Waterlogging and fire impacts on nitrogen availability and utilization in a subtropical wet heathland (wallum)

Protein, amino acids and ammonium were the main forms of soluble soil nitrogen in the soil solution of a subtropical heathland (wallum). After fire, soil ammonium and nitrate increased 90- and 60-fold, respectively. Despite this increase in nitrate availability after fire, wallum species exhibited uniformly low nitrate reductase activities and low leaf and xylem nitrate, During waterlogging soil amino acids increased, particularly gamma-aminobutyric acid (GABA) which accounted for over 50% of amino nitrogen. Non-mycorrhizal wallum species were significantly (P < 0.05) N-15-enriched (0.3-4.3 parts per thousand) compared to species with mycorrhizal associations (ericoid-type, ecto-, va-mycorrhizal) which were strongly depleted in N-15 (-6.3 to -1.8 parts per thousand). Lignotubers and roots had delta(15)N signatures similar to that of the leaves of respective species. The exceptions were fine roots of ecto-, ecto/va-, and ericoid type mycorrhizal species which were enriched in N-15 (0.1-2 4 parts per thousand). The delta(15)N signatures of delta(15)N(total soil N) and delta(15)N(soil NH4+) were in the range 3.7-4.5 parts per thousand, whereas delta(15)N(soil NO3-) was significantly (P < 0.05) more enriched in N-15 (9.2-9.8 parts per thousand). It is proposed that there is discrimination against N-15 during transfer of nitrogen from fungal to plant partner. Roots of selected species incorporated nitrogen sources in the order of preference: ammonium > glycine > nitrate. The exception were proteoid roots of Hakea (Proteaceae) which incorporated equal amounts of glycine and ammonium.

Bulk 15N, and 13Corg ratios, and calculation of unutilized nitrate of surface samples off the southwest coast of Africa (Table 1)

Bulk delta15N values in surface sediment samples off the southwestern coast of Africa were measured to investigate the biogeochemical processes occurring in the water column. Nitrate concentrations and the degree of utilization of the nitrate pool are the predominant controls on sedimentary delta15N in the Benguela Current region. Denitrification does not appear to have had an important effect on the delta15N signal of these sediments and, based on delta15N and delta13C, there is little terrestrial input.

Nitrogen use strategies of neotropical rainforest trees in threatened Atlantic Forest

The characteristics of nitrogen acquisition, transport and assimilation were investigated in species of an Atlantic Forest succession over calcareous soil in south-eastern Brazil. Differences in behaviour were observed within the regeneration guilds. Pioneer species showed high leaf nitrogen contents, a high capacity to respond to increased soil nitrogen availability, a high capacity for leaf nitrate assimilation and were characterized by the transport of nitrate + asparagine. At the other end of the succession, late secondary species had low leaf nitrogen contents, little capacity to respond to increased soil nitrogen availability, low leaf nitrate assimilation and were active in the transport of asparagine + arginine. The characteristics of nitrogen nutrition in some early secondary species showed similarities to those of pioneer species whereas others more closely resembled late secondary species. Average leaf delta(15)N values increased along the successional gradient. The results indicate that the nitrogen metabolism characteristics of species may be an additional ecophysiological tool in classifying tropical forest tree species into ecological guilds, and may have implications for regeneration programmes in degraded areas.

Sedimentology and Th, Pa, U, and N isotopic composition of core MD84-527

High-resolution records of opal, carbonate, and terrigenous fluxes have been obtained from a high-sedimentation rate core (MD84-527: 43°50'S; 51°19'E; 3269 m) by normalization to 230Th. This method estimates paleofluxes to the seafloor on a point-by-point basis and distinguishes changes in sediment accumulation due to variations in vertical rain rates from those due to changes in syndepositional sediment redistribution by bottom currents. We also measured sediment delta15N to evaluate the changes in nitrate utilization in the overlying surface waters associated with paleoflux variations. Our results show that opal accumulation rates on the seafloor during the Holocene and stage 3, based on 14C dating, were respectively tenfold and fivefold higher than the vertical rain rates, At this particular location, changes in opal accumulation on the seafloor appear to be mainly controlled by sediment redistribution by bottom currents rather than variations in opal fluxes from the overlying water column. Correction for syndepositional sediment redistribution and the improved time resolution that can be achieved by normalization to 230Th disclose important variations in opal rain rates. We found relatively high but variable opal paleoflux during stage 3, with two maxima centered at 36 and 30 kyr B.P., low opal paleoflux during stage 2 and deglaciation and a pronounced maximum during the early Holocene, We interpret this record as reflecting variations in opal production rates associated with climate-induced latitudinal migration of the southern ocean frontal system. Sediments deposited during periods of high opal paleoflux also have high authigenic U concentrations, suggesting more reducing conditions in the sediment, and high Pa-231/Th-230 ratios, suggesting increased scavenging from the water column. Sediment delta15N is circa 1.5 per mil higher during isotopic stage 2 and deglaciation. The low opal rain rates recorded during that period appear to have been associated with increased nitrate depletion. This suggests that opal paleofluxes do not simply reflect latitudinal migration of the frontal system but also changes in the structure of the upper water column. Increased stratification during isotopic stage 2 and deglaciation could have been produced by a meltwater lid, leading to lower nitrate supply rates to surface waters.

Calculated sea surface temperatures of two sediment cores

The reconstruction of ocean history employs a large variety of methods with origins in the biological, chemical, and physical sciences, and uses modern statistical techniques for the interpretation of extensive and complex data sets. Various sediment properties deliver useful information for reconstructing environmental parameters. Those properties that have a close relationship to environmental parameters are called ''proxy variables'' (''proxies'' for short). Proxies are measurable descriptors for desired (but unobservable) variables. Surface water temperature is probably the most important parameter for describing the conditions of past oceans and is crucial for climate modelling. Proxies for temperature are: abundance of microfossils dwelling in surface waters, oxygen isotope composition of planktic foraminifers, the ratio of magnesium or strontium to calcium in calcareous shells or the ratio of certain organic molecules (e.g. alkenones produced by coccolithophorids). Surface water salinity, which is important in modelling of ocean circulation, is much more difficult to reconstruct. At present there is no established method for a direct determination of this parameter. Measurements associated with the paleochemistry of bottom waters to reconstruct bottom water age and flow are made on benthic foraminifers, ostracodes, and deep-sea corals. Important geochemical tracers are d13C and Cd/Ca ratios. When using benthic foraminifers, knowledge of the sediment depth habitat of species is crucial. Reconstructions of productivity patterns are of great interest because of important links to current patterns, mixing of water masses, wind, the global carbon cycle, and biogeography. Productivity is reflected in the flux of carbon into the sediment. There are a number of fluxes other than those of organic carbon that can be useful in assessing productivity fluctuations. Among others, carbonate and opal flux have been used, as well as particulate barite. Furthermore, microfossil assemblages contain clues to the intensity of production as some species occur preferentially in high-productivity regions while others avoid these. One marker for the fertility of sub-surface waters (that is, nutrient availability) is the carbon isotope ratio within that water (13C/12C, expressed as d13C). Carbon isotope ratios in today's ocean are negatively correlated with nitrate and phosphate contents. Another tracer of phosphate content in ocean waters is the Cd/Ca ratio. The correlation between this ratio and phosphate concentrations is quite well documented. A rather new development to obtain clues on ocean fertility (nitrate utilization) is the analysis of the 15N/14N ratio in organic matter. The fractionation dynamics are analogous to those of carbon isotopes. These various ratios are captured within the organisms growing within the tagged water. A number of reconstructions of the partial pressure of CO2 have been attempted using d13C differences between planktic and benthic foraminifers and d13C values of bulk organic material or individual organic components. To define the carbon system in sea water, two elements of the system have to be known in addition to temperature. These can be any combination of total CO2 , alkalinity, or pH. To reconstruct pH, the boron isotope composition of carbonates has been used. Ba patterns have been used to infer the distribution of alkalinity in past oceans. Information relating to atmospheric circulationand climate is transported to the ocean by wind or rivers, in the form of minerals or as plant andanimal remains. The most useful tracers in this respect are silt-sized particles and pollen.

d15N of trap WR2

Temporal changes in d15N values of sinking particles collected with sediment traps in the Benguela upwelling regime off southwest Africa mirrored variations in the input of inorganic nitrogen to the surface water. Reductions in d15N (to as low as 2.5 per mil) corresponded to low sea surface temperatures during austral spring and late austral autumn/early winter, indicating increased nitrate availability due to the presence of recently upwelled water. High particulate fluxes accompanied the low d15N values and sea surface temperatures, reflecting increased productivity, fueled by the upwelled nutrients. High d15N values (up to 13.1 per mil) coincided with high sea surface temperatures and low particle fluxes. In this area, the seaward extension of upwelling filaments, which usually occurs twice yearly, brings nutrient-rich water to the euphotic zone and leads to elevated productivity and relatively lower d15N values of the particulate nitrogen. Satellite images of ocean chlorophyll show that productivity variations coincide with d15N changes. The observed isotopic pattern does not appear to have been caused by variations in the species composition of the phytoplankton assemblage. Calculations based on d15N of the sinking particulate nitrogen show that the surface nitrate pool was more depleted during late austral summer/early fall and mid-winter and that supply exceeded demand during the intense spring bloom and in late austral fall. The main uncertainty associated with these estimates is the effect of diagenesis on d15N and possible variability in preservation of the isotope signal between periods of high and low particle flux.

Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency

We present new nitrogen isotope data from the water column and surface sediments for paleo-proxy validation collected along the Peruvian and Ecuadorian margins between 1°N and 18°S. Productivity proxies in the bulk sediment (organic carbon, total nitrogen, biogenic opal, C37 alkenone concentrations) and 15N/14N ratios were measured at more than 80 locations within and outside the present-day Peruvian oxygen minimum zone (OMZ). Microbial N-loss to N2 in subsurface waters under O2 deficient conditions leaves a characteristic 15N-enriched signal in underlying sediments. We find that phytoplankton nutrient uptake in surface waters within the high nutrient, low chlorophyll (HNLC) regions of the Peruvian upwelling system influences the sedimentary signal as well. How the d15Nsed signal is linked to these processes is studied by comparing core-top values to the 15N/14N of nitrate and nitrite (d15N[NOx]) in the upper 200 m of the water column. Between 1°N and 10°S, subsurface O2 is still high enough to suppress N-loss keeping d15NNOx values relatively low in the subsurface waters. However d15N[NOx] values increase toward the surface due to partial nitrate utilization in the photic zone in this HNLC portion of the system. d15N[sed] is consistently lower than the isotopic signature of upwelled [NO3]-, likely due to the corresponding production of 15N depleted organic matter. Between 10°S and 15°S, the current position of perennial upwelling cells, HNLC conditions are relaxed and biological production and near-surface phytoplankton uptake of upwelled [NO3]- are most intense. In addition, subsurface O2 concentration decreases to levels sufficient for N-loss by denitrification and/or anammox, resulting in elevated subsurface d15N[NOx] values in the source waters for coastal upwelling. Increasingly higher production southward is reflected by various productivity proxies in the sediments, while the north-south gradient towards stronger surface [NO3]- utilization and subsurface N-loss is reflected in the surface sediment 15N/14N ratios. South of 10°S, d15N[sed] is lower than maximum water column d15N[NOx] values most likely because only a portion of the upwelled water originates from the depths where highest d15N[NOx] values prevail. Though the enrichment of d15N[NOx] in the subsurface waters is unambiguously reflected in d15N[sed] values, the magnitude of d15N[sed] enrichment depends on both the depth of upwelled waters and high subsurface d15N[NOx] values produce by N-loss. Overall, the degree of N-loss influencing subsurface d15N[NOx] values, the depth origin of upwelled waters, and the degree of near-surface nitrate utilization under HNLC conditions should be considered for the interpretation of paleo d15N[sed] records from the Peruvian oxygen minimum zone.

TOC, TN, and sedimentary d15N and d13C ratios of sediment core SO201-2-85

We present high-resolution records of sedimentary nitrogen (d15Nbulk) and carbon isotope ratios (d13Cbulk) from piston core SO201-2-85KL located in the western Bering Sea. The records reflect changes in surface nitrate utilization and terrestrial organic matter contribution in submillennial resolution that span the last 180 kyr. The d15Nbulk record is characterized by a minimum during the penultimate interglacial indicating low nitrate utilization (~62-80%) despite the relatively high export production inferred from opal concentrations along with a significant reduction in the terrestrial organic matter fraction (mterr). This suggests that the consumption of the nitrate pool at our site was incomplete and even more reduced than today (~84%). d15Nbulk increases from Marine Isotope Stage (MIS) 5.4 and culminates during the Last Glacial Maximum, which indicates that nitrate utilization in the Bering Sea was raised during cold intervals (MIS 5.4, 5.2, 4) and almost complete during MIS 3 and 2 (~93-100%). This is in agreement with previous hypotheses suggesting that stronger glacial stratification reduced the nutrient supply from the subeuphotic zone, thereby increasing the iron-to-nutrient ratio and therefore the nitrate utilization in the mixed surface layer. Large variations in d15Nbulk were also recorded from 180 to 130 ka BP (MIS 6), indicating a potential link to insolation and sea-level forcing and its related feedbacks. Millennial-scale oscillations were observed in d15Nbulk and d13Cbulk that might be related to Greenland interstadials.

(Table 1) Comparison of diatom-bound and bulk sedimentary d15N values of ODP Holes 175-1082A and 175-1084A

The Matuyama Diatom Maximum (MDM) is a time of peak opal accumulation from 2.6 to ~2.0 Ma within the Benguela Current upwelling system that was initiated by increased influence of Southern Ocean water on the eastern South Atlantic. We measured opal, total organic carbon (TOC), and CaCO3 fluxes and C and N stable isotopes in sediments deposited from 2.4 to 1.95 Ma at Sites 1082 and 1084 to explore the biogeochemical dynamics within the Benguela region. The infusion of Southern Ocean water delivered dissolved nutrients and Southern Ocean flora and fauna, resulting in local opal accumulation increasing up to 8 g/cm**2/ky and the production of diatom mats. Some d15N measurements of diatom-bound organic matter indicate that the mats grew within the Benguela region. The bulk sediment d15N records are taken to reflect changes in the d15N of nitrate in the incoming water, where lower values at 2.4 Ma reflect less nitrate utilization in the Antarctic. A long-term increase in relative nitrate uptake in the Southern Ocean is evidenced by the gradual increase in d15N toward 1.9 Ma.

Stable isotope composition of diatom-bound organic matter

Oceanic nutrient cycling in the Southern Ocean is supposed to have an important impact on glacial-interglacial atmospheric CO2 changes and global climate. In order to characterize such nutrient cycling over the last two climatic cycles we investigated carbon and nitrogen isotopic ratios of diatom-bound organic matter (d13Cdiat and d15Ndiat, respectively) in two cores retrieved form the Atlantic and Indian sectors of the Antarctic Ocean. The two cores show the same isotopic patterns. The d13Cdiat values are depleted during glacial periods and enriched during interglacial periods, indicating lower productivity during cold times. The d15Ndiat values are enriched during glacial periods and depleted during interglacial periods, arguing for greater nitrate utilization during cold times. Taken at face value, this apparent contradiction leads to opposite conclusions on the role of the Southern Ocean biological pump on the atmospheric CO2 changes. However, the two sets of data can be reconciled by a "sea ice plus mixing rate scenario" that calls upon a balance between the effect of cutting off gas transfer at the ocean-atmosphere boundary and the effect of reducing vertical transport of nutrients through the pycnocline.