Soil fluxes of CO2, CH4, and N2O after fertilization across a three million year old soil age gradient

Background/Question/Methods

The factors that control the flux rates of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) from soil are complex but are generally influenced by soil physical properties such as texture, soil nutrient availability, and soil water content/water potential. The three million year old semi-arid substrate age gradient of Arizona (SAGA) has a well-established gradient of soil texture, soil nutrients, and soil water holding capacity, while other factors controlling gas flux rates, such as parent material, climate, and vegetation, remain constant. The objective of this study was to determine the response of each gas to fertilization and how these responses may vary due to background soil nutrient concentrations and seasonal differences in soil temperature and water availability. We measured fluxes of CO₂, CH₄, and N₂O during four distinct seasons across the SAGA after six years of nitrogen (N), phosphorus (P), and N+P fertilization. We also measured pools of available N and P simultaneously with each flux measurement in each fertilization treatment, season, and site.

Results/Conclusions

We found that CO₂, CH₄, and N₂O consistently varied as a result of soil age and seasonal differences rather than in response to fertilizer addition. However, in certain situations, fertilizer significantly altered soil gas fluxes, though rarely in a consistent pattern. Preliminary results indicate that over all seasons and soil ages, CO₂ was significantly related to available P concentrations, water content, water potential, and temperature, yet was unrelated to N concentrations. There was no significant relationship between CH₄ and any soil nutrients or physical factors. Rates of N₂O flux were only significantly related to gravimetric water content. Soil gas fluxes at the SAGA exhibited similar idiosyncratic responses to fertilization seen in many ecosystems. Furthermore, these results 1) suggest that the effects of fertilization on gas fluxes did not vary due to background levels of nutrients, 2) support existing evidence that CH₄ uptake at the SAGA is controlled by factors other than those reported in more humid regions and 3) reflect the importance of seasonal variability on biogeochemical process rates in these semi-arid soils.