Soil microbes compete strongly with plants for soil inorganic and amino acid nitrogen in a semiarid grassland exposed to elevated CO2 and warming

Authors: CHEN, JANET - University Of Wyoming; CARRILLO, YOLIMA - Western Sydney University; PENDALL, ELISE - Western Sydney University; DIJKSTRA, FEIKE - University Of Sydney; EVANS, DAVE - Washington State University; MORGAN, JACK - Retired ARS Employee; WILLIAMS, DAVE - University Of Wyoming

Submitted to: Ecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2015
Publication Date: 4/2/2015
Citation: Chen, J., Carrillo, Y., Pendall, E., Dijkstra, F., Evans, D.R., Morgan, J., Williams, D. 2015. Soil microbes compete strongly with plants for soil inorganic and amino acid nitrogen in a semiarid grassland exposed to elevated CO2 and warming. Ecosystems. doi: 10.1007/s10021-015-9868-7.

Interpretive Summary: Rising concentrations of the greenhouse gas carbon dioxide (CO2) are important not only for their effect on climate change, but also through their direct impact on ecosystems via a generally positive effect on plant growth. In order to understand how CO2 plus climate change are affecting native grasslands, the Prairie Heating and CO2 Enrichment Experiment was developed to warm and artificially enrich air with CO2 in experimental plots of native mixed-grass prairie at the High Plains Grasslands Research Station near Cheyenne, Wyoming, USA. These artificial manipulations simulate the expected thermal and CO2 environments of the second half of this century. This study, and a supporting experiment conducted in growth chambers wherein light, CO2 and temperature could be controlled and plant nutrition more carefully controlled than in a field experiment, focused on the impact of high ambient CO2 and temperature on the soil presence and assimilation of N compounds by plants and soil microorganisms. In general, CO2 and temperature had moderate to small effects on soil N forms and plant/microbe uptake. While native plants had the capacity to assimilate both organic and inorganic forms of soil N, soil microorganisms outcompeted a native grass for N. The results of this and other work suggest that the N demands of native plants are likely met via the release of N by soil microorganisms and generally low rates of N uptake by plants. The results also suggest that short-term consequences of rising CO2 and temperature on plant/soil N cycling in this grassland are modest. Long-term consequences of higher temperatures and CO2 on ecosystem N cycling are unknown.

Technical Abstract: Free amino acids (FAAs) in soil are an important N source for plants, and abundances are predicted to shift under altered climate conditions such as elevated atmospheric CO2. Composition, plant uptake capacity and plant and microbial use of FAAs relative to inorganic N forms were investigated in a temperate semiarid grassland exposed to experimental warming and free-air CO2 enrichment. FAA uptake by two dominant grassland plants, Bouteloua gracilis and Artemesia frigida, was determined in hydroponic culture. Bouteloua gracilis and microbial N preference was then investigated in experimental field plots using isotopically labeled FAA and inorganic N sources. Alanine and phenylalanine had the highest concentrations in the field, and B. gracilis and A. frigida rapidly consumed these FAAs in hydroponic experiments. However, B. gracilis assimilated little isotopically labeled alanine, ammonium and nitrate in the field. Rather, soil microbes immobilized the majority of all three N forms. Elevated CO2 and warming did not affect plant or microbial uptake. FAAs are not direct sources of N for B. gracilis, and soil microbes outcompete this grass for organic and inorganic N when N is at peak demand within temperate semiarid grasslands.