Another study used GeoChip 2.0 to characterize microbial communities along an oil contaminant gradient and found a decreased number of functional genes as the contaminant levels increased although genes involved in the degradation of biphenyl, catechol, and protocatechuate increased in the more contaminated samples (Liang et al., 2009a) . The most important environmental factors in determining the microbial community structure were oil concentration and soil available nitrogen. Liang et al. (2011) collected contaminated and uncontaminated soils from five oil fields across China in order to determine whether oil contamination or geographic location played a larger role in determining the microbial community structure. Results from GeoChip 2.0 indicated that communities from uncontaminated sites had higher functional gene diversity than those from contaminated sites in the same geographical area. Overall, the microbial communities clustered based on geographic location; however, when only organic contaminant degradation genes were examined, the contaminated samples clustered together. Geographic location was able to explain ∼33% of the microbial community variation observed, oil explained ∼10% of the variation, and soil geochemistry explained another 12%, while the remainder (∼41%) was unexplained.