The Photophysiological Response of Nitrogen-Limited Phytoplankton to Episodic Nitrogen Supply Associated With Tropical Instability Waves in the Equatorial Atlantic

In the Equatorial Atlantic nitrogen availability is assumed to control phytoplankton dynamics. However, in situ measurements of phytoplankton physiology and productivity are surprisingly sparse in comparison with the North Atlantic. In addition to the formation of the Equatorial cold tongue in the boreal summer, tropical instability waves (TIWs) and related short-term processes may locally cause episodic events of enhanced nutrient supply to the euphotic layer. Here, we assess changes in phytoplankton photophysiology in response to such episodic events as well as short-term nutrient addition experiments using a pair of custom-built fluorometers that measure chlorophyll a (Chl a) variable fluorescence and fluorescence lifetimes. The fluorometers were deployed during a transatlantic cruise along the Equator in the fall of 2019. We hypothesized that the Equatorial Atlantic is nitrogen-limited, with an increasing degree of limitation to the west where the cold tongue is not prominent, and that infrequent nitrate injection by TIW related processes are the primary source alleviating this limitation. We further hypothesized phytoplankton are well acclimated to the low levels of nitrogen, and once nitrogen is supplied, they can rapidly utilize it to stimulate growth and productivity. Across three TIW events encountered, we observed increased productivity and chlorophyll a concentration concurrent with a decreased photochemical conversion efficiency and overall photophysiological competency. Moreover, the observed decrease in photosynthetic turnover rates toward the western section suggested a 70% decrease in growth rates compared to their maximum values under nutrient-replete conditions. This decrease aligned with the increased growth rates observed following 24 h incubation with added nitrate in the western section. These results support our hypotheses that nitrogen is the limiting factor in the region and that phytoplankton are in a state of balanced growth, waiting to “body surf” waves of nutrients which fuel growth and productivity.

Environmental control of marine phytoplankton stoichiometry in the North Atlantic Ocean

The stoichiometric coupling of carbon to limiting nutrients in marine phytoplankton regulates the magnitude of biological carbon sequestration in the ocean. While clear links between plankton C:N ratios and environmental drivers have been identified, the nature and direction of these links, as well as their underlying physiological and ecological controls, remain uncertain. We show, with a well-constrained mechanistic model of plankton ecophysiology, that while nitrogen availability and temperature emerge as the main drivers of phytoplankton C:N stoichiometry in the North Atlantic, the biological mechanisms involved vary depending on the spatiotemporal scale and region considered. We find that phytoplankton C:N stoichiometry is overall controlled by nitrogen availability below 40° N, predominantly driven by ecoevolutionary shifts in the functional composition of the phytoplankton communities, while phytoplankton stoichiometric plasticity in response to dropping temperatures and increased grazing pressure dominates at higher latitudes. Our findings highlight the potential of “organisms-to-ecosystems” modeling approaches based on mechanistic models of plankton biology accounting for physiology, ecology, and trait evolution to explore and explain complex observational data and ultimately improve the predictions of global ocean models.

Carbon and Nitrogen Dynamics in Mexican Management Forest: Simulation of Biomass Harvesting and C and N Amendments

Sustainable silvicultural management requires the maintenance of long-term ecosystem processes. We used the CENTURY model to simulate the impact of wood extraction and organic amendments on aboveground biomass, carbon (C) storage, and the availability of nitrogen (N) in the two dominant silvicultural methods in Mexico: the silvicultural development method (SDM) and irregular forest management (IFM). The values of the mean absolute percentage error for the SDM and IFM were 2.1% and 3.3% for C in aboveground biomass, 5.7% and 5.0% for soil organic carbon (SOC), and 14.9% and 21.6% for N, respectively. Simulation for the SDM (1967–2068) suggested a reduction of ~7% in C in soil, microbial biomass, and litter, 9% in aboveground biomass C, and ~20% in the mineral N available. For IFM, the simulation (2009–2019) suggested a reduction of 14% in the accumulation of aboveground biomass and 13% in the mineral N available. Simulation of the adoption of management practices suggested that N mineral availability would increase by 2%–3% without drastically reducing the SOC, improving aboveground biomass production by ~7%, in each management system. Study Implications In Mexico, current silvicultural management is causing alterations in the biological and chemical processes of the soil, but the future impacts on the production of forest wood and loss of fertility cannot be estimated by direct measurements. We simulated two silvicultural management alternatives with two rotation cycles and measured the response in terms of SOC, nitrogen availability, and aboveground biomass. The model shows that improving forest residue management by adding organic amendments to the soil would counteract changes in soil microbial activity, nitrogen availability, SOC, and aboveground biomass in the future. Managers should consider this information to reorient current crop residue management to achieve the objectives and the sustainability of forest management in Mexican temperate forests.

Site Specific Nutrient Management Options for Achieving Higher Yields in Cotton and Chickpea under Rainfed Condition

A study was undertaken to delineation of spatial variability of soil fertility status in order to prepare soil available nutrient maps for improved productivity in different crops grown in the study area of Kurnool revenue division in Kurnool district of Andhra Pradesh state using remote sensing and GIS techniques. The Knowledge of spatial-variability is critical for site specific nutrient management in soil fertility to obtain higher yields. Soil sample (350) were collected from surface from 350 selected sites for preparing precise digital maps using point, line and polygon tools of the Geographic Information System (GIS) with ArcGIS software 10.3 was used for database creation and for creating the union of various thematic maps. The spatial variability maps were generated and delineated into different zones for N, P and K. Soil available Nitrogen, Phosphorus and Potassium spatial variability values generated from the thematic maps of Kurnool division were used to establish fertilizer recommendations for cotton in kharif and Chickpea in rabi during 2018-19 seasons. The recommended doses of Nitrogen (RDN) that worked for cotton were 401 to 450, 351 to 400, > 450 and < 350 kg ha-1 for the areas with Nitrogen availability of 140 to 210, 210 to 280, <140 and > 280 kg/ha, respectively. The Phosphorous fertilizer recommendation for soils with available P of < 30 kg/ha and > 30 kg/ha was figured out as > 250 kg ha-1 and < 250 kg ha-1, respectively. For soil available Potassium recorded 230 to 560, < 230 and > 560 kg/ha, the K recommendation was figured out as 301 to 400, > 401 and < 300 kg/ha, respectively. Recommended doses of Nitrogen (RDN) was worked out for chickpea were 51 to 75, > 76 and < 50 kg/ha for the areas with available N ranges of 184 to 280, < 184 and > 280 kg/ha, respectively. The Phosphatic fertilizer recommendation for soils of available P of < 23.5 and 23.5 to 40 kg/ha was figured out as > 200 kg/ha and 171 to 200 kg/ha, respectively. For the soil available potassium recorded 253 to 412, 413 to 570, < 253 and > 570 kg/ha, the K recommendation was figured out as 66 to 100, 31 to 65, > 100 and < 30 kg/ha, respectively.