Identifying the predictors of mycorrhizal response under multiple fertilization regimes

Arbuscular mycorrhizal fungi (AMF) play an important role in crop productivity, but their response to different fertilization regimes is variable. Understanding the mechanisms governing mycorrhizal responses under various fertilization regimes is key to maximizing the mycorrhizal contribution to ecosystem functioning. Indigenous AMF communities and soil were collected from long-term field experiments with various fertilization regimes. A greenhouse bioassay was carried out to identify the key regulators of the mycorrhizal response to inorganic and organic fertilization. Different fertilization regimes generated continuous gradients for most soil properties and a full spectrum of mycorrhizal phenotypes. Mycorrhizal growth response (MGR) in particular showed complex shifts from positive to neutral to negative, while mycorrhizal phosphorus (P) response was consistently positive and mycorrhizal nitrogen (N) response was consistently negative in fertilized treatments. Soil available P and available N to P (N:P) ratio appeared to be key soil predictors of the mycorrhizal response. Also, the nutrient status of non-mycorrhizal (NM) plants, namely shoot N concentration, shoot P concentration and shoot N:P ratio, were plant predictors of mycorrhizal response. A positive MGR occurred when NM plants were P-limited, and neutral and negative MGR occurred when NM plants were N-limited or co-limited by N and P. Further, the thresholds of soil available P (c. 13 mg kg-1), NM shoot N concentration (c. 14 g kg-1), NM shoot P concentration (c. 0.7 g kg-1) and NM shoot N:P ratio (c. 18), respectively, were estimated to predict mycorrhizal response. We conclude that the relative availability of N and P in soil and NM plants are good predictors of mycorrhizal response in different fertilization regimes. The existing nutrient thresholds combining soil and plant allowed more accurate prediction of mycorrhizal response and this has important implications for optimizing the mycorrhizal association in some important crops by managing soil nutrients.