Professor Western University London, Ontario, Canada
Abstract: Increased nitrogen (N) deposition and climate warming are two global change factors likely to have substantial impacts on soil and plants in northern temperate ecosystems over the next century. Although both N addition and warming can increase plant productivity, chronic N addition can lead to soil N saturation, which can decrease soil N retention over the long term. Likewise, warming can impact soil N retention by reducing snow cover and increasing the frequency of soil freeze-thaw cycles. We added new N addition and warming plots to a pre-existing field experiment established in a grass-dominated old field to study the effects of N addition and warming on plant and soil N retention over both the short term (3 years) and the longer term (16 years). For the plants, we targeted the responses of Poa pratensis and Bromus inermis, the two dominant grasses at the field site. Bromus inermis responded significantly to the warming treatment with greater biomass production and greater retention of the 15N tracer compared to in the control plots. In contrast, Poa pratensis only responded significantly to the N addition treatment, with greater biomass production and higher percent tissue nitrogen compared to in the ambient N plots. The soils responded significantly to both treatments, with increased 15N retention in the warmed plots and increased 15N retention in the N addition plots. Notably, plot age did not play an important role in modulating the plant and soil 15N retention responses, which possibly could be explained by the two grass species remaining dominant in all of the treatment plots after 16 years (i.e. a lack of a shift in community composition or the relative abundance of plant species). These results demonstrate that in the absence of a shift in plant species composition, the effects of increased N deposition and warming on N deposition may not change substantially over the longer term.
