PARENT SESSION
Oral Session # 19: Biogeochemistry II: Grasslands.
Presiding: JT Lennon
Tuesday, August 5. 8:00 AM to 11:30 AM, SITCC Meeting Room 105.
Nitrogen losses and cycling affected by fire and vegetation in an oak savanna.
Dijkstra, Feike*,1, Hobbie, Sarah1, Reich, Peter1, 1 University of Minnesota, Saint Paul, Minnesota, USA
ABSTRACT- Long-term prescribed fires have increased woody canopy openness and reduced nitrogen (N) cycling (i.e. net N mineralization) in an oak savanna in Minnesota, USA. It is unclear if the reduced N cycling is caused by greater N losses with fire that reduce mineralizable N pools or by the conversion in vegetation from oak to C4 grass dominance, which reduces N mineralization per gram of soil N. We established oak-dominated plots in unburned sites, and oak- and grass-dominated plots in frequently burned sites (burned 2 out of 3 years on average during the last 40 years) that allowed us to make contrasts across fire history and between vegetation types. We measured in situ and potential net N mineralization, total soil N, and N losses through volatilization and leaching. In frequently burned sites, we also measured in situ and potential net N mineralization and total soil N underneath large single oaks or cluster of oaks that died 5, 10, or 15 years ago and that are now dominated by C4 grasses. Net N mineralization in the oak-dominated unburned plots were two times higher than in oak-dominated burned plots, which in turn were four times higher than in the grass-dominated burned plots. Nitrogen losses in the frequently burned plots were much higher than in the unburned plots and mainly occurred through volatilization that most likely reduced total soil N and thus net N mineralization in the frequently burned plots. Nitrogen losses were not different between the oak- and grass-dominated burned plots. Therefore, the conversion from oak to grass dominance must have further reduced net N mineralization in the burned plots. Net N mineralization in soils near dead trees, irrespective of the time of death, was as low as in the grass-dominated plots. The rapid reduction in net N mineralization after tree death suggests that this may be caused by the disappearance of living trees rather than by the increased abundance of grasses. Our results show that long-term prescribed fires directly reduce N cycling because of increased N losses and that the reduction in N cycling is exacerbated by a change in vegetation from oak trees to C4 grasses.
Key words: plant species effects, vegetation change, prescribed burning, nutrient cycling