Nitrogen loading affects free amino acid concentrations differently than inorganic N in temperate forest soils along a gradient of nitrogen deposition

We examined labile soil N pools from two oak/maple dominated forest sites along a gradient of nitrogen (N) deposition around the Chicagoland area. The western site, in Cook County, IL (CCI), receives roughly 20% less N deposition than the eastern site in Porter, IN (PI). We established two 10 x 10 m control and two supplemental N plots at each site. For the last five years, the supplemented plots have received monthly applications to raise the N levels three times the natural deposition rates. We measured soil concentration of seventeen free amino acids (AAs), nitrate and ammonium in 0-10 and 10-20 cm depth soil. N concentration of the total AAs were significantly higher than inorganic N concentrations at either site or soil depth. The top five AAs often accounted for more than 60% of the total AAs. Glycine concentration was the sixth lowest at all sites and depths. There were some notable shifts in predominant forms of AAs in response to N addition. N loading also resulted in roughly 40 and 20% increases in total AAs and nitrate in the top 10 cm of soils at the CCI forest site.  At this site, all N forms increased significantly in response to N addition in 10-20 cm depth. The PI site, however, showed no significant change in the top soil concentration of all N forms. At this site, total AAs decreased by 44% in 10-20 cm depth. We believe that the PI forest site had already reached N saturation stage thus it did not retain added N.