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, N-15 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 N-15 was rapidly immobilized with < 2% and 11% of the N-15 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 N-15 was initially recovered as soil organic N (oN), with a majority accounted for in microbial biomass. Plants contained similar to 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 N-15 was recovered from the unburned prairie, while 85% was recovered from burned prairie. The total N-15 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 N-15 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.