Fine-scale heterogeneity in soil carbon dynamics as a result of woody encroachment into tallgrass prairie

Background/Question/Methods

As woody plants spread through the tallgrass prairie, we expect changes in the microbial processes that affect carbon storage in soil. An unanswered question is whether woody encroachment of prairie will ultimately result in more carbon leaving soils than when grasses dominate. Specifically, this research aims to address how (1) microbial nutrient demand, (2) soil nutrient status, and (3) the rate of soil C mineralization change with woody encroachment. Cornus drummondii is a clonal woody shrub that grows outward from a center point of establishment. This creates a gradient in which soils at the shrub's edge have experienced encroachment most recently while soils at the shrub's center have experienced encroachment the longest. Along this gradient, we measured microbial demand for, and the availability of, soil carbon, nitrogen, and phosphorus. Microbial nutrient demand was assessed via potential enzymatic activity assays of β-glucosidase, NAGase, leucine aminopeptidase, and phosphatase. We measured C mineralization rate and δ¹³C-CO₂ with short-term incubations of collected soils, and applied an isotopic mixing model to determine the proportion of CO₂ that was attributable to the breakdown of older grass-derived organic matter versus newer shrub-derived organic matter.

Results/Conclusions

Using distance from shrub center as a proxy for time, soil microbial demand for carbon increases and inorganic nitrogen availability decreases the longer soils have experienced woody encroachment. Additionally, soil C mineralization rate decrease with time since woody encroachment. No other soil properties or processes significantly changed along the spatial gradient within C. drummondii islands. However, there was a strong correlation between δ¹³C-CO₂ and soil C mineralization rate. When microbes were breaking down proportionally more shrub-derived organic matter, the soil C mineralization rate was higher. Taken together these results suggest that there is enhanced fine root turnover and C exudation closer to the edges of these clonal shrubs. More labile root-derived inputs would increase microbial activity and C mineralization rates. This would also explain greater microbial demand for C in the center of C. drummondii islands. Overall, we conclude that woody encroachment creates spatial heterogeneity in soil properties and microbial processes on finer scales than are normally measured. While we cannot determine the overall effects of woody encroachment on soil carbon balance from this study alone, our results indicate that soils emit more CO₂ when microbes break down shrub-derived organic matter.