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).