Document: JAM-3-59-59

Canopy type effects on nitrogen flow in southwestern ponderosa pine forests.

ROBERTS, J.M.* and S.C.HART

Northern Arizona University, Flagstaff, AZ 86011, USA ¹

Abstract:
Since the late 1800's, ponderosa pine (Pinus ponderosa) forests of the American Southwest have undergone significant changes due to heavy grazing and fire suppression. The structure of these forests has changed from open, park-like stands of large trees with a diverse and abundant herbaceous understory to dense stands of pole-sized trees and very little understory biomass. A consequence of these human-induced changes in forest structure is increased competition for water and nutrients, which has negatively impacted forest health. Although we know that N availability limits plant growth in these forests, little experimental evidence exists concerning how changes in forest structure have altered rates of N release from litter and soil organic matter. We assessed the effects of old-growth pine and grass canopy types on inorganic N transformations and decomposition at the Camp Navajo Army National Guard base near Flagstaff, AZ. Five plots of each canopy type were randomly selected within four replicated stands. Aerobic laboratory incubations were used to assess canopy effects on soil organic matter quality and N mineralization potential. Initial rates of respiration and maximum respired C were significantly greater under pine canopies. Net N mineralization after the first 7 d and cumulatively over the 56-d incubation period was also greatest for the pine canopies. Grassy openings, however, had significantly greater nitrification after 7 d, but there were no canopy type differences after 56 d. The ratio of C respired to N mineralized was greatest for pine canopies, indicating a lower soil organic matter quality than in grassy openings. A cellulose-filled litterbag method was used to assess decomposition rates of soil organic matter. Litterbags were field-incubated for 10 months and analyzed for mass loss and inorganic N content. These results seem to suggest that pine canopies promote N cycling more than grassy openings; however, the relative abundance of these two canopy types will determine stand-level rates of N cycling.

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This abstract is being presented at: 8:30 AM in session:
Oral Session #41: N Dynamics: Additions, Retention and Transformations.