Patterns in the coupling of N and P uptake in stream networks

Background/Question/Methods

The most pressing environmental problems faced by society are manifestations of changes in biogeochemical cycles. The urgency of mitigating these problems has brought into sharp focus the need for a stronger mechanistic understanding of the factors that control biogeochemical cycles and how these factors affect multiple elements. Our overarching goal is to assess the strength of coupling between carbon, nitrogen, and phosphorus cycles in small headwater streams, including streams draining small watersheds in Northern California and the East Siberian Arctic. We have used a range of whole ecosystem approaches, rooted in nutrient spiraling theory, including plateau and pulsed nutrient enrichment experiments at a range of N:P ratios in heterotrophic and autotrophic streams. We use these experiments to calculate changes in nutrient spiraling metrics in response to changes in absolute and relative supply of N and P, and we use these results to infer the strength of the linkage between N and P cycles.

Results/Conclusions

California streams are N-limited, and the strength of N and P coupling depends on position along the stream network. In small heterotrophic streams, addition of N increased P uptake, suggesting strong coupling of N and P cycles. In autotrophic streams, N and P were weakly coupled, likely due to increased dominance of uptake by algae rather than heterotrophic bacteria, with concomitant changes in P storage. We observed small but consistent reductions in P uptake at high N:P supply in autotrophic streams, which may indicate suppression of N-fixation at high N. Arctic streams showed less consistency in response to nutrient enrichment, with some streams showing very little change in N or P uptake with changes in supply N:P, and others showing a decrease in N uptake in response to increased P addition. We speculate weak coupling of N and P uptake in streams in which physical sorption dominates P uptake. We hypothesize that differences between streams in the degree of coupling of N and P uptake reflect variation in the relative importance of abiotic and biotic mechanisms of nutrient uptake, including composition of the microbial community (autotrophs vs. heterotrophs), changes in transient storage of water, and physical P sorption.