Plant roots and associated mycelia enhance soil N transformation through different mechanisms in a karst plantation

Purpose Plant roots and associated mycelia play a crucial role in soil nitrogen (N) cycling. However, the underlying mechanisms by which roots and mycelia affect soil N transformation in karst soil remain unclear. Methods By using ingrowth cores, the present study focused on elucidating the underlying mechanisms by which roots and mycelia impact soil N transformation in a Cryptomeria fortune plantation in a karst region. Results Both roots and mycelia significantly increased the net N mineralization rate, with increases in magnitude of 9% and 25%, respectively, in soils of the Cryptomeria fortune plantation. Moreover, we found that this increase in N mineralization coincided with significant increases in the microbial biomass and extracellular enzyme activities, suggesting that both roots and mycelia could enhance N mineralization through their effect on microbial processes. Moreover, mycelia induced a significant decrease in metal-mineral organic complexes, on which roots had only a minor net effect, implying that mycelia could enhance N mineralization via their effects on the breakage of mineral-associated N complexes. Conclusions These combined results suggest that plant roots and mycelia accelerate soil N transformation through different mechanisms. In particular, mycelia presumably function through both biotic processes (microbial mineralization) and abiotic processes (disruption of stabilized mineral-proteinaceous complexes), whereas roots mainly function through biotic processes. Given that roots and mycelia are belowground symbionts, these two mechanisms are proposed to function together to promote N transformation and thus have significant ecological ramifications for N cycling in karst ecosystems.