Effect of nitrogen enrichment
Contrary to our expectations, we did not observe effects of nitrogen
enrichment sources on crop pollinators and pollination. It is possible
that the proxies used in our study do not adequately represent the real
nitrogen exposure levels in our study fields. Indeed, while pollinators
can be affected by local (i.e. within field) changes in nutrient
availability (David et al. 2019), our proxies for nitrogen levels
are taken at much broader scales. The amount of nitrogen that is in
reality deposited in a specific location of the biosphere may not be
well represented by the NO2 levels measured in the troposphere at much
larger spatial resolution of the available data from the Sentinel-5
satellite (i.e., 7×3.5 km). Similarly, the estimated mean application
rate of fertilizers at each study region (which is based on average
application levels for each crop at country level, and do not consider
personal decisions of landowners) may not be of a sufficient resolution
to detect changes in fertilization rate and its effects at the local
scale. For example, the presence of (semi-)natural habitats in the
landscape will inevitably decrease the estimated average fertilizer
application rate at 1km2 resolution, while a high
proportion of highly enriched crops, such as cereals, maize or
fertilised grasslands (e.g., ray grass) will tend to increase the
estimated average application rate. Future works involving farmer
interviews asking for the actual amount of fertilizer applied to better
characterize nutrient availability would be important. Moreover,
although we had a clear gradient of N fertilizer input across sites, all
study sites were located in landscapes with a critical positive surplus
of nitrogen inputs (that goes up to 20 t.km-2 for the
year 2010) (European Environment Agency, 2020). Consequently, it is
possible that throughout the study region pollinator communities are
dominated by nitrophilous species (Carvalheiro et al. 2020) well adapted
to high nitrogen conditions and the negative effects of nitrogen on
pollinators and their contribution to crop production are no longer
detectable in our specific study sites.
Finally, it is possible that functional composition of pollinators has
changed along the nitrogen availability gradient but with no net change
in pollinator abundance, or their contribution to crop production.
Indeed, N enrichment can have contrasting effects on pollinator species.
Pollinators with more diversified diets might be less affected by
landscape eutrophication potentially due to their ability to forage on a
higher diversity of flowers in a diverse set of habitats (Pöyry et
al. 2017; Carvalheiro et al. 2020). N deposition that changes
soil nutrient availability is an important driver of plant species
composition change and result in the decline of oligotrophic plant
species, such as nitrogen fixing Fabaceae species (Roth et al.2013, 2019). Fabaceae are the main food resource of most bumble bee
species and many other solitary bees (Goulson et al. 2008; Kleijn
& Raemakers 2008; Connop et al. 2010). Thus, species specialised
on Fabaceae (and other N sensitive plants), can have more difficulty in
finding adequate resources be more susceptible to the effects of N
enrichment than other pollinator species (Stevens et al. 2018).
But if, for the crops studied here, species that prefer nitrophilous
environments (see Carvalheiro et al. 2020) are equally efficient for
crop pollination than species which are negatively affected, pollinator
community compositional changes would not affect the net crop
pollination outcome.