Nematode communities respond more to N enrichment than to plant community changes over decades in tallgrass prairie

Temperate grasslands such as the North American tallgrass prairie are among the most endangered terrestrial ecosystems due to changes in climate and land-use practices. While belowground responses of terrestrial ecosystems to perturbations have received greater attention in recent years, there is a dearth of long-term studies documenting changes over decadal scales. The current study addresses the long-term effects of fire (annual burning or fire exclusion), mowing, and nitrogen (N) and phosphorus (P) fertilization on the structure and composition of a tallgrass prairie nematode community after 32 years of experimental treatments. Fire exclusion resulted in conversion of grassland to woodland, and a general decrease in nematode population densities, while annual prescribed fire maintained a grassland state. Although the change in vegetative state affected overall nematode abundance, vegetative structure was not the major driver of nematode community composition. Rather, changes in nitrogen availability appeared to be the dominant driver of nematode community dynamics. Responses of herbivorous taxa were dominated by interactions among burning, mowing, and N fertilization treatments and varied across taxonomic groups, but a general pattern of increasing relative abundances with N fertilization was observed, particularly in the presence of annual burning or mowing. In contrast, the relative abundance of the fungivorous Tylenchidae, the dominant nematode family in terms of abundance, declined from 44 % to 26 % after 32 years of N enrichment, while fire exclusion favored the opportunistic bacterivorous Rhabditidae. Although higher trophic level responses were generally uninformative, our results confirmed the value of nematode community analysis in soil food web diagnostics, with fungivore to bacterivore ratios and the maturity index (MI) identified as useful community indices.