PARENT SESSION
Poster Session # 13: Biogeochemistry, Photosynthesis, and Respiration.

Tuesday, August 5 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

Changes in soil organic carbon cycling as juniper forests invade prairie: A ¹³C isotope approach.

Smith, Dixie¹, Fagan, Roxane^*,2, Johnson, Loretta², ¹ Pittsburg State University, Pittsburg, Kansas, USA² Kansas State University, Manhattan, Kansas, USA

ABSTRACT- Woody plant expansion into grasslands has been occurring across the Great Plains. The purpose of this study was to investigate changes in ¹³C dynamics in bulk soils, soil particle size-fractions, and in CO₂ from soils collected from 40-60 year old closed-canopy juniper (Juniperus virginiana) forest and paired C₄ grasslands. We hypothesized that a lifeform shift to juniper will alter quality, quantity, and distribution of SOC. We quantified the vegetation origin and cycling of new forest C and the vegetation substrate for microbial respiration by analyzing the ¹³C of soils and respired CO₂. ¹³C of SOC in forest soils indicates a large input of juniper-derived C to 10cm. At 0-2.5cm, 38% of SOC of the bulk soil is juniper in origin. In contrast, the majority of SOC in bulk soil below 10cm is prairie-derived, residual carbon. Forest C is present in all size fractions but prevails in the >212micron fraction. In grassland soils, ¹³C of respired CO₂ generally reflects the isotopic composition of bulk soil. In contrast, the ¹³C of respired CO₂ from 0-10cm depth in forest soils initially reflected the microbial utilization of new juniper humus (-25‰) from bulk soil. Thus, microbes preferentially utilized recent, C₃-derived juniper SOC. In summary, ¹³C data from bulk soils, particle size fractions, and microbial incubations confirm that fundamental vegetation shifts are altering the quality, and distribution of SOC.

Key words: woody plant encroachment, tallgrass prairie, soil organic carbon, ¹³C