Nitrogen cycling and water pulses in semiarid grasslands: Are microbial and plant processes temporarily asynchronous?

Authors: DIJKSTRA, FEIKE - University Of Sydney; Augustine, David; BREWER, PAUL - Colorado State University; VON FISCHER, JOE - Colorado State University

Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2012
Publication Date: 5/10/2012
Citation: Dijkstra, F., Augustine, D.J., Brewer, P., Von Fischer, J. 2012. Nitrogen cycling and water pulses in semiarid grasslands: Are microbial and plant processes temporarily asynchronous?. Oecologia. 170:799-808.

Interpretive Summary: Following a thunderstorm in dry rangelands, plants and soil microbial organisms can respond in different ways over time. Mismatches in the timing of plant versus microbial activity can lead to losses of nitrogen, an important soil nutrient, in these rangelands. We studied whether the uptake and transformation of nitrogen by microbes corresponded to the timing of plant uptake of nitrogen from the soil. Specifically, we used labelling of the soil with an isotope of nitrogen to compare plant and microbial responses to simulated thunderstorms equivalent to 0.4 inches and 0.8 inches of rain. The processing of nitrogen by microbes increase 1-3 days after watering, with the largest response after 0.8 inches of rain. In contrast, plant uptake of nitrogen increased more after the 0.4 than after the 0.8 inch pulse of rain. Both microbial and plant responses went back to baseline levels within 10 days, indicating that both microbial and plant responses were short-lived. Thus, microbial and plant response largely occurred over the same time period following a simulated thunderstorm in this dry rangeland. However, the size of the storm substantially influenced whether plants or microbes were more effective in getting the nitrogen from the soil. Furthermore, losses of nitrogen increased after both small and large storms, indicating that changes in storm sizes with future climate change could cause increased nitrogen losses from these rangelands.

Technical Abstract: Precipitation pulses in arid ecosystems can lead to temporal asynchrony in microbial and plant processing of nitrogen (N) during drying/wetting cycles causing increased N loss. In contrast, more consistent availability of soil moisture in mesic ecosystems can synchronize microbial and plant processes during the growing season, thus minimizing N loss. We tested whether microbial N cycling is asynchronous with plant N uptake in a semiarid grassland. Using 15N tracers, we compared rates of N cycling by microbes and N uptake by plants after water pulses of 1 and 2 cm to rates in control plots without a water pulse. Microbial N immobilization, gross N mineralization and nitrification dramatically increased 1-3 days after the water pulses with greatest responses after the 2 cm pulse. In contrast, plant N uptake increased more after the 1 than after the 2 cm pulse. Both microbial and plant responses reverted to control levels within 10 days, indicating that both microbial and plant responses were short-lived. Thus, microbial and plant processes were temporally synchronous following a water pulse in this semiarid grassland, but the magnitude of the pulse substantially influenced whether plants or microbes were more effective in acquiring N. Furthermore, N loss increased after both small and large water pulses (as shown by a decrease in total 15N recovery), indicating that changes in precipitation event sizes with future climate change could exacerbate N losses from semiarid ecosystems.