Nutrients Come and Go, but C:N Ratios are Forever: A Continental Scale Perspective on Changing C:N Ratios From Land to Water

Carbon and nitrogen cycles are often tightly coupled due to the limiting nature of nitrogen for carbon fixation by plants. Different environmental factors, like climate and plant composition affect cycling of both elements whereas environmental disturbances ranging from storm events to fires to seasonal changes can lead to the decoupling of both cycles. However, the relative influence of different environmental factors on the transfer of both elements between terrestrial and aquatic pools within a single watershed is not well known. In this study we try to understand what large scale environmental factors influence changes in C:N ratios within a given watershed from leaves to soils to streams.

We combined data on stream, groundwater and soil chemistry, stand composition, and land use from twenty co-located aquatic and terrestrial NEON sites to understand changes in C:N ratios from plants to soils to streams. Preliminary analyses focus on the relationship between annual average stream nitrogen and carbon exports compared to plant, soil, and groundwater C:N ratios, nitrogen mineralization rates, mean annual precipitation and temperature, and land use. Plant C:N ratios varied from 16.1 to 55.0, whereas soil C:N ratios in the top 30 cm of soil ranged from 5.6 to 34.4, with significant correlations between both variables. Stream DOC:TDN ratios varied from 0.5 to 30.1 and were strongly correlated with groundwater and weakly correlated with soils. These results show that, relative to carbon, nitrogen increases from canopy to soils to streams and suggest nitrogen losses from watersheds as nutrients are cycled in the system, however the processes driving these changes are not clear. These results also suggest that the rate of transfer of nitrogen and carbon between environmental pools is consistent across a range of environmental conditions, as sites with low C:N ratios in vegetation also tend to have low C:N ratios in all other pools. Analysis of nutrient stoichiometry may provide new insight into nutrient transfer across terrestrial and aquatic systems across ecosystems and potential changes to these pools in light of CO2 fertilization and changing leaf stoichiometry.