Featured research (3)
Increasing the overall use efficiency of nitrogen (N) and phosphorus (P) resources in food production while minimizing losses to the environment are required to meet the dual challenge of food security and sustainability. Yet studies quantifying the overall performance of different agro-system types and how these have changed over time remain rare, although they are essential to propose solution pathways. Here, we reconstructed fluxes of N and P within 78 watersheds of the St. Lawrence Basin (SLB) of eastern Canada between 1901 and 2011, using the Generalized Representation of Agro-Food System model (GRAFS). This analysis allowed us to classify different agro-food system types and to evaluate how agricultural specialization influenced nutrient efficiencies and potential losses to the environment over time. Using a cluster analysis, we identified four agro-food system types with different overall outcomes in efficiencies and losses. We show that agricultural practices in the SLB were similar until the 1950's and deemed unsustainable in several watersheds by depleting agricultural soils of their nutrients (particularly N). With the advent of manufactured fertilizers and the intensification of livestock farming, the SLB then rapidly shifted through the 1970s and 1980s to more intensified and highly unsustainable agro-food system types, where, in 2011, ~77 % of N and ~ 94 % of P inputs were lost to the environment. We also show that nutrient pollution continued to increase despite gains in the nutrient use efficiency of animal farming due to higher nutrient throughput from intensive production. The increased proportion of confined animals, disconnected from croplands, indeed resulted in inefficient nutrient recycling. While nutrient use efficiency may mitigate nutrient pollution, reducing the absolute nutrient flux through agro-food systems should be a priority, likely through a reconnection of crop and animal farming and an overall reduction of meat production, specifically from concentrated, intensive livestock systems.
Diverse prokaryotic communities consume and transform a broad suite of molecules in the dissolved organic matter (DOM) pool, which controls major biogeochemical cycles. Despite methodological advancements that provide increasingly more detailed information on the diversity of both prokaryotic communities and DOM components, understanding how these two component parts are structured to influence ecosystem functioning remains a major challenge in microbial ecology. Using empirical data collected along a gradient of productivity in the Labrador Sea, we characterized relationships among DOM compounds, metabolic processing, and prokaryotic diversity by structuring prokaryotic communities using spatial abundance distribution (SpAD) modeling. We identified strong associations of different SpAD taxonomic groups with specific organic substrates as well as with metabolic rates. Amplicon sequence variants (ASVs) with more cosmopolitan distributions (i.e. normal‐like) such as Bacteroidia were related to fresher DOM substrates such as free and combined amino acids whereas rare ASVs (i.e. logistic) like δ‐proteobacteria were associated with complex forms of organic matter. In terms of ecosystem function, rates of respiration and production were most strongly predicted by the abundance of certain SpAD taxonomic groups. Given the importance and complexity of linking environmental conditions, prokaryotic community structure, and ecosystem function, we propose a framework to bridge the gap between prokaryotic diversity, microbial ecology, and biogeochemistry among methods and across scales. Our work suggests that SpAD modeling can be used as an intermediate step to link prokaryotic community structure to both finer DOM details and larger ecosystem scale processes.
The “freshwater pipe” concept has improved our understanding of freshwater carbon (C) cycling, however, it has rarely been applied to macronutrients such as nitrogen (N) or phosphorus (P). Here, we synthesize knowledge of the processing of C, N, and P together in freshwaters from land to the ocean. We compared flux estimates into and out of the N and P “pipes” and showed the net removal rates of N and P by inland waters were less than those for C. The C : N : P stoichiometry of inland water inputs vs. exports differed due to large respiratory C and N losses, and efficient P burial in inland waters. Residence time plays a critical role in the processing of these elements through the pipe, where higher water residence times from streams to lakes results in substantial increases in C : N, C : P, and N : P ratios.