COS 186-4 - Dead-N street: Fungal necromass chemistry controls carbon and nitrogen retention and the response to chronic nitrogen addition across forest types.

Abstract: Microbial necromass is a major component of slowly cycling soil carbon (C) and represents the majority of rapidly cycling soil nitrogen (N). This puts necromass at the nexus of processes influencing both soil C storage and plant productivity. Despite this, long-term direct measurements of necromass decomposition are rare, and previous work has largely ignored N dynamics. The objectives of this study were to understand how fungal necromass C and N dynamics are impacted by necromass chemistry; forest microbial and plant communities; and soil N availability. To test this, we decomposed four species of fungal necromass (Cenococcum geophilum, Cladosporium spp., Meliniomyces bicolor, Mortierella elongata) with varying chemistry for one year across four different forest stands (dominated by Acer saccharum, Pinus strobus, Populus grandidentata, and Quercus ellipsoidalis, respectively) containing a long-term N addition experiment. We hypothesized that 1) Necromass C and N dynamics would vary by both necromass chemistry as well as forest type; 2) Carbon and N loss from decomposing necromass would be regulated by the presence of recalcitrant compounds (e.g., melanin) that protect N from decomposition, and 3) Nitrogen addition would slow late-stage decomposition and N loss, but the magnitude of this effect would vary with necromass chemistry.

We found that necromass initially decomposed quickly (9-22% of mass lost per day), but a substantial fraction remained after 250 days (19-53% of initial mass). All species showed net N release throughout decomposition with a close correspondence to mass loss. Variation in mass and N loss were primarily driven by necromass chemistry (88% and 77%, respectively; p< 0.001 for both), with forest type taking a lesser, and necromass species-dependent role. However, the chemical factors governing species differences in N dynamics differed from those controlling mass loss. Specifically, N release was slower with higher initial necromass lipid content and C:N ratio, while late-stage mass loss was governed primarily by recalcitrant compounds such as melanin. Further, N addition inhibited late-stage mass loss more than N loss, although this was unrelated to initial melanin content. Taken together, these results stress the importance of the afterlife effects of microbial communities to C and N cycling, primarily stemming from differences in their chemical composition. Our results also suggest that these effects may be sensitive to anthropogenic N deposition.