Carbon, nitrogen, and phosphorus dynamics across a three million year substrate age gradient in northern Arizona, United States

Within a given climatic regime, soil properties exert a strong control over the cycling and storage of carbon (C), nitrogen (N), and phosphorus (P) - three elements that are essential to all forms of life. Current ecosystem theory suggests that C and N accumulate rapidly during early to mid-stages of soil development, but progressive P limitation to plant productivity at later stages leads to an overall reduction in both the storage and cycling rates of ecosystem C and N. This model is based on results from humid ecosystems, and may not apply to more arid systems where limited water availability and patchy vegetation distribution could significantly alter patterns of C, N, and P dynamics during ecosystem development. To better understand the mechanisms controlling ecosystem processes during soil development in semiarid ecosystems, I established a long substrate age gradient consisting of four sites dominated by semiarid pinon-juniper woodlands within the San Francisco volcanic field in northern Arizona, USA. These sites differ markedly in the age of their underlying substrate from 930 to 55,000, 750,000, and 3,000,000 years old. I found that the various fractions of soil P behaved largely as predicted; total soil P declined consistently with substrate age, as did the fraction of P held in primary mineral form, while organic phosphorus increased. Although the rates of change in P fractions are much slower along this semiarid substrate age gradient, the overall pattern remains consistent with more humid ecosystems. Soil organic C and N storage increased with substrate age up to the 750,000 y site and then declined to the oldest site, as did various measures of soil C and N transformation rates. In addition, turnover estimates of slowly cycling C pools suggests that the decline in C and N storage at the oldest site is most likely due to reduced productivity, possibly due to increased P limitation, and not to an increase in organic matter decomposition rates. These results suggest that biogeochemical theories of C, N, and P dynamics derived from humid environments are applicable to ecosystem development under more arid conditions.