Soil genomics and the nitrogen cycle across an elevation gradient in Northern Arizona

My group has been studying microbial communities and the nitrogen cycle in soils along a gradient in northern Arizona which spans 3000 m in elevation and includes four life zones.  Mean annual temperatures and precipitation range along this gradient from 3 to 15 °C and 100 to 700 mm respectively.  We have found that ammonia oxidizing archaea (AOA) are more abundant in these soils than ammonia oxidizing bacteria (AOB) and that the abundance AOB, but not AOA, correlated significantly with ammonium pool size and precipitation.  The ¹⁵N/¹⁴N  of DNA declined with higher elevation, reflecting soil ¹⁵N composition and indicating more N is retained at higher elevation sites.  We compared our results with analyses of soil DNA extracted from soils along an Hawaiian chronosequence where soil age ranges from 300 to 4,100,000 years.  Here only ammonia oxidizing archaeal genes could be detected.  The abundance of ammonia oxidizing archaea along both gradients was highly correlated with the δ¹⁵N of DNA suggesting that in many soils ammonia oxidizers control nitrogen retention and consequently ¹⁵N enrichment of ecosystems.