Decay rates and nitrogen dynamics of decomposing watercress (Nasturtium officinale R.Br.)

We examined the decomposition of watercress in the laboratory at 10° and 20 °C, and in the field. Rates varied from 0.058 g g^?1 day^?1 in the laboratory to 0.115 g g^?1 day^?1 in the field. There was a rapid generation of particles of size <500 µm. It is thought that washout of these from the litterbags in the field accounted for high field decomposition rate. Formation of dissolved nitrogen compounds during decomposition followed a time series from NH _inf4 ^sup+ to NO _in2 ^sup? to NO _inf3 ^sup? withdissolved organic nitrogen accumulating at the end of decomposition. Ammonification rates were 480 and 657 g NH₄-N g^?1 (dry wt) day^?1 and nitrification rates on the decomposing tissue were 640 and 571 µg NO₃-N g^?1 (dry wt) day^?1 at 10° and 20 °C respectively. Fifty-six per cent of the initial plant N was regenerated as NO₃-N 21% as DON and 25% remained as refractory particulate N.     

Relations between uptake and utilization of NO−3 in Pisum growing exponentially under nitrogen limitation

The utilization and translocation of nitrogen was investigated in exponentially growing, nitrogen-limited Pisum sativum L. cv. Marma. The plants were given N daily at exponentially increasing, although suboptimal, relative nitrogen addition rates (R_N) calculated to yield a relative increment in N of 0.06 day^?1 and 0.12 day^?1. After 10 days of NO^?₃ additions (26 days after sowing), the relative growth rate more or less equaled R_N. Uptake of NO^?₃ was several-fold higher than the N requirement for the growth rate set by R_N. The daily addition of NO^?₃ was taken up after 7 to 8 h, resulting in a cyclic behaviour in the NO^?₃ utilization. During the phase of net NO^?₃ influx, the filling phase (0 to 8 h), in vitro nitrate reductase activity (NR activity) and intracellular levels of soluble N in the root increased. In the phase of no net influx of NO^?₃ the depletion phase (8 to 24 h), the plants were entirely dependent on stored N. During this phase both in vitro NR activity and intracellular levels of soluble N decreased. Also the calculated actual rate of NO^?₃ reduction was high in the filling phase, while it was close to zero in the depletion phase. The pattern of these fluctuations indicates that the regulation of NO^?₃ utilization involves an interplay between transmembrane fluxes of NO^?₃, the cytosolic NO^?₃ concentration and NR activity. Cyclic fluctuations in N-containing compounds were also found in the xylem. Nitrogen was mainly transported as amino acids. The pattern of NO^?₃ transport in the xylem and the fluctuations in the shoot of in vitro NR activity indicate that a reasoning similar to that for the regulation of NO^?₃ assimilation in the root also applies for the shoot. The results also indicate a substantial supply of amino acids to the xylem through recirculation from the shoot.     

Multi-scale measurements and modeling of denitrification in streams with varying flow and nitrate concentration in the upper Mississippi River basin, USA

Denitrification is an important net sink for NO₃ ^? in streams, but direct measurements are limited and in situ controlling factors are not well known. We measured denitrification at multiple scales over a range of flow conditions and NO₃ ^? concentrations in streams draining agricultural land in the upper Mississippi River basin. Comparisons of reach-scale measurements (in-stream mass transport and tracer tests) with local-scale in situ measurements (pore-water profiles, benthic chambers) and laboratory data (sediment core microcosms) gave evidence for heterogeneity in factors affecting benthic denitrification both temporally (e.g., seasonal variation in NO₃ ^? concentrations and loads, flood-related disruption and re-growth of benthic communities and organic deposits) and spatially (e.g., local stream morphology and sediment characteristics). When expressed as vertical denitrification flux per unit area of streambed (U _denit, in μmol N m^?2 h^?1), results of different methods for a given set of conditions commonly were in agreement within a factor of 2–3. At approximately constant temperature (~20 ± 4°C) and with minimal benthic disturbance, our aggregated data indicated an overall positive relation between U _denit (~0–4,000 μmol N m^?2 h^?1) and stream NO₃ ^? concentration (~20–1,100 μmol L^?1) representing seasonal variation from spring high flow (high NO₃ ^?) to late summer low flow (low NO₃ ^?). The temporal dependence of U _denit on NO₃ ^? was less than first-order and could be described about equally well with power-law or saturation equations (e.g., for the unweighted dataset, U _denit ≈26 * [NO₃ ^?]^0.44 or U _denit ≈640 * [NO₃ ^?]/[180 + NO₃ ^?]; for a partially weighted dataset, U _denit ≈14 * [NO₃ ^?]^0.54 or U _denit ≈700 * [NO₃ ^?]/[320 + NO₃ ^?]). Similar parameters were derived from a recent spatial comparison of stream denitrification extending to lower NO₃ ^? concentrations (LINX2), and from the combined dataset from both studies over 3 orders of magnitude in NO₃ ^? concentration. Hypothetical models based on our results illustrate: (1) U _denit was inversely related to denitrification rate constant (k1_denit, in day^?1) and vertical transfer velocity (v _f,denit, in m day^?1) at seasonal and possibly event time scales; (2) although k1_denit was relatively large at low flow (low NO₃ ^?), its impact on annual loads was relatively small because higher concentrations and loads at high flow were not fully compensated by increases in U _denit; and (3) although NO₃ ^? assimilation and denitrification were linked through production of organic reactants, rates of NO₃ ^? loss by these processes may have been partially decoupled by changes in flow and sediment transport. Whereas k1_denit and v _f,denit are linked implicitly with stream depth, NO₃ ^? concentration, and(or) NO₃ ^? load, estimates of U _denit may be related more directly to field factors (including NO₃ ^? concentration) affecting denitrification rates in benthic sediments. Regional regressions and simulations of benthic denitrification in stream networks might be improved by including a non-linear relation between U _denit and stream NO₃ ^? concentration and accounting for temporal variation.     

Seasonal and geomorphic controls on N and P removal in riparian zones of the US Midwest

Riparian zones are an important strategy to mitigate N and P export to streams. However, their efficiency with respect to nitrate (NO₃ ^?), ammonium (NH₄ ⁺), or soluble reactive phosphorus (SRP) in groundwater remains uncertain in the US Midwest. This study investigates water table fluctuations and NO₃ ^?, NH₄ ⁺, and SRP concentration dynamics in two riparian zone types (outwash vs. glacial till) common in the upper US Midwest. During low water table periods, NO₃ ^? removal was 93 % at WR (outwash site), and 75 % at LWD (glacial till site); but during high water table periods, NO₃ ^? removal efficiencies dropped to 50 % at WR, and 14 % at LWD. Median seasonal mass fluxes of NO₃ ^? removed at WR (9.4–21.7 mg N day^?1 m^?1 of stream length) and LWD (0.4–1.9 mg N day^?1 m^?1) were small compared to other riparian zones in glaciated landscapes. The WR site was a small SRP sink (0.114 and 0.118 mg day^?1 m^?1 during the dry period and wet period, respectively), while LWD acted as a small SRP source to the stream (0.004 mg day^?1 m^?1 during the dry period; 0.075 mg day^?1 m^?1 during the wet period). Both LWD and WR acted as sources of NH₄ ⁺ to the stream with mass fluxes ranging from 0.17 to 7.75 mg N day^?1 m^?1. Although riparian zones in the US Midwest provide many ecosystem services, results suggest they are unlikely to efficiently mitigate N and P pollution in subsurface flow.     

Inorganic Nitrogen Leaching from Organic and Conventional Rice Production on a Newly Claimed Calciustoll in Central Asia

Characterizing the dynamics of nitrogen (N) leaching from organic and conventional paddy fields is necessary to optimize fertilization and to evaluate the impact of these contrasting farming systems on water bodies. We assessed N leaching in organic versus conventional rice production systems of the Ili River Valley, a representative aquatic ecosystem of Central Asia. The N leaching and overall performance of these systems were measured during 2009, using a randomized block experiment with five treatments. PVC pipes were installed at soil depths of 50 and 180 cm to collect percolation water from flooded organic and conventional paddies, and inorganic N (NH₄-N+NO₃-N) was analyzed. Two high-concentration peaks of NH₄-N were observed in all treatments: one during early tillering and a second during flowering. A third peak at the mid-tillering stage was observed only under conventional fertilization. NO₃-N concentrations were highest at transplant and then declined until harvest. At the 50 cm soil depth, NO₃-N concentration was 21–42% higher than NH₄-N in percolation water from organic paddies, while NH₄-N and NO₃-N concentrations were similar for the conventional and control treatments. At the depth of 180 cm, NH₄-N and NO₃-N were the predominant inorganic N for organic and conventional paddies, respectively. Inorganic N concentrations decreased with soil depth, but this attenuation was more marked in organic than in conventional paddies. Conventional paddies leached a higher percentage of applied N (0.78%) than did organic treatments (0.32–0.60%), but the two farming systems leached a similar amount of inorganic N per unit yield (0.21–0.34 kg N Mg⁻¹ rice grains). Conventional production showed higher N utilization efficiency compared to fertilized organic treatments. These results suggest that organic rice production in the Ili River Valley is unlikely to reduce inorganic N leaching, if high crop yields similar to conventional rice production are to be maintained.     

Changes in NO3− and K+ uptake by four species in flowing solution culture in response to increased irradiance

Net rates of NO₃^? and K⁺ uptake were compared for oilseed rape (Brassica napus L. cv. Jet neuf), perennial ryegrass (Lolium perenne L. cv. S23), Italian ryegrass (Lolium multiflorum Lam. cv. Augusta) and wheat (Triticum aestivum L. cv. Fen-man) in flowing solution culture during a 4-day sequence of low-low-high-high natural irradiance. Concentrations of NO₃^? (10 μM) and K⁺ (2.5 μM) in solutions were maintained automatically and hourly variation in net uptake of these ions was measured. During the 2 days of low irradiance (<1 MJ m^?2 day^?1) the uptake rates of both ions by all species were low at <1 mmol NO₃^?, m^?2 h^?1 and <0.4 mmol K⁺ m^?2 h^?1. Uptake increased in each species during the first day of high irradiance (7.90 MJ m^?2 day^?1) to >4 mmol NO₃^? m^?2 h^?1 and >1.4 mmol K⁺ m^?1 h^?1. These higher rates were maintained throughout the following night. The lag-time between maximum irradiance and the onset of the highest acceleration in uptake was greater for NO₃^? (5–8 h) than for K⁺ (≤1 h) in rape, wheat and Italian ryegrass. Uptake of NO₃^?, by perennial ryegrass showed an almost constant acceleration for 18 h following maximum irradiance. In all species the measured maximum inflows (uptake rate per unit root length) of both ions were greater than theoretical maximum potential inflows to a non-competing infinite-sink root in soil, by factors of 7 and 36, respectively, for NO₃^? and K⁺, averaged over all species.     

Nitrogen cycling in forest soils across climate gradients in Eastern China

A ¹⁵N tracing study was carried out to investigate the potential gross nitrogen (N) dynamics in thirteen forest soils in Eastern China ranging from temperate to tropical zones (five coniferous forests, six deciduous broad-leaf forests, one temperate mixed forest, one evergreen broad-leaf forests ecosystems), and to identify the major controlling factors on N cycling in these forest ecosystems. The soil pH ranged from 4.3 to 7.9 and soil organic carbon (SOC) ranged from 6.6 g?kg^?1 to 83.0 g?kg^?1. The potential gross N transformation rates were quantified by ¹⁵N tracing studies where either the ammonium or nitrate pools were ¹⁵N labeled in parallel treatments. Gross mineralization rates ranged from 0.915 μg N g^?1 soil day^?1 to 2.718 μg N g^?1 soil day^?1 in the studied forest soils. The average contribution of labile organic-N (M _Nlab ) to total gross mineralization (M _Nrec +M _Nlab ) was 86% (58% to 99%), indicating that turnover of labile organic N plays a dominant role in the studied forest ecosystems. The gross mineralization rates in coniferous forest soils were significantly lower (ranging between 0.915 and 1.228 μg N g^?1 soil day^?1) compared to broad-leaf forest soils (ranging from 1.621 to 2.718 μg N g^?1 soil day^?1) (p?<?0.01). Thus, the dominant vegetation may play an important role in regulating soil N mineralization. Nitrate production (nitrification) occurred via two pathways, oxidation of NH ₄ ⁺ and organic N the forest soils. Correlations with soil pH indicated that this is a key factor controlling the oxidation of NH ₄ ⁺ and organic N in theses forest ecosystems. NH ₄ ⁺ oxidation decreased with a decline in pH while organic N oxidation increased. The climatic conditions (e.g. moisture status) at the various sites governed the NO ₃ ^? -N consumption processes (dissimilatory NO ₃ ^? reduction to NH ₄ ⁺ (DNRA) or immobilization of NO ₃ ^? ). Total NO ₃ ^? consumption and the proportion of total NO ₃ ^? consumption to total NO ₃ ^? production decreased with an increase in the drought index of ecosystems, showing that strong interactions appear to exist between climatic condition (e.g. the drought index), N mineralization and the rate of DNRA. Interactions between vegetation, climatic conditions govern internal N cycling in these forests soils.     

Taking the pulse of snowmelt: in situ sensors reveal seasonal, event and diurnal patterns of nitrate and dissolved organic matter variability in an upland forest stream

Highly resolved time series data are useful to accurately identify the timing, rate, and magnitude of solute transport in streams during hydrologically dynamic periods such as snowmelt. We used in situ optical sensors for nitrate (NO₃ ^?) and chromophoric dissolved organic matter fluorescence (FDOM) to measure surface water concentrations at 30?min intervals over the snowmelt period (March 21–May 13, 2009) at a 40.5 hectare forested watershed at Sleepers River, Vermont. We also collected discrete samples for laboratory absorbance and fluorescence as well as δ¹⁸O–NO₃ ^? isotopes to help interpret the drivers of variable NO₃ ^? and FDOM concentrations measured in situ. In situ data revealed seasonal, event and diurnal patterns associated with hydrological and biogeochemical processes regulating stream NO₃ ^? and FDOM concentrations. An observed decrease in NO₃ ^? concentrations after peak snowmelt runoff and muted response to spring rainfall was consistent with the flushing of a limited supply of NO₃ ^? (mainly from nitrification) from source areas in surficial soils. Stream FDOM concentrations were coupled with flow throughout the study period, suggesting a strong hydrologic control on DOM concentrations in the stream. However, higher FDOM concentrations per unit streamflow after snowmelt likely reflected a greater hydraulic connectivity of the stream to leachable DOM sources in upland soils. We also observed diurnal NO₃ ^? variability of 1–2?μmol?l^?1 after snowpack ablation, presumably due to in-stream uptake prior to leafout. A comparison of NO₃ ^? and dissolved organic carbon yields (DOC, measured by FDOM proxy) calculated from weekly discrete samples and in situ data sub-sampled daily resulted in small to moderate differences over the entire study period (?4 to 1% for NO₃ ^? and ?3 to ?14% for DOC), but resulted in much larger differences for daily yields (?66 to +27% for NO₃ ^? and ?88 to +47% for DOC, respectively). Despite challenges inherent in in situ sensor deployments in harsh seasonal conditions, these data provide important insights into processes controlling NO₃ ^? and FDOM in streams, and will be critical for evaluating the effects of climate change on snowmelt delivery to downstream ecosystems.     

Root growth as a function of ammonium and nitrate in the root zone

We examined the effect of soil NH₄⁺ and NO₃^? content upon the root systems of field-grown tomatoes, and the influence of constant, low concentrations of NH₄⁺ or NO₃^? upon root growth in solution culture. In two field experiments, few roots were present in soil zones with low extractable NH₄⁺ or NO₃^?; they increased to a maximum in zones having 2μg-N NO₃^? g^?1 soil and 6 μg-N NO₃⁼ g^?1 soil, but decreased in zones having higher NH₄⁺ or NO₃^? levels. Root branching was relatively insensitive to available mineral nitrogen. Plants maintained in solution culture at constant levels of NH₄⁺ or NO₃^?, had similar shoot biomass, but all root parameters – biomass, length, branching and area – were greater under NH4 nutrition than under NO₃^?. These results suggest that the size of root system depends on a functional equilibrium between roots and shoots (Brouwer 1967) and on the balance between soil NH₄⁺ and NO₃^?.     

The nitrogen balance of Raphanus sativus X raphanistrum plants. I. Daily nitrogen use under high nitrate supply

Abstract Growth-chamber cultivated Raphanus plants accumulate nitrate during their vegetative growth. After 25 days of growth at a constant supply to the roots of 1 mol m^?3 (NO^?₃) in a balanced nutrient solution, the oldest leaves (eight-leaf stage) accumulated 2.5% NO^?₃-nitrogen (NO₃-N) in their lamina, and almost 5% NO₃-N in their petioles on a dry weight basis. This is equivalent to approximately 190 and 400 mol^?3 m^?3 concentration of NO^?₃ in the lamina and the petiole, respectively, as calculated on a total tissue water content basis. Measurements were made of root NO^?₃ uptake, NO^?₃ fluxes in the xylem, nitrate uptake by the mesophyll cells, and nitrate reduction as measured by an in vivo test. NO^?₃ uptake by roots and mesophyll cells was greater in the light than in the dark. The NO^?₃ concentration in the xylem fluid was constant with leaf age, but showed a distinct daily variation as a result of the independent fluxes of root uptake, transpiration and mesophyll uptake. NO^?₃ was reduced in the leaf at a higher rate in the light than in the dark. The reduction was inhibited at the high concentrations calculated to exist in the mesophyll vacuoles, but reduction continued at a low rate, even when there was no supply from the incubation medium. Sixty-four per cent of the NO^?₃ influx was turned into organic nitrogen, with the remaining NO^?₃ accumulating in both the light and the dark.     

Nitrogen isotope composition of tomato (Lycopersicon esculentum Mill. cv. T-5) grown under ammonium or nitrate nutrition

Studies that quantify plant δ¹⁵N often assume that fractionation during nitrogen uptake and intra-plant variation in δ¹⁵N are minimal. We tested both assumptions by growing tomato (Lycopersicon esculetum Mill. cv. T-5) at NH₄⁺ or NO^?₃ concentrations typical of those found in the soil. Fractionation did not occur with uptake; whole-plant δ¹⁵N was not significantly different from source δ¹⁵ N for plants grown on either nitrogen form. No intra-plant variation in δ¹⁵N was observed for plants grown with NH⁺₄. In contrast. δ¹⁵N of leaves was as much as 5.8% greater than that of roots for plants grown with NO^?₃. The contrasting patterns of intra-plant variation are probably caused by different assimilation patterns. NH⁺₄ is assimilated immediately in the root, so organic nitrogen in the shoot and root is the product of a single assimilation event. NO^?₃ assimilation can occur in shoots and roots. Fractionation during assimilation caused the δ¹⁵N of NO^?₃ to become enriched relative to organic nitrogen; the δ¹⁵N of NO^?₃ was 11.1 and 12.9% greater than the δ¹⁵N of organic nitrogen in leaves and roots, respectively. Leaf δ¹⁵N may therefore be greater than that of roots because the NO^?₃ available for assimilation in leaves originates from a NO^?₃ pool that was previously exposed to nitrate assimilation in the root.     

Localized and internal effect of nitrate on symbiotic dinitrogen fixation

Alfalfa (Medicago sativa L.) is a deeply rooted perennial legume which, under field conditions, may be exposed to varying NO₃^? concentrations with depth. Our objective was to characterize the effect of localized (deep vs shallow) exposure of alfalfa root systems to NO₃^? on symbiotic N₂ fixation and NO₃^?-N uptake. Cuttings of a single alfalfa plant were grown in vertical split root systems in a controlled environment chamber. The split root system was a rigid acrylic tube (5 cm diam. by 60 cm long) filled with silica sand and divided into upper and lower sections at the 30-cm depth by a 5-mm-thick wax layer. Roots penetrated the wax layer, but mixing of nutrient solutions between the sections was prevented. Nodulation was restricted to the upper section. The plants were subjected for 10 days to the following treatments: both sections of the split root system received nutrient solution containing either 0.5, 5.25, or 10 mM NO₃^?; the upper section received 0.5 mM NO₃^? while the lower section received 10 mM NO₃^?; or the upper section received 10 mM NO₃^? while the lower section received 0.5 mM NO₃^?. Increasing supply of NO₃^? in the nutrient solution to both sections resulted in higher NO₃^?-N uptake, lower nodule mass and lower specific nitrogenase activity. Although NO₃^?-N uptake did not differ, plants exposed to 10 mM NO₃^? for 10 days in the upper, nodulated section of the root system had a 20% lower nodule mass than plants exposed to the same NO₃^? concentration in the lower, non-nodulated section of the root system. Specific nitrogenase activity was not different between these two treatments. Therefore, we conclude that: (1) nodule mass was dependent on two factors, the amount of NO₃^?-N taken up and the concentration of NO₃^? within the nodulated root zone; and (2) specific nitrogenase activity was little affected by the concentration of NO₃^? surrounding the nodules, but was strongly inhibited by NO₃^?-N taken up.     

Soil fertility and fine root dynamics in response to 4 years of nutrient (N,P, K) fertilization in a lowland tropical moist forest,Panama

The question of how tropical trees cope with infertile soils has been challenging to address, in part, because fine root dynamics must be studied in situ. We used annual fertilization with nitrogen (N as urea, 12.5 g N m^?2 year^?1), phosphorus (P as superphosphate, 5 g P m^?2 year^?1) and potassium (K as KCl, 5 g K m^?2 year^?1) within 38 ha of old‐growth lowland tropical moist forest in Panama and examined fine root dynamics with minirhizotron images. We expected that added P, above all, would (i) decrease fine root biomass but, (ii) have no impact on fine root turnover. Soil in the study area was moderately acidic (pH = 5.28), had moderate concentrations of exchangeable base cations (13.4 cmol kg^?1), low concentrations of Bray‐extractable phosphate (PO₄ = 2.2 mg kg^?1), and modest concentrations of KCl‐extractable nitrate (NO₃ = 5.0 mg kg^?1) and KCl‐extractable ammonium (NH₄ = 15.5 mg kg^?1). Added N increased concentrations of KCl‐extractable NO₃ and acidified the soil by one pH unit. Added P increased concentrations of Bray‐extractable PO₄ and P in the labile fraction. Concentrations of exchangeable K were elevated in K addition plots but reduced by N additions. Fine root dynamics responded to added K rather than added P. After 2 years, added K decreased fine root biomass from 330 to 275 g m^?2. The turnover coefficient of fine roots <1 mm diameter ranged from 2.6 to 4.4 per year, and the largest values occurred in plots with added K. This study supported the view that biomass and dynamics of fine roots respond to soil nutrient availability in species‐rich, lowland tropical moist forest. However, K rather than P elicited root responses. Fine roots smaller than 1 mm have a short lifetime (<140 days), and control of fine root production by nutrient availability in tropical forests deserves more study.     

Nitrate uptake and utilization is modulated by exogenous γ-aminobutyric acid in Arabidopsis thaliana seedlings

Exogenously applied GABA modulates root growth by inhibition of root elongation when seedlings were grown in vitro on full-strength Murashige and Skoog (MS) salts, but root elongation was stimulated when seedlings were grown on 1/8 strength MS salts. When the concentration of single ions in MS salts was individually varied, the control of growth between inhibition and stimulation was found to be related to the level of nitrate (NO₃^?) in the growth medium. At NO₃^? concentrations below 40 mM (full-strength MS salts level), root growth was stimulated by the addition of GABA to the growth medium; whereas at concentrations above 40 mM NO₃^?, the addition of GABA to the growth medium inhibited root elongation. GABA promoted NO₃^? uptake at low NO₃^?, while GABA inhibited NO₃^? uptake at high NO₃^?. Activities of several enzymes involved in nitrogen and carbon metabolism including nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (NADH-GOGAT), NADP-dependent isocitrate dehydrogenase (NADP-ICDH), and phosphoenol pyruvate carboxylase (PEPCase) were regulated by GABA in the growth medium. Supplementing 1/8 strength MS medium with 50 mM GABA enhanced the activities of all of the above enzymes except ICDH activities in root tissues. However, at full-strength MS, GABA showed no inhibitory effect on the activities of these enzymes, except on GS in both root and shoot tissues, and PEPCase activity in shoot tissues. Exogenous GABA increased the amount of NR protein rather than its activation status in the tissues. This study shows that GABA affects the growth of Arabidopsis, possibly by acting as a signaling molecule, modulating the activity of enzymes involved in primary nitrogen metabolism and nitrate uptake.     

Effects of aluminium on nitrate uptake and assimilation

A study was conducted to examine the hypothesis that the effects of external Al on NO₃^? uptake and assimilation depend upon the concentration of Al present. Young soybean seedlings [Glycine max (L.) Merrill, cv. Essex], growing under moderate acidity stress at pH 4-2, were exposed to a range of {A1³⁺} in solution for 3d, and to labelled 99 atom %¹⁵NO₃^? during the final hour of Al exposure. Uptake of ¹⁵NO₃^?g^?1 root dry weight was increased by about 28% in the presence of Al at {A1³⁺} below 10 mmolm^?3, and NO₃^? uptake was decreased by about 12% when the {A1³⁺} increased to 44mmoln^?3. The stimulation phase closely paralleled stimulation of root elongation. At higher {A1³⁺}, the inhibition of root elongation was much more severe than that of NO₃^? uptake. There was no indication of a separate effect of Al on root ¹⁵NO₃^? reduction in situ, as the accumulation of reduced ¹⁵N in the root remained a similar percentage of ¹⁵NO₃^? uptake at all {A1³⁺}. At higher {A1³⁺}, the atom %¹⁵N enrichment of the insoluble reduced-N (protein) fraction of root tips increased. This suggests that the Al inhibition of root elongation did not result from disruption of the N supply to the root apex.     

The effects of land use change and irrigation water resource on nitrate contamination in shallow groundwater at county scale

The objective of this study was to assess the impacts of land use changes and irrigation water resource on the nitrate contamination in shallow groundwater. 394 water samples were sampled from the same irrigation wells during a period of five years (from 2002 to 2007) in Huantai County in the North China Plain. NO₃-N concentration in irrigation wells was measured. Geostatistical method combined with GIS technique was used to analyze the spatio-temporal distribution of groundwater NO₃-N concentrations in Huantai County. Land use type and irrigation water resource were combined with the variation of NO₃-N concentrations by statistical approach to investigate the relationship between them. The distribution map showed that the percentages of area increased by 13.06%, 14.37%, 12.23% and 3.85% for that had nitrate concentrations of 10–15, 15–20, 20–30 mg L^?1 and greater than 30 mg L^?1 for shallow groundwater, respectively, while decreased by 28.87% and 14.63% for 0–5 and 5–10 mg L^?1. In the well-irrigated field, the NO₃-N concentrations in shallow groundwater had increased for vegetables, wheat–vegetables and wheat–maize rotations. In contrast, fast-growing tree system could act as a buffer to retain shallow groundwater nitrate content which resulted in reduced NO₃-N concentrations. Under the same land use condition, irrigation with sewage, or well and sewage by turns would both enormously add nitrate to groundwater.     

The Effect of Exogenous IAA and Kinetin on Nitrate Reductase,Nitrite Reductase and Glutamate Dehydrogenase Activities in Excised Pea Roots

Nitrate reductase (NO₃R) activity, nitrite reductase (NO₂R) activity and NADH₂ dependent glutamate dehydrogenase (GDH) activity were followed in extracts from excised pea roots incubated under aseptic conditions for 9 and 24 h in nitrate containing nutrient medium to which IAA was added in concentrations promoting lateral root formation (1 × 10^?5; 3 × 10^?5; 5 × 10^?5 M) and kinetin in concentrations which reduce lateral root formation (0.1; 1; 5 mg 1^?1, that is 4.65 × 10^?7;4.65 × 10^?6 and 2.3 × 10^?5 M). NO₃R activity was not influenced by IAA, NO₂R activity was slightly depressed by IAA after 24 h incubation and GDH activity was slightly increased after 24 h incubation in the presence of IAA. Kinetin decreased NO₃R activity significantly both after 9 h and 24 h incubation, slightly increased NO₂R activity after 9 h incubation but slightly decreased it after 24 h incubation, and did not affect GDH activity after 24 h incubation. However, when applied together with IAA, kinetin abolished the promoting effect of IAA on GDH activity. IAA neither reversed nor accentuated the effect of kinetin on NO₂R activity. Nevertheless the depressing effect of kinetin on NO₃R activity was emphasized by the presence of IAA after 9 h incubation. The results obtained indicate that reduced nitrate assimilation due to the depression of nitrate reductase activity caused by kinetin probably contributes to the negative growth effect of kinetin in pea root segments grown in nitrate medium.     

KINETICS OF CELL DEATH IN THE CYANOBACTERIUM ANABAENA FLOS-AQUAE AND THE PRODUCTION OF DISSOLVED ORGANIC CARBON1

Kinetics of cell death and the production of dissolved organic carbon (DOC) were investigated in Anabaena flos-aquae (Lyngb.) Bréb grown on three different N sources (N₂nitrate, and ammonium) in a phosphorus (P)-limited chemostat. The fraction of live cells in the total population increased as growth rate increased with decreasing P limitation. Cell death was less in nitrate and ammonium media than in N₂. The specific death rate (γ), when calculated as the slope ofv^?1_x vs. D^?1, where v_xand D are live cell fraction (or cell viability) and dilution rate, respectively, was 0. 0082 day^?1 in N₂and 0.0042 day^?1 in nitrate. The slope of the plot in ammonium culture was not significant; however, the value of the live cell fraction was within the range for the NO^?₃culture. The fraction of live vegetative cells in N₂ culture was constant at all growth rates and the increase in the overall live cell fraction with growth rate was due entirely to an increase in live heterocysts. Live heterocysts comprised 3.5% of the total cells at a growth rate of 0.25 day^?1 and increased to 6.3% at 0.75 day^?1 with the ratio of live heterocysts to live vegetative cells linearly increasing with growth rate. The fraction of live vegetative cells was invariant in nitrate cultures us in N₂cultures. The live heterocysts fraction also increased with growth rate in nitrate cultures, along with the live heterocysts : live vegetative cells ratio, but the level was lower than in N₂cultures. DOC released from dead cells increased inversely with growth rate in N₂from 36.4% of the total DOC at a growth rate of 0.75 day^?1 to 54.15% at 0.25 day^?1. The contribution of cell death to the total DOC production in nitrate and ammonium media was significantly less than that under N₂DOC from dead cells consisted mainly of high-molecular-weight compounds, whereas DOC excreted from live cells was largely of low molecular weight.     

不同氮素水平对油麦菜栽培效果的综合评价分析

为探讨不同氮素水平的硝态氮与铵态氮对油麦菜(Lactuca sativa L.)生长及品质的影响,自主设计并制作了适合蔬菜水培的立体管道栽培系统,配制了NO3-：NH4+=10：0,NO3-：NH4+=7：1,NO3-：NH4+=7：3的营养液,通过模糊数学原理,采用层次分析法(AHP)确定品质的权重值,综合评价了不同氮素水平对油麦菜的栽培效果。结果表明,以NO3-：NH4+=7：3营养液的栽培效果最好,其次是NO3-：NH4+=7：1。当营养液的NO3-：NH4+=7：3时,油麦菜的产量最高,叶片叶绿素、可溶性糖、可溶性蛋白质含量最高,硝酸盐含量最少,其综合品质最好。层次分析法对油麦菜不同氮素水平营养液的栽培效果的综合评价有较好的适用性,有助于获得南方设施管道栽培油麦菜最佳的营养液配方。