Dominant roles but distinct effects of groundwater depth on regulating leaf and fine-root N, P and N:P ratios of plant communities

Aims As the determinant of water availability in drylands, groundwater plays a fundamental role in regulating vegetation distribution and ecosystem processes. Although considerable progress has been made over the past years in the relationship between environment stress and plant community-level traits, the potential influence of water stress induced by groundwater changes on plant community-level stoichiometry remains largely unclear. Here, we examined whether belowground and aboveground community-level stoichiometry responded differently to groundwater changes. Methods We measured nitrogen (N) and phosphorus (P) concentrations in plant leaves and fine-roots of 110 plots under a broad range of groundwater depths in a typical arid inland river basin. We examined the spatial patterns and drivers of community-level N:P stoichiometry in leaves and fine-roots. Important Findings Community-level leaf and fine-root N, P and N:P ratios were mainly determined by groundwater, vegetation types and species composition, among which groundwater played a dominant role. Groundwater indirectly regulated community-level N:P stoichiometry through affecting vegetation types and species composition. Vegetation types and species composition had significant direct influences on community-level N:P stoichiometry. Furthermore, groundwater depth had opposite influences on community-level leaf and fine-root N:P stoichiometry. Groundwater depth regulated vegetation types and further decreased leaf N, P but increased leaf N:P ratios and fine-root N. Groundwater depth had a positive indirect impact on fine-root P but a negative indirect impact on fine-root N:P ratios primarily by affecting species composition. Our findings indicate that groundwater rather than climate conditions effectively regulates community-level N:P stoichiometry, and below- and aboveground N:P stoichiometry has opposite responses to groundwater.