Agricultural management in boreal regions alters soil respiration burst profiles

Abstract

Land use conversion and climate change represent major threats to carbon (C) storage in boreal soils. Chapter I (literature review) of this dissertation shows that natural boreal soils are well-equipped to handle climate change, given that these ecosystems are already shaped by disturbance and possess natural mechanisms to resist or compensate for C loss under climate change. Much less is known about converted boreal soils under agricultural management, despite rapid and ongoing conversion. Chapters II-V of this dissertation thus combined four studies at different resolutions (within-field, within-farm, within-region, and global) to examine the fate of soil C in converted boreal soils through intensive soil abiotic property and soil respiration testing. Chapter II (within-field) examined a chronosequence of converted organic and podzolic soils and found that agricultural management rapidly homogenized soil to agricultural norms but also lost C rapidly under mineral management; surprisingly, high C presence resisted homogenization. Chapter III (within-farm) compared paired agricultural fields under normal management or amended with pulverized rock, and found that pulverization does not affect soil properties immediately; soil pedogenic type was more important in determining behavior. Chapter IV (within-region) compared soils under four land uses (LUs), and found that respiration patterns in each LU were distinct enough from each other (forest, agriculture, grassland) to allow for accurate prediction of LU. Chapter V (global) compared four LUs worldwide (agricultural, grassland, forest, and transitional) and found that global patterns of forest and agriculture could be accurately predicted via respiration, but grassland and transitional were too heterogeneous. Furthermore, respiration could indicate underlying management effects more accurately than abiotic factors on the global scale. This dissertation reveals that 1) boreal land conversion can cause <5yr shifts to agricultural norms accompanied by rapid C loss; 2) traditional management of boreal abiotic fertility fails to address questions of soil health and C cycling; 3) the status of boreal soils can be understood through LU-specific patterns of proportional respiration; and 4) conversion to agriculture creates anthropic soils with distinct behavior patterns that can be accurately predicted globally and must thus be managed differently from natural sites.