Lateral advection supports nitrogen export in the oligotrophic open-ocean Gulf of Mexico

In contrast to its productive coastal margins, the open-ocean Gulf of Mexico (GoM) is notable for highly stratified surface waters with extremely low nutrient and chlorophyll concentrations. Field campaigns in 2017 and 2018 identified low rates of turbulent mixing, which combined with oligotrophic nutrient conditions, give very low estimates for diffusive flux of nitrate into the euphotic zone (< 1 µmol N m−2 d−1). Estimates of local N2-fixation are similarly low. In comparison, measured export rates of sinking particulate organic nitrogen (PON) from the euphotic zone are 2 – 3 orders of magnitude higher (i.e. 462 – 1144 µmol N m−2 d−1). We reconcile these disparate findings with regional scale dynamics inferred independently from remote-sensing products and a regional biogeochemical model and find that laterally-sourced organic matter is sufficient to support >90% of open-ocean nitrogen export in the GoM. Results show that lateral transport needs to be closely considered in studies of biogeochemical balances, particularly for basins enclosed by productive coasts.

Massive export of diazotrophs across the tropical south Pacific Ocean

Diazotrophs are widespread microorganisms that regulate marine productivity in 60% of our oceans by alleviating nitrogen limitation. Yet, their contribution to organic carbon and nitrogen export fluxes has never been quantified, making an assessment of their impact on the biological carbon pump impossible. Here, we examine species-specific fates of several groups of globally-distributed unicellular (UCYN) and filamentous diazotrophs in the mesopelagic ocean. We used an innovative approach consisting of the combined deployment of surface-tethered drifting sediment traps, Marine Snow Catcher, and Bottle-net, in which we performed nifH sequencing and quantitative PCR on major diazotroph groups across the subtropical South Pacific Ocean. nifH sequencing data from sediment traps deployed at 170 m, 270 m and 1000 m provide clear evidence that cyanobacterial and non-cyanobacterial diazotrophs are systematically present in sinking particles down to 1000 m, with export fluxes being the highest for the UCYN-A1 symbiosis, followed by UCYN-B or Trichodesmium (depending on station and depth), Gamma A and UCYN-C. Specific export turnover rates (a metric similar to the export efficiency adapted to organisms) point to a more efficient export of UCYN groups relative to the filamentous Trichodesmium. This is further confirmed by Marine Snow catcher data showing that the proportion of sinking cells was significantly higher for UCYN compared to Trichodesmium. Phycoerythrin-containing UCYN-B and UCYN-C-like cells were indeed recurrently found embedded in large (> 50 micrometers) seemingly organic aggregates, or organized into clusters of tens to hundreds of cells linked by an extracellular matrix, facilitating the export. Overall, diazotrophs accounted for 6-13% (170 m) to 45-100% (1000 m) of the total particulate nitrogen export fluxes in our study. We thus conclude that diazotrophs are important contributors to carbon sequestration in the subtropical South Pacific Ocean and need to be considered in future studies to improve the accuracy of current regional and global estimates of export.

Nitrogen inputs from smallholder farming in a tropical montane catchment - current state and outlook

&lt;p&gt;Rising human populations increase the demand for food and lead to the intensification of agriculture and nitrogen fertilization to sustain productivity. In the tropical montane Mau Forest Complex in Kenya, the annual export of nitrogen from a catchment dominated by smallholder agriculture were reported to almost double those from the native forest. Despite the assumption that this excess nitrogen originates from fertilizer application, there are no studies that provide empirical information on the amount and spatial distribution of nitrogen inputs from smallholder agriculture into these catchments. Given the fact that the Mau Forest complex lost 25% of its forest cover to agriculture and other encroachment activities, such information is essential to better quantify the effect of smallholder farming practices on the nitrogen cycle and its contribution to catchment nitrogen export.&lt;/p&gt;&lt;p&gt;This study aimed at quantifying spatial distribution of fertilizer inputs in a smallholder catchment in the Mau Forest Complex using a household survey (n=185).&lt;/p&gt;&lt;p&gt;Results show that almost all farmers (99.4%) use inorganic fertilizers with an average nitrogen (N) application rate of 41&amp;#177;7.8 kg N ha&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt; diammonium phosphate (DAP). Among the DAP users, 16% apply in addition 79&amp;#177;3.9 kg N ha&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt; as NPK fertilizer, and 11% add 29&amp;#177;5.3 kg N ha&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt; as calcium ammonium nitrate (CAN). Overall, the average nitrogen input from inorganic fertilizers is 64&amp;#177;13.7 kg N ha&lt;sup&gt;-1&lt;/sup&gt; yr&lt;sup&gt;-1&lt;/sup&gt;. Only 6% of the cropland is fertilized using manure and other farmland residues with 79% of farmers anticipating to increase their inorganic fertilizer application rates in the future.&lt;/p&gt;&lt;p&gt;In conclusion, a future increase in nitrogen application rates on farmland in combination with continued conversion of natural forest to agricultural land raises a concern on whether nitrogen export will increase further, posing a threat to drinking water quality and the health of aquatic ecosystems downstream. To balance the trade-off between food production and the catchment nitrogen balance, there is a need to train farmers on appropriate methods, timing and optimal amounts of fertilizer application to prevent unnecessary losses.&lt;/p&gt;

Lateral Advection Supports Nitrogen Export in the Oligotrophic Open-Ocean Gulf of Mexico

In contrast to its productive coastal margins, the open-ocean Gulf of Mexico (GoM) is notable for highly stratified surface waters with extremely low nutrient and chlorophyll concentrations. Field campaigns in 2017 and 2018 indicate low rates of turbulent mixing, which combined with oligotrophic nutrient conditions, give very low estimates of diffusive flux of nitrate into the euphotic zone (< 1 µmol N m-2 d-1). Estimates of local N2-fixation are similarly low. In comparison, measured export rates of sinking particulate organic nitrogen (PON) from the euphotic zone are 2-3 orders of magnitude higher (i.e. 462 – 1144 µmol N m-2 d-1). We reconcile these disparate findings with regional scale dynamics inferred independently from remote-sensing products and a regional biogeochemical model and find that laterally-sourced organic matter is sufficient to support >90% of open-ocean nitrogen export in the GoM. Results show that lateral transport needs to be closely considered in studies of biogeochemical balances, particularly for basins enclosed by productive coasts.