Influence of light intensity on the distribution of carbon and consequent effects on mineralization of soil nitrogen in a barley (Hordeum vulgare L.)-soil system

A pot experiment was conducted in a ¹⁴C-labelled atmosphere to study the influence of living plants on organic-N mineralization. The soil organic matter had been labelled, by means of a 200-days incubation, with ¹⁵N. The influence of the carbon input from the roots on the formation of microbial biomass was evaluated by using two different light intensities (I). Mineralization of ¹⁵N-labelled soil N was examined by following its fate in both the soil biomass and the plants. Less dry matter accumulated in shoots and roots at the lower light intensity. Furthermore, in all the plant-soil compartments examined, with the exception of rhizosphere respiration, the proportion of net assimilated ¹⁴C was lower in the low-I treatment than in the high-I treatment. The lower rates of ¹⁴C and ¹⁵N incorporation into the soil biomass were associated with less root-derived ¹⁴C. During the chamber period (¹⁴CO₂-atmosphere), mineralized amounts of ¹⁵N (measured as plant uptake of ¹⁵N) were small and represented about 6.8 to 7.8% of the initial amount of organic ¹⁵N in the soil. Amounts of unlabelled N found in the plants, as a percentage of total soil N, were 2.5 to 3.3%. The low availability of labelled N to microorganisms was the result of its stabilization during the 210 days of soil incubation. Differences in carbon supply resulted in different rates of N mineralization which is consistent with the hypothesis that roots induce N mineralization. N mineralization was higher in the high-I treatment. On the other hand, the rate of mineralization of unlabelled stable soil N was lower than labelled soil ¹⁵N which was stabilized. The amounts of ¹⁵N mineralized in planted soil during the chamber period (43 days) which were comparable with those mineralized in unplanted soil incubated for 210 days, also suggested that living plants increased the turnover rate of soil organic matter.     

Labelling of animal manure nitrogen with15N

A sheep was fed on¹⁵N-labelled ryegrass hay during a period of 9 days in order to obtain¹⁵N-labelled manure. After 9 days of feeding, the total N in faeces contained 3.70 atom %¹⁵N excess, which was equivalent to 82% of the¹⁵N enrichment of the hay N. The easily-decomposable fraction of the faecal N was less labelled (2.89 atom %¹⁵N excess) than the slowly-decomposable fraction. The¹⁵N enrichment of mineralized faecal N did not change significantly during 32 weeks of incubation in sand. About 25% of the faecal N was water-soluble. This N had a higher¹⁵N enrichment than the total faecal N, indicating that a part of the water-soluble N was indigestible feed N. The faeces contained only small amounts of NH ₄ ⁺ -N, which had a¹⁵N enrichment similar to the¹⁵N enrichment of N mineralized during incubation in sand. It is suggested that the labelled faecal N obtained after a few days of feeding on labelled feed could be divided in two N pools: A decomposable N fraction (about 60%) with a¹⁵N enrichment similar to the enrichment of N mineralized in sand (2.89 ± 0.09 atom %¹⁵N excess), and a very slowly-decomposable N fraction (about 40%) with a¹⁵N enrichment similar to that of the feed (4.52 atom %¹⁵N excess).     

Increase in pH stimulates mineralization of ‘native’ organic carbon and nitrogen in naturally salt-affected sandy soils

Large amounts of terrestrial organic C and N reserves lie in salt-affected environments, and their dynamics are not well understood. This study was conducted to investigate how the contents and dynamics of ‘native’ organic C and N in sandy soils under different plant species found in a salt-affected ecosystem were related to salinity and pH. Increasing soil pH was associated with significant decreases in total soil organic C and C/N ratio; particulate (0.05–2 mm) organic C, N and C/N; and the C/N ratio in mineral-associated (<0.05 mm) fraction. In addition, mineral-associated organic C and N significantly increased with an increase in clay content of sandy soils. During 90-day incubation, total CO₂-C production per unit of soil organic C was dependent on pH [CO₂-C production (g kg⁻¹ organic C) = 22.5 pH – 119, R ² = 0.79]. Similarly, increased pH was associated with increased release of mineral N from soils during 10-day incubation. Soil microbial biomass C and N were also positively related to pH. Metabolic quotient increased with an increase in soil pH, suggesting that increasing alkalinity in the salt-affected soil favoured the survival of a bacterial-dominated microbial community with low assimilation efficiency of organic C. As a result, increased CO₂-C and mineral N were produced in alkaline saline soils (pH up to 10.0). This pH-stimulated mineralization of organic C and N mainly occurred in particulate but not in mineral-associated organic matter fractions. Our findings imply that, in addition to decreased plant productivity and the litter input, pH-stimulated mineralization of organic matter would also be responsible for a decreased amount of organic matter in alkaline salt-affected sandy soils.     

Soil nitrogen availability evaluations based on nitrogen mineralization potentials of soils and uptake of labeled and unlabeled nitrogen by plants

Summary Sudangrass [Sorghum sudanense (Piper) Stapf] was grown in a greenhouse pot experiment on 39 soils having a broad range of chemical and physical characteristics. Labelled N as sodium nitrate (9% excess N¹⁵) was applied at rates of 200 and 400 mg of N per pot (2kg of soil). After 6 weeks of growth, total N and N¹⁵ were determined on plant tops and roots and on the cropped soils. Maximum yield differed widely among the soils owing to variations in yield-limiting factors other than N. Despite the diversity of responses to N fertilizer, the experiment provided a meaningful basis for assessing soil nitrogen availability. Amounts of N taken up from soils were similar from pots receiving no fertilizer N and from pots receiving labeled N.Amounts of soil organic N mineralized during cropping plus the mineral N present initially in the soils correlated highly with amounts of soil N taken up by whole plants (tops and roots). Average recovery by whole plants of mineral N formed before and during the cropping period was about 85 per cent, a value corresponding closely to recovery of fertilizer N in this experiment. The similarity in recovery of N provided by soil and fertilizer suggests that mineral N from these sources comprised a common pool that behaved as an entity with respect to mineralization-immobilization relations or other reactions affecting N availability to plants.A-values, the amounts of soil N having an availability equivalent to that of applied fertilizer N, were similar for two levels of applied labeled N and for tops and whole plants. Moreover, A-values were similar to amounts of N mineralized before and during crop growth. This result is particularly significant, since amounts of N mineralized during crop growth were estimated from N mineralization potentials, taking into account the effects of temperature on the mineralization rate constant. Thus, the study provides preliminary evidence that the soil N mineralization potential offers a basis for reliably estimating amounts of soil N mineralized during selected periods of time under specified temperature regimes.     

Available soil nitrogen in relation to fractions of soil nitrogen and other soil properties

The available N of 27 soils from England and Wales was assessed from the amounts of N taken up over a 6-month period by perennial ryegrass grown in pots under uniform environmental conditions. Relationships between availability and the distribution of soil N amongst various fractions were then examined using multiple regression. The relationship: available soil N (mg kg^–1 dry soil)=(N_min×0.672)+(N_inc×0.840)+(N_mom×0.227)–5.12 was found to account for 91% of the variance in available soil N, where N_min=mineral N, N_inc=N mineralized on incubation and N_mom=N in macro-organic matter. The N mineralized on incubation appeared to be derived largely from sources other than the macro-organic matter because these two fractions were poorly correlated. When availability was expressed in terms of available organic N as % of soil organic N (N_ao) the closest relationship with other soil characteristics was: N_ao=[N_inc×(1.395–0.0347×CN_mom]+[N_mom×0.1416], where CN_mom=CN ratio of the macro-organic matter. This relationship accounted for 81% of the variance in the availability of the soil organic N.The conclusion that the macro-organic matter may contribute substantially to the available N was confirmed by a subsidiary experiment in which the macro-organic fraction was separated from about 20 kg of a grassland soil. The uptake of N by ryegrass was then assessed on two subsamples of this soil, one without the macro-organic matter and the other with this fraction returned: uptake was appreciably increased by the macro-organic matter.     

STUDIES OF THE MECHANISM OF DEMYELINATION REGIONAL DIFFERENCES IN MYELIN STABILITY IN VITRO1

—Incubation of slices of rat central nervous system in Krebs-Ringer bicarbonate buffer produced a lipoprotein fraction which floated on 10·5% sucrose after homogenization of the slices and centrifugation. This fraction was not found after homogenization and centrifugation of fresh tissue and appeared to depend upon incubation. The amount of the light fraction increased in the following order per 100-mg slice: cerebrum < thalamic area < cerebellum < brain stem < spinal cord. The lipid composition of this fraction was similar to that of myelin, but contained a lower protein content compared to myelin of the corresponding area. This fraction was termed ‘dissociated myelin’. Upon incubation of slices a portion of the basic protein was lost from myelin subsequently isolated, and the dissociated fraction was slightly enriched in basic protein. The distribution of myelin protein among the characteristic three groups (basic, proteolipid and high mol. wt.) was quite different in myelin from spinal cord compared to that from other CNS area. Spinal cord myelin contained about 17% protein compared to about 23% in cerebrum, with brain stem myelin intermediate (19%), and the difference appeared to be due to lesser amounts of proteolipid in the caudal areas. The amount of dissociation after incubation was about 3–5 per cent of the total myelin in the cerebral cortex, 10 per cent in the thalamic area, 20 per cent in cerebellum, 35 per cent in the brain stem, and around 45 per cent in spinal cord. The smaller amount of proteolipid protein in spinal cord myelin may result in a deficiency of cohesive forces holding lipids and proteins together, thus causing greater instability and dissociation. Myelin dissociation increased with time of incubation up to 3 h, was augmented by Ca²⁺, and was substantial at pH 11, reaching a peak at pH 7, then decreased in the acid range. A similar fraction has been isolated previously from fresh CNS tissue made edematous by chronic treatment of rats with triethyl tin. The possible relationship of swelling in the disease process and myelin dissociation are discussed.     

Long-term effects of elevated nitrogen on forest soil organic matter stability

Nitrogen addition may alter the decomposition rate for different organic-matter pools in contrasting ways. Using a paired-plot design, we sought to determine the effects of long-term elevated N on the stability of five organic-matter pools: organic horizons (Oe+a), whole mineral soil (WS), mineral soil fractions including the light fraction (LF), heavy fraction (HF), and a physically recombined fraction (RF). These substrates were incubated for 300 days, and respiration, mineralized N, and active microbial biomass were measured. Samples with elevated N gave 15% lower cumulative respiration for all five substrates. Over the 300-day incubation, the Oe+a gave twice the cumulative respiration (gCkg^–1 initial C) as the LF, which gave slightly higher respiration than the HF. Respiration was 35% higher for the WS than for the RF. Mineralized N was similar between N treatments and between the LF and HF. Net N mineralized by the LF over the course of the 300-day incubation decreased with higher C:N ratio, due presumably to N immobilization to meet metabolic demands. The pattern was opposite for HF, however, which could be explained by a release of N in excess of metabolic demands due to recalcitrance of the HF organic matter. Mineralized N increased with respiration for the HF but showed no pattern, or perhaps even decreased, for the LF. WS and RF showed decreasing active microbial biomass near the end of the incubation, which corresponded with decreasing respiration and increasing nitrate. Our results show that long-term elevated N stabilized organic matter in whole soil and soil fractions.     

Agricultural Management and Labile Carbon Additions Affect Soil Microbial Community Structure and Interact with Carbon and Nitrogen Cycling

We investigated how conversion from conventional agriculture to organic management affected the structure and biogeochemical function of soil microbial communities. We hypothesized the following. (1) Changing agricultural management practices will alter soil microbial community structure driven by increasing microbial diversity in organic management. (2) Organically managed soil microbial communities will mineralize more N and will also mineralize more N in response to substrate addition than conventionally managed soil communities. (3) Microbial communities under organic management will be more efficient and respire less added C. Soils from organically and conventionally managed agroecosystems were incubated with and without glucose (¹³C) additions at constant soil moisture. We extracted soil genomic DNA before and after incubation for TRFLP community fingerprinting of soil bacteria and fungi. We measured soil C and N pools before and after incubation, and we tracked total C respired and N mineralized at several points during the incubation. Twenty years of organic management altered soil bacterial and fungal community structure compared to continuous conventional management with the bacterial differences caused primarily by a large increase in diversity. Organically managed soils mineralized twice as much NO₃ ^? as conventionally managed ones (44 vs. 23 μg N/g soil, respectively) and increased mineralization when labile C was added. There was no difference in respiration, but organically managed soils had larger pools of C suggesting greater efficiency in terms of respiration per unit soil C. These results indicate that the organic management induced a change in community composition resulting in a more diverse community with enhanced activity towards labile substrates and greater capacity to mineralize N.     

Patern of nitrogen release during decomposition of some green manures in a tropical alluvial soil

Summary The pattern of release of ammonium and nitrate nitrogen during decomposition of glyricidia, sunflower, centrosema, calapagonium and crotolaria under aerobic and anaerobic conditions, in an alluvial soil over a period of 7 weeks was studied. Under aerobic conditions, the NH₄ ⁺–N production reached the maximum after the 4th week. Nitrate-N and total available-N increased in all cases throughout the incubation period except in sunflower. This showed a nitrification inhibitory effect and had a relatively high C/N ratio (11.0) and low total N content (2.8%). In general the increase in NH₄ ⁺–N and NO₃ ^––N was more rapid in the early stages of incubation.Under anaerobic conditions, the production of these nutrients was considerably low. Soil organic matter mineralized faster than the added organic material which started to decompose slowly after sometime. Nitrate-N tend to decrease during incubation attributable to denitrification.     

Organic matter and nitrogen in the unharvested fractions of grass swards in relation to the potential for nitrate leaching after ploughing

The amounts of organic matter in the stubble, litter, root and soil macro-organic matter fractions of two swards of perennial ryegrass that had received normal applications of either fertilizer or cattle urine were, on average for the four fractions, about 3000, 500, 11,500 and 8,800 kg ha⁻¹. The swards had been established 8 or 15 years previously and each was sampled at intervals over a period of about one year. The amounts of N contained in the four fractions were, on average, 68, 12, 249 and 240 kg ha⁻¹, a total of 569 kg N ha⁻¹. With other swards, increasing rates of application of fertilizer N were found to have little effect on the amounts of organic matter in stubble and roots. Concentrations of N in the organic matter of the stubble and roots, however, increased significantly with increasing rate of fertilizer application, though, with stubble, moderate rates of application had little effect. Assessments based on these data, together with other published information, indicate that the amount of N mineralized from the combined stubble, litter, root and macro-organic matter fractions during the first year after ploughing may range from about 40 kg to at least 360 kg N ha⁻¹ depending on the age of the sward and its recent management. The amount mineralized is likely to increase with age of sward, with increasing rate of fertilizer N and with utilisation by grazing rather than cutting.     

Modelling the change in soil organic C and N and the mineralization of N from soil in response to applications of slurry manure

A computer simulation model of the turnover of organic matter in soil was adapted to simulate the change in soil organic C and N contents of soil during several years following annual additions of farm slurry to maize fields. The model proved successful in estimating the build-up of both C and N in soil and the leaching of N to ground-water in response to applications of slurry ranging from 50 to 300 tons per hectare per year. The model was then used to estimate the build-up of organic matter in soil under crops of fodder maize that were grown using the excess of manure produced during the last 20 years in the Netherlands. The build-up of organic matter from these applications was estimated to lead to about 70 kg extra nitrogen mineralized ha^-1 yr^-1. As a result of legislation manure applications have decreased and are expected to decrease further in the immediate future. Calculations suggest that after 10 years of manure applied at rates no longer exceeding the amount needed to replace the phosphorus removed by crops, the extra mineralization of N will still be between 45 and 60 kg ha^-1 yr^-1. If manure applications cease altogether then the extra mineralization will be about 25–30 kg N ha^-1 yr^-1.     

Some significance of soil organic phosphorus mineralization in the phosphorus nutrition of cocoa in Ghana

Summary 1. Soil samples from a 2² NP:KMg factorial experiment, in which yield response of cocoa to NP was highly correlated to organic phosphorus content of the 0–2 layer, were studied.2. It was found that the organic phosphorus content ranged from 46.0 to 69.5 per cent of the total phosphorus and was correlated to the percentage total nitrogen, total phosphorus, organic carbon and pH.3. Incubation of the soils at 50 per cent of their water holding capacity and a temperature of 27°C for periods of 14, 28, 42, 56, and 70 days resulted in the average mineralization of organic phosphorus equivalent to 4,0. 13.6, 38.2, 50.0, and 54.0 pounds P per acre respectively.4. More organic phosphorus was mineralized at 50°C than at 27° or 40°C.5. The percentage of organic phosphorus mineralized was generally higher in the plots where NP had been applied.6. Laboratory application of nitrogen or phosphorus to the soils before incubation resulted in greater mineralization. The effect of nitrogen and phosphorus together was greater than in the presence of nitrogen alone but only in a few cases was it greater than in the presence of phosphorus alone. The effect of nitrogen and phosphorus was not additive.7. It is concluded that although the organic phosphorus increased in the NP-treated plots, increased mineralization occurred concurrently; and the effect of nitrogen and phosphorus application in increasing the mineralization of the soil organic phosphorus during laboratory incubation explained the high correlation found between cocoa yield response and organic phosphorus content of the top soil.     

长期施肥及撂荒对土壤氮素矿化特性及外源硝态氮转化的影响

以黄土高原南部17年长期定位试验不同处理土壤为研究对象,研究了不同肥料处理及撂荒条件下土壤氮素矿化特性、灭菌与不灭菌条件下不同肥力土壤对施入外源硝态氮转化的影响.结果表明：氮磷钾化肥和有机肥配施（MNPK）及长期撂荒处理显著提高了土壤有机质和全氮含量以及土壤氮素矿化量和矿化率;氮磷钾化肥（NPK）处理虽然提高了土壤无机氮含量,但对土壤有机质、全氮、土壤氮素矿化量和矿化率的影响相对较小.高温高压灭菌显著增加了土壤铵态氮含量,但对不同处理土壤硝态氮含量无明显影响;在灭菌土壤培养过程中,土壤铵态氮含量呈显著增加趋势.同一土壤类型,不论灭菌与否,培养过程中施入土壤的硝态氮含量保持相对稳定,说明在本研究培养条件下,生物因素和非生物因素对外源硝态氮在土壤中的转化无明显影响.     

Soil nitrogen dynamics along a gradient of long-term soil development in a Hawaiian wet montane rainforest

I analyzed the rates of net N mineralization and nitrification of soils from seven sites in a Hawaiian wet montane forest. The sites differ in age, ranging from 400 to 4,100,000 yr, but are comparable in other variables (all at 1200 miasl with 4000 mm or more mean annual rainfall), and the chronosequence simulated a development of soils from basaltic lava. Soils were incubated for 20 days at 17.5 °C, which is nearly equivalent to a mean field air temperature of the sites, and at an elevated temperature of 25.5 °C under three treatments: 1) field-wet without amendments, 2) air dried to a permanent wilting point, and 3) fertilized with phosphate (NaH₂PO₄) at the rate of 50 g P per g dry soil. Both mineralization and nitrification rates varied significantly among the sites at the field temperature (p<.00001). Fractions of the mineralized organic matter (indexed by the N produced per g organic C) increased sharply from the youngest to the 5000-yr site before declining abruptly to a near constant value from the 9000 to the 1,400,000-yr sites. Total organic C in the top soils (<15 cm deep) increased almost linearly with age across the sites. Consequently, net NH₄- and NO₃-N produced on an area basis (g m^-2 20 d^-1) increased sharply from 0.2 in the youngest site to 1.2 in the 5000-yr site, then both became depressed once but steadily increased again. The fraction of organic matter mineralized, and the net N turnover rates were outstandingly high in the oldest site where a large amount of organic matter was observed; the topsoil organic matter which was used in this analysis appeared to be highly labile, whereas the subsurface organic matter could be relatively recalcitrant. As suggested by earlier workers, the initial increase in N turnover seemed to correspond to the increasing quantity of N in the soils through atmospheric deposition and biological fixation. The later decline in fraction of organic matter mineralized seemed to relate to increasing soil C/N ratios, increasingly recalcitrant organic matter, and poorer soil drainage with age. The elevated temperature treatment produced significantly higher amounts of N mineralization, except for the youngest site where N was most limiting, and for two sites where soil waterlogging might be severe. P fertilization invariably resulted in slower N turnovers, suggesting that soil microbes responded to added P causing N immobilization. The youngest site did not significantly respond to added P. The magnitude of immobilization was higher in older than in younger soils, suggesting that P more strongly limits microbial populations in the older soils.     

Non-exchangeable binding of ammonium and amino nitrogen by Norway spruce raw humus

Summary The capacity of an originally acid Norway spruce raw humus to fix isotopically labelled ammonium and amino nitrogen in a form resistant to cold 1N HCl treatment was studied. The amount fixed was determined after a reaction period of 24 hours (the humus pretreated with propylene oxide), using the amount of labelled N in the HCl-leached humus residue as a basis for calculating the amount of added N fixed. The nitrogen sources used were ammonium chloride, glycine and cyanamide. It was found that the fixation of added ammonium and glycine N was exceedingly low in the H⁺-saturated raw humus (pH 3.3–3.4), but the fixation rate was rapidly increased by increasing the pH during the aerobic incubation. Maximum fixation was obtained at a final pH of 10–11. Within the acid range the fixation was constantly higher for added glycine-N than ammonium-N. On the alkaline side of the neutral point the amount of fixation tended to be similar for ammonium and glycine. In treatments with N¹⁵-labelled ammonium, it was shown that small but fully detectable amount of added N were present in the soluble organic N fraction of the HCl extract, the quantities increasing with increasing soil pH during the incubation. The fixation was increased by increasing temperature and decreased by oxidative pretreatment of the humus before the addition of N. In the nitrogen gas atmosphere the fixation figures were 40 to 50 per cent lower than for corresponding treatments in air atmosphere. When various N compounds were added in equimolar concentrations the highest fixation was recorded for cyanamide. In studying the stability of fixed N to acid hydrolysis, it was found that 54 per cent of the fixed N resisted eight hours' refluxing with 6N HCl, the corresponding figure for the native raw humus N being 19 per cent. About one third of the fixed N was liberated as ammonia during the acid hydrolysis.     

Tracer studies on the balance and chemical distribution of applied nitrogen under different moisture regimes using lysimeter

Summary Tracer studies were made on balance and chemical distribution of added fertilizer under field conditions using a modified type of lysimeter at different moisture regimes. A modified chemical method was also used for the determination of different forms of organic N.An average of 25 per cent of the isotope enriched nitrogen applied to soil could not be accounted for at the end of the 3 years of experiment. The amount of residual added N in soil was around 33 per cent of which 27 per cent was in 0–20 cm layers and only 6 per cent was found in 20–50 cm layers. The average crop recoveries were around 43 per cent. Only 0.18 per cent of NO₃–N was leached from the irrigated plots.The alkali-stable N (amino acid-N) fraction was higher for irrigated (19 per cent) than nonirrigated plots (15 per cent). There were no difference in the amounts of fixed NH₄, non-hydrolyzed and alkali-labile N fractions for irrigated and non-irrigated plots. Only an average of 1.5 per cent of total fertilizer N was found as fixed NH₄–N form but the total fixed NH₄–N was higher (10–13 per cent) than that reported by other workers for surface soil layers. The sum of different soil-nitrogen fractions were always higher than the total nitrogen in soil.     

GLUTAMYL, ASPARTYL AND AMIDE MOIETIES OF CEREBRAL PROTEINS: METABOLIC ASPECTS IN VITRO

Abstract— The total mixed proteins (excluding proteolipids) were isolated from cat cerebral cortex and subjected to acid and enzymic hydrolyses. Analyses on the hydrolysates were carried out by specific enzymic procedures to determine the glutamyl, glutaminyl, aspartyl and asparaginyl composition. The content of total glutamyl and total aspartyl residues was the same in all types of protein samples, with average values of 78 and 58 /miol/100 mg of protein, respectively. In biopsy samples approximately 45 per cent of each total was in the amide form. Preparation of slices of cerebral cortex for incubation was associated with deamidation in situ of 16 per cent of the protein-bound glutaminyl residues. The extent of deamidation was not increased by incubation or by prolonged hypoxia and was unaffected by prior anaesthesia or by incubation of slices with 10 mM-NH₄Cl or 40 mM-malonate. Slices prepared from animals intoxicated with methionine sulphoximine exhibited no deamidation. No deamidation was observed for slices of subcortical white matter, liver, kidney, testis or diaphragm of the cat. Cortical proteins from other species appeared to behave similarly to those of the cat. The 5-4 μmol of NH₃ released/g of fresh cortex could account for about 85 per cent of the endogenous free ammonia regularly encountered in such slices. Hence the labile fraction of protein-bound glutaminyl amide groups represents, as previously suspected, a major source of endogenous cerebral NH₃. Proteins isolated from cerebral cortical slices incubated with L-[U-¹⁴C]glutamic acid or L-[U-¹⁴C]glutamine contained 105 (±0.095) per cent of the total ¹⁴C metabolized. The ratios (x 100) of protein to free pool specific radioactivities (c.p.m.μmol) of glutamic acid and of glutamine were in the range 0-22 to 0-42, or of the same order as previously reported for other amino acids. Comparable results were obtained with proteins isolated from cerebral cortical slices incubated with 10 mM-¹⁵NH₄Cl or L-[amide-¹⁵N]glutamine or both. In the amide N of protein-bound glutaminyl residues the atoms per cent excess ¹⁵N ranged from 007 to 0-42. This degree of labelling could be accounted for completely by the turnover of the entire glutaminyl moiety, as indicated by the ¹⁴C studies. Simultaneous analyses of free pool NH₃ and glutamine suggested that transfer of glutamine from medium to slice involves deamidation as it is taken up and reamidation after entry.     

The influence of living plants on mineralization and immobilization of nitrogen

Summary Changes in the pattern of distribution of the nitrogen of the soil and seedling grass plants have been investigated when the grass plants were grown in pots of sandy soil, from a pasture, at pH 5.7. Net mineralization of soil nitrogen was not observed during an experimental period of one month in the absence of added nitrogenous fertilizer (Table 2). Addition of labeled nitrogen (as ammonium sulphate) to the soil at the beginning of the experimental period resulted in a negative net mineralization during this period (Table 4b). When none of the fertilizer nitrogen remained in its original form in the soil it was found that approximately 12 per cent of the labeled nitrogen had been immobilized in soil organic compounds. Clipping of the grass at this date was followed by a decrease in the amount of labeled soil organic nitrogen, indicating that mineralization was not depressed by living plants. The application of unlabeled ammonium sulphate subsequent to the utilization of the labeled nitrogen did not decrease the amount of immobilized labeled nitrogen in the soil organic matter, as would be expected if the organic nitrogen compounds of the soil had been decomposed to ammonia. This was thought to be due to the fact that decomposition of organic nitrogen compounds in permanent grassland results in the production of peptides, amino acids etc. which are utilized by microorganisms without deamination taking place. In pots with ageing grass plants, labeled organic nitrogen compounds were found to be translocated from the grass shoots to the soil (Table 7). Net mineralization of soil organic nitrogen was positive in the contents of pots containing killed root systems (Table 3b). About 8 per cent of the labeled nitrogen added to the contents of such pots, in the form of ammonium sulphate, was found to be present in soil organic nitrogen compounds approximately 4 weeks after application, while a total of about twice this amount of soil organic nitrogen was mineralized during that period. From the results obtained in this investigation, it is concluded that the constant presence of living plants is responsible for the accumulation of nitrogen in organic compounds in permanent grassland. No evidence was obtained that the decomposition of such compounds in the soil is inhibited by living plants.     

Decomposition of organic matter and mineralization of nitrogen in brookston silt loam and alfalfa green manure

Summary C¹⁴ and N¹⁵ doubly labelled alfalfa tissue at addition rates of 5 and 1 ton per acre was incubated in the laboratory for 72 days with virgin and cultivated Brookston silt loam. The alfalfa tissue was more extensively decomposed in the virgin soils than in cultivated soil, but retention of tissue carbon was not affected by rate of addition. Percentage decomposition of organic matter in the virgin soil was greater than that in the cultivated soil. Addition of alfalfa tissue reduced decomposition of soil organic matter in proportion to the rate of addition and resulted in a gain of carbon in the incubation mixture. No priming action was noted.An increase in the rate of tissue addition caused an increase in the amount of nitrogen mineralized from the tissue but had little effect on the amount of nitrogen mineralized from the soil. Nitrogen mineralized from the soil organic matter was preferentially immobilized during the latter part of the incubation period.It appears that the organic matter content of the soil as well as the rate of tissue addition may regulate the priming action of green manures.Published with approval of the Director, Ohio Agricultural Experiment Station as Journal Paper No.94-62.