Precipitation context reshapes the effects of global change factor number on grassland functioning via plant and nematode diversity

The number of co‐occurring global change factors (GCFs) is increasing, yet their combined effects on grassland functioning via plant–soil biotic interactions remain poorly understood, especially under contrasting precipitation contexts.

Using a long‐term multifactor experiment in the semi‐arid steppe of Inner Mongolia, we established a gradient in GCF number (0–3) through nitrogen addition, phosphorus addition and soil acidification. By integrating plant diversity with the taxonomic, functional and metabolic dimensions of nematode communities, we evaluated their effects on above‐ground net primary productivity (ANPP) and below‐ground ecosystem multifunctionality (BEMF) and tested whether these relationships were reorganized under contrasting precipitation years.

Increasing GCF number consistently reduced plant and nematode diversity, simplified nematode food‐web structure and suppressed nematode metabolic activity. ANPP increased with GCF number only in the wet year, whereas BEMF declined in both years. ANPP was primarily and positively associated with nematode metabolic footprints, whereas BEMF was jointly regulated by plant diversity and nematode metabolic footprints. Importantly, these pathways shifted with precipitation context. In the normal year, plant diversity and nematode metabolic footprints jointly sustained both ANPP and BEMF. In the wet year, a functional divergence emerged: above‐ground processes benefited from nutrient inputs, increasing ANPP, whereas below‐ground processes experienced cumulative stress from multiple GCFs, suppressing nematode metabolic footprints and reducing BEMF.

Synthesis and applications . Our findings demonstrate that the number of co‐occurring GCFs is a critical but underappreciated dimension of global change, and its ecological consequences depend strongly on precipitation context. Grassland management should consider cumulative nutrient and acidification pressures, maintain plant diversity and use nematode metabolic footprints as indicators of below‐ground functional change. Incorporating soil faunal functional traits into global change assessment frameworks is therefore essential for understanding and sustaining ecosystem multifunctionality under future environmental change.