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ARS Home » Northeast Area » University Park, Pennsylvania » Pasture Systems & Watershed Management Research » Research » Publications at this Location » Publication #143612
Title: LONG-TERM PARTITIONING OF NITROGEN IN TALLGRASS PRAIRIE

Author
item Dell, Curtis
item WILLIAMS, MARK - OREGAN STATE UNIV.
item RICE, CHARLES - KANSAS STATE UNIV.

Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/22/2004
Publication Date: 5/11/2005
Citation: Dell, C.J., Williams, M., Rice, C. 2005. Long-term partitioning of nitrogen in tallgrass prairie. Ecology. 86(5):1280-1287.

Interpretive Summary: Annual, spring burning of prairie rangeland generally increases forage production, but results in the loss of some nitrogen from the system. To determine how the prairie alters nitrogen availability, we injected nitrogen tracers (15N) into soil cores placed in both unburned and annually burned prairie and followed the fate of the applied nitrogen for five growing seasons. Cores were removed from the field six days after 15N application or at the end of one, two, or five growing seasons. Throughout the experiment, the greatest percentage of the applied nitrogen was found in the soil microbes and soil organic matter. Consistently more applied N was tied up in soil organic matter in burned prairie where less nitrogen was lost from the rootzone. Plants in both burned and unburned prairie immobilized about 20% of the applied N during the first six days and maintained that nitrogen through the first two growing seasons. Sampling at the end of each growing season showed that a majority of the plant nitrogen was located below ground (roots and rhizomes) which minimizes the amount of N that can be lost to fire. The experiment showed that plants have adapted to limited nitrogen availability with internal recycling. Also, increased microbial N demand in burned prairie leads to greater immobilization of N within the rootzone soil and counters losses of N when above plant material is burned.

Technical Abstract: The availability of N limits productivity in the tallgrass prairie with soil microbes controlling the immediate fate of inorganic N. Annual, spring burning has been shown to increase microbial N demand thus decreasing the quantity of N available to plants. In this study, the partitioning of 15NH4+ between plant and soil N pools was followed over five growing seasons in annually burned and unburned prairie. Six days after application, 90% (unburned) to 98% (burned) of the applied 15N was recovered with 70% (unburned) to 77% (burned) of the applied N recovered as soil organic N. Plants contained approximately 20% of the applied N with the largest portion recovered from roots. 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. Recovery of 15N from the soil organic fraction decreased to 33% (unburned) and 52% (burned) of the applied N. The total 15N 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 most of lost plant N was transferred to the soil organic pool and total recoveries were similar to those observed after one and two seasons. Conservation of N by plants and tight cycling of N within the rootzone suggest mechanisms by which prairie can be a highly productive ecosystem despite limited N availability. Microbial immobilization conserves N within the root zone and acts to counterbalance loss of N to fire. Despite yearly losses of N through the combustion of above ground biomass, annual burning of prairie significantly increased retention of applied N relative to unburned prairie.