Daily and seasonal changes in soil amino acid composition in a semiarid grassland exposed to elevated CO2 and warming

Soil amino acids are often an important source of nitrogen (N) for plants, and anticipated global changes, including climate warming and rising atmospheric CO2 levels, have the potential to alter plant and microbial production and consumption of this N source in soils. We determined soil amino acid composition over a 1-year period at diurnal and seasonal time scales in a multi-factor global change experiment with elevated CO2 and warming in native semiarid grassland. Soil amino acids were collected in April, May and June of 2011 and April 2012 using a soil water perfusion and extraction method that minimized soil disturbance. This was a particular advantage when taking diurnal measurements. The extracts were analyzed by ultra performance liquid chromatography. We detected 16 different soil amino acids throughout the study, and glutamine/glutamate (glu-x), arginine, serine and asparagine/aspartate (asp-x) were consistently at highest relative concentrations, comprising 3-41, 6-20, 2-22 and 7-24 % of total amino acids, respectively. No direct effects of experimental warming or elevated CO2 on soil amino acid composition were observed. However, the relative abundance of individual soil amino acids shifted diurnally and seasonally with changes in soil temperature and soil moisture. Glu-x and arginine increased and serine decreased with higher temperature, while asp-x and serine increased and arginine decreased with higher moisture. Overall, the relative abundances of soil amino acids responded more strongly to both diurnal and seasonal changes in temperature and soil moisture than to elevated atmospheric CO2 and experimental warming.