitrogen (BFN) through legumes has been suggested as a strategy for nitrous oxide (N2O) mitigation from intensively managed grasslands. While current literature provides evidence for an N2O emission reduction effect due to reduced fertiliser input, little is known about the effect of increased legume proportions potentially offsetting these reductions, i.e. by increased N2O emissions from plant residues and root exudates. In order to assess the overall effect of this mitigation strategy on permanent grassland, we performed an in situ experiment and quantified net N2O fluxes and biomass yields in two differently managed grass–clover mixtures. We measured N2O fluxes in an unfertilised parcel with high clover proportions vs. an organically fertilised control parcel with low clover proportions using the eddy covariance (EC) technique over 2 years. Furthermore, we related the measured N2O fluxes to management and environmental drivers. To assess the effect of the mitigation strategy, we measured biomass yields and quantified biologically fixed nitrogen using the 15N natural abundance method. The amount of BFN was similar in both parcels in 2015 (control: 55±5 kg N ha−1 yr−1; clover parcel: 72±5 kg N ha−1 yr−1) due to similar clover proportions (control: 15 % and clover parcel: 21 %), whereas in 2016 BFN was substantially higher in the clover parcel compared to the much lower control (control: 14±2 kg N ha−1 yr−1 with 4 % clover in DM; clover parcel: 130±8 kg N ha−1 yr−1 and 44 % clover). The mitigation management effectively reduced N2O emissions by 54 % and 39 % in 2015 and 2016, respectively, corresponding to 1.0 and 1.6 t ha−1 yr−1 CO2 equivalents. These reductions in N2O emissions can be attributed to the absence of fertilisation on the clover parcel. Differences in clover proportions during periods with no recent management showed no measurable effect on N2O emissions, indicating that the decomposition of plant residues and rhizodeposition did not compensate for the effect of fertiliser reduction on N2O emissions. Annual biomass yields were similar under mitigation management, resulting in a reduction of N2O emission intensities from 0.42 g N2O-N kg−1 DM (control) to 0.28 g N2O-N kg−1 DM (clover parcel) over the 2-year observation period. We conclude that N2O emissions from fertilised grasslands can be effectively reduced without losses in yield by increasing the clover proportion and reducing fertilisation.

Legumes increase grassland productivity with no effect on nitrous oxide emissions

“…As a result our study was unable to determine the legume proportion which would maximise plant productivity and mitigate N 2 O emissions. Studies have found that oversowing clover without any fertilisation compared to low clover proportion with organic fertilisation reduced N 2 O emissions by 54% and 39% in year 1 and 2 of a field experiment (Fuchs et al 2018). However, this reduction in emissions is more likely to be related to the difference in fertilisation between treatments rather than clover proportions per se.…”

Grazing-related nitrous oxide emissions: from patch scale to field scale

“…Thus they still include N inputs from plant residues and atmospheric deposition. Background emissions are also often regarded as a late effect of fertilization events from previous years (Bouwman, 1996;Gu et al, 2009). On pastures, background emissions may additionally result from trampling of the cows that can further stimulate the N 2 O production via denitrification due to soil compaction (Bhandral et al, 2007).…”

“…The EC technique was already applied successfully to quantify N 2 O emissions from pastures and grasslands (Jones et al, 2011). Some studies also tried to compare different systems (e.g., intensive -extensive, different crops, land/lake) with one EC tower (e.g., Biermann et al, 2014;Fuchs et al, 2018) by partitioning the fluxes based on wind direction and system geometry, but typically one tower for each system is preferable. In order to understand and quantify the emissions of a pasture, the combined approach of EC measurements and chambers is regarded as the best solution (Cowan et al, 2015).…”

Eddy covariance flux measurements of gaseous elemental mercury over a grassland

“…In this study, NEE of Hg 0 and CO 2 was measured concurrently with two independent EC systems over the intensively managed grassland used for forage production. Details on grassland species composition, harvest, and fertilization practices are described in Zeeman et al (2010), Merbold et al (2014) and Fuchs et al (2018). The tower for long-term EC greenhouse gas measurements was located between two adjacent grassland parcels (Fig.…”

“…The 30 min CO 2 flux was quantified in the conventional way established in ecosystem studies (see Aubinet et al, 2012) using the EddyPro ® (LI-COR Inc., Lincoln, NE, USA) software (see Fuchs et al, 2018, for specific information related to the Chamau field site). For each 30 min CO 2 flux interval, a flux quality control (QC) flag was determined following Mauder and Foken (2004): 0 (best data quality for detailed investigations), 1 (good data for longer-term studies) and 2 (poor quality).…”