Interactions of nitrogen fertilizers and forest humus—II. Beech and oak

Laboratory incubation experiments with and without added urea or NH₄NO₃ were performed on humus from stands of beech (Fagus silvatica) grown on soils from limestone, schists, flysch and peridotites and on humus from oak (Quercus conferta) stands on soils from limestone and schists.Beech and oak humus from stands grown on soils from limestone and flysch showed considerable nitrification with a concurrent high mobilization rate of the nutrient elements Ca, Mg and K, especially in the presence of increasing urea concentrations, but no net humus N mineralization was observed. Beech humus from stands grown on soils from schists and peridotites showed no nitrification and increasing concentrations of added urea did not modify their inability to nitrify. Non-nitrifying types of humus showed considerable ammonification but their Ca, Mg and K mobilization rates were about one-tenth those observed in nitrifying humus and were inversely correlated with urea concentrations.Exchangeable Al³⁺ and extractable Mn were present in high concentrations in the underlying inorganic soils in all cases where nitrification was absent from the overlying humus but addition of 500 parts Al³⁺ and 1000 parts Mn/10⁶ separately or in combination to a nitrifying humus failed to inhibit nitrification.An interpretation of these findings is attempted with reference to the possibility of absence of nitrification in climax vegetations and the preference of certain forest species for NH⁺₄ or NO^?₃.     

Leaching of nitrogen forms from controlled‐release nitrogen fertilizers

Abstract

Application of soluble forms of nitrogen (N) fertilizers to sandy soils may cause leaching of nitrate N (NO₃‐N) resulting in contamination of groundwater. The leaching loss of N may be reduced to a certain extent by the use of controlled‐release N formulations. A leaching column study was conducted to evaluate the leaching of urea, ammonium N (NH₄‐N), and NO₃‐N forms from selected urea‐based controlled‐release formulations (Meister, Osmocote, and Poly‐S) and uncoated urea under eight cycles of intermittent leaching and dry conditions. Following leaching of 1,760 mL of water (equivalent to 40 cm rainfall) through the soil columns, the recovery of total N (sum of all forms) in the leachate accounted for 28, 12, 6, or 5% of the total N applied as urea, Poly‐S, Meister, and Osmocote, respectively. Loss of urea‐N from all fertilizer sources was pronounced during the initial leaching events (with the exception of Meister). Cumulative leaching of urea‐N was 10% for uncoated urea while <1.7% for the controlled‐release formulations. Cumulative leaching of NH₄‐N was 6.2% for uncoated urea while <0.5% for the controlled‐release formulations. Cumulative leaching loss of NO₃‐N was 3.78% for Osmocote, 4.6% for Meister, 10.4% for urea, and 10.5% for Poly‐S. This study demonstrates a significant reduction in leaching of N forms from controlled‐release formulations as compared to that from the soluble form.     

Carbon and nitrogen turnover in response to warming and nitrogen addition during early stages of forest litter decomposition—an incubation experiment

Purpose

Little is known about the interactive effects of temperature, nitrogen (N) supply, litter quality, and decomposition time on the turnover of carbon (C) and N of forest litter. The objective of this study was to investigate the interactive effects of warming, N addition and tree species on the turnover of C and N during the early decomposition stage of litters in a temperate forest.

Materials and methods

A 12-week laboratory incubation experiment was carried out. The leaf litters including two types of broadleaf litters (Quercus mongolica and Tilia amurensis), a needle litter (Pinus koraiensis), and a mixed litter of them were collected from a broad-leaved Korean pine mixed forest ecosystem in northeastern China in September 2009. Nine treatments were conducted using three temperatures (15, 25, and 35 °C) combined with three doses of N addition (equal to 0, 75, and 150 kg?·?ha^?1?a^?1, respectively, as NH₄NO₃).

Results and discussion

After 12 weeks of incubation, the mass loss ranged between 12 and 35 %. The broadleaf litters had greater mass loss and cumulative CO₂–C emission than the needle litter. Temperature and N availability interacted to affect litter mass loss and decomposition rate. The dissolved organic carbon (DOC) and nitrogen (DON) concentrations in litter leachate varied widely with litter types. DOC increased significantly with increased temperature but decreased significantly with increased N availability. DON increased significantly with increased N availability but showed a higher level at the moderate decomposition temperature. The amounts of CO₂ and N₂O emission were significantly higher at 25 °C than those at 15 and 35 °C, and were significantly increased by the N addition.

Conclusions

The present study indicated relatively intricate temperature and N addition effects on C and N cycling during early stages of litter decomposition, implying that future increases in temperature and N deposition will directly affect C and N cycling in broad-leaved Korean pine mixed forest ecosystem, and may indirectly influence the ecosystem composition, productivity, and functioning in NE China. It is, therefore, important to understand the interactive effects of biotic and abiotic factors on litter decomposition in field conditions in order to assess and predict future ecosystem responses to environmental changes in NE China.     

The effects of volcanic ash influenced soils and nitrogen fertilization on douglas‐fir seedlings

Abstract

An experiment was conducted to help understand the contributions of different soil layers to soil fertility, plant growth, and response to fertilization. Douglas‐fir (Pseudotsuga menziesii) seedlings were used in a pot bioassay to delimit the effects of volcanic ash soils, urea fertilization at 100 and 200 ppm, and the technique of using undisturbed soil as a growth medium. Volcanic ash horizons contained more available phosphorus and mineralized more nitrogen than underlying horizons. Best seedling growth occurred in ash horizons fertilized with 200 ppm of urea‐N. Fertilization decreased soil pH, mycorrhizae formation and foliar‐P levels but increased foliar‐N. The technique of using undisturbed soil had little effect on seedling growth in the weak structured ash horizons but did decrease root weights in the moderate structured sub‐ash layers.     

Effect of nitrogen and sulphur fertilizers and seed inoculation with rhizobium phaseoli on the nitrogen‐sulphur relationships of bean (phaseolus vulgaris L.)

A greenhouse experiment with beans (Phaseolus vulgaris L.) was performed in order to investigate the effect of nitrogen and sulphur application and seed inoculation on the yield, leaf area, distribution of different nitrogen and sulphur fractions and N/S ratio in shoot, fruit and root.

Inoculation of plants together with nitrogen or sulphur application produces an increase in the concentration of total nitrogen and a decrease in the accumulation of nitrate‐nitrogen and sulphate‐sulphur in shoot, fruit and root. Leaf area increased more with nitrogen than with sulphur application while the highest amounts of fruit dry matter were obtained with sulphur application.

N: S ratios obtained were different according to the part of the plant tested. Sulphur fertilization decreased the N: S ratios in shoot, fruit and root. The data obtained indicate that and adequate N: S ratio can insure maximum production of yield.     

Recent advances in the chemistry of nitrogen,phosphorus and potassium as fertilizers in soil: A review

The advent of civilization has made humans dependent on plants for food and medicine, leading to the intensification of agricultural production. The intense cultivation of crops has resulted in the depletion of available nutrients from soil, thereby demanding the application of excess nutrients to soil to improve yield. Thus, mineral fertilizer discovery and application have, in many ways, contributed greatly to meeting global food demands. However, aside from the positive effects of mineral fer...     

Studies of nitrogen immobilization and mineralization in calcareous soils—IV. Changes in the organic nitrogen of light and heavy subfractions of silt- and fine clay-size particles during nitrogen turnover

A calcareous clay nd a calcareous sand, were fractionated densimetrically by dispersion in organic liquids of sp. gr. 1.59–2.06. The N contents of the light fractions decreased with increasing densities of the suspending liquids and were up to 18–23 times higher than those of the whole soils. Light fraction organic-N of both the sandy and clay soils was obtained mainly from silt-size components. However, the efficiency, with which light fraction material was obtained from the two whole soils, varied. With the clay soil, the total yield of light fraction organic-N was increased markedly by applying the densimetric technique to particle size components, rather than to the whole soil.Silt-size and fine clay-size particles from soils, sampled during rapid metabolism of microbial organic-[¹⁵N], were further fractionated densimetrically in “Nemagon”, sp. gr. 2.06. The organic-[¹⁵N] of the light and heavy subtractions changed markedly (P < 0.05) during periods of net ¹⁵N immobilization and mineralization, including a period after soil fumigation when extensive decomposition of [¹⁵N]-labelled microbial biomass occurred. Changes in the ¹⁵N of complementary light and heavy subfractions followed similar trends. Light subtraction organic-[¹⁵N] usually showed the greater relative change but the differences between the subtractions were not statistically significant. It is concluded that when small proportions only of soil organic-N are associated with macroorganic debris, as in these two soils amended wth glucose and ¹⁵NO^?₃, densimetric fractionation at a sp. gr. as high as 2.06 will yield light and heavy fractions, whose nitrogenous components are similarly available to biological attack. Enhanced metabolism of light fraction material is more likely to be demonstrated when such material consists mainly of obvious plant residues, and this may be more easily achieved by fractionation in liquids of sp. gr. <2.     

Interactions of gamma‐irradiated sewage,nitrogen and phosphorus for Sorghum on a calcareous soil

Abstract

The interaction of gamma‐irradiated sewage sludge, nitrogen and phosphorus fertilizers on dry matter production, phosphorus concentration and phosphorus uptake of sorghum was examined. Three crops of sorghum were grown in the greenhouse in 19 1 plastic buckets. The phosphorus and sludge treatments were applied at the initiation of the experiment only. Nitrogen was applied to the corresponding nitrogen treatment pots before each of the three croppings.

There was a significant nitrogen x sewage interaction for dry matter production, phosphorus concentration and phosphorus uptake in each harvest. There was a significant phosphorus sewage sludge interaction in the first harvest for phosphorus uptake. All other possible interactions were not statistically significant. The 67 metric ton/ha sludge rate produced nearly the same yield as the nitrogen treatment. In the first harvest, sludge significantly increased plant phosphorus uptake from the fertilizer phosphorus. The phosphorus uptake in sorghum from 472 and 944 kg P/ha from the sludge treatments was comparable to that from 1299 and 2598 kg P/ha from triple superphosphate.     

Community level physiological profiles of microbial communities from forest humus polluted with different amounts of Zn,Pb, and Cd—Preliminary study with BIOLOG ecoplates

Abstract

Heavy metals may alter the structure and metabolic functions of soil microbial community. The objective of our study was to compare the community level physiological proffies (CLPPs) of microbial communities from forest humus polluted with different amounts of Zn, Pb, and Cd to test whether the addition of soluble Zn and Cd may affect the CLPPs of microbial communities. The samples were taken at 18 locations in southern Poland referred to as unpolluted (UP), slightly polluted (SP), and heavily polluted sites (HP). The contents of heavy metals were measured after wet digestions in concentrated HNO₃. Microbial communities were extracted using 0.96% NaCl solution. In order to test heavy metal tolerance of microbial communities from UP sites the extracts from these sites were additionally treated with Zn (50 mg L^-1; UP + Zn) and Cd (1 mg L^-1; UP + Cd). Metabolic functions of the microbial communities were analyzed using BIOLOG Ecoplates method. The contents of Zn, Pb and Cd were the highest at HP sites (4,740, 1,120, 41.0 mg kg^-1, respectively) followed by SP (830, 509, 9.2 mg kg^-1, respectively), and UP (173, 93, 2.1 mg kg^-1, respectively) sites. Principal components analysis (PCA) indicated that CLPPs at all sites were similar. This suggests that microbial community from SP and HP sites revealed tolerance to heavy metals. Addition of Zn affected CLPPs of microbial communities from UP sites as indicated by significantly (p < 0.05) higher value of PC1 score. The addition of Cd did not affect CLPPs of microbial communities from these sites.     

Studies of nitrogen immobilization and mineralization in calcareous soils—II: Mineralization of immobilized nitrogen from soil fractions of different particle size and density

¹⁵NO^?₃ was immobilized in a calcareous sandy soil and a calcareous clay soil each incubated with glucose and wheat straw. Net mineralization of organic-¹⁵N was more rapid in the sandy soil, irrespective of C amendment, and in soils amended with glucose. Intermittent drying and wetting of soils during incubation stimulated mineralization of ¹⁵N-labelled and native soil organic-N in all treatments. The availability (percentage mineralization) of recently-immobilized ¹⁵N consistently exceeded that of the native soil N. Ratios of the availability of labelled and unlabelled N were similar in the sandy and clay soils but varied according to C amendment, drying and wetting cycle and incubation period.Changes in the distribution of immobilized N amongst soil extracts and soil fractions of different particle size and density were determined during periods of net N mineralization. In straw-amended soils, the organic-¹⁵N of a light fraction, sp.gr. < 1.59, decomposed relatively rapidly during the late mineralization period. Decreases of organic ¹⁵N of the fine clay fraction were also recorded. In glucose-amended soils, net N mineralization was accompanied by significant decreases in the concentrations of organic-¹⁵N of the silt and fine clay fractions.Drying and rewetting of soils hastened or magnified changes occurring in the organic-¹⁵N of soil fractions, but qualitatively, the pattern of change was similar to that observed with soils incubated under uniformly-moist conditions.The percentage distribution of labelled and unlabelled N suggested that in the long term, the silt fraction will accumulate an increasing proportion of the more stable nitrogenous residues.     

Studies of nitrogen immobilization and mineralization in calcareous soils—I: Distribution of immobilized nitrogen amongst soil fractions of different particle size and density

¹⁵NO₃^? was immobilized in a calcareous clay and a calcareous sandy soil during incubation of each soil with glucose and wheat straw. Changes in the distribution of immobilized ¹⁵N amongst soil extracts and soil fractions of different particle size and density were determined during periods of net N immobilization.The nature of the organic-C amendment, but not soil type, significantly influenced both the distribution of the immobilized ¹⁵N and the pattern of changes of the organic-¹⁵N of soil fractions with time. In straw-amended soils, approx. 20% of the organic-¹⁵N became associated with a light fraction, sp. gr. < 1.59, the remainder becoming distributed mainly amongst the silt and clay fractions. In glucoseamended soils, very little (< 1.2%) of the ¹⁵N was immobilized in the light fraction, sp. gr. < 1.59, most being rapidly distributed amongst the silt and clay fractions. During a period of complete immobilization, organic-¹⁵N was transferred from the fine clay to the silt and coarse clay fractions.Silt, coarse clay and fine clay components from glucose-amended soils sampled at the end of the net immobilization phase were further fractionated densimetrically into light (sp. gr. < 2.06) and heavy (sp. gr. > 2.06) subfractions. The organic-¹⁵N of respective light subfractions accounted for 43–64% of the total organic-¹⁵N of the silt, 1–9% of that of the coarse clay and 19–21% of that of the fine clay fractions.     

Ecological studies on coccoid bacteria in a pine forest soil—II. growth of bacteria introduced into soil

The growth of five typical but phenetically distinct cocci and Arthrobacter strains, isolated from a pine forest soil, has been investigated. Soil reaction was found to have a marked effect on growth and unless naturally acidic soils were made more alkaline they did not support growth, even in the presence of added nutrients. However, in the presence of fungi which could use the added nutrients, e.g. chitin, mycelial fragments etc., bacterial growth was possible and could be correlated with a decrease in acidity, especially around particles of organic matter. Where the pH rose above 7.9, bacterial growth again decreased. All the bacterial strains tested reacted in the same way, suggesting that they occupied similar microenvironments in both the acidic and alkaline soil horizons examined. Some explanations for the occurrence of nonsporing bacteria in soils in which apparently they cannot grow are suggested.     

Reducing nitrogen leaching‐losses from containerized plants: The effectiveness of controlled‐release fertilizers

Abstract

To evaluate the effectiveness of controlled‐release fertilizer (CRF) for reducing nitrogen (N) leaching‐losses from containerized greenhouse crops, three experiments were conducted where CRFs were applied in different ways and compared to water‐soluble fertilizer (WSF). In each experiment, ‘First Lady’ marigold (Tagetes erecta L.) plants in 0.5‐liter pots of a soilless growth medium were fertilized with the same amount of ? from 20N‐4.3P‐16.6K WSF, Osmocote 14N‐6.2P‐11.6K CRF, or Nutricote 14N‐6.2P‐11.6K CRF fertilizers. The volume of irrigation water applied to all treatments was the same in each experiment. Nitrogen content, as NH₄‐N and NO₃‐N in container leachates, and plant growth were measured and used to compare WSF with CRFs incorporated in the growth medium, or as applied to the surface, in either one large application or two small doses. A single large application of CRF at planting resulted in as much or more ? leaching than the regular application of WSF. Effectiveness of CRFs in limiting ? leaching was greatly increased by making two smaller applications, the first at planting and the second 15 to 35 days later. More ? was recovered in the leachate when CRFs were incorporated in the growth medium compared to surface application. Regardless of fertilizer type, application method, timing of application, or for each individual experiment, NO₃‐N was the predominant ? form found in the leachate and more than one‐half of the total amount of ? leached during each experiment was recovered within 30 days of planting.     

Determination of dissolved organic nitrogen using persulfate oxidation and conductimetric quantification of nitrate‐nitrogen

Abstract

Nitrogen (N) in forest soil extracts and surface waters may be dominantly in organic compounds as dissolved organic nitrogen (DON). Due to various difficulties associated with measuring total N (as TKN) by the Rjeldahl digest, this important vehicle for nutrient movement is rarely monitored. By coupling two relatively new methods and optimizing them for use in soil studies, we developed an alternative method for measuring DON. Analysis of pure compounds and field samples shows that persulfate oxidation combined with conductimetric quantification of nitrate (NO₃) provides a highly accurate measure of dissolved N content. With relatively inexpensive equipment and reagents, a single technician can digest and assay over a hundred samples a day. This rapid, simple, and accurate assay may make it possible to routinely monitor DON where it had previously been impractical. This in turn could substantially enhance understanding about the form and quantity of N involved in nutrient fluxes.     

Microsite assessment of forest soil nitrogen,phosphorus, and potassium supply rates in‐field using ion exchange membranes

Abstract

A new method for microsite assessment of soil nutrient supply in forest soil was developed. The method involves the use of ion exchange membranes to assess differences in soil nitrogen (N), phosphorus (P), and potassium (K) supply rates in‐field over small depth increments in the forest floor (i.e., the L, F, and H horizons). Ion exchange membranes were buried and retrieved from the forest floor in an aspen forest stand in Saskatchewan, Canada. Small (6 mm diameter) sections of the membrane were cut out and ion concentration on the sections measured to provide a nutrient supply rate at that location. Soil nutrient supply rates at the site ranged from 4.6–6.0, 7.3–8.5, 11.6–21.5, and 122–196μg 10 cm²#lb2 h^‐1 for NH₄ ⁺‐N, NC₃ ^‐‐N, P, and K, respectively. On average, the highly humified H horizon had the highest N and P supply rates, followed by the F horizon, with the surface litter (L horizon) having the lowest N supply rates. The simplicity and sensitivity of the procedure make this method appropriate for in‐field assessment of differences in soil nutrient supply over small vertical and horizontal distance and was especially appropriate for the forest floor horizons in forest soils.     

Control of soil phosphatase activities at millimeter scales in a mixed paper birch – Douglas-fir forest: The importance of carbon and nitrogen

Organic P can serve as an important source of P for plants and microbes when mineralized by extracellular phosphatases. Substrate induction, end-product repression and/or resource limitation regulate activities of phosphatase in bulk soils. Yet, factors controlling enzyme activities in fine-scale microsites may differ from those observed at larger scales. Understanding such differences is needed to improve estimates of global models of biogeochemical cycling. Imprinting of soil profiles using cellulose sheets infused with chromogenic substrates allows study of extracellular enzymes at mm scales under naturally occurring soil temperatures, with minimal disturbance to soil microbial communities. In this study, we used a soil imprinting approach to investigate soil chemical characteristics associated with mm-scale regions of high in situ phosphatase activities in a mixed paper birch – Douglas-fir forest in the southern interior of British Columbia. In addition, we tested whether the addition of simple (ammonium chloride plus sodium acetate) and complex (cellulose, collagen, chitin) forms of carbon (C) and/or nitrogen (N) to 1 cm² microplots on soil profiles influenced in situ phosphatase activity. In unamended microplots, percent C was 30% higher on average (P = 0.05) and percent N was about 15% higher (P = 0.05) in high-phosphatase microsites. Extractable P did not differ between high and low-phosphatase microsites, regardless of the form of P measured. Within the first 24 h, no difference in imprintable phosphatase was observed between C and N addition treatments, but after 72 h, microplots receiving any substrate containing N had higher phosphatase activities than those receiving only water (P < 0.001). The results from both of our studies support a role for resource limitation in regulating phosphatase activities at this site because either (i) P became limiting in microsites with higher amounts of C and N, and/or (ii) microsites with higher C and N were the ones where these nutrients were in sufficient supply to allow microbes to excrete extracellular enzymes. We conclude that phosphatase excretion occurs in C + N-enriched soil microsites, but that any such phosphatase-active microsites located beyond the rhizosphere are unlikely to supply P to roots because of the low diffusion rates of orthophosphate.     

Radiocarbon pollution and self-purification of humus in chernozems of the East-European plain in 1900–2008

The dynamics of the ¹⁴C content in the humus of chernozems in 1900?C2008 are considered. The elevated ¹⁴C content in the atmosphere because of nuclear weapons tests has led to the contamination of humus with bomb radiocarbon. In 1966?C1968, the ¹⁴C reserves in the profiles of chernozems exceeded the background ones by 15%; in 1978, by 12%; and, in 1998, by 2%. By the year of 2008, its reserves became equal to the background ones. The ¹⁴C distribution along the soil profiles changed. By 1978, the 0- to 30-cm-thick soil layer became free from radiocarbon due to its self-purification; however, at depths of 40?C70 and 100?C115 cm, its weak accumulation was registered. By 2008, the whole soil profile was free from ¹⁴C. The main mechanism of the soil self-purification from radiocarbon is suggested to be the constant substitution of fragments of humus compound structures for those of fresh organic matter entering the soils with the ¹⁴C content being in equilibrium with the atmospheric one, i.e., due to the renewal of the carbon in the humus. The rate of the carbon renewal and its migration in the soils are assed based on the ¹⁴C concentrations in the humus.     

Effects of supplemental nitrogen on nitrogen‐assimilation enzymes,free amino nitrogen,soluble sugars,and crude protein of rice

Abstract

An upland rice variety IAC‐47 was grown in a greenhouse to determine the effect of foliar nitrogen (N) supplementation during grain development on the activity of the N assimilation enzymes, nitrate reductase (NR) and glutamine synthetase (GS), on free amino‐N content and leaf soluble sugars, and on grain crude protein content. At 10 and 20 days after anthesis (DAA), the leaves were fertilized with a liquid fertilizer containing 32% N as 12.8% urea, 9.6% ammonium (NH₄), and 9.6% nitrate (NO₃) in increasing rates corresponding to 0,20+20, 40+40, and 60+60 kg N ha^‐1. Leaves were collected twice (at 12 DAA and 14 DAA for GS activity, sugar and amino‐N content, and at 11 and 13 DAA for NRA) after each application of leaf N. The late foliar application of N increased significantly grain crude protein without a corresponding decrease in grain weight. The NR activity (NRA) increased after the foliar application of N. In the flag leaf, 60+60 kg N ha^‐1 (21 DAA) resulted in higher NRA (20x over the control), while GS activity was smaller than the control. At 22 DAA there was an increase in GS activity in the flag leaf at 20+20 N level. However, the GS activity decreased as applied N levels increased. Also at the 20+20 level, there were increases in free amino‐N in the flag leaf and second leaf at the final harvest. Throughout the experiment, plants at the 60+60 N level had the lowest levels of soluble sugars. Increases in crude protein were highest at 40+40 N level (27.9%), followed by 60+60 (18.7%).     

Interactions of oxygen‐nitrogen‐salinity stresses on plant growth and mineral content of sunflower (Helianthus annuus L.) in sand culture

Reports relating the separate and combined influences of soil aeration, nitrogen and saline stresses on the germination, growth and ion accumulation in sunflowers are lacking in the literature. The sunflowers of this report were grown in sand culture in the greenhouse. Separate and combined treatments of two levels of aeration, three levels of nitrogen and three levels of NaCl were applied to plants which were harvested at 40 and 56 days. Seed germination was excellent in all treatments. Plant height and dry weight decreased with each type of stress. Low oxygen (0.20 μg O₂ cm^‐2 min^‐1) and nitrogen (10 ppm) combined with 70 meq/1 NaCl caused the greatest reduction of plant growth. Leaf number was reduced by low nitrogen and excess salt. Low oxygen reduced the accumulation of K, Ca and Mg and increased the Na and N‐NO₃ content of sunflower leaves. Potassium to sodium ratios in plant on sunflower growth could be partially ameliorated with the tissue were decreased by greater EC values of the nutrient solution and low soil ODR. The adverse effects of salt and oxygen stresses addition of nitrogen. Ion accumulation was an earlier indicator of plant stress than plant growth and both parameters provided excellent methods for assessing plant tolerance to soil aeration, nitrogen and saline stresses.