Forests losing large quantities of nitrogen have elevated 15N:14N ratios

Summary Urea (U) and ammonium nitrate (AN) had been applied to a Scots pine (Pinus sylvestris L.) forest in northern Sweden for 18 consecutive years at four doses resulting in total N applications ranging from 0 to 1980 kg ha^–1. The ¹⁵N abundance ( ¹⁵N) of the grass Deschampsia flexuosa (L.) Trin. increased linearly (from –0.7 to 11.0) with application rate in the case of U. The response to AN was in the same direction but smaller. While others have shown that the initial response of nitrogen-limited systems to additions of N is a change of ¹⁵N abundance towards that of added N, this study shows that further and excessive additions leads to a retention of ¹⁵N. Monitoring ¹⁵N abundance over time in dose-response trials of this type thus opens new possibilities to estimate critical loads of N and the point of nitrogen saturation.     

Assessment of genetic variability for N2 fixation between and within provenances of Leucaena leucocephala and Acacia albida estimated by 15N labelling techniques

Nitrogen fixed in 13 provenances of Acacia albida and 11 isolines of Leucaena leucocephala inoculated with effective Rhizobium strains was measured by ¹⁵N techniques and the total N difference method. In the test soil, on the average, L. leucocephala derived about 65% of its total N from atmospheric N₂ fixation compared to about 20% by A. albida. Significant differences in the percentage of N derived from atmospheric N₂ (% Ndfa) occurred, between provenances or isolines within species. The % Ndfa ranged from 37 to 74% within L. leucocephala and from 6 to 37 within A. albida; (equivalent to 20–50 mg N plant^–1 and 4–37 mg N plant^–1 for the two species over three months, respectively) and was correlated with the nodule mass (r=0.91). The time course of N₂ fixation of three selected provenances (low, intermediate and good fixers) was followed at 12 weekly intervals over a 36 week period. The % Ndfa of all provenances and isolines increased with time; and except for one of the L. leucocephala provenances, % Ndfa was similar within species at the 36 weeks harvest. There was a significant correlation between % Ndfa and the amount of N₂ fixed (r=0.96). Significant interactions occurred between provenances and N treatments and often growth of uninoculated but N fertilized plants was less variable than for inoculated unfertilized plants.     

Influence of salt stress on primary metabolism of Zea mays L. seedlings of model genotypes

Short-term studies for comparing some primary metabolic and growth-responses to salt stress in seedlings of two maize genotypes differing in drought resistance were carried out under controlled conditions. Both genotypes revealed high yielding ability in favourable environments. Treatments: Control (Hoagland-Arnon No 1 solution) and salt stress (Hoagland-Arnon solution plus NaCl, _s = –0.84MPa). It was found that in both genotypes the activity of the principal metabolic pathway supplying reduced nitrogen (¹⁵N) for the synthesis of amino acids and proteins as well as the assimulatory number (¹⁴CO₂—assimilation relation rate per chlorophyll unit) were decreased under the effect of the stress. These effects were more marked in the resistant genotype. In this genotype the stress induced metabolic activity decline was accompanied by a corresonding reduction of the relative growth rate. Conversely, continuing growth, resulting probably from accumulation of solutes, was observed in the susceptible genotype.On the basis of these and other observations it is assumed that the resistant genotype manifests short-term energy saving stress reactions.     

Nitrogen fixation and growth of soybean as influenced by varying the methods of inoculation with Bradyrhizobium japonicum

The effects of inoculating soil with a water suspension of Bradyrhizobium japonicum (i) at seeding, (ii) 7, or (iii) 14 days after planting (DAP), (iv) seed slurry inoculation and (v) seed slurry supplemented with postemergence inoculation of a water suspension of Bradyrhizobium at 7 or (vi) 14 DAP, on nodulation, N₂ fixation and yield of soybean (Glycine max. [L.] Merrill) were compared in the greenhouse. The ¹⁵N isotope dilution technique was used to quantify N₂ fixed at flowering, early pod filling and physiological maturity stages (36, 52 and 70 DAP, respectively). On average, the water suspension inoculation formed the greatest number of nodules, and seed plus postemergence inoculation formed slightly more nodules than the seed-only inoculated plants (27, 19 and 12 nodules/plant respectively at physiological maturity). Seed slurry inoculation followed by postemergence inoculation at 14 DAP gave the highest nodule weight, with the plants fixing significantly more (P<0.05) N₂ (125 mg N plant⁻¹ or 56% N) than any other treatment (mean, 75 mg plant⁻¹ or 35% N). However, the higher N₂ fixation was not translated into higher N or dry matter yields. Estimates of N₂ fixed by the ostemergence Bradyrhizobium inoculations as well as plant yield were not significantly different from those of the seed slurry inoculation. Thus, delaying inoculation (e.g., by two weeks as in this study) did not reduce the symbiotic ability of soybean plants.     

Comparative effects of organic and inorganic nitrogen sources applied to a flooded soil on rice yield and availability of N

A pot experiment was conducted to study the effect of organic and inorganic nitrogen (N) sources on the yield and N uptake of rice from applied and native soil-N. The residual effect of these N sources on a succeeding wheat crop was also studied. Organic N was applied in the form of ¹⁵N-labelled Sesbania aculeata L., a legume, and inorganic N in the form of ¹⁵N-labelled ammonium sulphate. The two sources were applied to the soil separately or together at the time of transplanting rice. Recovery of N by rice from both the applied sources was quite low but both sources caused significant increases in biomass and N yield of rice. Maximum increase was recorded in soil treated with organic N. The residual value of the two materials as source of N for wheat was not significant; the wheat took up only a small fraction of the N initially applied. Loss of N occurred from both applied N sources, the losses being more from inorganic N. Both applied N sources caused a substantial increase in the availability of soil-N to rice and wheat; most of this increase was due to organic N and was attributed to the so-called ‘priming’ effect or ANI (added nitrogen interaction) of the applied material.     

15N estimates of nitrogen fixation by white clover (Trifolium repens L.) growing in a mixture with ryegrass (Lolium perenne L.)

The ¹⁵N isotope dilution technique and the N difference method were used to estimate N₂ fixation by clover growing in a mixture with ryegrass, in a field experiment and a controlled environment experiment. Values obtained using N difference were approximately 25% lower than those estimated using ¹⁵N isotope dilution. In the field experiment there was a measured N benefit to grass growing with clover, equivalent to 42.7 kgN ha^-1. The grass in the mixture had a lower atom %¹⁵N content and a higher N content than grass in a monoculture; therefore values for N₂ fixation were different depending on choice of control plant i.e. monoculture or mixture grass. In the controlled environment experiment there were no significant differences between either the atom %¹⁵N contents or the N contents of monoculture grass and grass growing in a mixture with clover. It is concluded that there is a long term indirect transfer of N from clover to associated grass which can lead to errors in estimates of N₂ fixation.     

Nutrients and mass in litter and top soil of ten tropical tree plantations

The importance of litter to nutrient and organic matter storage and the possible influence of species selection on soil fertility in ten stands each consisting of a separate tree species were examined in this study. The plantations had been grown under similar conditions in an arboretum in the Luquillo Experimental Forest, Puerto Rico. The species involved were: Anthocephalus chinensis, Eucalyptus × patentinervis, E. saligna, Hernandia sonora, Hibiscus elatus, Khaya nyasica, Pinus caribaea var. hondurensis, P. elliottii var. densa, Swietenia macrophylla, and Terminalia ivorensis. After 26 yr, litter mass ranged from 5 mg ha^-1 in the H. sonora stand to 27.2 Mg ha^-1 in the P. caribaea stand. Nutrients in the litter (N, P, K, Ca, and Mg) also varied widely, but stands were ranked in different order when ranked by nutrients in the litter than then ranked according to accumulation of mass. Only E. saligna and A. chinensis stands were ranked similarly in accumulation of both nutrients and mass, and the stand of H. elatus was ranked higher with respect to nutrient accumulation than to accumulation of mass. The nutrient concentration in standing leaf litter generally increased in the order of recently fallen <old intact< fragmented. Nutrient concentration of standing leaf litter appears to increase with age and depth in the litter layer. The amount of nutrients stored in the litter compartment of these plantations was in the same order of magnitude as the quantity of available nutrients in the top 10-cm of mineral soil. Total litter mass was negatively correlated with the mass-weighted concentration of N, K, and Mg. The same relationship was found for Ca in the leaf litter and N in the fine wood litter compartments. In some stands (notably P. caribaea, P. elliottii, and E. saligna), leaf litter derived from species other than the species planted in that particular stand had higher nutrient concentration than leaf litter from the planted species. Soils of the 10 stands were classified in the same soil series and had similar texture (clay soils). However, significantly different chemical characteristics were found. Results obtained by analysis of covariance and by limiting comparisons to adjacent stands with similar soil texture, indicate that different species have had different influences on the concentration of available nutrients in soil.     

Mechanism of Agonist-Induced Down-Regulation and Subsequent Recovery of Muscarinic Acetylcholine Receptors in a Clonal Neuroblastoma X Glioma Hybrid Cell Line

The mechanisms of carbachol-induced muscarinic acetylcholine receptor (mAChR) down-regulation, and recovery following carbachol withdrawal, were studied in the neuroblastoma x glioma hybrid NG108-15 cell line by specific ligand binding assays. N-[3H]Methylscopolamine ([3H]NMS) and [3H]quinuclidinyl benzilate ([3H]QNB) were used as the ligands for the cell surface and total cellular mAChRs, respectively. Exposure of cells to 1 mM carbachol for 16 h decreased the specific binding of [3H]NMS and [3H]QNB by approximately 80%. Bacitracin (1-4 mg/ml) and methylamine (1-15 mM), inhibitors of transglutaminase and of endocytosis, prevented agonist-induced loss of surface mAChRs. Pretreatment of cells with the antimicrotubular agents nocodazole (0.1-10 microM) and colchicine (1-10 microM) prevented carbachol-induced loss of [3H]QNB binding, but not that of [3H]NMS binding. These results indicate that agonist-induced mAChR down-regulation occurs by endocytosis, followed by microtubular transport of receptors to their intracellular degradation sites. When carbachol was withdrawn from the culture medium following treatment of cells for 16 h, receptors recovered and were incorporated to the surface membrane. This recovery process was antagonized by monovalent ionophores monensin (0.1 microM) and nigericin (40 nM), which interfere with Golgi complex function. Receptor recovery was also prevented by the antimicrotubular agent nocodazole. Thus, recovery of receptors appears to be mediated via Golgi complex and microtubular transport to the surface membrane.     

GABAA Receptor Complex in an Experimental Model of Hepatic Encephalopathy: Evidence for Elevated Levels of an Endogenous Benzodiazepine Receptor Ligand

The involvement of the gamma-aminobutyric acidA (GABAA) receptor complex in the pathogenesis of hepatic encephalopathy was examined in thioacetamide-treated rats with fulminant hepatic failure. Partially purified extracts from encephalopathic rat brain were approximately three times more potent in inhibiting [3H]Ro 15-1788 binding to benzodiazepine receptors than identically prepared extracts from control rats. High levels of inhibitory activity were also found in extracts of plasma, heart, and liver from thioacetamide-treated rats. The inhibition of [3H]Ro 15-1788 binding by brain extracts appeared to be competitive and reversible and was unaffected by treatment with either proteolytic enzymes or boiling. Further, GABA significantly enhanced the potency of these extracts in inhibiting [3H]flunitrazepam binding. In contrast, no differences were found in radioligand binding to the constituent recognition sites of the GABAA receptor complex in well-washed brain membranes prepared from control and encephalopathic animals. These findings suggest that the recognition-site qualities of the constituent proteins of the GABAA receptor complex are unchanged in an experimental model of hepatic encephalopathy. However, significant elevations in the level of a substance or substances with neurochemical properties characteristic of a benzodiazepine receptor agonist may contribute to the electrophysiological and behavioral manifestations of hepatic encephalopathy.     

Chronic Exposure of NG 108-15 Cells to Opiate Agonists Does Not Alter the Amount of the Guanine Nucleotide-Binding Proteins Gi and GO

We have characterized the pertussis toxin substrate in NG 108-15 cell membranes using site-specific antisera and ADP-ribosylation. Cell membranes contain two pertussis toxin-sensitive guanine nucleotide-binding protein alpha-subunits (G alpha) whose Rf values in gel electrophoresis coincide with those of G alpha o and G alpha i2. The total quantity of Gi and Go immunoreactivity amounted to 24.3 +/- 2.8 pmol/mg, whereas only 1.5 +/- 0.2 pmol/mg are capable of undergoing ADP-ribosylation catalyzed by pertussis toxin. Pretreatment of cells with the agonist [D-Ala2,D-Leu2]-enkephalin (DADLE) for 24 h and DADLE or morphine for 72 h did not alter the incorporation of ADP-ribose or the immunoreactive amount of Gi and Go subunits. However, pretreatment for 72 h with naloxone increased the incorporation of ADP-ribose without an apparent change in affinity or in the immunochemically determined protein levels of Gi and Go. This indicates that the process of down-regulation and desensitization of the delta-opioid receptor neither requires quantitative alterations in the levels of Gi and Go nor changes in the degree of coupling among their subunits. In contrast, chronic exposure to antagonists seems to alter the degree of precoupling between alpha- and beta-subunits of Gi and/or Go.