Partitioning of Nitrogen over Five Growing Seasons in Tallgrass Prairie

Annual spring burning of tallgrass prairie increases plant biomass production despite losses of N and lower net N mineralization. To better understand how burning influences the cycling of N in prairie, '5N was injected to soil as NH4+, and the partitioning between plant and soil N pools was followed over five growing seasons in annually burned and unburned prairie. Applied '5N was rapidly immobilized with <2% and 11% of the '5N remaining in inorganic forms six days after application in burned and unburned prairie, respectively. Seventy-seven percent (burned prairie) and 70% (unburned prairie) of the applied '5N was initially recovered as soil organic N (oN), with a majority accounted for in microbial biomass. Plants contained --20% of the applied N with the largest portion recovered from roots regardless of burning. At the end of the first growing season, only 55% of the applied 15N was recovered from the unburned prairie, while 85% was recovered from burned prairie. The total 'SN content of the plants changed little during the first growing season, but the portion recovered in the rhizomes increased, indicating belowground N storage. Total recovery and distribution of applied N changed little from the end of the first to the end of the second season growing season. Accumulations of 15N within the plants decreased greatly between the second and fifth growing seasons, but N lost from plants was accounted for in oN. Conservation of N by plants and tight cycling of N within the root zone suggest mechanisms by which prairie can be a highly productive ecosystem despite limited N availability. The immobilization of potentially mineralized N within the root zone increased with burning, offsetting the loss of N to fire probably by reducing leaching and denitrification losses.