Author(s): Yajun Hu 1 , Matthias C. Rillig 2 , 3 , Dan Xiang 1 , Zhipeng Hao 1 , Baodong Chen 1 , *
Arbuscular mycorrhizal (AM) fungi are ubiquitous symbionts of higher plants and important components of most terrestrial ecosystems . AM fungi can efficiently take up mineral nutrients, especially phosphate, from soil, and then deliver them to the host plant; in exchange, 4-20% of plant photosynthates are directly transferred from plant to AM fungi to support development of the symbiosis . Therefore, these symbiotic soil fungi are recognized as critical links between the above- and belowground parts of ecosystems .
The occurrence of the AM symbiosis is influenced not only by host plant species, but also by the environmental factors, such as temperature , pH , , and soil fertility . The influences of particular environmental factor on AM fungi have been extensively examined by quantifying root colonization, hyphal length density or spore density in soil along environmental gradients at different scales, including aridity , salinity , nutrient , land degradation  and temperature  gradients. For instance, Heinemeyer and Fitter (2004) demonstrated that higher temperature could stimulate colonization of AM fungi on host plants and development of extraradical mycelium . As commonly recognized, AM fungi facilitate plant acquisition of limited soil resources; AM plants would therefore be more abundant in ecosystems with less available soil nutrients . On the other hand, soil pH was reported to be a key factor influencing the abundance and distribution of soil fungi and bacteria . Positive correlation between soil pH and root colonization by AM fungi was recorded in both acid and alkaline soils under low available phosphate conditions , -. However, despite of established relationships between AM fungal occurrence and particular environmental factors, few studies to date have examined the effects of multiple environmental factors on AM fungal abundance in a natural ecosystem.
AM fungi promote the growth of host plants, by means of providing mineral nutrients and water ,  and up-regulating photosynthesis . In this way, AM fungi are also involved in soil carbon cycling, as plant photosynthates are the original source for soil carbon pools, and better plant growth would subsequently lead to more carbon input into soil ecosystem. Moreover, extraradical mycelium (ERM) of AM fungi and their products (including glomalin-related soil protein, GRSP) could potentially stimulate soil aggregation , while the soil aggregates would provide protection for organic carbon from rapid degradation by microbes . Several studies revealed that soil warming could promote ERM development , . AM fungi are therefore supposed to increase carbon inputs to soil carbon pool under climatic warming. However, Rillig et al.  reported that GRSP content and soil aggregate stability decreased under higher temperature, despite of increasing ERM abundance, which would potentially lead to a reduction in soil carbon sequestration. Obviously, more systematic investigation is required to reveal the relationship between AM fungal abundance and soil carbon sequestration along environmental gradients.
In the present study, the arid and...