Influence of the N and P status of plant material and of added N and P on the mineralization of C from 14C-labelled ryegrass in soil

Ryegrass was grown under conditions of low N, low P, or high N and P nutrient supply in an atmosphere containing ¹⁴CO₂ and then incubated in soil supplemented with or without N or P fertilizer. Determined in fresh plant tissue, the persistency of residual labelled C after 6 months was in the order low-N plants>low-P plants>high-N and-P plants. The addition of N conserved C, particularly when there was additional P present. Hydrolysable labelled C (12M/0.5M H₂SO₄) showed similar trends. In analyses of freeze-dried plant tissue, the main effect was also the increased persistency of C from low-N plants compared to high-N plants. The addition of N fertilizer increased the persistence of plant residue C, but only with grass containing low P. The addition of P fertilizer had no effect. In freeze-dried low-P plant tissue, sampled after 1.5, 6, and 12 months, the conserving effect of adding fertilizer N was confirmed. The addition of P, in contrast, enhanced the rate of decomposition. After 6 months, about a third of the C remained, and after 12 months, about one-quarter. It is concluded that P, whether intrinsic or added, can increase the rate of decomposition of organic residues in soil, but there is a strong interaction with N, which has a predominant influence. The effects of N depend on the form it is in. Increased intrinsic tissue N can increase the rate of C loss, whereas added inorganic N can decrease the rate of C loss during decomposition.     

Formation and remobilisation of soil microbial residue. Effect of clay content and repeated additions of cellulose and sucrose

In two studies, we assessed the mass balance of added ¹⁴C-labelled sucrose and ¹⁵NH₄¹⁵NO₃ by measuring ¹⁴CO₂, ¹⁴C and ¹⁵N in soil microbial biomass (SMB) and ¹⁴C and ¹⁵N in soil solution. Specifically, we assessed the potential of recently added ¹⁴C to be re-mobilised by cryptic growth using subsequent additions of sucrose and cellulose and the effect of physical protection on the stabilisation of the labelled substrate. We used both a constructed soil with low soil organic matter content and varied the clay content as well as a natural soil. We observed a substantial initial as well as a later stage transfer of ¹⁴C into unidentifiable form, hypothesised to be microbial residues. When using a standard k _EC value of 0.45, only roughly 50% of the added labelled substrates were accountable and therefore we explored the full range of reported k _EC values to assess the mass balance. Subsequent application of unlabelled sucrose and cellulose did not substantially increase turnover ¹⁴C and ¹⁵N. Contrary to our expectation, there was no effect of clay content on the amount of unidentified ¹⁴C and ¹⁵N. The unidentified ¹⁴C and ¹⁵N is ascribed to formation of soil microbial residue. The low recovery of added isotope suggests that our mechanistic models are missing a large and important pool in order to realistically simulate organic matter turnover in soil.     

Nitrogen fixation by Azospirillium brasilense in soil and the rhizosphere under controlled environmental conditions

Summary Acetylene reduction activity by Azospirillum brasilense, either free-living in soils or associated with wheat roots, was determined in a sterilised root environment at controlled levels of O₂ tension and with different concentrations of mineral N. In an unplanted, inoculated soil nitrogenase activity remained low, at approximately 40 nmol C₂H₄ h^-1 per 2kg fresh soil, increasing to 300 nmol C₂H₄ h^-1 when malic acid was added as a C source via a dialyse tubing system. The N₂ fixation by A. brasilense in the rhizosphere of an actively growing plant was much less sensitive to the repressing influence of free O₂ than the free-living bacteria were. An optimum nitrogenase activity was observed at 10 kPa O₂, with a relatively high level of activity remaining even at an O₂ concentration of 20 kPa. Both NO _inf3 ^sup- and NH _inf4 ^sup+ repressed nitrogenase activity, which was less pronounced in the presence than in the absence of plants. The highest survival rates of inoculated A. brasilense and the highest rates of acetylene reduction were found in plants treated with azospirilli immediately after seedling emergence. Plants inoculated at a later stage of growth showed a lower bacterial density in the rhizosphere and, as a consequence, a lower N₂-fixing potential. Subsequent inoculations with A. brasilense during plant development did not increase root colonisation and did not stimulate the associated acetylene reduction. By using the ¹⁵N dilution method, the affect of inoculation with A. brasilense in terms of plant N was calculated as 0.067 mg N₂ fixed per plant, i.e., 3.3% of the N in the root and 1.6% in the plant shoot were of atmospheric origin. This ¹⁵N dilution was comparable to that seen in plants inoculated with non-N₂-fixing Psudomonas fluorescens.     

Influence of mycorrhiza vs. soluble phosphate on growth,nodulation, and N2 fixation (15N) in alfalfa under different levels of water potential

Summary The legume Medicago sativa (+Rhizobium melilott) was grown under controlled conditions to study the interactions between soluble P in soil (four levels), or a mycorrhizal inoculum, and the degree of water potential (four levels) in relation to plant development and N₂ fixation. ¹⁵N-labelled ammonium sulphate was added to each pot for a qualitative estimate of N₂ fixation, in order to rank the effects of the different treatments.Dry-matter yield, nutrient content and nodulation increased with the amount of plant-available P in the soil, and decreased as the water stress increased, for each P-level. The mycorrhizal effect on dry matter, N yield, and on nodulation was little affected by the water potential. Since P uptake was affected by the water content in mycorrhizal plants, additional mechanisms, other than those mediated by P, must be involved in the mycorrhizal activity.There was a positive correlation between N yield and nodulation for the different P levels and the mycorrhizal treatment at all water levels. A high correlation between plant unlabelled N content and atom% ¹⁵N excess was also found for all levels of P. In mycorrhizal plants, however, the correlation between unlabelled N yield and ¹⁵N was lower. This suggests that mycorrhiza supply plants with other N sources in addition to those derived from the improvement on N₂ fixation.     

Soil microbial, fungal, and nematode responses to soil fumigation and cover crops under potato production

Sodium N-methyldithiocarbamate (metam sodium) and 1,3 dichloropropene are widely used in potato production for the control of soil-borne pathogens, weeds, and plant parasitic nematodes that reduce crop yield and quality. Soil fumigation with metam sodium has been shown in microcosm studies to significantly reduce soil microbial populations and important soil processes such as C and N mineralization. However, few published data report the impact of metam sodium on microbial populations and activities in potato production systems under field conditions. Fall-planted white mustard (Brassica hirta) and sudangrass (Sorghum sudanense) cover crops may serve as an alternative to soil fumigation. The effect of metam sodium and cover crops was determined on soil microbial populations, soil-borne pathogens (Verticillium dahliae, Pythium spp., and Fusarium spp.), free-living and plant-parasitic nematodes, and C and N mineralization potentials under potato production on five soil types in the Columbia Basin of Eastern Washington. Microbial biomass C was 8–23% greater in cover crop treatments compared to those fumigated with metam sodium among the soil types tested. Replacing fumigation with cover crops did not significantly affect C or N mineralization potentials. Cumulative N mineralized over a 49-day laboratory incubation averaged 18 mg NO₃-N kg⁻¹ soil across all soil types and treatments. There was a general trend for N mineralized from fumigated treatments to be lower than cover-cropped treatments. Soil fungal populations and free-living nematode levels were significantly lowered in fumigated field trials compared to cover-cropped treatments. Fumigation among the five soil types significantly reduced Pythium spp. by 97%, Fusarium spp. by 84%, and V. dahliae by 56% compared to the mustard cover crop treatment. The percentage of bacteria and fungi surviving fumigation was greater for fine- than coarse-textured soils, suggesting physical protection of organisms within the soil matrix or a reduced penetration and distribution of the fumigants. This suggests the potential need for a higher rate of fumigant to be used in fine-textured soils to obtain comparable reductions in soil-borne pathogens.     

Tuber formation as influenced by the C : N ratio in potato plants

Increasing the tuber number per plant and promoting tuber formation are important goals for potato production. Thus, it is of significance to understand how environmental factors affect tuber formation, which relates to the development of agronomical practices. The objective of this research was to test whether changing the CO₂ or N supply level could alter the C : N ratio in potatoes, and whether a change of the C : N ratio in potatoes closely correlates with tuber formation in potato plants. Potato plantlets were grown in greenhouse with varied CO₂ or N levels. The C and N concentrations of plants were measured, and the tuber number and tuber weight were recorded after different growth times. The results show that the C : N ratios in the potato plants increased with increasing CO₂ concentration and that potato plants treated with higher CO₂ concentrations form tubers earlier. A decrease in the N supply also resulted in a higher C : N ratio and earlier tuber formation. Therefore the hypothesis proposed previously is supported by the present work. In addition, the results that C concentration in plants remained stable under different CO₂ or N levels imply that the variation of C : N ratio in the potato plants is mainly due to a change of N concentrations which decreased as the environmental CO₂ concentration increased, while increased as the N supply level increased.     

Nickel phytotoxicity in relationship to soil ph manipulation and chelating agents

Abstract

The main objective of this study was to ascertain effects of some edaphic factors on the uptake and influence of Ni on plant growth sinee Ni is a common trace element contaminant as well as an important component of serpentine soils. Corn (Zea mays L. inbred Ys₁/Ys₁) was rown in Yolo loam soil amended to give soil pH values of 4.2, 5.6, 7.5, and 8.2. A level of 100 μg Ni/g soil was not toxic to the corn. Shoot concentrations of Ni increased as soil pH decreased for both application rates of Ni. A level of 250 μg Ni/g soil decreased yields more at soil pH below 7 than above 7. Iron, Zn, and Mn levels in shoots did not appear to be directly related to the Ni applications although Fe levels tended to increase as a result of smaller plant size. PI54619–5–1 soybeans (Glycine max L. ) were grown in soil at two different pH values (with and without CaCO₃) and with and without a level of 1000 μg Ni/g added as the sulfate and thoroughly mixed with the soil and equilibrated for 1 month prior to transplanting the soybeans with and without application of a chelating agent, DTPA (diethylene triamine pentaacetic acid), commonly used to correct Fe deficiency in plants. Plants were killed in the soil of pH 6.2 when the 1000 μg Ni/g soil was added. The pH 7.2 soil decreased the toxicity of Ni. The DTPA had little effect on yields, but increased the amount of Ni in plants. Nickel decreased the Fe, Zn, Cu, and Mn concentrations of the plants. Stems contained less Ni than did leaves. In another experiment, EDTA (ethylenediamine tetraacetate) greatly increased Ni concentrations in bush beans (Phaseolus vulgaris L. C.V. Improved Tendergreen) and in barley (Hordeum vulgare L.C.V. Atlas 57) grown in Yolo loam soil, and simultaneously increased Fe concentrations. Lime (CaCO₃ or MgCO₃) decreased toxicity of Ni in bush beans. DTPA increased Ni transport in bush beans and increased the ratio of Ni in leaves to that in stems at soil pH 7.5 and 8.2, but not at pH 4.0 and 5.8.     

Fate and interaction with native soil N of ammonium N applied to wetland rice (Oryza sativa L.) grown under saline and non-saline conditions

Summary The effect of salts on the balance of fertilizer N applied as ¹⁵N-labelled ammonium sulphate and its interaction with native soil N was studied in a pot experiment using rice (Oryza sativa L.) as a test crop. The rice crop used 26%–40% of the applied N, the level of applied N and salts showing no significant bearing on the uptake of fertilizer N. Losses of fertilizer N ranged between 54% and 68% and only 5%–8% of the N was immobilized in soil organic matter. Neither the salts nor the rate of N application had any significant effect on fertilizer N immobilization. The effective use of fertilizer N (fertilizer N in grain/fertilizer N in whole plant) was, however, better in the non-saline soil. The uptake of unlabelled N (N mineralized from soil organic matter and that originating from biological N₂ fixation in thes rhizosphere) was inhibited in the presence of the salts. However, in fertilized soil, the uptake of unlabelled N was significantly enhanced, leading to increased A values [(1-% Ndff/% Ndff)x N fertilizer applied, where Ndff is N derived from fertilizer], an index of interaction with the added N. This added N interaction increased with increasing levels of added N. Since the extra unlabelled N taken up by fertilized plants was greater than the fertilizer N immobilized, and the root biomass increased with increasing levels of added N, a greater part of the added N interaction was considered to be real, any contribution by an apparent N interaction (pool substitution or isotopic displacement) to the total calculated N interaction being fairly small. Under saline conditions, for the same level of fertilizer N addition, the added N interaction was lower, and this was attributed to a lower level of microbial activity, including mineralization of native soil N, rootdriven immobilization of applied N, and N₂ fixation.     

Assessment of the relative uptake of added and indigenous soil nitrogen by nodulated legumes and reference plants in the 15N dilution measurement of N2 fixation: Derivation of method

Current methods for measuring N₂ fixation by nodulated legumes involve the addition of small amounts of ¹⁵N-labelled plant-available N compounds to soil so that plant N derived from the soil may be identified. All such methods assume that the proportions of added N and indigenous soil N assimilated by N₂-fixing and non-fixing plants grown in the same soil are the same, irrespective of the amount of soil N assimilated. The development of a method for assessing these proportions is described.Nodulated legumes and reference plants are grown in soils receiving none or one rate of addition of labelled N compound containing several (two or more) concentrations of ¹⁵N. The proportions of added and indigenous N assimilated, are determined from the intercepts and slopes of regression lines relating isotopic composition of plant N to that of added N, together with some other readilly-obtainable plant N measurements.     

Transgenic Bt plants decompose less in soil than non-Bt plants

Bt plants are plants that have been genetically modified to express the insecticidal proteins (e.g. Cry1Ab, Cry1Ac, Cry3A) from subspecies of the bacterium, Bacillus thuringiensis (Bt), to kill lepidopteran pests that feed on corn, rice, tobacco, canola, and cotton and coleopteran pests that feed on potato. The biomass of these transgenic Bt plants (Bt+) was decomposed less in soil than the biomass of their near-isogenic non-Bt plant counterparts (Bt−). Soil was amended with 0.5, 1, or 2% (wt wt⁻¹) ground, dried (50 °C) leaves or stems of Bt corn plants; with 0.5% (wt wt⁻¹) ground, dried biomass of Bt rice, tobacco, canola, cotton, and potato plants; with biomass of the near-isogenic plants without the respective cry genes; or not amended. The gross metabolic activity of the soil was determined by CO₂ evolution. The amounts of C evolved as CO₂ were significantly lower from soil microcosms amended with biomass of Bt plants than of non-Bt plants. This difference occurred with stems and leaves from two hybrids of Bt corn, one of which had a higher C:N ratio than its near-isogenic non-Bt counterpart and the other which had essentially the same C:N ratio, even when glucose, nitrogen (NH₄NO₃), or glucose plus nitrogen were added with the biomass. The C:N ratios of the other Bt plants (including two other hybrids of Bt corn) and their near-isogenic non-Bt counterparts were also not related to their relative biodegradation. Bt corn had a significantly higher lignin content than near-isogenic non-Bt corn. However, the lignin content of the other Bt plants, which was significantly lower than that of both Bt and non-Bt corn, was generally not statistically significantly different, although 10-66% higher, from that of their respective non-Bt near-isolines. The numbers of culturable bacteria and fungi and the activity of representative enzymes involved in the degradation of plant biomass were not significantly different between soil amended with biomass of Bt or non-Bt corn. The degradation of the biomass of all Bt plants in the absence of soil but inoculated with a microbial suspension from the same soil was also significantly less than that of their respective inoculated non-Bt plants. The addition of streptomycin, cycloheximide, or both to the soil suspension did not alter the relative degradation of Bt+ and Bt− biomass, suggesting that differences in the soil microbiota were not responsible for the differential decomposition of Bt+ and Bt− biomass. All samples of soil amended with biomass of Bt plants were immunologically positive for the respective Cry proteins and toxic to the larvae of the tobacco hornworm (Manduca sexta), which was used as a representative lepidopteran in insect bioassays (no insecticidal assay was done for the Cry3A protein from potato). The ecological and environmental relevance of these findings is not clear.     

Nutrient uptake and growth of potato: Arbuscular mycorrhiza symbiosis interacts with quality and quantity of amended biochars

Aims : The aim of this study was to explore interactive effects between quality (types) and quantity (application rates) of biochar as well as of arbuscular mycorrhiza (AM) symbiosis on the growth of potato plants. Methods : A low P sandy loam soil was amended with 0%, 1.5%, or 2.5% (w/w) of either of 4 types of biochar, which were produced from wheat straw pellets (WSP) or miscanthus straw pellets (MSP) pyrolyzed at temperatures of either 550°C or 700°C. Potato plants grown in pots containing the soils or soil biochar mixture were inoculated with or without AM fungus (AMF), Rhizophagus irregularis. The experiment was carried out under fully irrigated semi‐field conditions and plants were harvested 101 days after planting. Results : Application of high temperature biochar decreased growth, biomass and tuber yield of potato plants, while the low temperature biochar had a similar effect on yield as plants grown without biochar amendment. Total biomass of potato plants were decreased with the increasing rate of biochar. Arbuscular mycorrhizal fungus inoculation stimulated the growth of potato plants in all organs, increased tuber biomass significantly in 1.5% MSP700 amended plants, and to a lesser degree for WSP700, MSP550, and WSP550. In addition, plant biomass gain was linearly related to N, P, and K uptake, the ratio of P to N in the leaf of plants indicated that all treatments were mainly P‐limited. A multiple linear regression using P uptake and biochar rate as independent variables explained 91% of the variation in total biomass. The single effect of AMF inoculation, type and rate of biochar affected plant N, P and K uptake similarly. While AMF inoculation significantly increased P uptake in potato plants grown in soil with WSP700 or MSP700 despite of the rate of biochar. In general, application of biochar significantly increased AMF root colonization of potato plants. Conclusions : The application of MSP550 at 1.5% combined with AMF stimulated growth of potato the most. Furthermore, the results indicated that the interactive effect of AMF inoculation, biochar type and application rate on potato growth to a large extent could be explained by effects on plant nutrient uptake.     

Effects of temperature,glucose and inorganic nitrogen inputs on carbon mineralization in a Tibetan alpine meadow soil

High levels of available nitrogen (N) and carbon (C) have the potential to increase soil N and C mineralization. We hypothesized that with an external labile C or N supply alpine meadow soil will have a significantly higher C mineralization potential, and that temperature sensitivity of C mineralization will increase. To test the hypotheses an incubation experiment was conducted with two doses of N or C supply at temperature of 5, 15 and 25 °C. Results showed external N supply had no significant effect on CO₂ emission. However, external C supply increased CO₂ emission. Temperature coefficient (Q₁₀) ranged from 1.13 to 1.29. Significantly higher values were measured with C than with N addition and control treatment. Temperature dependence of C mineralization was well-represented by exponential functions. Under the control, CO₂ efflux rate was 425 g CO₂–C m^?2 year^?1, comparable to the in situ measurement of 422 g CO₂–C m^?2 year^?1. We demonstrated if N is disregarded, microbial decomposition is primarily limited by lack of labile C. It is predicted that labile C supply would further increase CO₂ efflux from the alpine meadow soil.     

Enhancement of maize plant growth with inoculation of phosphate-solubilizing bacteria and biochar amendment in soil

Maize plant has an absolute requirement of nutrients (N, P, and K) for growth and development. The microbial application can facilitate in addressing limited access to chemical fertilizer concern. Moreover, biochar and phosphorus-solubilizing bacterial (PSB) community can contribute together in nutrient availability. Both have the P-supply potential to the soil, but their interaction has been tested less under semiarid climatic conditions. The purpose of the study was to evaluate the potential of biochemically tested promising PSB strains and biochar for maize plant growth and nutritional status in plant and soil. Therefore, two isolated PSB strains from maize rhizosphere were biochemically tested in vitro and identified by 16S rDNA gene analysis. The experiment was conducted in the greenhouse where the plant growth and nutrient availability to the plants were observed. In this regard, all the treatments such as PSB strain-inoculated plants, biochar-treated plants, and a combination of PSBs + biochar-treated plants were destructively sampled on day 45 (D₄₅) and day 65 (D₆₅) of sowing with four replications at each time. PSB inoculation, biochar incorporation, and their combinations have positive effects on maize plant height and nutrient concentration on D₄₅ and D₆₅. In particular, plants treated with sawdust biochar + Lysinibacillus fusiformis strain 31MZR inoculation increased N (32.8%), P (72.5%), and K (42.1%) against control on D₆₅. Besides that, only L. fusiformis strain 31MZR inoculation enhanced N (23.1%) and P (61.5%) than control which shows the significant interaction of PSB and biochar in nutrient uptake. PSB and biochar have the potential to be used as a promising amendment in improving plant growth and nutrient absorption besides the conventional approaches.     

Evolution of dinitrogen and nitrous oxide from the soil to the atmosphere through rice plants

Summary It is commonly assumed that a large fraction of fertilizer N applied to a rice (Oryza sativa L.) field is lost from the soil-water-plant system as a result of denitrification. Direct evidence to support this view, however, is limited. The few direct field, denitrification gas measurements that have been made indicate less N loss than that determined by ¹⁵N balance after the growing season. One explanation for this discrepancy is that the N₂ produced during denitrification in a flooded soil remains trapped in the soil system and does not evolve to the atmosphere until the soil dries or is otherwise disturbed. It seems likely, however, that N₂ produced in the soil uses the rice plants as a conduit to the atmosphere, as does methane. Methane evolution from a rice field has been demonstrated to occur almost exclusively through the rice plants themselves. A field study in Cuttack, India, and a greenhouse study in Fort Collins, Colorado, were conducted to determine the influence of rice plants on the transport of N₂ and N₂O from the soil to the atmosphere. In these studies, plots were fertilized with 75 or 99 atom % ¹⁵N-urea and ¹⁵N techniques were used to monitor the daily evolution of N₂ and N₂O. At weekly intervals the amount of N₂+N₂O trapped in the flooded soil and the total-N and fertilized-N content of the soil and plants were measured in the greenhouse plots. Direct measurement of N₂+N₂O emission from field and greenhouse plots indicated that the young rice plant facilitates the efflux of N₂ and N₂O from the soil to the atmosphere. Little N gas was trapped in the rice-planted soils while large quantities were trapped in the unplanted soils. N losses due to denitrification accounted for only up to 10% of the loss of added N in planted soils in the field or greenhouse. The major losses of fertilizer N from both the field and greenhouse soils appear to have been the result of NH₃ volatilization.     

Winter cover cropping influence on nitrogen mineralization,presidedress soil nitrate test,and corn yields

The mineralization and availability of cover crop N to the succeeding crop are critical components in the management of soil N to reduce N leaching. The effects of several leguminous and non-leguminous cover crops on soil N availability, N mineralization potential, and corn (Zea mays L.) yield were examined. The cover crops had variable effects on soil N availability and corn yield and N uptake. Because of the rapid mineralization of the cover crops following incorporation, the inorganic N levels in the soil sampled in mid-May 1992 (4 weeks after incorporation of cover crops), rather than the potentially mineralizable N, rate constants, initial potential mineralization rate, or cumulative N mineralized over 14 weeks, correlated well with N concentrations, C:N ratios, or the N added in the cover crops. However, the inclusion of potentially mineralizable N with inorganic N in a multiple regression improved the variability in the corn yield and the N uptake accounted for. Since extensive mineralization had occurred before the 21 May sampling, the potentially mineralizable N was affected more by the soil organic N and C than by the N concentrations of the cover crops. The presidedress NO₃ ^--N test levels were well predicted by the inorganic and potentially mineralizable N (R ²=0.89, P<0.01), although the test levels were better in predicting corn yield and N uptake. If the available soil N test needs to be made earlier than recommended by the presidedress NO₃ ^--N test, both inorganic and potentially mineralizable N are needed to better predict the corn yield and N uptake in the soils.     

The influence of pH,soil type and time on adsorbtion and uptake by plants of Cd added to the soil

Adsorption of Cd by two soils and its uptake by perennial ryegrass (Lolium perenne) and winter rape (Brassica napus) as a function of pH (pH 4 to 7) and the amount of Cd added to the soil (0 to 5 mg kg^?1 soil) were studied in a 2-yr pot experiment. In the soils, the more soluble fractions of Cd increased as the pH was lowered. Increasing the pH from 5 to 7 by adding CaO invariably reduced the Cd-content of ryegrass plants, but this decrease was less consistent where the pH had only been increased to 6. In some cases, acidifying the soil with S to reach a pH of 4 also led to a decrease in plant Cd-content. The Cd-content of rapeseed plants was markedly higher at pH 4 than at pH 5. Plant damage at low pH was observed in this crop. Water-leachable and CaCl₂-extractable soil Cd levels as well as plant uptake were higher in the sand soil than in the clay soil, whereas 1M NH₄AcO (buffered at pH 4.8 and 7) extracted roughly equal amounts from both soils. Adding more Cd to the soil did not change the relation between Cd levels in soil and those in plants; instead the amounts of Cd in both increased in direct proportion to the amounts added. Fixation of added Cd apparently did not occur continuously at any pH or Cd-level during the 2-yr period, but seasonal variations in solubility and uptake were observed.     

Influence of Azolla (Azolla microphylla Kaulf.) compost on biogenic gas production,inorganic nitrogen and growth of upland kangkong (Ipomoea aquatica Forsk.) in a silt loam soil

Azolla microphylla Kaulf. (Azolla) biomass was composted to create a high nitrogen (N) organic matter amendment (Azolla compost). We examined the effect of this Azolla compost on carbon (C) and N mineralization and the production of biogenic gases, nitrous oxide (N₂O) and carbon dioxide (CO₂), in a soil incubation experiment. A pot experiment with upland kangkong (Ipomoea aquatica Forsk.) examined plant growth in silt loam soil treated with three levels of Azolla compost. The results showed that N₂O production from soil increased with urea amendment, but not with Azolla compost treatments. The Azolla-amended soil showed enhanced CO₂ production throughout the 4-week incubation. The Azolla-treated soils showed a 98% lower global warming potential compared to urea treatment over the 4-week incubation. However, Azolla-amended soil had higher nitrate (NO₃^–) levels compared to urea-fertilized soil at 1 week of incubation, and these were maintained until the fourth week. Soils amended with Azolla compost showed lower ammonium nitrogen (NH₄-N) levels than those in the urea-fertilized soils. The height and dry weight of upland kangkong fertilized with Azolla compost were similar to plants receiving urea fertilization. Therefore, the use of Azolla compost as a substitute for urea fertilizer would be beneficial for reducing the production of N₂O while maintaining plant growth.     

The relationship of soil and leaf nutrients to rice leaf oranging

A symptom called leaf‐oranging, indicating a deficiency of many nutrients, occurs in paddy rice (Oryzasativa L.) when production expands into some upland soils. Rice (Gui Chou cv.) was grown in culture pots in a flooded, weathered, upland soil (Nacogdoches) and compared to rice growth in a flooded soil currently used for paddy rice production (Dacosta) in Texas to understand the soil and plant factors involved in leaf‐oranging. Fertilizer rates of 0, 10, and 100 mg N/kg as (NH₄)₂SO₄ were applied to each soil along with phosphorus (P) and potassium (K) fertilizer. The orange Leaf Index (OLI), a measure of leaf‐oranging, was determined weekly and increased to 60–70% for plants grown in the upland soil but its progression was delayed by higher N treatments. No leaf‐oranging was observed in the paddy soil. The soil evoking leaf‐oranging was low in silicon (Si) and high in iron (Fe). In addition, analysis of leaves from these plants showed 19–25% higher leaf ammonium‐nitrogen (NH₄‐N), 9–137% higher manganese (Mn) levels and lower total N:NH₄ concentration compared to normal rice leaves four weeks after transplanting. This inferred that leaf‐oranging probably was associated with some degree of NH₄‐N toxicity and antagonism with K. Leaf‐oranging was also associated with low calcium (Ca) assimilation or Ca uptake inhibition because of the heavy Fe‐oxide coating of the roots of the affected rice plants. In this experiment, leaf‐oranging was not associated with toxic levels of Fe or Mn.     

Tree girdling provides insight on the role of labile carbon in nitrogen partitioning between soil microorganisms and adult European beech

Nitrogen (N) cycling in terrestrial ecosystems is complex since it involves the closely interwoven processes of both N uptake by plants and microbial turnover of a variety of N metabolites. Major interactions between plants and microorganisms involve competition for the same N species, provision of plant nutrients by microorganisms and labile carbon (C) supply to microorganisms by plants via root exudation. Despite these close links between microbial N metabolism and plant N uptake, only a few studies have tried to overcome isolated views of plant N acquisition or microbial N fluxes. In this study we studied competitive patterns of N fluxes in a mountainous beech forest ecosystem between both plants and microorganisms by reducing rhizodeposition by tree girdling. Besides labile C and N pools in soil, we investigated total microbial biomass in soil, microbial N turnover (N mineralization, nitrification, denitrification, microbial immobilization) as well as microbial community structure using denitrifiers and mycorrhizal fungi as model organisms for important functional groups. Furthermore, plant uptake of organic and inorganic N and N metabolite profiles in roots were determined.Surprisingly plants preferred organic N over inorganic N and nitrate (NO₃⁻) over ammonium (NH₄⁺) in all treatments. Microbial N turnover and microbial biomass were in general negatively correlated to plant N acquisition and plant N pools, thus indicating strong competition for N between plants and free living microorganisms. The abundance of the dominant mycorrhizal fungi Cenococcum geophilum was negatively correlated to total soil microbial biomass but positively correlated to glutamine uptake by beech and amino acid concentration in fine roots indicating a significant role of this mycorrhizal fungus in the acquisition of organic N by beech. Tree girdling in general resulted in a decrease of dissolved organic carbon and total microbial biomass in soil while the abundance of C. geophilum remained unaffected, and N uptake by plants was increased. Overall, the girdling-induced decline of rhizodeposition altered the competitive balance of N partitioning in favour of beech and its most abundant mycorrhizal symbiont and at the expense of heterotrophic N turnover by free living microorganisms in soil. Similar to tree girdling, drought periods followed by intensive drying/rewetting events seemed to have favoured N acquisition by plants at the expense of free living microorganisms.     

Combined effects of soil waterlogging and compaction on rice (Oryza sativa L.) growth, soil aeration, soil N transformations and 15N discrimination

 The combined effects of soil compaction and soil waterlogging on the growth of two rice cultivars (Oryza sativa L., cultivars Kanto 168 and Koshihikari) and soil N transformations were studied in pots. Although waterlogging eliminated initial differences in mechanical resistance between compacted and loose soils, Kanto 168 and Koshihikari roots had, respectively, less biomass and a lower porosity if soil was compacted prior to waterlogging. The cause for this was probably established before waterlogging. Redox values showed that upland soils were well aerated. Loose waterlogged soils contained oxic sites, but compacted waterlogged soils did not. Potential denitrification was stimulated by waterlogging and, to a larger extent, by plant presence. Waterlogging lowered potential nitrifying capacities, by competition between plants and micro-organisms for NH₄ ⁺ rather than by oxygen shortage. Compaction prior to waterlogging benefited the potential nitrifying capacity of soils with either cultivar and the potential denitrifying capacity for soils with Koshihikari. Compaction had no effect on nitrification or denitrification in upland soils. N recoveries were low, especially in pots without plants, as a result from sampling strategy and N loss. On day 42/43 after potting, total δ¹⁵N values of waterlogged pots were positive, whereas after 22 days all pots had negative total δ¹⁵N values. Final δ¹⁵N values of plant parts from waterlogged and upland soils were positive and negative, respectively. Although the δ¹⁵N values generally accorded well with the other results, they did not support higher N losses from compacted waterlogged soils than from loose waterlogged soils with plants, as suggested by potential denitrifying activities. Received: 4 February 2000