Long-term nitrogen fertilization inhibits carbon and nitrogen loss during late stage fungal necromass decomposition depending on necromass chemistry

Fungal necromass is increasingly recognized as a key component of in soil carbon (C) and nitrogen (N) cycling. However, how C and N loss from fungal necromass during decomposition are impacted by global change factors such as anthropogenic N addition and changes to soil C supply (e.g. via changing root exudation and rhizosphere priming) remains unclear and understudied relative to plant tissues. To address these gaps, we conducted a year-long decomposition experiment with four species of fungal necromass incubated across four forested sites in plots that had received inorganic N and/or labile C fertilization for decades in Minnesota, USA. We found that necromass chemistry was the primary driver of C and N loss from fungal necromass as well as response to fertilization. Specifically, N addition suppressed late-stage decomposition, but this effect was weaker in melanin-rich necromass, contrary to the hypothesis based on plant litter dynamics that N addition should suppress decomposition of more complex organic molecules. Labile C addition had no effect on either the early or late stages of necromass decomposition. Nitrogen release from necromass also varied among species, with N-poor necromass having lower N release after controlling for differences in mass loss via regression. The relatively minor effects of N fertilization on the proportion of initial necromass N released suggests that N demand by decomposers is the primary control on N loss during fungal necromass decomposition. Together, our results stress the importance of the afterlife effects of fungal chemical composition to forest soil C and N cycles. Further, they demonstrate that C and N release from this critical pool can be reduced by ongoing anthropogenic N addition.