Carbon structure and enzyme activities in alpine and forest ecosystems

The chemical structure of soil organic matter fractions and its relationship to biological processes remains uncertain. We used pyrolysis-gas chromatography/mass spectrometry to analyze the molecular structure of light and heavy fraction C from soils in the San Juan Mountains, Colorado. The soil samples, each replicated three times, were from two elevations (alpine and low forest) within two geochemically distinct basins (igneous and sedimentary). We also analyzed whether variation in the activity of nine enzymes that mediate soil organic matter turnover and nutrient cycling could explain differences in C structure. We found that, across basins and elevation, light fraction and heavy fraction C had distinct chemistries. The light fraction was characterized by an abundance of plant lignin biomarkers, including phenol, 2-methoxy-4-vinyl-(vinylguaiacol) and phenol, 2-methoxy-(guaiacol); in contrast heavy fraction had very little unaltered lignin but an abundance of polysaccharides, such as furfural, and proteins such as pyrrole. In alpine sites, light fraction was less abundant (4.27 versus 31.79 g kg⁻¹) and had a lower C/N ratio (17.25 versus 32.01) than in forests. The alpine sites also had higher activities of phosphatase, β-d-1,4-cellobiosidase, β-1,4-glucosidase, l-leucine aminopeptidase, and β-1,4-xylosidase. Protein abundance in the heavy fraction was correlated with peptidase, β-1,4-glucosidase, and phosphatase activities; in the light fraction, protein abundance was correlated with peptidase, xylosidase, and β-d-1,4-cellobiosidase activities. β-1,4-N-acetyl-glucosaminidase was negatively correlated with polysaccharides in the light and heavy fractions and positively correlated with lignin in the light fraction. However, there were not always significant correlations between enzymes and substrates. We suggest that this is likely because soil organic matter chemistry reflects long-term decomposition processes while enzyme dynamics fluctuate with current conditions or due to the presence of a pool of sorbed enzymes in the heavy fraction. While alpine and forest ecosystem C distribution and enzyme activities varied, substantial depletion of lignin derivatives in the heavy fraction across sites suggest that these compounds do not persist in stable soil C pools.