In tallgrass prairie, periodic spring fires often result in enhanced aboveground net primary productivity (ANPP) that exceeds the productivity of either annually burned or unburned sites. This study evaluated two alternate hypotheses for the "pulse" in productivity following an infrequent fire: (1) enhanced ANPP results from increased net N mineralization rates due to the removal of surface litter and elevated soil temperatures following fire (the enhanced mineralization hypothesis) or (2) enhanced ANPP results from a transient release from both light and N limitation during a nonequilibrium period as a switch from energy to N limitation occurs (the transient maxima hypothesis). The former hypothesis predicts greater N availability following an infrequent fire, relative to either annually burned or unburned prairie. The latter predicts that N availability following an infrequent fire will decline to intermediate levels, relative to unburned and annually burned prairie, and continue to decline with successive annual fires. To test these hypotheses, I measured inorganic soil N, net N mineralization rates, and plant productivity and N content at Konza Prairie in sites with several different burn histories (unburned, annually burned, infrequently burned). Inorganic soil N and cumulative net N mineralization rates were greatest on the unburned sites, lowest in annually burned sites, and intermediate in infrequently burned sites. Net N mineralization rates and plant tissue N content both declined with successive spring burning. These results did not support the enhanced mineralization hypothesis but indicated that enhanced ANPP following an infrequent fire resulted from an accumulation of inorganic and mineralizable N in the absence of fire which, under conditions of adequate light availability, was utilized following a spring fire. This is consistent with the transient maxima hypothesis and suggests that nonequilibrium responses to multiple, variable resources (light, energy, N) are an important aspect of tallgrass prairie ecosystem dynamics.
