Does soil type affect soil nitrogen response to changing CO2 concentrations?

Background/Question/Methods

The response of terrestrial ecosystems to changing CO₂ concentrations is of great interest because of the uncertainty of how these systems will respond to this environmental change over long periods of time. Many of the current elevated CO₂ studies focus primarily on a single site, which makes it difficult to discern how other environmental conditions, such as soil type, may affect ecosystem processes. In this study we monitored soil nitrogen concentrations in grassland plots that were exposed to a continuous linear gradient of CO₂ concentration ( 250 to 500 ppm) since 2006 in Temple, TX. The CO₂ gradient was applied to plots from three soil series that differed considerably in texture and hydrology: two silty-clay soils, Austin (mollisol) and Houston (vertisol), and a sandy loam, Bastrop (alfisol). Each of the plots supported established perennial grassland vegetation representative of the native grassland plant communities. Soil nitrogen (NO₃, NH₄ and DON) concentrations were monitored monthly during the 2008 growing seasons by sampling soil water at 15 cm depth using suction lysimeters. Additional measurements, such as extracellular enzyme activity, were taken from the different soil types at points along the CO₂ gradient.
Results/Conclusions

Initial measurement of inorganic nitrogen concentrations from shallow lysimeter samples did not vary with CO₂ concentration during the 2008 growing season. There were differences in inorganic nitrogen levels among the different soil types, with higher concentrations in the silty-clay soils. DON concentrations were greater in the sandy loam soil exposed to high CO₂ concentrations relative to those exposed to subambient CO₂. This response of DON concentrations to CO₂ was not present in the silty-clay soils, indicating that there are distinct differences in nitrogen processes among these soils. Additionally we saw great extracellular enzyme activity, such as aminopeptidase, in one of the silty-clay soils (Austin series) when compared to the sandy loam (Bastrop). These differences in nitrogen processes among the soil types may help explain why the response of other ecosystem processes, such as aboveground net primary productivity, to the CO₂ gradient varies with soil type. The variation among soils in texture can be as important as atmospheric CO₂ concentration in determining N availability and carbon cycling responses in grassland.