Using a hydropedological framework to identify carbon and nitrogen cycling hotspots in a northern hardwood forest

Background/Question/Methods

Spatial heterogeneity in soils is so great that it can mask patterns in ecosystem processes and impede attempts to scale up across the landscape. The hydropedologic approach to soil unit classification highlights the central role that water flowpaths play in soil development. Recent advances using a hydropedological approach to characterize soils at the Hubbard Brook Experimental Forest in NH based on topographic position, proximity to bedrock outcrops, and hydrologic regime led to the description and mapping of five distinct soil functional groups called hydropedologic soil units (HPU) at the catchment scale. This research also suggested that soil, soil solution, and groundwater chemistry vary systematically by HPU. In this study, we evaluated C and N and their isotopes in bulk soils and soils passed through five sieves sizes (from 500 µm to 63 µm), nitrification, and N and O isotopes in nitrate in groundwater and tension lysimeters from five representative HPUs. Our overall objective was to better identify location, timing, and mechanisms for C and N retention and loss in soils. Specifically, we sought to evaluate whether transects from bedrock outcrops to deeper soils immediately downslope (E to Bhs podzols) were areas of significant C and N cycling in the catchment.

Results/Conclusions

We observed different C and N patterns in the E and Bhs compared to the typical podzols. %C and %N were higher in the Bhs and lower in the E compared to the typical podzols.  δ¹⁵N , which can serve as a measure of the extent of microbial processing of N was higher in the Bhs and E compared to the typical podzols. We found that both δ¹⁵N and δ¹³C in soil tended to increase with decreasing sieve size for most HPUs. Nitrification was highest in the Bhs and E compared to the typical podzols. In groundwater wells and soil solution, DOC and DON concentrations were highest in the E, while the highest nitrate concentrations were observed in the Bhs podzols. These preliminary results, along with isotopic measures of δ¹⁵N and δ¹⁸O in nitrate in solution suggest distinct patterns of C and N cycling as a function of HPU. The flashy response to precipitation events and subsequent wetting and drying cycles may enhance nitrification in the E and Bhs podzols.  The hydropedogical approach provides a strong framework for the use of fine-scale data to quantify C and N cycling at the catchment scale.