Nitrogen Mobilization Research Articles

Reduced atmospheric CO2 inhibits nitrogen mobilization in Festuca rubra.

In defoliated grasses, where photosynthesis is reduced due to removal of leaf material, it is well established that remobilization of nitrogen occurs from both older remaining leaves and roots towards the younger growing leaves. In contrast, little is known about the movement of nitrogen within intact grass plants experiencing prolonged inhibition of photosynthesis. We tested the following hypotheses in Festuca rubra L. ssp. rubra cv. Boreal: that both reduction of the atmospheric CO2 concentration and defoliation (1) induce mobilization of nitrogen from roots and older leaves towards growing leaves and (2) elicit similar directional change in the abundance of proteins in roots and older leaves relevant to the process of nitrogen mobilization including, glutamine synthetase (GS), EC 6.3.1.2; papain, EC 3.4.22.2; chymopapain, EC 3.4.22.6; ribulose bisphosphate carboxylase/oxygenase (Rubisco), EC 4.1.1.39; and the light harvesting complex of photosystem II (LHCPII). After growth at ambient atmospheric CO2 concentration, plants of F. rubra were subject to atmospheres containing either ambient (350 micro l l-1) or deplete (< 20 micro l l-1) CO2. Concurrently, plants were either left intact or defoliated on one occasion. Steady state 15N labelling coupled with a series of destructive harvests over a 7-day period enabled changes in the nitrogen dynamics of the plants to be established. Proteins pertinent to the process of nitrogen mobilization were quantified by immunoblotting. Irrespective of defoliation, plants in ambient CO2 mobilized nitrogen from older to growing leaves. This mobilization was inhibited by deplete CO2. Greater concentration of Rubisco and reduced chymopapain abundance in older remaining leaves of intact plants, in deplete compared with ambient CO2, suggested the inhibition of mobilization was due to inhibition of protein degradation, rather than to the export of degradation products. Both deplete CO2 and defoliation induced nitrogen mobilization from roots to growing leaves. In plants at ambient CO2, defoliation did not affect nitrogen uptake or its allocation. Therefore in F. rubra nitrogen mobilization can occur independently of any downregulation of nitrogen uptake. This suggests either different signal compounds may act to downregulate uptake and upregulate mobilization, or if one particular signalling compound is used its concentration threshold differs for induction of mobilization and downregulation of uptake. The abundance of the cysteine proteases papain and chymopapain was low in roots suggesting that they were not involved in protein degradation in this tissue.

Urea uptake and nitrogen mobilization by apple leaves in relation to tree nitrogen status in autumn

SummaryBench-grafted Fuji/M.26 trees were fertigated with one of seven nitrogen concentrations (0, 2.5, 5, 7.5, 10, 15 or 20 mM), using a modified Hoagland’s solution from 30 June to 1 September. In mid-October, plants in each N treatment were randomly divided into three groups. One group was sampled destructively to determine background tree N status before foliar urea applications. The second was painted with a 3% 15N-enriched urea solution twice, at weekly intervals, on both sides of all leaves. The third group served as the control. All the fallen leaves from both the 15N-treated and the control trees were collected during leaf abscission. The trees were harvested at the end of natural leaf fall. Nitrogen fertigation during the growing season resulted in a wide range of tree N status, as indicated by leaf N (1.3–3.5.g m–2) in the autumn. The percentage of N partitioned into the foliage increased linearly with increasing leaf N content up to 2.2 g m–2, then reached a plateau of 50–55% with further rise in leaf N. 15N uptake and mobilization per unit leaf area and the percentage of 15N mobilized from the leaves decreased with increasing leaf N content. Of the 15N mobilized back to the tree, the percentage of 15N partitioned into the root system decreased with increasing tree N status. For the control trees, N mobilization per unit leaf area increased with increasing leaf N up to 3 g m–2, then levelled off with any further increase in N content. The percentage of N mobilization remained at approximately 60% until leaf N reached 3 g m–2, then declined with a further increase in leaf N content. Foliar 15N-urea applications reduced mobilization of endogenous leaf N regardless of tree N status. On a whole-tree basis, foliar 15N-urea applications increased the total amount of N across N fertigation treatments. We conclude that trees with low N status are more efficient in absorbing and mobilizing N from foliar urea than those with high N status. Likewise, more N derived from foliar urea is partitioned into the root systems of low N trees than of high N trees.

Irrigation Decreases Loss of Leaf Nitrogen and Rubisco During Pod Development in Chickpea

Chickpea (Cicer arietinum L., var. Pusa 256) plants raised under unirrigated and irrigated field conditions showed a decrease in leaf nitrogen and soluble protein content after flowering during pod development. This was found to be associated with a decrease in Rubisco content after flowering. Leaf nitrogen, soluble protein and Rubisco content, however, were higher in irrigated than in unirrigated plants. The Rubisco content at the flowering and post‐flowering stages was 29.43 and 16.59 %, respectively, of leaf soluble protein in unirrigated plants. Under irrigated conditions, the Rubisco content was 49.91 and 37.99 %, respectively, at the flowering and post‐flowering stages. These results therefore indicated a decrease in the mobilization of leaf nitrogen by irrigation in chickpea. The findings are discussed in relation to the decrease in seed yield and harvest index by irrigation commonly observed in this crop under north Indian conditions.

Impact of nitrogen application on the performance of wheat (Triticum aestivum) and nitrogen use efficiency under different dates of sowing

Field experiments were conducted during rabi seasons of 2003-04 and 2004-05 to find out the effect of sowing time and nitrogen schedules on perofrmance of wheat and N balance in sandy loam soils of Hisar, Haryana .The results showed that 40 days delayed sowing resulted in 24.5% reduction in wheat grain yield. However, there was increase in nitrogen content in grains with the delayed sowing whereas, nitrogen uptake decreased with delayed sowing. Maximum mobilization of nitrogen to grain was recorded on 25 December and minimum on 15 November sown crop. There was decrease in physiological efficiency (PE), agronomic efficiency (AE), nitrogen recovery (NR) and nitrogen use efficiency (NUE) by 8.4, 37, 9 and 8% respectively with delay in sowing from 15 November to 25 December. There was 4.14% increase in grain yield when N was applied as 45.3% of recommended N (150 kg) at sowing (F ) followed by 50% at first irrigation and 4.7% as foliar application @ 3% concentration over recom- 4 mended (F ) schedule (50% at sowing and 50% at first irrigation) and 58.4% increase over no nitrogen application. 1 About 60 kg/ha increase in nitrogen uptake was recorded in different nitrogen schedules over control. The highest nitrogen mobilization was observed in treatment where no nitrogen was applied and minimum in recommended schedule. NR, NUE and AE were 15.7%, 8.0% and 12% more under F when compared with F treatment. How- 4 1 ever, there was decrease of 6% in PE under F than F treatment. The highest B:C ratio was also recorded in early 4 1 sowing and in F treatment . 4

Numerical modeling of interstitial diffusion in binary systems. Application to iron nitriding

The purpose of the present work is to provide a numerical model for nitrogen diffusion in iron accounting for the concentration dependence of the intrinsic diffusivity of nitrogen. Fick's second equation is solved using a finite volume method for the typical example of nitriding of pure iron. The non-linearity of the problem is treated without any specific assumption concerning the interface position. The thermodynamic factor is determined by means of a regular solution model. The surface boundary conditions are not required to be constant. Comparison of the results of the present work with experimental data available in the literature reveals a number of discrepancies. In particular, the nitrogen depth-concentration profiles for the ε-Fe 2N 1− x and γ′-Fe 4N phases are significantly different from those predicted by other models. The mobility and self-diffusion coefficient of nitrogen in ε-Fe 2N 1− x and γ′-Fe 4N, respectively, have therefore been re-evaluated. It is found that M N (ε)=4.0×10 −14 exp(−66 400/ RT) mol m 2 s −1 J −1 and that D N (γ′)*=2.63×10 −10 exp(−83 500/ RT) m 2 s −1.

Urea is a product of ureidoglycolate degradation in chickpea. Purification and characterization of the ureidoglycolate urea-lyase.

A ureidoglycolate-degrading activity was analyzed in different organs of chickpea (Cicer arietinum). Activity was detected in all the tissues analyzed, but highest levels of specific activity were found in pods, from which it has been purified and characterized. This is the first ureidoglycolate-degrading activity that has been purified to homogeneity from any photosynthetic organism. Only one ureidoglycolate-degrading activity was found during the purification. The enzyme was purified 1,500-fold, and specific activity for the pure enzyme was 8.6 units mg(-1), which corresponds with a turnover number of 1,600 min(-1). The native enzyme has a molecular mass of 180 kD and consists of six identical or similar-sized subunits of 31 kD each. The enzyme exhibited hyperbolic, Michaelian kinetics for (-) ureidoglycolate with K(m) values of 6 and 10 microM in the presence or absence of Mn(2+), respectively. Optimum pH was between 7 and 8 and maximum activity was found at temperatures above 70 degrees C, the enzyme being extremely stable and resistant to heat denaturation. The activity was inhibited by EDTA and enhanced by several bivalent cations, thus suggesting that the enzyme is a metalloprotein. This enzyme has been characterized as a ureidoglycolate urea-lyase (EC 4.3.2.3), which catalyzes the degradation of (-) ureidoglycolate to glyoxylate and urea. This is the first time that such an activity is detected in plant tissues. A possible function for this activity and its implications in the context of nitrogen mobilization in legume plants is also discussed.

Evaluation of Compost as an Amendment to Commercial Mixes used for Container-grown Golden Shrimp Plant Production

Growth of golden shrimp plant (Pachystachys lutea Nees.) transplants was evaluated in media containing 0%, 25%, 50%, 75%, or 100% compost derived from biosolids and yard trimmings. A commercial coir- or peat-based media was amended with compost. As compost composition in the peat or coir-based media increased from 0% to 100%, carbon/nitrogen ratios decreased; and media stability, nitrogen mobilization, pH, and electrical conductivity increased. Bulk density, particle density, air-filled porosity, container capacity, and total porosity increased as more compost was added to either peat- or coir-based media. Plants grown in media with high volumes of compost (75% or 100%) had less leaf area and lower shoot and root dry weight compared to the controls (no compost). Regardless of percentage of compost composition in either peat or coir-based media, all plants were considered marketable after 8 weeks.

Nitrogen deposition and leaching from two forested catchments in Southwest China--preliminary data and research needs.

Increased nitrogen deposition has resulted in increased nitrogen pools and nitrogen leaching in European and North American forest soils. The development in Asia in general, and China in particular, suggests increased deposition of reduced nitrogen from changes in agricultural practices and of oxidized nitrogen from rapid growth of the transportation sector. Decreased nitrogen retention in forested areas in the future may cause increased NO3– leaching and, thus, acidification and eutrophication in surface waters. The differences in climate, ecosystems, land use, and deposition history make direct application of knowledge from studies in Europe and North America difficult. In Southwest China the potential for nitrogen mobilization from forest soils may be high because of the warm and humid climate, resulting in high decomposition rates of soil organic matter. However, there are very few data available for quantifying the suspected potential for increased nitrogen leaching in forest ecosystems. Here we present data from two forested catchments, dominated by Masson pine (Pinus massoniana), near Guiyang and Chongqing, respectively, in Southwest China. The present nitrogen deposition is moderate, estimated in the range from 10 to 40 kg N ha–1 year–1. The C/N ratios of the soils are generally below 15. Nitrate concentrations in soil water are rather variable in space, with highest values of several hundred microequivalents per liter. The turnover rate of nitrogen in the forest ecosystem is quite high compared to the atmospheric deposition rate. At present, nitrate runoff from the catchments is low and intermediate in Guiyang and Chongqing, respectively. More research is needed to improve our ability to predict future nitrogen leaching from subtropical Asian coniferous forests.

Recent advances in exploring physiology and biodiversity of ectomycorrhizas highlight the functioning of these symbioses in ecosystems.

Ectomycorrhizas, the dominating mycorrhizal symbiosis in boreal, temperate and some tropical forests, are formed by 5000-6000 species of the asco- and basidiomycetes. This high diversity of fungal partners allows optimal foraging and mobilisation of various nitrogen and phosphorus forms from organic soil layers. In this review, two approaches to study the functioning of this multitude of symbiotic associations are presented. On selected culture models, physiological and molecular investigations have shown that the supply of hexoses has a key function in controlling the plant-fungus interaction via partner-specific regulation of gene expression. Environmental factors which affect fungal carbon supply, such as increased nitrogen availability, also affect mycorrhiza formation. Based on such laboratory results, the adaptative capability of ectomycorrhizas to changing field conditions is discussed. The second approach consists of analysing the distribution of mycorrhizas in ecosystem compartments and to relate distribution patterns to variations of ecological factors. Recent advances in identification of fungal partners in ectomycorrhizas by analysing the internal transcribed spacer of ribosomal DNA are presented, which can help to resolve sampling problems in field studies. The limits of the laboratory and the field approaches are discussed. Despite some problems, this combined approach is the most promising. Direct investigation of gene expression, which has been introduced for soil bacteria, will be difficult in the case of mycorrhizal fungi which constitute organisms with functionally varying structures.

Mobility of Soil Nitrogen and Microbial Responses following the Sudden Death of Established Turf

AbstractThe stability of nitrogen within a turf‐soil ecosystem is important both for efficient turf management and preventing the contamination of ground water by nitrate. The objective of this study was to quantify responses of the microbial community and the mobility of soil nitrogen following the sudden death of established turf. Twelve‐year‐old turf plots comprising four cool‐season turfgrass species fertilized with five N sources were maintained on an Enfield silt loam (coarse‐silty over sandy or sandy‐skeletal, mixed, active, mesic Typic Dystrudept) at Kingston, RI. Half of the plots were killed with glyphosate in early September and any regrowth was removed mechanically. Measurements of soil physical, chemical, and microbiological properties and nitrate leaching in killed and healthy plots were compared for 12 mo. Turf death did not alter soil moisture, temperature, pH, or extractable ammonium. Nitrate levels were higher in both the root zone and at 60 cm following turf death and this difference persisted for the sampling year. Carbon mineralization and microbial biomass C were not different between soils from healthy and killed plots. Killed plots leached three times more nitrate than healthy plots but this amounted to less than 10% of total soil N present. Retention of nitrate in a turf‐soil system depends on absorption by living grass roots, although reasonable N stability is also provided by N cycling within the soil microbiota. Protecting ground water from nitrate contamination is optimized by maintaining a vigorous turfgrass cover.

Specific lipoxygenase isoforms accumulate in distinct regions of soybean pod walls and mark a unique cell layer.

Developing seeds constitute a strong sink for the plant and rely on the turnover and mobilization of carbon and nitrogen assimilates to supply the nutrients needed for their maturation. In large part these nutrients emanate from the vegetative organs including leaves and pod walls. Vegetative lipoxygenases (VLXs) accumulate in the paraveinal mesophyll cell layer of soybean (Glycine max L.) leaves where individual isoforms are proposed to play a role(s) as active enzymes or as transient storage proteins. VLXs also are prominent proteins in soybean pod walls, representing approximately 12% of the total soluble protein. Examining the temporal, tissue, and subcellular patterns of individual VLX isoform accumulation and of lipoxygenase activity through pod wall development indicates that VLXD is the principal VLX isoform playing a role in storage in this organ. The major accumulation of VLXD occurs just prior to seed fill within the endocarp middle zone, and protein extracted from this region shows relatively low levels of lipoxygenase activity, suggesting the middle zone may act as a storage tissue. Three other VLX isoforms, VLXA, VLXB, and VLXC colocalize to the cytoplasm of a single discrete cell layer in the mesocarp. Thus, the patterns of VLX cellular and subcellular localization in pod walls suggest independent functions for these different isoforms while also serving as specific markers for a novel cell layer in the pod wall.

Transformation and mineralization of synthetic 14C-labeled humic model compounds by soil-feeding termites

The majority of termite species are considered to be humivorous, but the exact nature of their carbon and energy source, the mechanisms involved in digestion and the impact of this feeding habit on the humification of soil organic matter are largely obscure. We performed feeding trials with soil-feeding termites (Termitidae: Termitinae), using 14C-labeled humic model compounds as substrates. In the case of Cubitermes orthognathus, the components of chemically identical synthetic humic acids (HA), labeled either in their proteinaceous or aromatic building blocks, and synthetic fulvic acids (FA), labeled in the aromatic building blocks, all had similarly low mineralization rates (2.5, 2.2 and 2.7%, respectively) when incubated in soil for 10 days in the absence of termites. When termites were present, the mineralization rate of the aromatic component of HA and FA increased only slightly but significantly (2.4 and 3.1%, respectively; P < 0.05), whereas that of the proteinaceous component increased more than 10-fold (30.8%). Similar results were obtained when the protein was peptonized prior to polymerization (9.7 vs. 27.4% mineralization). Mineralization of HA was accompanied by a transformation of the residual peptide label to FA, whereas the aromatic label of HA and FA was partly transformed to humin during gut passage. High-performance gel permeation chromatography showed a strong shift in the size-class distribution of peptide label towards low-molecular-weight products, especially of the material ingested by the termites; the smallest molecules were recovered from the termite bodies. Similar results were obtained in feeding trials with Cubitermes umbratus and Thoracotermes macrothorax. Together with previous findings, the current results provide strong evidence that during gut passage, the combined action of extreme alkalinity in the anterior hindgut, autoxidative processes, and probably also proteolytic activities, renders a large reservoir of potential substrates accessible. While peptidic components of humic substances are selectively digested, aromatic components are apparently not an important food source for soil-feeding termites. These findings have important implications for the mobilization of organic nitrogen in tropical soils.

Inhibition of plant asparagine synthetase by monoterpene cineoles.

Asparagine (Asn) synthetase (AS) is the key enzyme in Asn biosynthesis and plays an important role in nitrogen mobilization. Despite its important physiological function, little research has been done documenting inhibitors of plant AS. Plant growth inhibition caused by the natural monoterpene 1,4-cineole and its structurally related herbicide cinmethylin was reversed 65% and 55%, respectively, by providing 100 microM Asn exogenously. Reversion of the phytotoxic effect was dependent on the concentration of Asn. The presence of either 1,4-cineole or cinmethylin stimulated root uptake of [(14)C]Asn by lettuce (Lactuca sativa) seedlings. Although the physiological responses suggested that both compounds affected Asn biosynthesis, biochemical analysis of AS activity showed that the natural monoterpene was a potent inhibitor (I(50) = approximately 0. 5 microM) of the enzyme, whereas the commercial product was not inhibitory up to levels of 10 mM. Analysis of the putative metabolite, 2-hydroxy-1,4-cineole, showed that the cis-enantiomer was much more active than the trans-enantiomer, suggesting that the hydroxyl group was involved in the specific ligand/active site interaction. This is the first report that AS is a suitable herbicide target site, and that cinmethylin is apparently a proherbicide that requires metabolic bioactivation via cleavage of the benzyl-ether side chain.

Carbon and nitrogen mobilization in Larix x eurolepis leafy stem cuttings assessed by dual (13)C and (15)N labeling: relationships with rooting.

Changes in use of both stored and newly synthesized sources of carbon (C) and nitrogen (N) were investigated during rooting of leafy cuttings of Larix x eurolepis A. Henry. We used dual (13)C and (15)N long-term labeling of reserves of stock plants and followed isotope dilution of the labels in the cuttings to determine the respective proportions of C and N derived from stock plant reserves (Q(C,old), Q(N,old)) and from newly synthesized sources (Q(C,new), Q(N,new)). We also assessed their partitioning into the growing needles and roots. Because rooting development occurred over several months, destructive harvests were made during three periods (t(0): April 9; t(1): May 15 to June 5; t(2): June 12 to June 26). Total C content (Q(C)) of cuttings did not change with time, whereas total N content (Q(N)) decreased significantly between t(0) and t(2). In rooted cuttings, unlike unrooted cuttings, total N concentration decreased less markedly in needles and in the lower stem component between t(1) and t(2) than between t(0) and t(2). The lower stem of rooted cuttings contained more Q(N,old) than that of unrooted cuttings and showed lower C:N and (13)C:(15)N ratios. The increase in Q(N,new) in the lower stem was positively correlated with rooting status. At t(1), new roots appeared and grew mainly at the expense of Q(C,new) and Q(N,old). By t(2), root growth was mainly dependent on new assimilates. We conclude that in leafy cuttings the initial amount of N reserves, rather than C reserves, may constitute a limiting factor for rooting.

504 Urea Uptake and Nitrogen Mobilization by Apple Leaves in Relation to Tree Nitrogen Status in the Fall

Bench-grafted Fuji/M26 trees were fertigated with seven nitrogen concentrations (0, 2.5, 5.0, 7.5, 10, 15, and 20 mm) by using a modified Hoagland solution from 30 June to 1 Sept. In Mid-October, plants in each N treatment were divided into three groups. One group was destructively sampled to determine background tree N status before foliar urea application. The second group was painted with 3% 15N-urea solution twice at weekly interval on both sides of all leaves while the third group was left as controls. All the fallen leaves from both the 15N-treated and control trees were collected during the leaf senescence process and the trees were harvested after natural leaf fall. Nitrogen fertigation resulted in a wide range of tree N status in the fall. The percentage of whole tree N partitioned into the foliage in the fall increased linearly with increasing leaf N content up to 2.2 g·m–2, reaching a plateau of 50% to 55% with further rise in leaf N. 15N uptake and mobilization per unit leaf area and the percentage of 15N mobilized from leaves decreased with increasing leaf N content. Of the 15N mobilized back to the tree, the percentage of 15N partitioned into the root system decreased with increasing tree N status. Foliar 15N-urea application reduced the mobilization of existing N in the leaves regardless of leaf N status. More 15N was mobilized on a leaf area basis than that from existing N in the leaves with the low N trees showing the largest difference. On a whole-tree basis, the increase in the amount of reserve N caused by foliar urea treatment was similar. We conclude that low N trees are more effective in utilizing N from foliar urea than high N trees in the fall.

Nitrogen mobilization in Asia

The addition of anthropogenic N by food and energy production to the global environment contributes to the greenhouse effect, acid deposition, photochemical smog, stratospheric ozone depletion and eutrophication of fresh and marine waters. On a global basis, anthropogenic N mobilization is greater then natural sources of bio-reactive N. Currently, Asia is a hotspot of N mobilization and distribution to downwind and downstream environments, primarily due to food production. Asia's contribution will continue to increase, not only due to population growth, but also to increases in the per capita consumption of food and energy. This paper provides an overview of the global N cycle, presents an analysis of N dynamics within agroecosystems, examines N mobilization in Asia and discusses possible rates of N mobilization in the future.

DIRECT AND INDIRECT EFFECTS OF HERBIVORES ON NITROGEN DYNAMICS: VOLES IN RIPARIAN AREAS

Herbivores can directly increase nitrogen mobility by increasing the quality of organic matter entering the decomposition cycle, but they also may decrease nitrogen mobility by decreasing the biomass of high-nitrogen species in the plant community. We assessed effects of voles (Microtus) on nitrogen dynamics using exclosures in two riparian meadows (Crystal Bench and Blacktail Deer Creek) in Yellowstone National Park (USA). At both sites, the quantity of plant litter was decreased by herbivory following a vole population peak in 1992. At Crystal Bench, removal of voles caused a decrease in the nitrogen concentration and an increase in the C:N ratio of plant litter over the four years of the study. The higher quality litter produced in the presence of voles at Crystal resulted in a larger pool of potentially mineralizable nitrogen in soil from control plots relative to soils from plots that had not been accessible to voles. At Crystal, vole removal did not cause a change in plant community composition. How...

Collembola populations in an organic crop rotation: Population dynamics and metabolism after conversion from clover-grass ley to spring barley

Summary Population dynamics of earthworms, enchytraeids, mites and collembolans were assessed over two years as part of a collaborative research project in an organic crop rotation at the agricultural Research Centre Foulum, Jutland, Denmark. The Collembola study presented here is restricted to the first growth season of a two-year study, i.e from just before conversion of a two year old clover-grass ley to seedbed until harvest of the barley crop. The results refer mostly to the four most abundant species. The project included experimental reductions of earthworm populations to test the effects on interactions between earthworms and mesofauna. Expulsion by electro-shocking resulted in modest reductions in earthworm density and biomass. No significant effect of the reduction was observed on the collembolan populations. Total collembolan density and density, mean weight per specimen, biomass and population metabolism of the four most abundant species decreased markedly between the first and second sampling date as an effect of the mechanical disturbance, and inversion of soil layers caused by soil preparation. The depth distribution for all Collembola species was inverted from highest concentration in the 0–10 cm layer to highest concentration in the 10–20 cm soil layer. After a population minimum in May, the density, mean weight, biomass and metabolism increased more or less regularly according to species. The net loss of biomass after conversion implied a mobilization of 5 mg N.m−2 from dead collembolan tissues. Together with an estimate for nitrogen due to excretion, the contribution of Collembola to nitrogen mobilization through the growth season was estimated tentatively as 10 mg.m−2. This amount was negligible compared with the contribution of earthworms. The possibility that indirect catalytic effects of collembolan activity could have any significant effect on nutrient availability for plants was not resolved but the collembolan population metabolism showed no obvious synchrony with the nutrient requirements of the crop.

Nitrogen Mobility in Biosolid-Amended Glaciated Soil

The potential leaching and availability of nitrogen in biosolids and biosolid-amended glaciated soil were investigated using mineralization and column studies. Aerobic and anaerobic mineralization studies were conducted to assess nitrogen mineralization potential from biosolid-amended glaciated soils and estimate long- and short-term mineralization rates of products. Semicontinuous column studies were conducted to assess the leachability of nitrogen from biosolid-amended soil under simulated unsaturated soil conditions. This study used a Paxton soil because its characteristics are typical of glaciated soil found in New England. Three types of biosolids, which are representative of biosolids currently available in Connecticut, were evaluated: composted biosolids (compost), cement kiln dust stabilized (CKD) biosolids, and heat-dried biosolids (pellet). Results indicate that the pellet product had the greatest mineralization potential (based on unmixed biosolids) at 5 317 mg N/kg, followed by compost (324 mg N/kg) and CKD biosolids (58 mg N/kg). Nitrogen mineralized for the anaerobic test (mixed with soil) varied from 56 to 90% of the total nitrogen and for the aerobic test varied from 19 to 57%. The CKD biosolids percolate consisted primarily of nitrate and nitrite, whereas the predominant nitrogen species found in compost and pellet biosolids percolate was ammonium. Results suggest that one-time application of biosolids at agronomic or liming rates (whichever is limiting) will cause minimal effects to groundwater.

Ion-induced electron emission as a means of studying energy- and angle-dependent compositional changes of solids bombarded with reactive ions: II. Nitrogen bombardment of silicon

Target current measurements were used to determine ion-induced electron yields γ of silicon bombarded with 2–10 keV N + 2 ions at impact angles θ between 0° (normal incidence) and 84°, both for sputter-cleaned samples and under conditions of full loading with nitrogen. At normal incidence, the electron yields of nitrogen saturated samples γ sat 0 were much higher than the yields γ 0 0 of clean samples. The ratios r 0= γ sat 0/ γ 0 0 decreased with increasing energy, from ∼9 at 2 keV to ∼6 at 10 keV. This energy dependence of r 0 is attributed to preferential loss of nitrogen. With increasing impact angle, the yields of nitrogen-saturated samples first increased and then decreased rather rapidly at some critical angle θ c. After having passed through a minimum between 50° (10 keV) and 70° (2 keV) the stationary yields increased in a manner similar to the yields of clean silicon. Compared with O + 2 bombardment, several important differences were observed with primary N + 2 ions. (i) The fall-off in electron yield occurred at larger angles the higher the beam energy, 25–27° at 2 keV and 30–32° at 10 keV, the half-width of the fall-off being independent of the energy. (ii) Even at the most oblique impact angles the stationary yields were significantly higher than with clean silicon (12–26% higher at 80°). (iii) There was no correlation of θ c with the angles at which the sputtering yield equals the silicon-to-nitrogen ratio of Si 3N 4, i.e. Y=3/4 Si atoms/N atom. (iv) At oblique N + 2 incidence, i.e. between ∼38 and ∼75°, ripples started to grow at rather small depths of erosion, as low as 30 nm at 5 keV (topography of crater bottoms investigated by atomic force microscopy). Ripple formation strongly enhances the electron yields. The results are discussed with reference to the very low mobility of nitrogen in Si 3N 4 which allows ‘supersaturation’ of the sample within the range of the implanted ions.