An increasing body of evidence suggests that microorganisms are far more sensitive to heavy metal stress than soil animals or plants growing on the same soils. Not surprisingly, most studies of heavy metal toxicity to soil microorganisms have concentrated on effects where loss of microbial function can be observed and yet such studies may mask underlying effects on biodiversity within microbial populations and communities. The types of evidence which are available for determining critical metal concentrations or loadings for microbial processes and populations in agricultural soil are assessed, particularly in relation to the agricultural use of sewage sludge. Much of the confusion in deriving critical toxic concentrations of heavy metals in soils arises from comparison of experimental results based on short-term laboratory ecotoxicological studies with results from monitoring of long-term exposures of microbial populations to heavy metals in field experiments. The laboratory studies in effect measure responses to immediate, acute toxicity (disturbance) whereas the monitoring of field experiments measures responses to long-term chronic toxicity (stress) which accumulates gradually. Laboratory ecotoxicological studies are the most easily conducted and by far the most numerous, but are difficult to extrapolate meaningfully to toxic effects likely to occur in the field. Using evidence primarily derived from long-term field experiments, a hypothesis is formulated to explain how microorganisms may become affected by gradually increasing soil metal concentrations and this is discussed in relation to defining “safe” or “critical” soil metal loadings for soil protection.

Toxicity of heavy metals to microorganisms and microbial processes in agricultural soils: a review.

This paper reviews the current knowledge of microbial processes affecting C sequestration in agroecosystems. The microbial contribution to soil C storage is directly related to microbial community dynamics and the balance between formation and degradation of microbial byproducts. Soil microbes also indirectly influence C cycling by improving soil aggregation, which physically protects soil organic matter (SOM). Consequently, the microbial contribution to C sequestration is governed by the interactions between the amount of microbial biomass, microbial community structure, microbial byproducts, and soil properties such as texture, clay mineralogy, pore-size distribution, and aggregate dynamics. The capacity of a soil to protect microbial biomass and microbially derived organic matter (MOM) is directly and/or indirectly (i.e., through physical protection by aggregates) related to the reactive properties of clays. However, the stabilization of MOM in the soil is also related to the efficiency with which microorganisms utilize substrate C and the chemical nature of the byproducts they produce. Crop rotations, reduced or no-tillage practices, organic farming, and cover crops increase total microbial biomass and shift the community structure toward a more fungal-dominated community, thereby enhancing the accumulation of MOM. A quantitative and qualitative improvement of SOM is generally observed in agroecosystems favoring a fungal-dominated community, but the mechanisms leading to this improvement are not completely understood. Gaps within our knowledge on MOM-C dynamics and how they are related to soil properties and agricultural practices are identified.

/pdf/bacterial-and-fungal-contributions-to-carbon-sequestration-3i9a4yeaey.pdf

Bacterial and Fungal Contributions to Carbon Sequestration in Agroecosystems

It is generally accepted that the low quality of soil carbon limits the amount of energy available for soil microorganisms, and in turn the rate of soil carbon mineralization. The priming effect, i.e. the increase in soil organic matter (SOM) decomposition rate after fresh organic matter input to soil, is often supposed to result from a global increase in microbial activity due to the higher availability of energy released from the decomposition of fresh organic matter. Work to date, however, suggests that supply of available energy induces no effect on SOM mineralization. The mechanisms of the priming effect are much more complex than commonly believed. The objective of this review was to build a conceptual model of the priming effect based on the contradictory results available in the literature adopting the concept of nutritional competition. After fresh organic matter input to soils, many specialized microorganisms grow quickly and only decompose the fresh organic matter. We postulated that the priming effect results from the competition for energy and nutrient acquisition between the microorganisms specialized in the decomposition of fresh organic matter and those feeding on polymerised SOM.

The priming effect of organic matter: a question of microbial competition

Equilibrium and kinetic isotope fractionations during incomplete reactions result in minute differences in the ratio between the two stable N isotopes, 15N and 14N, in various N pools. In ecosystems such variations (usually expressed in per mil [δ15N] deviations from the standard atmospheric N2) depend on isotopic signatures of inputs and outputs, the input–output balance, N transformations and their specific isotope effects, and compartmentation of N within the system. Products along a sequence of reactions, e.g. the N mineralization–N uptake pathway, should, if fractionation factors were equal for the different reactions, become progressively depleted. However, fractionation factors vary. For example, because nitrification discriminates against 15N in the substrate more than does N mineralization, NH4+ can become isotopically heavier than the organic N from which it is derived.Levels of isotopic enrichment depend dynamically on the stoichiometry of reactions, as well as on specific abiotic and biotic conditions. Thus, the δ15N of a specific N pool is not a constant, and δ15N of a N compound added to the system is not a conservative, unchanging tracer. This fact, together with analytical problems of measuring δ15N in small and dynamic pools of N in the soil–plant system, and the complexity of the N cycle itself (for instance the abundance of reversible reactions), limit the possibilities of making inferences based on observations of 15N abundance in one or a few pools of N in a system. Nevertheless, measurements of δ15N might offer the advantage of giving insights into the N cycle without disturbing the system by adding 15N tracer.Such attempts require, however, that the complex factors affecting δ15N in plants be taken into account, viz. (i) the source(s) of N (soil, precipitation, NOx, NH3, N2-fixation), (ii) the depth(s) in soil from which N is taken up, (iii) the form(s) of soil-N used (organic N, NH4+, NO3−), (iv) influences of mycorrhizal symbioses and fractionations during and after N uptake by plants, and (v) interactions between these factors and plant phenology. Because of this complexity, data on δ15N can only be used alone when certain requirements are met, e.g. when a clearly discrete N source in terms of amount and isotopic signature is studied. For example, it is recommended that N in non-N2-fixing species should differ more than 5‰ from N derived by N2-fixation, and that several non-N2-fixing references are used, when data on δ15N are used to estimate N2-fixation in poorly described ecosystems.As well as giving information on N source effects, δ15N can give insights into N cycle rates. For example, high levels of N deposition onto previously N-limited systems leads to increased nitrification, which produces 15N-enriched NH4+ and 15N-depleted NO3−. As many forest plants prefer NH4+ they become enriched in 15N in such circumstances. This change in plant δ15N will subsequently also occur in the soil surface horizon after litter-fall, and might be a useful indicator of N saturation, especially since there is usually an increase in δ15N with depth in soils of N-limited forests.Generally, interpretation of 15N measurements requires additional independent data and modelling, and benefits from a controlled experimental setting. Modelling will be greatly assisted by the development of methods to measure the δ15N of small dynamic pools of N in soils. Direct comparisons with parallel low tracer level 15N studies will be necessary to further develop the interpretation of variations in δ15N in soil–plant systems. Another promising approach is to study ratios of 15N[ratio ]14N together with other pairs of stable isotopes, e.g. 13C[ratio ]12C or 18O[ratio ]16O, in the same ion or molecules. This approach can help to tackle the challenge of distinguishing isotopic source effects from fractionations within the system studied.

Tansley Review No. 95 15N natural abundance in soil-plant systems

The kEC value (=extractable part of microbial biomass C) of the fumigation-extraction (FE) method was assessed on the basis of 153 soils (94 arable, 46 grassland and 13 forest soils) by indirect calibration using the fumigation-incubation (FI) method. Sixty-six soils were investigated for the first time and the data on a further 87 soils were obtained from the literature. The single kEC values ranged from 0.23 to 0.84. A split according to the form of land use resulted in a significantly (Scheffe, P = 0.05) lower kEC value for the arable soils (0.42; n = 94) in comparison to those for the grassland (0.49; n = 46) and the forest soils (0.51; n = 13). This difference is mainly due to the significant effects of the respiration rate measured in non-fumigated control samples of the FI method which was used for calibration of the kEC value. For that reason, I investigated the effects of incubation temperature (22°, 25° and 28°C) on biomass C data obtained by the FI method, and thus on the kEC value of the FE method, and discuss further problems of direct and indirect calibration. Based on experimental and literature data, I conclude that the kEC values of Vance et al. (Soil Biology & Biochemistry19, 703–707, 1987) and Wu et al. (Soil Biology & Biochemistry22, 1167–1169, 1990) remain valid. A kEC value of 0.38 can be recommended for C analysis by dichromate consumption and a kEC value of 0.45 for that by UV-persulfate or oven oxidation.

The fumigation-extraction method to estimate soil microbial biomass: Calibration of the kEC value

Lentil (Lens culinaris Medikus) is being grown increasingly on the Canadian prairies as a pulse or green manure crop, and may increase N availability to a succeeding crop. This study was designed to compare the effects of lentil green manure, lentil straw, and wheat (Triticum aestivum L.) straw on plant-available N during the growing season after application. The fate of ¹⁵N from fall-applied (1988) lentil green manure, lentil straw, and wheat straw and spring-applied (1989) fertilizer (NH₄)₂ SO₄ was determined four times during the 1989 growing season at a field site located at Outlook, Saskatchewan, Canada, on a Bradwell sandy loam (Typic Boroll). Denitrification and leaching losses of ¹⁵N from added lentil and wheat straw were negligible, but 24 and 30% of the ¹⁵N in lentil green manure and fertilizer, respectively, were lost in the 6-wk period after planting (8 May 1989). By wheat harvest (8 Aug. 1989), 7% of the ¹⁵N in lentil and wheat straw and 37% of the ¹⁵N in lentil green manure were mineralized. Addition of green manure increased net mineralization of indigenous soil N at the time of planting by 0.4 g m⁻², equivalent to 10% of added green manure N. Immobilization of soil and fertilizer N was similar for lentil and wheat straw. The smaller fraction of ¹⁵N assimilated from green manure (19%) than from fertilizer (34%) by wheat was due solely to less net mineralization of green-manure N rather than net immobilization of fertilizer N. Of the ¹⁵N added in lentil and wheat straw, 5.5% was assimilated by wheat. Thus, lentil straw was not a significant source of N in this study, while ≈40% of the N in lentil green manure was potentially available for plant uptake. Contribution no. R688 of the Saskatchewan Institute of Pedology.

Plant-available nitrogen from lentil and wheat residues during a subsequent growing season

Microbial utilization of glucose-14C by soil microbes was investigated in two laboratory experiments. In the first experiment, 14C-labelled glucose was added to two soils at seven rates ranging from 36 to 2304 μg C g−1 soil. An average of 42% of added 14C was mineralized by day 3 at glucose rates ⩾288 μgCg−1 soil in both soils, but this proportion declined at lower rates. Only 30% of added 14was mineralized at the lowest rate of glucose addition. The fraction of soil 14C released by fumigation-extraction (FE-14C) ranged from 11 to 36%, and decreased linearly with the proportion of 14C mineralized in both soils. The effects of addition rate persisted until the end of the experiment, at 35 days. In the second experiment, addition of unlabelled glucose at 6, 24 or 72 h after addition of glucose-14C at 30 μg C g−1 soil did not appreciably affect the proportion of 14C mineralized, while addition 72 h before or immediately after 14C addition increased 14C mineralization by about 50%. Fumigation-labile 14C was reduced in all cases by addition of unlabelled glucose, with the greatest reduction when unlabelled glucose was added immediately after glucose-14C. We conclude that assimilated glucose-14C was incompletely metabolized at low rates of glucose addition unless soil microorganisms were ‘activated’ by a prior addition of glucose. The proportion of 14C mineralized at low rates of glucose addition to soils may be useful as an indicator of C availability: a small proportion mineralized may indicate low C availability whereas a large proportion mineralized may indicate high C availability.

Microbial utilization of 14C[U]glucose in soil is affected by the amount and timing of glucose additions.

The objective of this study was to investigate the effect of variable rates of substrate addition and length of incubation period on in situ estimates of the proportion of microbial C and N released by a chloroform-fumigation-extraction method. A Bradwell silty loam soil containing ca 340 μg microbial C g−1 soil was amended with [14C(U)]glucose and [15N](NH4):SO4 at 30 and 2. 300 and 2, and 300 and 20μg of C and N. respectively, g−1 soil. The following measurements were taken at 0.25, 0.5. 1. 3 and 7 days after the addition of substrates: respired 14C, mineral 15N, total 15N and increase in extractable 14C and 15N upon fumigation. 14C-efficiency, defined as the proportion of 14C in the microbial biomass relative to the total amount of 14C removed from the soil solution, was >70% in all treatments at 0.25 and 0.5 days and in the treatment receiving the lowest rate of glucose at all sampling times. The proportion of microbial 14 released by fumigation-extraction was >40% when the efficiency was >70%. but was 34% when the efficiency was lower. In the treatment receiving the low rate of glucose microbial N accounted for ca 50% of added 15N, of which >40% was released by fumigation-extraction; in the treatments receiving the high rate of glucose microbial N accounted for an average of 81% of added 15N, of with only 24% was released by fumigation-extraction at 3 and 7 days after the addition of substrates. Based on these observations, it was concluded that at low rates of glucose addition microbial 14C and 15N remained present among cytoplasmic constituents and little if any was utilized in biosynthesis; sufficient glucose to stimulate microbiul growth and a sufficient period of incubation would be required to obtain reliable estimates of the proportion of microbial C and N released upon fumigation-extraction.

Extractability of microbial 14C and 15N following addition of variable rates of labelled glucose and (NH4)2SO4 to soil

A study was conducted to determine the effects of grinding, added N, and the absence of soil on C mineralization from agricultural plant residues with a high C:N ratio. The evolution of CO2 from ground and unground wheat straw, lentil straw, and lentil green manure, with C:N ratios of 80, 36, and 9, respectively, was determined over a period of 98 days. Treatments with added N were included with the wheat and lentil straw. Although the CO2 evolution was initially much faster from the lentil green manure than from the lentil or wheat straw, by 98 days similar amounts of CO2 had evolved from all residues incubated in soil with no added N. Incubation of plant residues in the absence of soil had little effect on CO2 evolution from the lentil green manure or lentil straw but strongly reduced CO2 evolution from the wheat straw. Grinding did not affect CO2 evolution from the lentil green manure but increased CO2 evolution from the lentil straw with no added N and from the wheat straw. The addition of N increased the rate of CO2 evolution from ground wheat straw between days 4 and 14 but not from unground wheat straw, and only slightly increased the rate of CO2 evolution from lentil straw during the initial decomposition. Over 98 days, the added N reduced the amounts of CO2 evolved from both lentil and wheat straw, due to reduced rates of CO2 evolution after ca. 17 days. The lack of an N response during the early stages of decomposition may be attributed to the low C:N ratio of the soluble straw component and to microbial adaptations to an N deficiency, while the inhibitory effect of N on CO2 evolution during the later stages of decomposition may be attributed to effects of high mineral N concentrations on lignocellulolytic microorganisms and enzymes.

Carbon dioxide evolution from wheat and lentil residues as affected by grinding, added nitrogen, and the absence of soil

The dynamics of soil microbial biomass after the addition of plant residues have a considerable influence on nutrient availability for plants, and can be quantified using the chloroform-fumigation-extraction method. The dynamics of microbial C and N following addition of ¹⁴C- and ¹⁵N-labeled lentil (Lens culinaris Medik.) green manure, lentil straw, and wheat (Triticum aestivum L.) straw were investigated under field conditions at a site located at Outlook, Saskatchewan, on a Bradwell sandy loam (Typic Boroll). Plant residues were incorporated into microplots on 5 Oct. 1988, and the fraction of added ¹⁴C and ¹⁵N in microbial biomass was determined on four dates during the 1989 growing season. Maximum levels of labeled and unlabeled microbial biomass were observed at the time of planting (8 May) in 1989. Of added ¹⁴C, 26 and 15% was in the microbial biomass in the green manure and straw treatments, respectively, on 8 May; greater microbial accumulation of green-manure ¹⁴C was due to a higher proportion of ¹⁴C being available rather than to a higher efficiency of ¹⁴C assimilation. Microbial ¹⁵N accounted for 65 to 81% of added residue ¹⁵N on 8 May. Plant-residue ¹⁵N was readily available to decomposer microorganisms from all residue types, whereas ¹⁴C was more available from green manure than straw. During the 1989 growing season, microbial ¹⁴C declined by 51 and 40% in the green manure and straw treatments, respectively, while microbial ¹⁵N declined by 54% in all treatments. The decline in microbial ¹⁵N during the 1989 growing season was approximately five times greater than the amount of ¹⁵N mineralized in all sampling periods except the first for the green-manure treatment. The highest levels of labeled and unlabeled microbial biomass observed at the time of planting indicates that microbial biomass may reduce losses of N and other nutrients during periods of low crop demand, and may act as a source of nutrients during active crop growth. Contribution no. R690 of the Saskatchewan Institute of Pedology.

Seasonal Microbial Biomass Dynamics after Addition of Lentil and Wheat Residues

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