process or the other depending on their physiological restrictions and mode of dispersion. Microbial dispersal mechanisms include wind, water, and hitchhiking on more mobile macrobes. Microbial dispersion is difficult to study, and little is known about its effect on microbial community assembly relative to the effect of abiotic and biotic assembly mechanisms, particularly in roots. For this reason only assembly mechanisms that fit within the niche hypothesis are discussed below. The taxa within root microbial communities seem to be drawn from the surrounding soil, though the relative abundance of various taxa may differ greatly from those found in bulk soil due to unique niches in the root and rhizosphere. Biotic assembly mechanisms Different parts of the root are associated with different microbial communities. For example, fine roots, root tips, and the main root are all associated with different communities, and the rhizosphere, root surface, and root tissue are all associated with different communities, likely due to the unique chemistry and nutrient status of each of these regions. Additionally different plant species, and even different cultivars, harbor different microbial communities, probably due to host specific immune responses and differences in carbon root exudates. Host age affects root microbial community composition, likely for similar reasons as host identity. The identity of neighboring vegetation has also been shown to impact a host plant's root microbial community composition. Abiotic assembly mechanisms Abiotic mechanisms also affect root microbial community assembly because individual taxa have different optima along various environmental gradients, such as nutrient concentrations, pH, moisture, temperature, etc. In addition to chemical and climatic factors, soil structure and disturbance impact root biotic assembly. Succession The root microbiome is dynamic, fluid within the constraints imposed by the biotic and abiotic environment. As in macroecological systems, the historical trajectory of the microbiotic community may partially determine the present and future community. Due to antagonistic and mutualistic interactions between microbial taxa, the taxa colonizing a root at any given moment could be expected to influence which new taxa are acquired, and therefore how the community responds to changes in the host or environment. While the effect of initial community on microbial succession has been studied in various environmental samples, human microbiome, and laboratory settings, it has yet to be studied in roots. See also Mangrove root microbiome References Soil biology Microbiology Microbiomes