PARENT SESSION
Contributed Oral Session 60: Biogeochemistry: Soil Nutrient Dynamics; Decomposition
Tuesday, August 9, 1:30 PM - 5:00 PM, Meeting Room 516 D, Level 5, Palais des congrès de Montréal

Effects of Soil C and N dynamics at a fragmented afforested site in a previously herbaceous field.

Gaydess, Elizabeth^*,1, Billings, Sharon¹, ¹ University of Kansas, Lawrence, KS, USA

ABSTRACT- Fragmented woody plant encroachment is occurring in many grasslands. This area was once dominated by tallgrass prairie. Land management changes, such as fire suppression and limited grazing, have helped to facilitate species composition change from herbaceous to woody plants in many regions. Assessment of soil organic matter transformations in the forests of different sizes is needed to understand the impacts of fragmented afforestation on C and N dynamics. The University of Kansas has a fragmented afforestation site that has three replicated treatments, 4x8m, 12x24m, and 50x100m, separated by a grassy interstitial area. Several soil C and N parameters have been measured in the large vs. the small plot treatments. SOM inputs have lower C:N ratios in the small treatments, indicating higher quality litter material being incorporated into the small plots. In soil incubations cumulative N mineralization in the large patches (23.02g/g) was only 47% of that which was mineralized in the small patches (48.95g/g). Moreover, higher gross rates of nitrification were exhibited in soils from the small plots compared to the large plots. Soils from both treatments were fractionated by sizes (212-2000m, 63-212m, and <63m) and analyzed for %C and %N, C:N ratio, and 13C and 15N. The most pronounced trends were seen in the 212-2000m fraction. The small treatments were more depleted for 13C (-24.9±0.05 ‰) v. large treatments (-23.1± 0.7‰), suggesting greater incorporation of C3 forest vegetation to these formerly C4 supporting soils in small patches. Consistent with this, the small plots had a higher %C and %N in the 212-2000m v. this fraction in the large plots (3.72±0.15% vs. 2.06±0.43% and 0.26±0.02% vs. 0.16±0.03%, respectively). The small patches' 212-2000m fraction likely has more C and N because newer litter inputs are being relatively quickly incorporated into SOM.

Key words: afforested grassland, N mineralization, litter quality, soil size fractionation