80
Bromus may affect N fi xation, red istribut ion of N in soil, leaching losses of N, and
denitrifi cation and could thereby infl uence longer-term trends of soil N where it has
become dominant. High rates of N mineralization and nitrifi cation relative to rates of
N immobilization in mesic ecosystems typically lead to increased N loss through
leaching or trace N gas emissions (Stark and Hart 1997 ). However, this pattern may
not occur in semiarid and arid ecosystems because high nitrifi cation could conserve
N by depleting NH 4 + , which is otherwise readily lost as NH 3 in high pH soils. Also,
while nitrifi cation is normally a key step toward denitrifying losses of NO, N 2 O, or
N 2 gases, denitrifi cation is suppressed by the low C and water availability in semiarid
soils (Smart et al. 1999 ). Moreover, precipitation is scarce enough in the area
Fig. 3.7 Seasonal changes in soil nitrate concentrations in the surface soils (0–10-cm layer)
beneath Bromus tectorum , Agropyron desertorum , and Artemisia tridentata ssp. wyomingensis
vegetation in Rush Valley, UT. Values represent means and SE ( n = 4) for each vegetation type
(compiled from Stark et al. 2006 ; Norton et al. 2012 ; Morris et al. accepted )
M.J. Germino et al.