Graduate Thesis Or Dissertation
 

Quantifying C and N contents and isotope signatures of SOM pools in the H. J. Andrews DIRT plots

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  • The mechanisms governing short- and long-term belowground carbon dynamics need to be understood. As part of a larger project developed to assess the effect of quantity and quality of litter inputs on the rate of soil organic matter (SOM) formation, I examined SOM in the H. J. Andrews Detritus Input and Removal Treatments (DIRT) plots. This study was designed to: (1) determine how five years of treatment had changed the SOM in the reduced input plots and the added input plots relative to the control plots; (2) determine if the more labile (light) fraction of the soil had changed more from the manipulations than the more recalcitrant (heavy) fraction of the SOM; (3) document how the light and heavy fractions changed with depth in this coniferous forest relative to the published trends in other forest types; and, (4) determine if density fractionation conserved the C and N of the sample, or if the method resulted in any losses or transformations that might yield the method untrustworthy. To accomplish these objectives, I measured carbon and nitrogen concentrations and isotope values of SOM at different depths in the soil profile and by density fraction because previous work has shown that these parameters are good indicators of soil age! recalcitrance. I separated soil into labile (light) and more recalcitrant (heavy, mineral-bound) fractions from three depths in each of 18 treatment plots at the H. J. Andrews Forest, Cascades, OR using sodium polytungstate. Soil light fraction averaged 5.3% of whole soil at 0-5 cm, 3.3% at 5-10 cm, and 1.3% at 10-20 cm. Light fraction from Control plots contained less carbon than light fraction from No Inputs and No Roots plots, and No Litter plots contained less carbon than No Roots plots. No other treatment differences were observed. In a second set of analyses, the treatments were combined and treated as replicate samples to quantify depth and density trends. Light fraction C and N concentrations were greater than heavy fraction concentrations. For example, the light fraction contained 25.6% more C and 0.3% more nitrogen at 0-5 cm than the heavy fraction. The heavy fraction was more '3C and 15N enriched than the light fraction (P<0.001). Heavy fraction 6'3C values ranged from -26.5 to -25.3%, whereas light fraction values ranged from -27.0 to -26.8% with increasing depth. Heavy fraction ö'5N values ranged from 3.1 to 6.3% with depth compared to a range of 0.1 to 0.7% in the light fraction. Bulk soil carbon and nitrogen concentrations generally decreased with depth while bulk C and N isotope values increased with depth. Lower concentrations of more isotopically enriched carbon and nitrogen indicate more decomposed soil organic matter. Because the density trends followed the bulk depth trends (and were even more pronounced), I conclude that heavy fraction SOM is more decomposed than light fraction material. In summary, the findings were: (1) five years of treatment did not change the SOM in the reduced input plots and the added input plots relative to the control plots, with the one exception of light fraction carbon concentration differences between control and rootless plots, suggesting a root mediated priming effect; (2) the light fraction did not change more from the manipulations than the heavy fraction of the SOM (with the one exception mentioned above), indicating that the turnover times of both pools are greater than five years; (3) the trends with depth in this coniferous forest were similar to the published trends in other forest types despite the fact that this forest had greater carbon concentrations at all depths than SOM from other forests; and, (4) based on mass balance analysis, density fractionation conserved the C and N of the samples, so the method can be used to separate the labile and recalcitrant pools of SOM.
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