Monday, August 3, 2009

PS 16-130: Do source-sink relationships influence nitrogen transfer between conspecific Pinus ponderosa saplings in a Northern California woodland?

Meagan Hynes1, Xinhua He2, Robert Zasoski1, and William Horwath1. (1) University of California, Davis, (2) Michigan Tech University

Background/Question/Methods

We investigated N transfers from pine saplings to nearby pines, forbs, grasses and legumes in a pine-oak woodland in northern California’s Sierra Nevada foothills. Using 15N isotopic pulse-labeling, we evaluated effects of source-sink relationships on N movement between fifteen pairs of naturally regenerated Pinus ponderosa saplings that were ~0.6 m apart.  Applied treatments were: (1) 15N donor treatment where K15NO3- applied to the needles of “donor” saplings; (2) K15NO3-  applied to donor sapling needles and K14NO3- applied to “receiver” sapling needles; (3) control treatment where donor and receiver pines received no 15N or 14N. After 6 weeks, entire pine saplings were harvested. Herbaceous species were sampled along a 300 cm transect that was located between the pine pairs.

Results/Conclusions

Although application of 14N to receiver pines resulted in lower 15N enrichment in receivers, this trend was not statistically significant. Effects of supplying N to receivers may have been diminished by relatively high N already present in sapling foliar tissue. Donor needles and stem had the greatest 15N enrichment. Herbaceous plants adjacent to donors were enriched with 15N.  Ectomycorrhizal colonization of donor roots was negatively correlated with δ15N found in roots of herbaceous species sampled at +100 cm. In receiver roots, ectomycorrhizal colonization correlated negatively with 15N concentrations in root tissue. Along the 300 cm transect, herbaceous plants located nearest to the pine pairs had the greatest enrichment suggesting that herbaceous species benefited from proximity to the donor pine. Grasses and forbs were more enriched than legumes. The higher 15N concentrations in forb roots harvested near the pine pairs was negatively correlated with 15N in the receiver needles; uptake by forbs corresponds with reduced transfer to the receiver pine. This study demonstrated that in a pine-oak woodland, N transfers from pine saplings to other saplings and herbaceous species are strongly affected by multiple source-sink relationships.