Do enzymes link microbial communities to soil organic matter chemistry? A cross site synthesis using py-gc/ms

Background/Question/Methods

Enzymes are the biochemical link between microbial communities and soil organic matter dynamics but the degree to which enzymes and soil organic matter chemistry influence each other in the field remains uncertain. Many studies over the last several decades have examined soil enzyme activities and many others have examined soil organic matter chemistry using a range of methods.  Very few studies, however, have attempted to understand the relationship between enzyme activities and particular chemical constituents of soil organic matter.  Indeed, this is an inherently difficult endeavor because soil organic matter chemistry reflects long-term decomposition processes while enzyme dynamics fluctuate with current conditions.  Moreover, because of sorption processes enzyme potentials in the lab may not reflect in situ activities.  We have conducted a synthesis of soil organic matter chemistry in whole soils and soil fractions and its relationship to enzyme activities from five sites and more than 100 soil samples.  The sites include high alpine ecosystems in Colorado, different land use intensities in the South Carolina Piedmont, and agricultural and forest soils in Michigan.  Soil organic matter chemistry was analyzed using pyrolysis-gas chromatography/mass spectroscopy, a method that provides insights into the individual compounds in soil organic matter.

Results/Conclusions

We found consistent and predictable trends in soil organic matter chemistry and its relationship to enzyme activities across all of our sites.  The relative abundance of lignin derivatives was generally positively correlated with sand content while the abundance of nitrogen containing compounds, polysaccharides and lipids decreased in sandy soils and increased in soils with high clay content.  Differences in chemistry among size fractions were greater than the differences within size fractions across sites.  In general, nitrogen containing compounds were important drivers of hydrolytic and nitrogen-cycle enzymes and were negatively related to fungal/bacterial ratios.  Most lignin derivatives, including guaiacol and 4-vinylguaiacol, showed no relationship to enzyme activity and were positively related to fungal/bacterial ratios.  Our synthesis indicates that: 1) soil organic matter chemistry varies among sites but there are also consistent patterns related to edaphic soil properties and size fractions; 2) across a range of different ecosystems consistent trends emerge with respect to composition of soil organic matter and its relationship to enzyme activities; and 3) human activities can modify soil organic matter chemistry and enzyme activities in ways that could alter long-term soil C dynamics.