Dead or alive: Fungal controls on decomposition and soil carbon storage in northern temperate forests
DeLancey, L. C . 2025.
Abstract
Soils store more carbon (C) than living vegetation and the atmosphere combined. Therefore, understanding the factors that regulate soil C storage and loss (decomposition) will be critical in understanding C cycling under ongoing climate change. Fungi play key roles in decomposition and C storage in forests by being the primary decomposers (saprotrophs) and acting as plant mutualists (ectomycorrhizal fungi; ECM). It has been
hypothesized that these ECM fungi, which receive C from their host plant, can outcompete saprotrophs for nitrogen (N), reducing decomposition. However, it is unclear a) whether this suppressive effect of ECM fungi on decomposition leads to increased C storage and b) how widespread it is among temperate pine forests. Further, recent research has found a majority of soil C and N is made up of dead fungal hyphae
(necromass), yet the chemical controls on C and N loss from necromass during decomposition and how these interact with global change factors like N addition, are poorly understood. The goals of this dissertation are therefore to elucidate the consequences and variation of ECM-saprotroph interactions and to clarify how N fertilization impacts fungal necromass decomposition. In chapter 1 I found that the presence of ECM fungi decreased the decomposition of belowground tissues and increased soil C stocks by 10%. This latter effect was strongest in the top 5 cm of soil and in fast-cycling, unprotected soil C. In chapter 2 I found that ECM suppression of leaf litter decomposition is not as widespread across temperate pine forests as previously theorized. Rather, ECM fungi either increase or do not affect pine litter decomposition in pine forests in California, Minnesota, and Florida. In chapter 3 I found that necromass C
loss, N loss, and responses to fertilization varied with necromass chemistry. Specifically, N addition inhibited late stage necromass decomposition, but this effect was weakest in melanin-rich necromass, contrary to hypotheses based on plant litter. Together, these
results add to our understanding of how fungi, dead and alive, influence soil C and N
cycling and highlight the diversity and importance of fungal chemistry and interguild
interactions.
