Epsilon Open Archive

Abstract

Tree growth in boreal systems is frequently limited by nitrogen (N) availability, and a significant portion of photosynthetically assimilated carbon (C) is partitioned to N-acquisition by roots and mycorrhizal fungi.
A new method for reversibly halting tree belowground C flux was developed. The method, termed stem compression, consists of applying pressure around the stem to collapse the conducting phloem and was shown to efficiently stop belowground C transport in Scots pine trees. Stem compression reduced tree N uptake by 32%, but N uptake of uncompressed trees also suffered - if they were surrounded by compressed trees. Conversely, a single compressed tree did not significantly affect N uptake by nearby uncompressed trees. This indicates that belowground C transport mediates N uptake, but also that within-community C status influences competition for N.
In the second part of the thesis, a method using the oxygen isotope discrimination technique for partitioning between the cytochrome oxidase (COX) and alternative oxidase (AOX) respiration was developed and applied to Scots pine roots from two forest stands. All plants examined to date contain AOX, as well as many other organisms. Its energy yield is only 1/3 of what is gained from COX, and pathway partitioning can shift in response to stress, such as nutrient deficiency.
Alternative oxidase partitioning was measured at two temperatures (6°C and 20°C). At 6°C significant fraction of fine root respiration (c. 20%) occurred via the AOX pathway. This fraction was found to decrease to c. 12% in response to elevated temperature and improved soil N availability. The potential influence of AOX on C sequestration coupled with responsiveness to environmental conditions make AOX relevant in light of changes in climate and forest management.
Taken together, the results point toward two feedback systems where C and N acquisition can act to reinforce each other: One mechanism where belowground C partitioning mediates uptake of soil/mycorrhizal N; and another where high soil N availability can trigger a shift in AOX partitioning, leading to increased biosynthetic efficiency of roots, potentially allowing more efficient use of the belowground C flux.