Humans have altered ecosystems over millennia through their construction of settlements and wide-ranging agricultural practices. Numerous studies have shown that past land use is important in structuring modern ecological communities in terrestrial systems. But are all prehistoric activities equal in their long-term impacts, or are there some which “disappear” from the ecological canvas over time? Ecosystem processes in the arid US southwest are influenced by soil properties such as texture that control water availability to plant communities. Thus, prehistoric agricultural activities that alter soil texture may leave long-lasting ecological legacies. We explored variation in soil properties, nutrient cycling, and annual plant communities in a prehistoric agricultural settlement in the northern Phoenix basin, AZ that was occupied and farmed between 800-1200 CE. In particular, we compared seasonal soil properties and spring herbaceous plant production and composition in sites that span a range of prehistoric agricultural intensity, including sites that have little evidence of prehistoric human occupation or use (low intensity), fields used for dry farming with rock alignments (medium intensity), and irrigated agricultural fields (high intensity).
Prehistoric dry farming in this Sonoran Desert ecosystem left few ecological legacies in soils or plant communities. Soil nitrogen cycling varied across the year, with highest concentrations of inorganic nitrogen occurring in September, after the summer monsoon season. Annual herbaceous vegetation emerged after winter but not summer rains and was similar in growth and composition between control and dry farmed areas. In contrast, soil texture was significantly altered in prehistoric irrigated fields (silty) compared to non-farmed areas (more sandy), and it matched the fine soil textures that occurred in the dry farmed area. However, despite high water-holding capacity and soil organic matter content, irrigated silt fields contained lower concentrations of inorganic nitrogen, cycled nitrogen slower, and supported less annual plant growth than soils of similar texture in dry farmed sites. Lack of subsurface horizon development in built, irrigated fields may limit water availability for ecosystem processes, despite their fine soil texture. Alternatively, long-term prehistoric cultivation of maize may have depleted soil nutrients in this soil over time.