4.4 AM fungi and soil properties
P input alone reduced mycorrhizal root colonisation at the plant
community level. Furthermore, P input alone and N + P addition together
suppressed AM fungal colonisation. Our results are in accordance with
other field studies, in which high soil P availability usually decreased
mycorrhizal root colonisation and the extra-radical biomass of AM fungi
(Avolio et al., 2014; G. Yang et al., 2016; G. W. Yang et al., 2014).
The potential mechanism underlying these observations may be that
relative high soil P availability generally decreases belowground C
allocation, directly reducing C supply for AM fungi (Kiers et al.,
2011).
Soil available P and inorganic N may alter the response of ANPP and
plant diversity to AM fungi (Johnson et al., 2015; G. Yang et al.,
2016). Our results indicated that both N and P inputs effectively
enhanced the amount of soil available P and inorganic N regardless of
inter-annual precipitation fluctuation. Our results are accordance with
those found by Avolio et al. (2014) and Jiang et al. (2018). In our
study, soil available P was higher in P alone and N + P plots than that
in controls, and presumably reduced mycorrhizal root colonisation.
Changes in plant-available N observed in our study were less than the
changes in plant-available P, especially in 2019 (normal year). This may
be attributable to the nutrient absorption ability of C.
aristatum (a fast-growing annual forb), which can absorb
plant-available N very effectively, leading to soil N depletion
(Ehleringer, Schwinning, & Gebauer, 2000).
Fungicide (benomyl) addition in contrasting precipitation conditions
considerably suppressed mycorrhizal root colonisation. A growing number
of field studies indicated that fungicide (benomyl) input may promote
ANPP by enhancing soil available N (Chen & Edwards, 2001; G. W. Yang et
al., 2014). Nevertheless, the current study found that fungicide
(benomyl) addition did not affect soil inorganic N, which is in
agreement with a field study by Qiao et al. (2019).