Understanding effects of elevated atmospheric CO2 and climate warming on plant nitrogen availability is important for predicting plant and ecosystem productivity responses to future global changes. Soil amino acids can be present in concentrations comparable to those of inorganic nitrogen and are a potential plant nitrogen source, especially when N availability is low. Production and consumption rates of soil amino acids are known to shift under elevated atmospheric CO2 and warming and likely result in shifts in pools of soil amino acids as well. For my dissertation I characterized soil amino acid composition and plant uptake in plots of the Prairie Heating and CO2 Enrichment (PHACE) experiment in a semiarid grassland exposed to elevated atmospheric CO2 and warming to determine the role of amino acids as a plant nitrogen source. To determine the role of amino acids as a plant nitrogen source, soil amino acid composition was first characterized using a classic method of extraction, involving field collection of soils and laboratory processing, and a novel field method of extraction with minimal soil disturbance. Soil extracts collected with the novel method more closely represent soil pore water and contained glutamate, glutamine and arginine in the highest relative amounts. In contrast, extracts collected with the classic method of extraction represent the entire soil amino acid content, including that from microbial lysis and soil aggregate breakdown, and contained alanine and phenylalanine in the highest concentrations. No significant effect of elevated CO2 or warming on soil amino acid composition was found. However, soil amino acid composition shifted over diurnal and seasonal timescales and these changes correlated with soil moisture and temperature. To determine if plants are capable of consuming amino acids present in soil, alanine and phenylalanine were then used in an uptake capacity experiment using two plant species common at the PHACE field site, Bouteloua gracilis and Artemesia frigida. Hydroponically cultured seedlings were fed alanine or phenylalanine as a sole nitrogen source. Within 30 minutes of feeding, all plants consumed over 99% of fed amino acids, demonstrating high plant uptake capacity. No effect of elevated CO2 was observed. After identifying soil amino acids present in the semiarid grassland and plant species capable of amino acid uptake, I performed a nitrogen preference and microbial competition study in field using B. gracilis to determine if plants use amino acids as a nitrogen source in the semiarid grassland. Plants were fed 15N (and 13C) labeled ammonium, nitrate and alanine in a nitrogen uptake and preference experiment in PHACE plots. After a 3 h feeding, little to no 15N or 13C was recovered in plant biomass regardless of the labeled nitrogen form. Although elevated CO2 and warming treatments separately increased overall plant nitrogen uptake compared to ambient and elevated CO2 plus warming treatments, increased uptake was minimal compared to microbial consumption. The majority (40-70%) of all fed 15N and 13C was observed in microbial biomass. High microbial sequestration of nitrate, ammonium and alanine and low plant uptake suggests that plants do not take up soil amino acids in the semiarid grassland due to strong microbial competition. Nitrogen preference, elevated CO2 and warming did not have an effect on microbial uptake. Although amino acids are present in soils and plants have the physiological capacity to consume this nitrogen source rapidly in hydroponic culture, B. gracilis plants in PHACE plots did not use soil amino acids as a direct nitrogen source. Furthermore, B. gracilis had very low inorganic nitrogen uptake in the field after our short-term feeding experiment indicating weak short-term competition for nitrogen. Rapid microbial sequestration of alanine as well as inorganic nitrogen suggests that strong microbial nitrogen competition is the main cause for reduced plant nitrogen uptake in field versus hydroponic culture. Grassland plant nitrogen demands are therefore likely met through low but consistent uptake and storage of inorganic or organic nitrogen released by microbes maintained over longer time scales not investigated in my dissertation.
Author : Janet Chen Release : 2013 Publisher : ISBN : 9781303444623 File Size : 22.79 MB Format : PDF, Docs Download : 821 Read : 476