Effect of incubation time and substrate concentration on N-uptake rates by phytoplankton in the Bay of Bengal

International audienceWe report here the results of three experiments, which are slight variations of the 15N method (JGOFS protocol) for determination of new production. The first two test the effect of (i) duration of incubation time and (ii) concentration of tracer added on the uptake rates of various N-species (nitrate, ammonium and urea) by marine phytoplankton; while the third compares in situ and deck incubations from dawn to dusk. Results indicate that nitrate uptake can be underestimated by experiments where incubation times shorter than 4h or when more than 10% of the ambient concentration of nitrate is added prior to incubation. The f-ratio increases from 0.28 to 0.42 when the incubation time increases from two to four hours. This may be due to the observed increase in the uptake rate of nitrate and decrease in the urea uptake rate. Unlike ammonium [y{=}2.07x{-}0.002\, (r2=0.55)] and urea uptakes [y{=}1.88x{+}0.004 (r2=0.88)], the nitrate uptake decreases as the concentration of the substrate (x) increases, showing a negative correlation [y{=}-0.76x+0.05 (r2=0.86)], possibly due to production of glutamine, which might suppress nitrate uptake. This leads to decline in the f-ratio from 0.47 to 0.10, when concentration of tracer varies from 0.01 to 0.04? M. The column integrated total productions are 519 mg C m-2 d-1 and 251 mg C m-2 d-1 for in situ and deck incubations, respectively. The 14C based production at the same location is ~200 mg C m-2 d-1, which is in closer agreement to the 15N based total production measured by deck incubation

Measurement of marine productivity using 15N and 13C tracers: Some methodological aspects

Various experiments involving the measurement of new, regenerated and total productivity using 15N and 13C tracers were carried out in the Bay of Bengal (BOB) and in the Arabian Sea. Results from 15N tracer experiments indicate that nitrate uptake can be underestimated by experiments with incubation time <4 hours. Indirect evidence suggests pico- and nano-phytoplankton, on their dominance over microphytoplankton, can also influence the f-ratios. Difference in energy requirement for assimilation of different nitrogen compounds decides the preferred nitrogen source during the early hours of incubation. Variation in light intensity during incubation also plays a significant role in the assimilation of nitrogen. Results from time course experiments with both 15N and 13C tracers suggest that photoinhibition appears significant in BOB and the Arabian Sea during noon. A significant correlation has been found in the productivity values obtained using 15N and 13C tracers

Nitrogen uptake rates and f-ratios in the Equatorial and Southern Indian Ocean

We report data on nitrate, ammonium and urea uptake rates from the Equatorial and Southern Indian Oceans. Productivity (0.81–2.23 mmol N m–2 d–1) over the Equatorial Indian Ocean was low, but the f-ratio (0.13–0.45) was relatively high. In the Southern Indian Ocean total N-uptake rate varied from 1.7 to 12.3 mmol Nm–2 d–1; it was higher in the Antarctic coast (69S) and lower over most of the Southern Ocean, the lowest being at 58S. The f-ratio also showed significant spatial variation, but was higher compared to values at the Equatorial Indian Ocean. The mean f-ratio in the Southern Indian Ocean was 0.50. The nitrate-specific uptake rates and f-ratios appear to have increased significantly in the recent past relative to earlier estimates. While productivity in the Southern Ocean is comparable to that in the Equatorial Indian Ocean, higher f-ratios in the former underscore its importance in the uptake of CO

Intra-annual oxygen isotope variations in Central Indian teak cellulose: possibility of improved resolution for past monsoon reconstruction

A clear seasonal cycle has been detected in the intraannual oxygen isotope variations (δ18O) of cellulose from several annual growth rings of teak trees that grew in central India. This persistent cycle is marked by higher δ18O values at the ring boundaries and lower δ18O values at intermediate parts. The amplitude of this seasonal cycle varies up to 6.8‰. Based on the pattern of teak growth reported in the literature and a plant physiological model that interprets the δ18O of plant cellulose, it seems possible to identify subsections of a ring that formed during pre-monsoon, peak-monsoon and post-monsoon. Comparison of the δ18O profile of a ring (year AD 1971), analysed with the highest resolution, and a model profile based on concurrent local meteorological data indicates the possibility of achieving a ~20 day resolution in monsoon reconstruction by intra-annual δ18O measurements

Natural isotopic composition of nitrogen in suspended particulate matter in the Bay of Bengal

International audienceWe present the first measurement of nitrogen isotopic composition (?15N) in suspended particulate matter (SPM) of the surface Bay of Bengal (BOB) at 24 different locations during pre- (April?May 2003) and post- (September?October 2002) monsoon seasons. The ?15N of particulate organic nitrogen (PON) in surface suspended matter of coastal as well as northern open BOB shows signatures of a two end-member mixing between continental inputs and marine sources. Dilution by the organic and detrital continental material brought in by rivers leads to consistently lower ?15N, evident from the relationship between surface salinity and ?15N. ?15N of surface PON of open ocean locations during both seasons, and also at coastal locations during pre-monsoon suggest the nitrate from deeper waters as a predominant source of nutrient for planktons. The depth profiles of ?15N of SPM during pre-monsoon season at nine different locations are also presented. These indicate an increase in ?15N by a maximum of 2.8? between euphotic depth and 300 m, which is lower than that observed in the eastern Indian Ocean, indicating the role of higher sinking rates of particles ballasted by aggregates of organic and mineral matter in BOB

Nitrogen Use Efficiency in Tomato (Lycopersicon esculentum L.) and French Bean (Phaseolus vulgaris L.) as Influenced by Coating of Urea with Neem Oil and Graded Levels of Nitrogen

In a pot-culture study, 'Arka Shrestha' tomato and 'Arka Komal' French bean were raised on red sandy-loam to compare urea coated with neem oil (2% w/w, NOCU) and prilled urea (PU) applied at 60, 80 and 100% of recommended N dose. To facilitate direct measurement of N use parameters, urea enriched with 15N (1 atom per cent excess) was used as the source of N. Compared to 'no urea' control, the application of N significantly increased dry matter production, fruit/pod yield as well as the parameters of N use. Prilled urea coated with neem oil (NOCU) was superior to PU in both the crops and produced 21% and 9% higher yield compared to the latter. Increasing the dose of N significantly increased dry matter production, yield and all parameters of N use. However, the interaction effects showed that N applied as NOCU at 80% the of recommended dose produced fruit/pod yield at par with that obtained at 100% of the recommended dose applied as PU in both crops. Corresponding fertilizer utilization achieved was 14.9% and 59.0% when 80% of N was applied as NOCU compared to 11.5% and 30.1 obtained when 100% of N was applied as PU in tomato and French bean, respectively

Wireless sensor networks for applied research on rain-fed farming in India: an exploratory user experiment

This document describes a user experiment that was conducted between October 2007 and February 2008 around a wireless sensor network for rain-fed agriculture. The experiment was organized jointly by UAS, the CK Trust and the EPFL. The participants came from an Indian university of agriculture, an Indian non-governmental organization active in rain-fed farming sustainability, and the Kanrnataka department of agriculture

Reconstruction of Late Quaternary Climate From a Paleo-Lacustrine Profile in the Central (Kumaun) Himalaya: Viewing the Results in a Regional Context

In this paper, we reconstruct the climatic changes starting from the late Pleistocene to the early Holocene as recorded from a fluvio-lacustrine section located within the Kumaun Central Himalaya. The results suggest two major climatic events corresponding with the Last Glacial Maximum (LGM) and Older Dryas (OD). The values of carbon isotopes vary between -23‰ and -14‰, along with a shift in vegetation pattern. The lower part of the section shows prevalence of C3 type vegetation, indicating warm and moist conditions at around 25,000 years BP, possibly coinciding with the intensification of the Indian Summer Monsoon. The onset of cold and arid phase is evident in the gradual shift in vegetation pattern from C3 to C4 plants, which is prominently observed in the middle part of the paleolake profile. Eventually, as the value of δ13C during this time interval confirms, a prolonged phase of cold and arid climate sets in, coinciding with the strengthening of winter westerlies. This cold phase is dated at ∼19,000 years BP and the extended phase of cold interval observed at Dwarahat profile correlates well with previous results elsewhere from the Himalaya. The profile also shows that the LGM phase gradually transforms into a warm and moist climate. This transition registered at 200 cm above the base of the profile, marks the end of glacial period. The short, yet a clear warm spike could be related to the oscillation of Bølling-Allerød interstadial at ∼15,000 years BP. A significant negative excursion marked by an abrupt increase in δ13C values from -20‰ to -14‰ observed toward the top part of the profile, however, is reflective of the reduced monsoon precipitation, corresponding possibly with OD. The topmost part of the profile that registers a depleted trend in δ13C values with dominance of C3 vegetation marks the return of the warm and moist climate

Effect of high level iron enrichment on potential nitrogen uptake by marine plankton in the Southern Ocean

Iron fertilization of the Southern Ocean is believed to counter the increasing CO 2 concentration in the atmosphere and the consequent global warming. Though a number of large scale iron enrichment experiments have been done in the recent past in different parts of the world ocean, little effort has been made to understand the effect of iron enrichment on nitrogen uptake rates and f-ratios. Here we assess the effect of iron addition on N-uptake rates and f-ratio in the Indian sector of the Southern Ocean