Microbial contributions to the increased nitrogen mineralization after air-drying of soils

The amounts of mineral-nitrogen (NH₄−N+NO₃−N) extracted by 2MKCL and the net amounts of N mineralized (δ Min-N) during a 10-day incubation of field-moist soils, air-dried then rewetted samples, and chloroform-fumigated samples, were measured in a range of 20 topsoils from grasslands. Air-drying generally increased extractable-N and the δ Min-N of the remoistened soils, but decreased the Min-N flush after fumigation. The C∶N ratios (CO₂−C production: net Min-N production) over 10 days decreased significantly from an average of 25 to 12 after initial air-drying, suggesting that substrates of low C∶N ratio, such as microbial cells, were contributiong to the extra N mineralized after the air-drying treatment. A procedure to quantify the contribution from microbial-N to the increased δ Min-N after air-drying was only partially successful, but indicated a large proportion of this increase was derived from microbial cells killed by desccation.     

Microbial biomass and growth kinetics of microorganisms in chernozem soils under different land use modes

The carbon content of microbial biomass and the kinetic characteristics of microbial respiration response to substrate addition have been estimated for chernozem soils under different land use: arable lands used for 10, 46, and 76 years, mowed meadow, natural forest, and forest shelter belt. Microbial biomass and the content of microbial carbon in humus (C_mic /C_org) decreased in the following order: soils under forest cenoses—mowed meadow—10-year arable land—46- and 75-year arable land. The amount of microbial carbon in the long-plowed horizon was 40% of its content in the upper horizon of natural forest. Arable soils were characterized by a lower metabolic diversity of microbial community and by the highest portion of microorganisms able to grow directly on glucose introduced into soil. The effects of different scenarios of carbon sequestration in soil on the amounts and activity of microbial biomass are discussed.     

不同氮输入对湿地草甸沼泽土N_2O排放和有机碳矿化的影响

通过室内培养实验,研究了不同氮输入梯度下(N0:0mg·g-1,N1:0.1mg·g-1,N2:0.2mg·g-1,N3:0.5mg·g-1)湿地草甸沼泽土N2O排放和有机碳矿化特征,并分析了土壤微生物量碳、氮变化规律。整个培养期(23d)内,N0、N1、N2和N3处理N2O排放总量分别为91.12、133.02、147.75和303.45μg.kg-1,随氮输入量增大而增大,表明氮输入对N2O排放产生促进作用;氮输入处理的有机碳矿化速率在整个培养期除最后培养阶段外均低于对照,表明氮输入对有机碳矿化有一定的抑制作用;各氮输入处理土壤微生物量碳降低,与对照差异显著(P0.05),但各处理间差异未达到显著水平,土壤微生物量氮随氮输入量增大呈线性增加,各处理间差异显著(P0.05),表明氮输入影响土壤微生物结构和组成,具体影响机理须进一步探讨。     

Biochemical properties of soils of undisturbed and disturbed mangrove forests of South Andaman (India)

Studies on soil quality of mangrove forests would be of immense use in minimizing soil degradation and in adopting strategies for soil management at degraded sites. Among the various parameters of soil quality, biological and biochemical soil properties are very sensitive to environmental stress and provide rapid and accurate estimates on changes in quality of soils subjected to degradation. In this study, we determined the general and specific biochemical characteristics of soils (0-30 cm) of inter-tidal areas of 10 undisturbed mangrove forest sites of S. Andaman, India. In order to determine the effects of disturbance, soils from the inter-tidal areas of 10 disturbed mangrove forest sites were also included in the study. The general biochemical properties included all the variables directly related to microbial activity and the specific biochemical parameters included the activities of extracellular hydrolytic enzymes that are involved in the carbon, nitrogen, sulfur and phosphorus cycles in soil. The pH, clay, cation exchange capacity, Al₂O₃ and Fe₂O₃ levels exhibited minimum variation between the disturbed and undisturbed sites. In contrast, organic C, total N, Bray P and K levels exhibited marked variation between the sites and were considerably lower at the disturbed sites. The study also revealed marked reductions in microbial biomass and activity at the disturbed sites. In comparison to the undisturbed sites, the levels of all the general biochemical parameters viz., microbial biomass C, microbial biomass N, N flush, basal respiration, metabolic quotient (qCO₂), ATP, N mineralization rates and the activities of dehydrogenase and catalase were considerably lower at the disturbed sites. Similarly, drastic reductions in the activities of phosphomonoesterase, phosphodiesterase, ß-g1ucosidase, urease, BAA-protease, casein-protease, arylsulfatase, invertase and carboxymethylcellulase occurred at the disturbed sites due mainly to significant reductions in organic matter/substrate levels. The data on CO₂ evolution, qCO₂ and ATP indicated the dominance of active individuals in the microbial communities of undisturbed soils and the ratios of biomass C:N, ATP:biomass C and ergosterol:biomass C ratios indicated low N availability and the possibility of fungi dominating over bacteria at both the mangrove sites. Significant and positive correlations between soil variables and biochemical properties suggested that the number and activity of soil microorganisms depend mainly on the quantity of mineralizable substrate and the availability of nutrients in these mangrove soils.     

Elevated CO2 increases nitrogen fixation and decreases soil nitrogen mineralization in Florida scrub oak

We report changes in nitrogen cycling in Florida scrub oak in response to elevated atmospheric CO₂ during the first 14 months of experimental treatment. Elevated CO₂ stimulated above-ground growth, nitrogen mass, and root nodule production of the nitrogen-fixing vine, Galactia elliottii Nuttall. During this period, elevated CO₂ reduced rates of gross nitrogen mineralization in soil, and resulted in lower recovery of nitrate on resin lysimeters. Elevated CO₂ did not alter nitrogen in the soil microbial biomass, but increased the specific rate of ammonium immobilization (NH₄⁺ immobilized per unit microbial N) measured over a 24-h period. Increased carbon input to soil through greater root growth combined with a decrease in the quality of that carbon in elevated CO₂ best explains these changes. These results demonstrate that atmospheric CO₂ concentration influences both the internal cycling of nitrogen (mineralization, immobilization, and nitrification) as well as the processes that regulate total ecosystem nitrogen mass (nitrogen fixation and nitrate leaching) in Florida coastal scrub oak. If these changes in nitrogen cycling are sustained, they could cause long-term feedbacks to the growth responses of plants to elevated CO₂. Greater nitrogen fixation and reduced leaching could stimulate nitrogen-limited plant growth by increasing the mass of labile nitrogen in the ecosystem. By contrast, reduced nitrogen mineralization and increased immobilization will restrict the supply rate of plant-available nitrogen, potentially reducing plant growth. Thus, the net feedback to plant growth will depend on the balance of these effects through time.     

Incubation experiments on nitrogen mineralization in loess and sandy soils

Summary In aerobic incubation experiments, nitrogen mineralization was investigated in agricultural loess and sandy soils. Fresh, fieldmoist samples were used for incubation. Using an optimization procedure the N-mineralization was split into two nitrogen fractions: A resistant, slowly decomposable organic N-fraction (index rpm) and a fast decomposable N-fraction (index dpm).Loess- and sandy soils showed similar mean reaction coefficients for N-mineralization. The results also indicated that the amount of mineralizable nitrogen in the resistant N-fraction depended directly on clay content.Soil sampling at different times during crop growing period gave different mineralization amounts and courses.Effect of added plant residues on N-mineralization, was also studied by incubation. Variation of type and quantity of added residues changed the net N-mineralization in a characteristic way: Sugar beet leaves, added in minced form, caused an increase in mineralization; while straw caused a temporary immobilization, followed by remineralization.Incubation experiments on undisturbed soil columns showed nearly linear mineralization with time.This paper was presented in part at the 1983 Congress of the German Soil Science Society held at Trier.     

Effect of copper activity on carbon and nitrogen mineralization in field-aged copper-enriched soils

Summary Nitrogen and C mineralization were investigated in five soils which had received anthropogenic Cu inputs many years ago, and compared to control soils. Copper activity was used to assess the active or toxic form of Cu; the activity of Cu in the soils ranged from approximately 10^–10 to 10^–7M. No effect of the Cu was observed on the mineralization rates, indicating that either the Cu levels were not toxic or that microbial adaptation to Cu inputs had occurred at the sites. The ion activity product ((Cu)(OH)²) is suggested as a possible indicator of plant available Cu.     

Soil restoration under pasture after topsoil removal: Some factors influencing C and N mineralization and measurements of microbial biomass

Factors influencing rates of C and N mineralization of soil and plant materials, and the reliability of different procedures for estimating microbial biomass, were measured in a soil (Typic Dystrochrept) that had been restored under grazed pasture in a temperate environment for 10–11 years after 20 cm of the original topsoil had been removed by stripping. Rates of net N mineralization were appreciably lower, but CO₂-C production higher, in the stripped than in the unstripped soil. These activities were not influenced directly by levels of soil mineral-N, but they were influenced by differences in plant composition. Herbage and litter, and roots, from the stripped plots were generally mineralized more readily to CO₂-C, but more slowly to net mineral-N, than were the corresponding materials from the unstripped plots. Rates of mineralization of herbage and litter, or roots, were mainly indistinguishable in stripped and unstripped soil, whereas rates of mineralization of all standing dead material were lower in stripped soil. Measurements of extractable-C flush, and of CO₂-C flush (using a fumigated soil control) and mineral-N flush by fumigation-incubation procedures, indicated that microbial biomass in stripped soil had recovered to at least 88 percent of the levels in unstripped soil. Substrate-induced respiration also generally indicated high levels of recovery of microbial biomass. The fumigation-incubation procedure appeared to under-estimate microbial biomass markedly in stripped soil when unfumigated soil controls were used; the used of a large soil inoculum (20 percent w/w) only sometimes overcame this problem. Possible reasons for apparent anomalies in estimation of microbial C are discussed.     

Net nitrogen mineralization and net nitrification along a tropical forest-to-pasture chronosequence

Soil inorganic nitrogen pools, net mineralization and net nitrification rates were compared during the dry season along a chronosequence of upland (terra firme) forest, 3-, 9- and 20-year-old pastures in the western Brazilian Amazon Basin state of Rondônia to investigate the influence of forest conversion to pasture on soil nitrogen cycles. Surface soil (0 to 10 cm) from forest had larger extractable inorganic nitrogen pools than pasture soils. In the forest, NO ₃ ^– pools equaled or exceeded NH ₄ ⁺ pools, while pasture inorganic N pools consisted almost exclusively of NH ₄ ⁺ . Rates of net N mineralization and net nitrification in seven -day laboratory incubations were higher in the seven - day forest than in the pastures. Net N mineralization rates did not differ significantly among different-aged pastures, but net nitrification rates were significantly lower in the 20-year-old pasture. Higher net N mineralization and net nitrification rates were measured in laboratory and in situ incubations of sieved soil, compared with in situ incubations of intact soil cores. Rates calculated in seven-day incubations were higher than determined by longer incubations. Sieving may increase N mineralization and/or decrease N immobilization compared with intact cores. We concluded that 7-day laboratory incubation of sieved soil was the most useful index for comparing N availability across the chronosequence of forest and pasture sites. High net nitrification rates in forest soils suggest a potential for NO ₃ ^– losses either through leaching or gaseous emissions.     

Reappraisal of drying and wetting effects on C and N mineralization and fluxes in soils

In the next decades, many soils will be subjected to increased drying/wetting cycles or modified water availability considering predicted global changes in precipitation and evapotranspiration. These changes may affect the turnover of C and N in soils, but the direction of changes is still unclear. The aim of the review is the evaluation of involved mechanisms, the intensity, duration and frequency of drying and wetting for the mineralization and fluxes of C and N in terrestrial soils. Controversial study results require a reappraisal of the present understanding that wetting of dry soils induces significant losses of soil C and N. The generally observed pulse in net C and N mineralization following wetting of dry soil (hereafter wetting pulse) is short‐lived and often exceeds the mineralization rate of a respective moist control. Accumulated microbial and plant necromass, lysis of live microbial cells, release of compatible solutes and exposure of previously protected organic matter may explain the additional mineralization during wetting of soils. Frequent drying and wetting diminishes the wetting pulse due to limitation of the accessible organic matter pool. Despite wetting pulses, cumulative C and N mineralization (defined here as total net mineralization during drying and wetting) are mostly smaller compared with soil with optimum moisture, indicating that wetting pulses cannot compensate for small mineralization rates during drought periods. Cumulative mineralization is linked to the intensity and duration of drying, the amount and distribution of precipitation, temperature, hydrophobicity and the accessible pool of organic substrates. Wetting pulses may have a significant impact on C and N mineralization or flux rates in arid and semiarid regions but have less impact in humid and subhumid regions on annual time scales. Organic matter stocks are progressively preserved with increasing duration and intensity of drought periods; however, fires enhance the risk of organic matter losses under dry conditions. Hydrophobicity of organic surfaces is an important mechanism that reduces C and N mineralization in topsoils after precipitation. Hence, mineralization in forest soils with hydrophobic organic horizons is presumably stronger limited than in grassland or farmland soils. Even in humid regions, suboptimal water potentials often restrict microbial activity in topsoils during growing seasons. Increasing summer droughts will likely reduce the mineralization and fluxes of C and N whereas increasing summer precipitation could enhance the losses of C and N from soils.     

Seedling establishment after endozoochory in disturbed and undisturbed grasslands

Local plant community composition and structure may be largely influenced by germination and seedling establishment from seeds dispersed in animal dung, through seed input, gap creation and nutrient enrichment. With an experimental approach we assessed (1) what the effect is of dung deposition on the number of seedlings in the plant community 3 months and 1 year after dung deposition, (2) what the effect is of this seedling establishment on the local plant community characteristics such as species richness and (3) if this effect interacts with large-scale soil disturbance which removes the close canopy, such as sod-cutting. Viable seeds of monocotylous species were abundantly present in the dung, and dung deposition led to a higher number of monocotylous seedlings after 3 months. However, this effect was no longer significant after 1 year. Moreover, the proportion of viable monocotylous seeds that effectively established in the field after 3 months was less than 5%. A lower number of viable seeds of the less-dominant dicotylous species was dispersed in the dung but they had a higher cover and species richness after 1 year. This resulted in an increased total small-scale species richness and diversity after dung deposition through a decreasing dominance of monocotylous species. Sod-cutting had a pronounced effect on seedling emergence: viable seeds dispersed by dung had a higher probability of successful establishment when the dung was deposited in large gaps. This indicates that an increase of safe sites associated with disturbance strengthens the effects of seed dispersal and gap creation by dung deposition.     

Seasonal and annual variation in nitrogen mineralization and nitrification along an elevational gradient in New Mexico

Patterns and amounts of nitrogen loss from disturbed ecosystems vary widely. The mineralization of organic nitrogen to ammonium and then nitrification to nitrate are important processes regulating nitrogen cycling rates and nitrogen losses. Nitrification is a significant process because of the production of the nitrate anion which is easily leached or denitrified. Most studies of these processes do not evaluate their seasonal and yearly variations. This study demonstrates that marked seasonal and yearly variations can occur in these processes in different ecosystems and suggests that nitrogen loss or other system properties correlated with one arbitrarily selected collection can be misleading. Spruce-fir and ponderosa pine ecosystems demonstrated little actual orpotential nitrification. Aspen and mixed conifer ecosystems demonstrated distinct seasonal patterns with increased rates of mineralization and nitrification during spring and summer months and a precipitous decline in both rates coincident with autumn foliage litterfall.The relative availability of soil nitrogen along with the amount of nitrogen circulating annually in litterfall prior to disturbance are useful predictors of the potential for nitrate production and loss following disturbance. However, other controls, including regulation by organic compounds, appear important in determining seasonal and annual variation in actual nitrification rates.     

In situ studies of soil mineral N fluxes: Some comments on the applicability of the sequential soil coring method in arable soils

Is the sequential in situ incubation of undisturbed soil cores, developed for forest stands applicable to arable soils? The incubation of covered and uncovered soil cores allows the estimation of net nitrogen mineralization (NNM), plant nitrogen uptake (N_uptake) and potential leaching losses (N_trans). The amounts and temporal dynamics of these N fluxes were determined at four arable soils in a two-year study. Results suggest that: (i) the method can not be recommended for the estimation of N uptake and leaching losses, but (ii) it is suitable for the estimation of NNM; (iii) incubations should preferably be started when soil is moist; (iv) the length of incubation periods should be reduced (<4 weeks); (v) dynamics of NNM is mainly determined by temperature and moisture conditions if there is no interference by agricultural management. Inputs of straw, manure, slurry or green manure strongly influence the amount and the dynamics of NNM.     

Effects of soil frost on nitrogen net mineralization, soil solution chemistry and seepage losses in a temperate forest soil

Freezing and thawing may alter element turnover and solute fluxes in soils by changing physical and biological soil properties. We simulated soil frost in replicated snow removal plots in a mountainous Norway spruce stand in the Fichtelgebirge area, Germany, and investigated N net mineralization, solute concentrations and fluxes of dissolved organic carbon (DOC) and of mineral ions (NH₄⁺, NO₃⁻, Na⁺, K⁺, Ca²⁺, Mg²⁺). At the snow removal plots the minimum soil temperature was −5 °C at 5 cm depth, while the control plots were covered by snow and experienced no soil frost. The soil frost lasted for about 3 months and penetrated the soil to about 15 cm depth. In the 3 months after thawing, the in situ N net mineralization in the forest floor and upper mineral soil was not affected by soil frost. In late summer, NO₃⁻ concentrations increased in forest floor percolates and soil solutions at 20 cm soil depth in the snow removal plots relative to the control. The increase lasted for about 2–4 months at a time of low seepage water fluxes. Soil frost did not affect DOC concentrations and radiocarbon signatures of DOC. No specific frost effect was observed for K⁺, Ca²⁺ and Mg²⁺ in soil solutions, however, the Na⁺ concentrations in the upper mineral soil increased. In the 12 months following snowmelt, the solute fluxes of N, DOC, and mineral ions were not influenced by the previous soil frost at any depth. Our experiment did not support the hypothesis that moderate soil frost triggers solute losses of N, DOC, and mineral ions from temperate forest soils.     

Effects of clear-cutting and burning on characteristics of nitrogen mineralization and microbes in the forest soil of a Pinus massoniana plantation in Southern China

The aim of this study was to clarify the effects of clear-cutting and burning (CCB) on soil fertility in a Pinus massoniana (Masson pine) plantation after CCB in Fujian Province, China. We investigated changes in nitrogen (N) mineralization potential (N₀), N mineralization rate constant (k) and the apparent activation energy (Ea) of the soil, with a mathematical analysis using a kinetics model based on the results of in vitro incubation. Changes in the amount of microbial biomass nitrogen (MBN), as well as the number of heterotrophic and nitrite-oxidizing bacteria, were also investigated. The N₀, MBN and the number of fungi and actinomycetes in forest soil was reduced for at least 18 months after CCB. The number of heterotrophic and nitrite-oxidizing bacteria increased, and k, Ea and N mineralization became greater after 6 months of CCB, compared with the control plots. Because there were few young trees planted, which would have taken up mineralized N in the post-CCB site, it is probable that a high proportion of the mineralized N that accumulated in the soil may have been lost during the summer rainy season. Therefore, it is suggested that CCB led to a deficiency in available N during short rotations, which resulted in soil degradation.     

Sulphur mineralization and adsorption in soils

Summary Studies were conducted to determine the comparative sulphur mineralizing capacity of selected Malaysian and Iowan soils and to determine the amounts of available and adsorbed sulphate in a number of Malaysian soils. Results of the mineralization study indicated that more sulphur mineralised from Malaysian soils although their average contents of total sulphur were lower compared to Iowan Soils. For both sets of soils, significant correlations between contents of organic carbon and total sulphur existed indicating that most of the sulphur was in organic combination. Phosphate solution consistently extracted higher quantities of sulphate in comparison to chloride solution in the Malaysian surface soils implying that a portion of the sulphate existed in adsorbed form. Adsorption of sulphate in soils was found to be dependent on concentration of sulphate added and followed the Langmuir adsorption isotherm.     

Temporal variability in fish assemblages from disturbed and undisturbed streams

Fish assemblages were sampled by electrofishingover a two- to ten-year period in undisturbedand anthropogenically disturbed South Carolinacoastal plain streams. Jaccard similarity,Bray–Curtis similarity, and Spearman rankcorrelations among samples collected from thesame sites over time were significantly greaterat undisturbed sites than at disturbed sites,suggesting greater fish assemblage persistenceand stability at the undisturbed sites. TheIndex of Biotic Integrity (IBI) also exhibitedsignificantly less variation over time atundisturbed sites than at disturbed sites.Physical habitat structure changed more overtime at disturbed sites than at undisturbedsites, and this variability was directlyrelated to temporal variability in fishassemblage structure. Comparisons betweenmultiple and single pass electrofishing samplessuggested that only a small proportion of thetemporal variability observed at the studysites was caused by inefficient sampling.Assessment of temporal variation in fishassemblage structure can serve as an indicatorof environmental disturbance and facilitate thedistinction of substantive ecological changefrom normal background variation.     

Net nitrogen mineralization and net nitrification rates in soils following deforestation for pasture across the southwestern Brazilian Amazon Basin landscape

Previous studies of the effect of tropical forest conversion to cattle pasture on soil N dynamics showed that rates of net N mineralization and net nitrification were lower in pastures compared with the original forest. In this study, we sought to determine the generality of these patterns by examining soil inorganic N concentrations, net mineralization and nitrification rates in 6 forests and 11 pastures 3 years old or older on ultisols and oxisols that encompassed a wide variety of soil textures and spanned a 700-km geographical range in the southwestern Brazilian Amazon Basin state of Rondônia. We sampled each site during October-November and April-May. Forest soils had higher extractable NO₃ ^?-N and total inorganic N concentrations than pasture soils, but substantial NO₃ ^?-N occurred in both forest and pasture soils. Rates of net N mineralization and net nitrification were higher in forest soils. Greater concentrations of soil organic matter in finer textured soils were associated with greater rates of net N mineralization and net nitrification, but this relationship was true only under native forest vegetation; rates were uniformly low in pastures, regardless of soil type or texture. Net N mineralization and net nitrification rates per unit of total soil organic matter showed no pattern across the different forest sites, suggesting that controls of net N mineralization may be broadly similar across a wide range of soil types. Similar reductions in rates of net N transformations in pastures 3 years old or older across a range of textures on these soils suggest that changes to soil N cycling caused by deforestation for pasture may be Basin-wide in extent. Lower net N mineralization and net nitrification rates in established pastures suggest that annual N losses from largely deforested landscapes may be lower than losses from the original forest. Total ecosystem N losses since deforestation are likely to depend on the balance between lower N loss rates from established pastures and the magnitude and duration of N losses that occur in the years immediately following forest clearing.     

Microcalorimetric assessment of microbial activity in long-term fertilization experimental soils of Southern China

Microcalorimetry, plate count and PCR–denaturing gradient gel electrophoresis (DGGE) were employed to investigate microbial diversity and activity in soils from the Red Soil Experimental Station of the Chinese Academy of Agricultural Sciences, Hunan Province, China, where a wheat–corn rotation with 12 fertilization treatments was established in 1990. Fertilization greatly increased microbial biomass carbon (C) and nitrogen (N) (C_mic and N_mic) as well as the activities of phosphatase, urease, invertase, protease, catalase and dehydrogenase. Manure alone (M) enhanced the number of denitrifying and aerobic bacteria by 54.4% and 20.5%, respectively, whereas fallow (H) increased the number of aerobic cellulose decomposing bacteria by 31.4%. Fallow and soils amended with mineral fertilizers plus pig manure or straw increased both the DGGE band patterns and the Shannon index compared with mineral fertilizers or the control. Mineral treatments with lower bacterial numbers enhanced the values of the peak time ( t _max) more than did organic treatments. The peak height ( P _max) was positively correlated ( P <0.01), with soil enzymes, C_mic and N_mic, and the number of microorganisms, whereas the peak time ( t _max) was negatively connected ( P <0.01) with these parameters. The microbial growth rate constant ( k ) was linked to bacteria ( P <0.01), actinomycetes ( P <0.05) and catalase ( P <0.05). The total heat evolution ( Q ) had no relationships with any soil microbial properties (except for catalase). We propose that P _max and t _max could be used as indices of soil microbial activity, while the values of k and Q are poor indicators.