Abstract. It is well established that plant species influence ecosystem processes, but we
have little ability to predict which vegetation changes will alter ecosystems, or how the effects
of a given species might vary seasonally. We established monocultures of eight plant species in
a California grassland in order to determine the plant traits that account for species impacts
on nitrogen and phosphorus cycling. Plant species differed in their effects on net N
mineralization and nitrification rates, and the patterns of species differences varied seasonally.
Soil PO4
and microbial P were more strongly affected by slope position than by species.
Although most studies focus on litter chemistry as the main determinant of plant species
effects on nutrient cycling, this study showed that plant species affected biogeochemical
cycling through many traits, including direct traits (litter chemistry and biomass, live-tissue
chemistry and biomass) and indirect traits (plant modification of soil bioavailable C and soil
microclimate). In fact, species significantly altered N and P cycling even without litter inputs.
It became particularly critical to consider the effects of these multiple traits in order to account
for seasonal changes in plant species effects on ecosystems. For example, species effects on
potential rates of net N mineralization were most strongly influenced by soil bioavailable C in
the fall and by litter chemistry in the winter and spring. Under field conditions, species effects
on soil microclimate influenced rates of mineralization and nitrification, with species effects on
soil temperature being critical in the fall and species effects on soil moisture being important in
the dry spring.
Overall, this study clearly demonstrated that in order to gain a mechanistic, predictive
understanding of plant species effects on ecosystems, it is critical to look beyond plant litter
chemistry and to incorporate the effects of multiple plant traits on ecosystems.