Nitrogen addition weakens drought-driven coupling between plant, arthropod, and soil nematode functional groups

The concurrent increase in drought and atmospheric nitrogen deposition has
profoundly impacted multitrophic biodiversity and ecosystem functioning in
grasslands. Despite the well-documented individual effects of reduced
precipitation and nitrogen enrichment, their interactive effects,
especially on multitrophic cascading responses (e.g., plant, nematode, and
arthropod communities), remain largely unknown. Using a four-year field
experiment in a typical steppe of Inner Mongolia, we explored the effects
of three drought scenarios (intense drought, excluding 100% of rainfall in
June; reduced precipitation frequency, reducing rainfall events by 50%
without changing total rainfall from June to August; and chronic drought,
excluding 50% of each rainfall event from June to August) and nitrogen
addition (+10 g N m–2 yr–1) on species diversity, functional group
abundance, and functional group associations within and between trophic
levels, including plant, ground-dwelling arthropod and soil nematode
communities, as well as their relationships with grassland productivity.
We found that: (1) Drought and nitrogen addition had contrasting effects
on multitrophic species diversity, functional group abundance, and
grassland productivity. Chronic drought significantly reduced productivity
independent of nitrogen, while nitrogen addition enhanced it. Intense
drought increased the abundance of bacterivorous and fungivorous
nematodes, but this trend was absent with nitrogen addition. Reduced
precipitation frequency had no significant effect on multitrophic
communities or productivity under any nitrogen condition. (2) Drought,
particularly chronic drought, enhanced positive associations between
ground-dwelling arthropod and soil nematode functional groups, whereas
nitrogen addition was accompanied by a weakening of these functional group
interconnections. (3) Increased productivity with nitrogen addition was
associated with reduced positive associations within plant functional
groups and between arthropod and nematode functional groups, along with
increased soil nitrogen availability. Drought was related to lower
productivity overall, although it also coincided with reduced associations
within plant functional groups, which were related to higher productivity.
Synthesis. Our results indicate that nitrogen deposition under drought
scenarios is linked to adverse effects on the abundance and
interconnections of multitrophic functional components, highlighting the
importance of species interactions across trophic levels for understanding
grassland responses to environmental change.