Abundance and productivity mediate invader effects on nitrogen dynamics in a California grassland

Abstract

Soil nitrogen (N) transformations have been shown to be influenced by plant community composition. Identifying species traits that control nitrogen dynamics is more straightforward when species dramatically differ in N input via litter (e.g., N-fixing invaders in a non-fixing community) or in litter carbon:N or lignin:N ratios. Cases where invaders and residents are more similar for such traits are more challenging to evaluate. In these settings, a species' relative abundance and its contribution to overall ecosystem productivity are likely to contribute significantly to the development of effects on N availability and cycling. We compared soil N dynamics in experimental grassland communities dominated by native perennial grasses (NP), exotic annual grasses (EA), and exotic perennial grasses (EP), as well as mixtures of the native perennial grasses with each exotic grass group (NP + EA and NP + EP). These groups differ from each other in subtle ways in traits likely to influence soil N cycling including annual productivity, allocation to roots versus shoots, litter production, litter chemistry, and degree of summertime activity. We found that ecosystem N dynamics were significantly different between the various species groups with the greatest differences occurring between EA plots and other community types: soils in EA plots had significantly lower rates of net N mineralization, net nitrification, and microbial biomass-N compared to either NP or NP + EA plots, and lower extractable nitrate in the spring compared to either NP or EP plots. The higher the proportion of productivity in a plot that derived from exotic species, particularly exotic annual species, the lower were the measured rates of net N mineralization. Stepwise regression analysis showed that vegetation productivity was the best predictor of N cycling metrics: the higher the productivity, the higher the rates of net mineralization and nitrification, and microbial biomass-N. We conclude that species' abundance and productivity were strong controlling factors in the development of differences in ecosystem N dynamics between our experimental treatments. Inclusion of relative proportion of productivity contributed by community members in models of the development of species effects will likely aid in predicting when and where invasive species will alter ecosystem N dynamics.