Changes in Soil Structure Resulting From Elevated Carbon Dioxide and Nitrogen Levels in a Pasture System.

Research on soil responses to global change has emphasized short-term dynamics, such as nutrient cycling, shifts in microbial communities, and root growth. The effects of global change on soil dynamics that change more slowly, like soil structure and related hydraulic properties, are virtually unknown. The objective of this work was to assess changes in soil pore structure and water retention in response to multi-year manipulations of atmospheric CO₂ concentration and nitrogen deposition. We collected soil from an experiment conducted at the USDA-ARS National Soil Dynamics Laboratory in Alabama. In this experiment, Bahia grass was grown on a loamy sand (Blanton series) for 6 years under ambient or elevated CO₂ (550 ppm), and at low or moderate levels of fertilization. Aggregates were sampled from surficial (0-10 cm) and deeper (10-20 cm) soils, and analyzed for the fractal dimension of mass (a measure of soil pore structure), as well as their organic carbon and nitrogen content. Water retention was measured from soil cores using the evaporation method. Fertilization strongly reduced fractal dimension under elevated CO₂, whereas fertilization did not affect fractal dimension under ambient CO₂. This pattern was observed in both surficial and deeper soils, but was more pronounced at the surface. Statistical separation for soil C, N, and water retention was dependent on depth and the treatments considered, but was generally most extreme with elevated CO₂ plus fertilizer addition. These results are consistent with elevated CO₂ and fertilization stimulating root growth and microbial activity, such that they generated more and larger soil pores than under more limiting conditions. As atmospheric CO₂ levels rise into the future, soils that are fertilized, whether intentionally or not, may have increasingly altered structure and hydraulic properties. Defining the bounds of such changes may be critical to managing the effects of global change on soils.