Relationships Between Arbuscular Mycorrhizal Symbiosis and Soil Fertility Factors in Citrus Orchards Along an Altitudinal Gradient

Arbuscular mycorrhizal (AM) symbionts are able to greatly affect soil fertility. However, the relationships between AM symbiosis development levels and citrus mycorrhizosphere soil fertility remain weakly known in field. In our study, AM colonization, spore density, hyphal length density, and glomalin-related soil protein (GRSP) content in citrus (Robertson naval orange grafted on Citrus reticulata Blanco) orchards along an altitudinal gradient were investigated seasonally in southern China. The results showed that AM colonization and abundances of spore and hyphae fluctuated significantly in different seasons and altitudes. The highest AM colonization (83.03%) was observed in orchards at 200 m above sea level in summer, spore density (16.8 spores g−1 soil) in orchards at 400 m in autumn, and hyphal length density (2.36 m g−1 soil) in orchards at 600 m orchards in summer; while the lowest values (43.60%, 2.7 spores g−1 soil and 0.52 m g−1 soil of AM colonization, spore density, and hyphal length density, respectively) were all observed in orchards at 800 m in winter. Correlation analyses demonstrated that the soil properties such as soil organic matter, alkali-hydrolyzable N, available P, and pH were significantly (P < 0.05) positively correlated with either citrus total AM colonization or the abundances of spore and hyphae. GRSP was significantly (P < 0.05) positively correlated with soil organic matter and pH. Redundancy analysis supported that soil environmental factors such as altitude, GRSP, soil organic matter, and alkali-hydrolyzable N severely (Monte Carlo permutation tests, P = 0.002) influenced AM colonization and abundances of spore and hyphae in citrus orchards. Our data demonstrated that soil environmental factors are vital in determining AM symbiosis development in citrus orchards.