Fine-scale phosphatase activities are associated with carbon and nitrogen-rich microsites in soils of a mixed Douglas fir and paper birch stand

Background/Question/Methods

Phosphorus plays an important role in driving primary production in terrestrial ecosystems. However, this nutrient is commonly bound within complex organic molecules, which are difficult for plants to access. Soil fungi, plant roots, and microbes produce extracellular phosphatase enzymes that mineralize organic phosphorus, thereby releasing orthophosphate ions that can be assimilated. This study focuses on the distribution of soil enzymes at mm scales in order to better understand the chemical and microbial features associated with high activities.  High soil phosphatase activities are thought to co-locate with areas of high microbial activity.  We hypothesized that high activities would also be associated with microsites high in carbon (C) and nitrogen (N) and low in inorganic phosphorus (P). A high C:P or N:P ratio would be predicted to indicate microsites where adequate C and N was available for synthesizing enzymes and where P was limiting microbial growth. In this study, an enzyme imprinting method was used to detect mm-scale phosphatase activity from soil profiles in a mixed Douglas fir and paper birch stand in British Columbia. Small (0.05 g) soil samples were removed from areas of high and low phosphatase activity at five root windows.

Results/Conclusions

Total extractable P (p=0.954), inorganic phosphate (p=0.867), and soluble organic P (p=0.200) were not different between areas of high and low phosphatase activity across all windows, suggesting that P availability alone was not important in driving phosphatase activity. As predicted, however, the percent total C (p=0.021) and percent total N (p=0.018) were higher in microsites with high phosphatase activity. This implies that higher levels of C and N, especially relative to P, stimulated phosphatase activity. The C:N ratio was higher in areas of high phosphatase activity (p=0.053), suggesting that C was likely a more important driver of phosphatase activity than N at this site.  These results confirm that soil imprinting, combined with careful targeted sampling, can be used to detect meaningful differences in soil chemistry at fine scales and increase our understanding of the drivers of soil processes at these scales.