New evidence that high potential nitrification rates occur in soils during dry seasons: Are microbial communities metabolically active during dry seasons?

Emerging research is challenging existing paradigms of nitrogen (N) cycling in arid and semiarid ecosystems that experience distinct seasonal patterns in precipitation. We measured equal or greater potential nitrification rates in the dry season than the wet season in chemically and physically distinct soils along a three million year substrate age gradient in Arizona. These surprising and counterintuitive results are supported by recently published work in California Mediterranean grasslands. Considered together, these studies call attention to the need to measure process rates during the dry, non-growing season of xeric ecosystems. Several mechanisms may be responsible for these patterns, but we highlight the importance of understanding the contribution of archaeal ammonia oxidizers to nitrification in these seasonally dry ecosystems.

► We measured potential nitrification rates on a 3 million year substrate age gradient.
► Potential nitrification rates were as high or higher in the dry season than in the wet season.
► We emphasize the potential importance of ammonia oxidizing archaea.
► These results indicate poor understanding of dry season biogeochemistry.