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

Abstract 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 4‐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 year −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.