FIRE AND TOPOGRAPHIC EFFECTS ON DECOMPOSITION RATES AND N DYNAMICS OFBURIED WOOD IN TALLGRASS PRAIRIE

Decay rates and N dynamics of wood in soils of annually burned and unb urned tallgrass prairie were measured over a 3-y period. Wooden dowels were placed at upland, mid-slope and lowland sites in two annually bu rned and two unburned watersheds. After 3 y, an average of only 15% of initial wood mass remained in burned watersheds, while 34% remained i n unburned watersheds. Topographic position also significantly affecte d decay rates, with dowels decaying faster in the shallow-soil, upland sites and slope sires than in the deep-soil, lowland sites. This patt ern is opposite of that generally observed for plant productivity (i.e . greater at lowland sites compared to uplands), and suggests that the controls of belowground decomposition and plant productivity are diss imilar. Dowels in both burned and unburned watersheds showed significa nt increases in N concentration over 3 y. Topographic position did not affect N concentration in the residual dowel material. Burn treatment , however, did affect N concentration, with dowels decomposing in burn ed watersheds having a higher average N concentration (0.5% after 3 y exposure) than dowels in unburned watersheds (0.43%). Relatively rapid decay rates resulted in net release of N, despite increased N concent ration in the residual material. Faster net N release on the annually burned watershed was due to faster mass loss, since there were no diff erences in the rate of increase in N concentration per unit mass lost. Surface soil temperatures on burned prairie following spring fire usu ally exceed those on unburned prairie. However, average monthly summer soil temperatures (May-August) at a 10 cm depth in burned and unburne d plots during the study were not statistically different and could no t explain decay rate differences. Additionally, one of our unburned wa tersheds was accidentally burned during the first year of the study. S urprisingly, there were no significant differences in rates of wood de cay between that watershed and the other unburned watershed. This sugg ests that indirect effects of annual fire (i.e. changes in the composi tion of soil flora and fauna) may override the short-term effects of f ire (i.e. changes in soil temperature and moisture) on belowground dec omposition in tallgrass prairie.