Graduate Thesis Or Dissertation
 

Nitrogen dynamics in red alder

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  • Red alder (Alnus rubra Bong.) is recognized as an important source of nitrogen to ecosystems that it inhabits. I examined N dynamics within alder trees, alder leaf litter, and the soil beneath alder leaf litter. ¹⁵Nitrogen, a stable isotope of N, was used as a tracer to follow the movement of N through the various systems of interest. Red alder trees were labeled with ¹⁵NO₃⁻ and ¹⁵NH₄⁺ using the stem-injection method. Leaves were sampled 3 and 15 mo subsequent to injection within several crown positions, including top, bottom, proximal, medial, and distal. Stem injection of both ¹⁵NH₄⁺ and ¹⁵NO₃⁻ at levels approaching one percent of crown N effectively labeled red alder trees. Although more variable, ¹⁵NO₃⁻ may have been more efficient in initial labeling. The distribution of ¹⁵N was uniform at the time of the first sampling, but was diluted in the distal and top positions by the second sampling. There was a clear increase in total N concentration toward the periphery of the tree. This increase became more pronounced with increasing crown size and crown closure. Crown position with respect to light availability may be the most important determinant in crown N allocation in red alder foliage. To study the transfer of N from red alder trees to the soil, ¹⁵N-labeled red alder foliage was allowed to decompose in the field for 21 mo. The concentration of ¹⁵N was measured in remaining detritus and at 0-5 and 5-15 cm depths in four soil fractions below the detritus. The soil fractions investigated included the light- and heavy-fractions of the soil, the chloroform-labile (microbial biomass) pool, and the whole-soil. Some recovery of ¹⁵N was noted in vegetation growing in the plots. The alder litter lost 78% of its mass, 77% of the total initial N, and only 64% of the total initial ¹⁵N. Although the heavy-fraction contained 77 to 88% of the total nitrogen, the concentration of N in the light-fraction was 3.5 times that in the heavy-fraction. Whole-soil recoveries were higher than the summed fractions for total N and for ¹⁵N in the top 5 cm. Light-fractions exhibited higher percent recoveries of ¹⁵N than heavy-fractions. Percent recovery of ¹⁵N in the chloroform-labile N fraction was not significant. The majority of nitrogen released from the leaves was concentrated within the top five centimeters of soil. After 21 mo of decomposition, alder detritus acted as a net source of N, most of which remained in the labile pools of the fractionated soil.
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