Effects of water additions, chemical amendments, and plants on in situ measures of nutrient bioavailability in calcareous soils of southeastern Utah, USA

Abstract

We used ion-exchange resin bags to investigate effects of water additions, chemical amendments, and plant presence on in situ measures of nutrient bioavailability in conjunction with a study examining soil controls of ecosystem invasion by the exotic annual grass Bromus tectorum L. At five dryland sites in southeastern Utah, USA, resin bags were buried in experimental plots randomly assigned to combinations of two watering treatments (wet and dry), four chemical-amendment treatments (KCl, MgO, CaO, and no amendment), and four plant treatments (B. tectorum alone, the perennial bunchgrass Stipa hymenoides R. & S. alone, B. tectorum and S. hymenoides together, and no plants). Resin bags were initially buried in September 1997; replaced in January, April, and June 1998; and removed at the end of the study in October 1998. When averaged across watering treatments, plots receiving KCl applications had lower resin-bag NO3− than plots receiving no chemical amendments during three of four measurement periods—probably due to NO3− displacement from resin bags by Cl− ions. During the January–April period, KCl application in wet plots (but not dry plots) decreased resin-bag NH4+ and increased resin-bag NO3−. This interaction effect likely resulted from displacement of NH4+ from resins by K+ ions, followed by nitrification and enhanced NO3− capture by resin bags. In plots not receiving KCl applications, resin-bag NH4+ was higher in wet plots than in dry plots during the same period. During the January–April period, resin-bag measures for carbonate-related ions HPO42−, Ca2+, and Mn2+ tended to be greater in the presence of B. tectorum than in the absence of B. tectorum. This trend was evident only in wet plots where B. tectorum densities were much higher than in dry plots. We attribute this pattern to the mobilization of carbonate-associated ions by root exudates of B. tectorum. These findings indicate the importance of considering potential indirect effects of soil amendments performed in␣conjunction with resource-limitation studies, and they suggest the need for further research concerning nutrient acquisition mechanisms of B.␣tectorum.