More exceptions than rules: Variable effects of ectomycorrhizal fungi on leaf litter decomposition in temperate pine forests
DeLancey, L.C., M.J. McCarthy, K. Peterson, J.J Yeam, L. Kaminsky, M.E. Smith, K.G. Peay, S. E. Hobbie and P.G. Kennedy. 2025.
Abstract
1. Ectomycorrhizal (ECM) fungi have long been thought to reduce litter decomposition
in nitrogen (N)- limited ecosystems by outcompeting saprotrophs for litter N (a
phenomenon known as the 'Gadgil effect'). However, recent research has called
the generality of this effect into question, by demonstrating that ECM fungi can
increase or decrease organic matter decomposition in different forests. The
ecological factors driving such variation in the size and direction of ECM fungal
effects on decomposition remain unclear.
2. Here, we tested the hypothesis that ECM fungi would suppress decomposition
of N- poor, recalcitrant litter more in forests with lower N- availability by
exacerbating saprotrophic N limitation. We conducted an in situ ECM fungal and
root reduction experiment (via soil trenching) in nine pine forests across three
US states, which varied in soil and litter N content, climate and pine host (Pinus
muricata in California, P. elliottii in Florida and P. resinosa in Minnesota). In each
site, we decomposed needle litter from (1) a pine species native to that site and (2)
a common litter, P. strobus, for 1 year.
3. Contrary to our expectations, ECM fungi either stimulated (California) or had
no effect on (Florida and Minnesota) pine needle decomposition. Across sites,
ECM fungal stimulation of decomposition did increase with total soil N content,
but was unrelated to inorganic N availability. Furthermore, despite previous
work suggesting that competition for N structures ECM fungal-saprotroph
interactions, trenching effects on decomposition did not differ between pine
litter types, despite large differences in initial litter C:N ratios, recalcitrance and
net litter N immobilization.
4. Synthesis. Taken together, our results add to a growing body of evidence that the
'Gadgil effect' is not universal, even in the N- poor litter of temperate pine forests
where it was first described and is often invoked. Furthermore, the inconsistency
of relationships between trenching effects with different metrics of decomposer
N supply and demand calls into question the central role of N in structuring fungal
interguild interactions.
Key Words
ectomycorrhizal fungi, Gadgil effect, litter decomposition, plant-soil (below- ground) interactions, priming effect, saprotrophic fungi, trenching
