Document: JOH-3-59-61

Do pines fix nitrogen?

MH KNOPS, J.*

University of Nebraska, Lincoln, NE 68583 USA ¹

Abstract:
I explored competition between trees and grasses in an experimental garden and found that the pine tree, Pinus strobus, competitively excluded the grasses Agropyron repens and Schizachyrium scoparium. The oak tree, Quercus ellipsoidalis, competitively excluded Agropyron and coexisted with Schizachyrium. Schizachyrium plots retained nitrogen at a similar rate, e.g. 1.24 (S.D. = 0.22) g N.m^-2.yr^-1, as nitrogen accumulates within old fields at Cedar Creek, which implies that Schizachyrium plots retain all atmospheric nitrogen inputs. Agropyron plots, in contrast, only accumulated nitrogen at a rate of 0.51 0.05 and, most likely, did not retain all nitrogen inputs. Quercus plots accumulated nitrogen at a rate of 2.15 :plusmn: 0.56, which was not significantly different from Schizachyrium plots. Pinus plots accumulated 6.62 0.31 g N.m^-2.yr^-1, which is five times more than atmospheric deposition. This accumulation, together with a higher C:N ratio of its tissue and the total C:N ratio in the plot allowed it to obtain a larger biomass than the other species. The unaccounted nitrogen input within the Pinus plots is in the order of 50 kg N.ha^-1.yr^-1 over a 6-year period, and has a different ¹⁵N signature than the grasses and Quercus. This nitrogen accumulation is key in Pinus competitive ability. Two possible unaccounted input sources are asymbiotic nitrogen fixation or a much larger belowground Afootprint of Pinus that allows Pinus to exploit temporal and spatial patterns in soil mineral nitrogen availability. Thus, Pinus competitively excludes grasses because of higher biomass accumulation, which is ultimately controlled by access to different nitrogen pools and, in addition, partly by a higher rate of carbon accumulation per unit of acquired nitrogen.

Keywords: nitrogen cycling, tree - grass competition, nitrogen fixation

This abstract is being presented at: 3:30 PM in session:
Oral Session #13: N Fixation and Biochemical Patterns.