As a rule, applications of P fertilizers reduce mycorrhizal
colonization of plant roots (Jackson et al. 2002;
Ryan et al. 2008; Marschner 2012) although at extremely
P-deficient conditions, low levels of P application
may also have a positive impact (Bolan et al. 1984).
The effects of Zn additions to soil on mycorrhizal colonization
of roots can be negative, positive or neutral,
depending on the conditions (Cavagnaro 2008). In
many studies dealing with non-toxic levels of Zn fertilizers,
the mycorrhizal colonization was unaffected by Zn treatments (McIlveen and Cole 1979; Ortas et al.
2002; Subramanian et al. 2009). Here, in agreement
with the literature, the mycorrhizal colonization in
non-autoclaved soil was drastically suppressed by
higher levels of P but unaffected by Zn fertilization
(Fig. 2), which was necessary for preventing Zn deficiency
(Fig. 1) and maximizing grain yield (Table 1). It
is also well-documented that sterilization of soil by
autoclaving eliminates native AM (Smith and Smith
1981; Endlweber and Scheu 2006). Therefore, any indirect
effects of P fertilization mediated by changes in
mycorrhizal activity should disappear in autoclaved soil.