Plant species effects on nutrient cycling: 20 (or so) years later

Background/Question/Methods

Understanding how plant species influence nutrient cycling is important in a variety of contexts, from elucidating the ecosystem and community effects of invasive species, to informing managment, to understanding the consequences of species range shifts in response to global environment change. In a review published in Trends in Ecology and Evolution over two decades ago, I asserted that along fertility gradients, plant traits change in predictable ways that reinforce patterns of fertility by creating positive feedbacks to rates of ecosystem nutrient cycling. I reviewed what has been learned in the ca. 20 years following the publication of Hobbie (1992) regarding the effects of plant species on nutrient cycling. Specifically I evaluated current evidence that species traits reinforce patterns of nutrient cycling along fertility gradients by evaluating two key assumptions: 1) that plant species traits important to nutrient cycling vary predictably along fertility gradients, and 2) that the pervasive emphasis on aboveground traits (especially leaf nitrogen) informs understanding of plant influences on soil processes. I also considered the need to the move beyond leaf litter feedbacks to a more comprehensive understanding of how species affect nutrient cycling through their effects on soil organic matter dynamics.

Results/Conclusions

I found that, despite enormous amounts of data synthesized for plant traits, few published studies related foliar traits to soil properties, including measures of soil fertility, indicating a need for more rigorous tests of the assumption that plant leaf traits change predictably across fertility gradients. I found evidence that traits other than leaf nitrogen may be more important drivers of decomposition under particular situations, such as where soil fauna are dominant decomposers. Furthermore, other components of plant production besides leaves (root biomass, stem biomass, root exudates and secretions, mycorrhizae) are significant fractions of detritus production and thus are equally important to carbon and nutrient cycling as leaves. Yet, their influence on decomposition and nutrient cycling cannot always be predicted from understanding of leaf traits. Finally, recent research indicates that variation in biochemical recalcitrance of detritus is unimportant in conferring long-term (> decadal scale) stability to soil organic matter. Although these soil organic matter pools turn over more slowly than litter, they can contain more nutrients than leaf litter at the ecosystem scale.  These observations suggest that a broader suite of plant traits and biogeochemical processes, i.e. beyond litter chemistry and litter decomposition, are important to carbon and nutrient cycling at the ecosystem scale.