Landscape and hillslope controls over soil biogeochemical properties in a northern hardwood forest

Background/Question/Methods

As part of a hydropedologic study of a forested catchment in Hubbard Brook Experimental Forest (New Hampshire, USA), we sought to investigate the connections between hydropedologic units and soil biogeochemical properties. In August 2010, we collected soils from three transects (high, medium, and low elevation) in the catchment, sampling three distinct hillslope positions and associated soils in each transect:  umbrepts, typical spodosols, and bimodal spodosols. We measured microbial biomass carbon(C), microbial respiration (as respiration rate during chloroform fumigation-incubation), and denitrification potential (denitrification enzyme assay) in the three surface horizons (forest floor-Oie; humic horizon-Oa/A; and mineral, illuvial horizon-Bh). We hypothesized that umbrepts would be biogeochemical hotspots along hillslopes, as zones where hydrologic flow paths converge, and that this would hold true for all soil horizons.  There was particular interest in the biological potential of Bh horizons which are carbon rich horizons, relatively deep in the profile with high potential for intercepting hydrologic flow paths. 

Results/Conclusions

We found similar patterns for microbial biomass C and microbial respiration. These variables were driven more by elevation than by hillslope position, such that the highest elevation transect had the highest microbial biomass C (mean of 90.1 ± 33.6 mg C/g soil) and microbial respiration (mean of 1.83 ± 6.10 mg C/g soil/day). Results for denitrification potential overall showed no differences between transects (0.97 – 1.59 µg N/g soil/hour). Patterns across soil horizons varied by transect for all three variables. In the highest elevation transect, the mineral illuvial horizon had the highest values, whereas in the mid- and low-elevation transects, results were higher in the forest floor compared to the humic horizon. Other work in this catchment has shown that the higher elevation locations have higher soil moisture and nitrate levels, suggesting that broader-scale distributions of moisture and nutrients are more important environmental filters than finer-scale hillslope characteristics in this catchment.