Abundance of arbuscular mycorrhizal fungi in the rhizosphere of healthy and declining citrus in East Nusa Tenggara, Indonesia

Citrus is an important commercial crop in Timor Tengah Selatan, East Nusa Tenggara, Indonesia, but many trees are suffering a decline in health. As citrus is heavily dependent on arbuscular mycorrhizal fungi (AMF), this study investigated the abundance of AMF in the rhizosphere of healthy and declining trees in citrus orchards at 12 geographical locations. In each orchard, 6 soil and 6 root samples representing 3 healthy and 3 declining trees were collected. The soil was analyzed for AMF spore abundance as well as physical (texture) and chemical properties (organic carbon, nitrogen, phosphorus, potassium, pH, and soil exchangeable capacity), while the fine roots of citrus were assessed for colonization. The data were analyzed using analysis of variance (ANOVA) where the health condition of the trees was under the geographical location/site factor. The results showed that the abundance of AMF spores was significantly affected by the geographical location from where the soils were collected, but the health condition of the trees had no effect. However, AMF colonization was significantly affected by both site and tree health. The number of AMF morphotypes tended to be higher under healthy trees than under declining trees. Soil analysis indicated that soil fertility (N and organic C) may be important for tree health. These results provided a new perspective on the possible involvement of AMF and soil nutrients in citrus decline. Further studies are required to define the interactions between AMF, soil fertility