Influences of organic fertilization and solarization in a greenhouse on particle-size fractions of a Mediterranean sandy soil

 The effects of a composted organic amendment and solarization on the organic matter (OM) of a sandy soil were determined by means of particle-size fractionation and analysis of carbon and nitrogen contents. After 2 years, total soil carbon increased under organic fertilization but did not significantly change with solarization. As a consequence of the climatic conditions in the greenhouse, the carbon concentrations (g kg^–1 fraction) of the particle-size fractions were lower than those found for temperate soils and closer to those for tropical soils. The carbon amounts (g kg^–1 soil) and carbon:nitrogen ratios, which were highest in fractions >200 μm, reflected the short-term influence of the industrially processed organic amendment, rich in composted coarse plant debris. In contrast, the characteristics of the OM associated with each fraction were not significantly affected by solarization. In comparison with other coarse-textured temperate or tropical soils, carbon concentrations in fine silt (2–20 μm) and clay (0–2 μm) fractions were very low. This suggests a "greenhouse effect", together with a high rate of carbon mineralization affecting fine silt and clay fractions. Received: 19 November 1999     

Simultaneous effects of plants and earthworms on mineralisation of 15N-labelled organic compounds adsorbed onto soil size fractions

 The simultaneous impact of three successive crops of wheat (Triticum aestivum L.) and of the earthworm (Lumbricus terrestris L.) on the mineralisation of ¹⁵N-labelled organic compounds adsorbed to different soil size fractions (sand and organic residues >50 μm; silt 50–2 μm; coarse clay 2–0.2 μm and fine clay <0.2 μm) was studied under controlled conditions in the greenhouse. Unplanted soils (UPS) were used as controls. In planted soils without earthworm (PS) total plant biomass decreased with each cropping by up to 50%. However, in planted soils with earthworms (PES) the total plant biomass loss was only 17%. This pattern was explained by the earthworm effect. Compared to the unplanted soils, the planted soils had an increased (mean +37%) mineralisation of ¹⁵N adsorbed onto fine clays and a partial transfer of ¹⁵N to silt and coarse clay. The quantities of ¹⁵N mineralised and transferred were higher in the planted soils with earthworms, indicating an amplification of the phenomenon in the presence of earthworms. The simultaneous effect of the rhizosphere and the drilosphere did not lead to increased mineralisation of N adsorbed onto coarse clays and silts but instead a greater transfer of N associated with the fine fractions towards the coarser fractions. Received: 25 April 2000     

Influence of heavy metals on the functional diversity of soil microbial communities

Three soil types-Calcaric Phaeozem, Eutric Cambisol and Dystric Lithosol-in large container pots were experimentally contaminated with heavy metals at four different levels (light pollution: 300 ppm Zn, 100 ppm Cu, 50 ppm Ni, 50 ppm V and 3 ppm Cd; medium pollution: twofold concentrations; heavy pollution: threefold concentrations; uncontaminated control). We investigated the prognostic potential of 16 soil microbial properties (microbial biomass, respiration, N-mineralization, 13 soil enzymes involved in cycling of C, N, P and S) with regard to their ability to differentiate the four contamination levels. Microbial biomass and enzyme activities decreased with increasing heavy metal pollution, but the amount of decrease differed among the enzymes. Enzymes involved in the C-cycling were least affected, whereas vartous enzyme activities related to the cycling of N, P and S showed a considerable decrease in activity. In particular, arylsulfatase and phosphatase activities were dramatically affected. Their activity decreased to a level of a few percent of their activities in the corresponding unpolluted controls. The data suggest that aside from the loss of rare biochemical capabilities-such as the growth of organisms at the expense of aromatics (Reber 1992)-heavy metal contaminated soils lose very common biochemical propertities which are necessary for the functioning of the ecosystem. Cluster analysis as well as discriminant analysis underline the similarity of the enzyme activity pattern among the controls and among the polluted soils. The trend toward a significant functional diversity loss becomes obvious already at the lowest pollution level. This implies that concentrations of heavy metals in soils near the current EC limits will most probably lead to a considerable reduction in decomposition and nutrient cycling rates. We conclude that heavy metal pollution severely decreases the functional diversity of the soil microbial community and impairs specific pathways of nutrient cycling.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday     

The distribution of nematodes and soil microbial communities across soil aggregate fractions and farm management systems

We hypothesized that nematode and microbial communities vary between soil aggregate fractions due to variations in physical and/or resource constraints associated with each fraction and that this, in turn, contributes to management impacts on whole soil food webs. Nematode and microbial communities were examined within three soil fractions: large macroaggregates (LM; >1000 μm), small macroaggregates (SM; 250-1000 μm) and inter-aggregate soil and space (IS; <250 μm) isolated from soils of four agricultural management systems: conventional tomato (CON), organic tomato (ORG), a minimum till grain-legume intercrop with continuous cover (CC) and an unmanaged riparian corridor (RC). Aggregate fractions appeared to influence nematode assemblages more than did management system. In general the IS and LM fractions contained higher densities of all nematode trophic groups than did SM. Management × fraction interactions for bacterivores and fungivores, however; suggested a non uniform trend across management systems. The IS fraction exhibited stronger trophic links, per the nematode structure index (SI), while the LM and SM fractions had more active fungal decomposition channels as indicated by the channel index (CI). Higher adult to juvenile ratios in the LM and IS than the SM fraction, and a positive correlation between nematode density in the IS fraction and the proportion of macroaggregates in the soil, indicated an association between soil structure and nematode distribution. Microbial communities varied across both aggregate fractions and management systems. Phospholipid fatty acid (PLFA) analysis suggested that the LM fraction contained greater microbial biomass, gram positive bacteria, and eukaryotes than the IS fraction, while SM contained intermediate PLFA associated with these groups. Total PLFA was greater under RC and ORG than under CC or CON. Total PLFA was positively correlated with % C in soil fractions while nematode abundance exhibited no such relationship. Our findings suggest that microbial communities are more limited by resource availability than by habitable pore space or predation, while nematode communities, although clearly resource-dependent, are better associated with habitable pore space for the soil fractions studied here.     

Transformations and recovery of residue and fertilizer nitrogen-15 in a sandy Lixisol of West Africa

 The fate of ¹⁵N-labeled plant residues from different cover-cropping systems and labeled inorganic N fertilizer in the organic, soil mineral, microbial biomass and soil organic matter (SOM) particle-size fractions was investigated in a sandy Lixisol. Plant residues were from mucuna (legume), lablab (legume), imperata (grass), maize (cereal) and mixtures of mucuna or lablab with imperata or maize, applied as a surface mulch. Inorganic N fertilizer was applied as ¹⁵N-(NH₄)₂SO₄ at two rates (21 and 42 mg N kg^–1 soil). Total N release from mucuna or lablab residues was 2–3 times higher than from the other residues, whereas imperata immobilized N throughout the study period. In contrast, ¹⁵N was mineralized from all the plant residues irrespective of the mineralization–immobilization pattern observed for total N. After 168 days, 69% of soil mineral N in mucuna- or lablab-mulched soils was derived from the added residues, representing 4–8% of residue N, whereas 9–30% of inorganic N was derived from imperata, maize and the mixed residues. At the end of the study, 4–19% of microbial biomass N was derived from the added residue/fertilizer-N, accounting for 1–3% of added residue-N. Averaged across treatments, particulate SOM fractions accounted for less than 1% of the total soil by weight but contained 20% of total soil C and 8% of soil N. Soils amended with mucuna or lablab incorporated more N in the 250–2000 μm SOM pool, whereas soil amended with imperata or the mixed residues incorporated similar proportions of labeled N in the 250–2000 μm and 53–250 μm fractions. In contrast, in soils receiving the maize or inorganic fertilizer-N treatments, higher proportions of labeled N were incorporated into the 53–250 μm than the 250–2000 μm fractions. The relationship between these differences in residue/fertilizer-N partitioning into different SOM particle-size fractions and soil productivity is discussed. Received: 12 March 1999     

Soil texture affects soil microbial and structural recovery during grassland restoration

Many biotic and abiotic factors influence recovery of soil communities following prolonged disturbance. We investigated the role of soil texture in the recovery of soil microbial community structure and changes in microbial stress, as indexed by phospholipid fatty acid (PLFA) profiles, using two chronosequences of grasslands restored from 0 to 19 years on silty clay loam and loamy fine sand soils in Nebraska, USA. All restorations were formerly cultivated fields seeded to native warm-season grasses through the USDA’s Conservation Reserve Program. Increases in many PLFA concentrations occurred across the silty clay loam chronosequence including total PLFA biomass, richness, fungi, arbuscular mycorrhizal fungi, Gram-positive bacteria, Gram-negative bacteria, and actinomycetes. Ratios of saturated:monounsaturated and iso:anteiso PLFAs decreased across the silty clay loam chronosequence indicating reduction in nutrient stress of the microbial community as grassland established. Multivariate analysis of entire PLFA profiles across the silty clay loam chronosequence showed recovery of microbial community structure on the trajectory toward native prairie. Conversely, no microbial groups exhibited a directional change across the loamy fine sand chronosequence. Changes in soil structure were also only observed across the silty clay loam chronosequence. Aggregate mean weighted diameter (MWD) exhibited an exponential rise to maximum resulting from an exponential rise to maximum in the proportion of large macroaggregates (>2000 μm) and exponential decay in microaggregates (<250 μm and >53 μm) and the silt and clay fraction (<53 μm). Across both chronosequences, MWD was highly correlated with total PLFA biomass and the biomass of many microbial groups. Strong correlations between many PLFA groups and the MWD of aggregates underscore the interdependence between the recovery of soil microbial communities and soil structure that may explain more variation than time for some soils (i.e., loamy fine sand). This study demonstrates that soil microbial responses to grassland restoration are modulated by soil texture with implications for estimating the true capacity of restoration efforts to rehabilitate ecosystem functions.     

Carbon mineralization and microbial activity in a field site trial used for 14C turnover experiments over a period of 30 years

 Long-term experiments on different crop management systems provide essential information about turnover of soil organic matter and changes in microbial properties over a period of time. A long-term field site trial, which was established in 1967 near Vienna, Austria, to document the fate of ¹⁴C-labelled manure (straw and farmyard) under different crop management systems (crop rotation, spring wheat and bare fallow), was investigated. Soil samples were taken in 1997 and separated into size fractions (>250 μm, 250–63 μm, 63–2 μm, 2–0.1 μm and <0.1 μm) after aggregate dispersion using low-energy sonication. Organic C, total N and ¹⁴C content were measured in the bulk soil and the size fractions and microbial properties were analysed in the bulk soil. Additionally, C mineralization in bulk soil samples was monitored at 20 °C over a period of 28 days, and subsequently ¹⁴C-CO₂ content was analysed. The distribution of organic C and N within the size fractions was similar between crop rotation and spring wheat; the highest amounts of organic C and N were found in the clay-sized fraction. The amounts of C and N were significantly smaller in the bare fallow, which was depleted of organic matter in the coarse-sized fractions. ¹⁴C distribution differed significantly from unlabelled C distribution, labelled C was accumulated in the silt-sized fraction, indicating weak humification of the applied manure C. The highest rate of C mineralization was measured in the crop rotation and spring wheat, whereas the emission rate of the bare fallow was about 40% lower. The higher ¹⁴C:C ratio of the bulk soil in comparison to the emitted CO₂ indicated that labelled C compounds still remained mineralizable after a period of 30 years. Microbial properties showed a great difference between crop management systems and bare fallow, particularly regarding urease and xylanase activity. Received: 31 May 1999     

Effect of drying and rewetting on mineralization and distribution of bacterial constituents in soil fractions

Summary Mineralization of ¹⁴C- and ¹⁵N-labelled whole bacteria, cytoplasm, and cell walls and their distribution in different soil fractions were studied during 211 days of incubation including two drying and rewetting cycles. With any of these three soil amendments, almost 60% of C derived from cellular constituents was released as CO₂, 15% was incorporated into the living microbial biomass and 25% was distributed into protected microbial metabolites or recalcitrant microbial products. The distribution of C and N derived from the amendments in the different soil fractions showed that constituents adsorbed on fine clay (<0.2 m were more rapidly decomposed than those adsorbed on silt (50-2 ) and coarse clay (2–0.2 ), indicating a faster organic matter turnover in fine clay than in silt and coarse clay. Although alternate soil drying and rewetting cycles did not significantly affect the mineralization of bacterial constituents, the cycles did have an important effect on the size and specific activities of newly formed microbial biomass. This suggests the presence of an active and a dormant fraction of soil biomass.     

Soil microbial community structure affected by 53 years of nitrogen fertilisation and different organic amendments

The Ultuna long-term soil organic matter experiment in Sweden (59′82° N, 17′65° E) was started in 1956 to study the effects of different N fertilisers and organic amendments on soil properties. In this study, samples were taken from 11 of the treatments, including unfertilised bare fallow and cropped fallow, straw with and without N addition, green manure, peat, farmyard manure, sawdust, sewage sludge, calcium nitrate and ammonium sulphate, with n = 4 for each treatment. Samples were taken from topsoil (0–20 cm) and subsoil (27–40 cm depth) and analysed for concentrations of phospholipid fatty acids (PLFAs), organic C, total N and pH. The results showed that the subsoil samples reflected the total PLFA content of the topsoil, but not the microbial community structure. Total PLFA content was well correlated with total organic C and total N in both topsoil and subsoil. Total PLFA content in topsoil samples was highest in the sewage sludge treatment (89 ± 22 nmol PLFA g dw⁻¹). This contradicts earlier findings on microbial biomass in this sewage sludge-treated soil, which indicated inhibition of microorganisms, probably by heavy metals added with sludge. A switch towards microbial growth and faster decomposition of organic matter occurred around 2000, coinciding with lowered heavy metal content in the sludge. According to the PLFA data, the microbial community in the sewage sludge treatment is now dominated by Gram-positive bacteria. A lack of Gram-negative bacteria was also observed for the ammonium sulphate treatment, obviously caused by a drop in pH to 4.2.     

Effects of Cd,Zn, or both on soil microbial biomass and activity in a clay loam soil

We investigated Cd, Zn, and Cd + Zn toxicity to soil microbial biomass and activity, and indigenous Rhizobium leguminosarum biovar trifolii, in two near neutral pH clay loam soils, under long-term arable and grassland management, in a 6-month laboratory incubation, with a view to determining the causative metal. Both soils were amended with Cd- or Zn-enriched sewage sludge, to produce soils with total Cd concentrations at four times (12 mg Cd g⁻¹ soil), and total Zn concentrations (300 mg Zn kg⁻¹ soil) at the EU upper permitted limit. The additive effects of Cd plus Zn at these soil concentrations were also investigated. There were no significant differences in microbial biomass C (B _C), biomass ninhydrin N (B _N), ATP, or microbial respiration between the different treatments. Microbial metabolic quotient (defined as qCO₂ = units of CO₂–C evolved unit⁻¹ biomass C unit⁻¹ time) also did not differ significantly between treatments. However, the microbial maintenance energy (in this study defined as qCO₂-to-μ ratio value, where μ is the growth rate) indicated that more energy was required for microbial synthesis in metal-rich sludge-treated soils (especially Zn) than in control sludge-treated soils. Indigenous R. leguminosarum bv. trifolii numbers were not significantly different between untreated and sludge-treated grassland soils after 24 weeks regardless of metal or metal concentrations. However, rhizobial numbers in the arable soils treated with metal-contaminated sludges decreased significantly (P < 0.05) compared to the untreated control and uncontaminated sludge-treated soils after 24 weeks. The order of decreasing toxicity to rhizobia in the arable soils was Zn > Cd > Cd + Zn.     

Critical sulphur concentration and sulphur requirement of microbial biomass in a glucose and cellulose-amended regosol

 The critical S concentration and S requirement of the soil microbial biomass of a granitic regosol was examined. S was applied at the rate of 0, 5, 10, 20, 30 and 50 μg S as MgSO₄·7H₂O, together with either 3000 μg glucose-C or 3333 μg cellulose-C, 400 μg N, and 200 μg P g ^–1 soil and 200 μg K g^–1 soil. Microbial biomass, inorganic SO₄ ^2–-S, and CO₂ emission were monitored over 30 days during incubation at 25  °C. Both glucose and cellulose decomposition rates responded positively to the S made available for microbial cell synthesis. The amounts of microbial biomass C and S increased with the level of applied S up to 10 μg S g^–1 soil and 30 μg S g^–1 soil in the glucose- and cellulose-amended soil, respectively, and then declined. Incorporated S was found to be concentrated within the microbial biomass or partially transformed into soil organic matter. The concentration of S in the microbial biomass was higher in the cellulose- (4.8–14.2 mg g^–1) than in the glucose-amended soil (3.7–10.9 mg g^–1). The microbial biomass C:S ratio was higher in the glucose- (46–142 : 1) than in the cellulose-amended soil (36–115 : 1). The critical S concentration in the microbial biomass (defined as that required to achieve 80% of the maximum synthesis of microbial biomass C) was estimated to be 5.1 mg g^–1 in the glucose- and 10.9 mg g^–1 in the cellulose-amended soil. The minimum requirement of SO₄ ^2–-S for microbial biomass formation was estimated to be 11 μg S g^–1 soil and 21 μg S g^–1 soil for glucose- and cellulose-amended soil, respectively. The highest levels of activity of the microbial biomass were observed at the SO₄ ^2–-S concentrations of 14 μg S g^–1 soil and 17 μg S g^–1 soil, for the glucose and cellulose amendments, respectively, and were approximately 31–54% higher during glucose than cellulose decomposition. Received: 20 October 1999     

Microbial-induced soil aggregate stability under different crop rotations

 Changes in the quantity and quality of soil organic carbon, and their effect on soil aggregate stability as a result of growing different crops in rotation with wheat, were investigated on a red earth (Oxic Paleustalf) in Wagga Wagga, New South Wales, Australia. After two cycles of the 1 : 1 rotation, while the total organic carbon in the 0–5 cm soil depth was similar (15.1 g/kg), significant differences in water stable aggregation were observed in the order: wheat/lupin=wheat/barley >wheat/canola>wheat/field pea. Using a selective extraction technique, significant differences in the quality (composition) of the soil organic carbon were detected in the soils from the different rotations. Soil from the lupin rotation had the highest salt- and acid-extractable carbon whereas that from the barley rotation had the highest level of hot-water-extractable carbon and microbial biomass carbon. Rather than total carbon or other extractable fractions, the observed differences in aggregate stability were only significantly (P<0.05) related to microbial biomass carbon, which made up only 1.3–1.7% of the total carbon pool. Multiple linear regression analysis indicated that with the exception of salt-extractable carbon, inclusion of any other of the less labile fractions failed to improve the correlation relationship. The labile nature of the microbial biomass carbon therefore accounted for the transient existence of the differences in aggregate stability under different rotation crops. The latter was found to be transient and disappeared at the end of the subsequent wheat crop. Received: 5 November 1998     

Ratios of microbial biomass estimates to evaluate microbial physiology in soil

Soil microbial biomass data derived from fumigation–extraction (FE), substrate-induced respiration (SIR) and ATP estimations differed significantly and were significantly correlated, which agrees to previous studies. In a second step, the SIR/FE, ATP/FE and SIR/ATP ratios were calculated to evaluate the glucose-responsive and active component of the microbial (active and resting) biomass and the glucose-responsive component of the active microbiota. Soils were sampled along gradients within and between associated ecosystems in Northern Germany, Denmark and along a gradient of heavy metal pollution in Finland. The ratios indicated that the active portion and glucose-responsive component decreased with proceeding litter decomposition, higher degree of sustainable land management practices and higher degree of heavy metal contamination. This work was presented at the workshop ‘Non-molecular manipulation of soil microbial communities’ at the University of Udine, Udine, Italy, 17–20 October 2004; convened by P.C. Brookes and M. De Nobili and supported by European Science Foundation.     

Bacteria and protozoa in soil microhabitats as affected by earthworms

The effects of incorporation of elm leaves (Ulmus glabra) into an agricultural sandy loam soil by earthworms (Lumbricus festivus) on the bacterial and protozoan populations were investigated. Three model systems consisting of soil, soil with leaves, and soil with leaves and earthworms, respectively, were compared. The total, viable, and culturable number of bacteria, the metabolic potentials of bacterial populations, and the number of protozoa and nematodes were determined in soil size fractions. Significant differences between soil fractions were shown by all assays. The highest number of microorganisms was found in microaggregates of 2–53 μm and the lowest in the <0.2μm fraction. A major part of the bacteria in the latter fraction was viable, but non-culturable, while a relatively higher number of culturable bacteria was found in the macroaggregates. The number of colony-forming units and 5-cyano-2,3-ditolyl tetrazolim chloride (CTC)-reducing bacteria explained a major part of the variation in the number of protozoa. High protozoan activity and predation thus coincided with high bacterial activity. In soil with elm leaves, fungal growth is assumed to inhibit bacterial and protozoan activity. In soil with elm leaves and earthworms, earthworm activity led to increased culturability of bacteria, activity of protozoa, number of nematodes, changed metabolic potentials of the bacteria, and decreased differences in metabolic potentials between bacterial populations in the soil fractions. The effects of earthworms can be mediated by mechanical mixing of the soil constituents and incorporation of organic matter into the soil, but as the earthworms have only consumed a minor part of the soil, priming effects are believed partly to explain the increased microbial activity. Received: 7 January 1996     

Do nitrogen isotope patterns reflect microbial colonization of soil organic matter fractions?

Different theories have been brought forward to explain the commonly observed δ¹⁵N enrichment with depth in soil profiles, including the discrimination against ¹⁵N during N decomposition and the buildup of ¹⁵N-enriched microbial residues. A combination of soil organic matter (SOM) size and density fractionations, ¹⁵N determinations, and phospholipid fatty acid (PLFA) analyses was conducted on soils from a pristine N-limited Nothofagus forest in southern Chile. The purpose of this study was to investigate which SOM fractions mostly reflect the ¹⁵N-enrichment pattern and to link ¹⁵N SOM enrichment with microbial community composition. Nitrogen-15 enrichments were greater for the microaggregate (<150 μm) than for the macroaggregate (>150 μm) size fraction, with Rayleigh isotope enrichment factors averaging −8.5‰ and −3.7‰, respectively. The macro-organic matter density fractions (>150 μm) showed intermediate enrichment factors of −5.1‰ and −7.3‰ for the light (<1.37 g cm⁻³) and heavy (>1.37 g cm⁻³) fraction, respectively. The abundance of fungal and bacterial PLFAs was significantly higher in the microaggregate compared to the macroaggregate size fraction, but their relative abundance did not change between aggregate size fractions. Our data link differential ¹⁵N enrichment of SOM fractions to “total” microbial abundance and, as such, corroborates existing theories of microbial-induced ¹⁵N enrichment. Isotopic fractionation during microbial N decomposition processes alone could not explain the large ¹⁵N enrichment in the microaggregate size fraction (−8.5‰) and the heavy density fraction (−7.3‰). We therefore suggest that microbial turnover and accretion of ¹⁵N-enriched microbial (especially fungal) compounds was an additional driver for ¹⁵N enrichment of this soil profile.     

Near-infrared spectroscopy for analysis of chemical and microbiological properties of forest soil organic horizons in a heavy-metal-polluted area

In industrial areas, heavy metals may accumulate in forest soil organic horizons, affecting soil microorganisms and causing changes in the chemical composition of the accumulated organic matter. The objectives of this study were to test the ability of near-infrared spectroscopy (NIRS) to detect heavy metal effects on the chemical composition of forest soil O horizons and to test whether NIRS may be used to quantitatively determine total and exchangeable concentrations of Zn and Pb (Zn_t, Pb_t, Zn_ex, Pb_ex) and other chemical and microbial properties in forest soil O horizons polluted with heavy metals. The samples of O horizons (n = 79) were analyzed for organic C (C_org), total N and S (N_t, S_t), Zn_t, Pb_t, Zn_ex, Pb_ex, basal respiration (BR), microbial biomass (C_mic) and C_mic-to-C_org ratio. Spectra of the samples were recorded in the Vis-NIR range (400–2,500 nm). To detect heavy-metal-induced changes in the chemical composition of O horizons principal components (PC1–PC7) based on the spectral data were regressed against Zn_t + Pb_t values. A modified partial least squares method was used to develop calibration models for prediction of various chemical and microbial properties of the samples from their spectra. Regression analysis revealed a significant relationship between PC3 and PC5 (r = −0.27 and −0.34, respectively) and Zn_t + Pb_t values, indicating an effect of heavy metal pollution on the spectral properties of the O horizons and thus on their chemical composition. For quantitative estimations, the best calibration model was obtained for C_org-to-N_t ratio (r = 0.98). The models for C_org, N_t, and microbial properties were satisfactory but less accurate. NIRS failed to accurately predict S_t, C_org-to-S_t, Zn_t, Pb_t, Zn_ex, and Pb_ex.     

Nitrogen mineralization in soil layers, soil particles and macro-organic matter under grassland

 A study was conducted to determine mineralization rates in the field and in different soil layers under three grassland managements (viz. a reseeded sward, a permanent sward with a conventional N management, and a long-term grass sward with 0 N (0-N) input). Potential mineralization rates of soil particles (sand, silt and clay) and macro-organic matter fractions of different sizes (i.e. 0.2–0.5, 0.5–2.0 and >2 mm) were also determined in the laboratory. In the reseeded plots, net mineralization was unchanged down to 40 cm depth. In the undisturbed conventional-N swards, mineralization rates were substantially higher in the top layer (0–10 cm) than in the deeper layers. In plots which had received no fertilizer N, mineralization was consistently low in all the layers. There was more macro-organic matter (MOM) in the 0-N plots (equivalent to 23 g kg^–1 soil for 0–40 cm) than in the two fertilized plots (i.e. conventional-N and reseeded) which contained similar amounts (ca. 15 g kg^–1 soil). C and N contents of separated soil particles did not differ amongst the treatments, but there were large differences with depth. Potential mineralization in the bulk soil was greatest in the 0–10 cm layers and gradually decreased with depth in all the treatments. Separated sand particles had negligible rates of potential mineralization and the clay component had the highest rates in the subsurface layers (10–40 cm). MOMs had high potential rate of mineralization in the surface layer and decreased with soil depth, but there was no clear pattern in the differences between different size fractions. Received: 17 November 1997     

Estimating the active and total soil microbial biomass by kinetic respiration analysis

 A model describing the respiration curves of glucose-amended soils was applied to the characterization of microbial biomass. Both lag and exponential growth phases were simulated. Fitted parameters were used for the determination of the growing and sustaining fractions of the microbial biomass as well as its specific growth rate (μ _max). These microbial biomass characteristics were measured periodically in a loamy silt and a sandy loam soil incubated under laboratory conditions. Less than 1% of the biomass oxidizing glucose was able to grow immediately due to the chronic starvation of the microbial populations in situ. Glucose applied at a rate of 0.5 mg C g^–1 increased that portion to 4–10%. Both soils showed similar dynamics with a peak in the growing biomass at day 3 after initial glucose amendment, while the total (sustaining plus growing) biomass was maximum at day 7. The microorganisms in the loamy silt soil showed a larger growth potential, with the growing biomass increasing 16-fold after glucose application compared to a sevenfold increase in the sandy loam soil. The results gained by the applied kinetic approach were compared to those obtained by the substrate-induced respiration (SIR) technique for soil microbial biomass estimation, and with results from a simple exponential model used to describe the growth response. SIR proved to be only suitable for soils that contain a sustaining microbial biomass and no growing microbial biomass. The exponential model was unsuitable for situations where a growing microbial biomass was associated with a sustaining biomass. The kinetic model tested in this study (Panikov and Sizova 1996) proved to describe all situations in a meaningful, quantitative and statistically reliable way. Received: 19 July 1999     

Recovering decomposing plant residues from the particulate soil organic matter fraction: size versus density separation

 A detailed size separation of particulate organic matter (POM) from soils amended with straw from Hordeum vulgare or Vicia sativa revealed that the loss of C during the first 56 days of incubation mainly occurred from particles >2,000 μm, without a concomitant reduction in the size of these large particles. Preliminary studies of POM from non-amended soil had shown that the stable heavy (>1.4 g cm^–3) POM fraction was mainly (>80%) composed of particles <400 μm, whereas the light fraction was dominated by larger particles (>80%). Therefore we decided to compare the POM <1.4 g cm³ with POM >400 μm. There was a very close relationship between POM>400 μm and POM <1.4 g cm^–3 with regard to amounts of C and N, as well as the appearance of these fractions under the microscope. Similarly there was a close relationship between changes in the C content of the POM fractions and the CO₂ respired, and this was also the case when comparing changes in POM-N with net N mineralization. This indicated that the biological activity during decomposition was actually localized in the POM. Due to the lighter workload and lower expenditure for reagents in connection with size separation of POM, we recommend the size separation procedure in connection with studies of residue decomposition in arable systems. Received: 23 May 2000     

Spatial changes of soil fungal and bacterial biomass from a sub-alpine coniferous forest to grassland in a humid, sub-tropical region

 Fungal and bacterial biomass were determined across a gradient from a forest to grassland in a sub-alpine region in central Taiwan. The respiration-inhibition and ergosterol methods for the evaluation of the microbial biomass were compared. Soil fungal and bacterial biomass both significantly decreased (P<0.05) with the shift of vegetation from forest to grassland. Fungal and bacterial respiration rates (evolved CO₂) were, respectively, 89.1 μl CO₂ g^–1 soil h^–1 and 55.1 μl CO₂ g^–1 soil h^–1 in the forest and 36.7 μl CO₂ g^–1 soil h^–1 and 35.7 μl CO₂ g^–1 soil h^–1 in the grassland surface soils (0–10 cm). The fungal ergosterol content in the surface soil decreased from the forest zone (108 μg g^–1) to the grassland zone (15.9 μg g^–1). A good correlation (R ²=0.90) was exhibited between the soil fungal ergosterol content and soil fungal CO₂ production (respiration) for all sampling sites. For the forest and grassland soil profiles, microbial biomass (respiration and ergosterol) declined dramatically with depth, ten- to 100-fold from the surface organic horizon to the deepest mineral horizon. With respect to fungal to bacterial ratios for the surface soil (0–10 cm), the forest zone had a significantly (P<0.05) higher ratio (1.65) than the grassland zone (1.05). However, there was no fungal to bacterial ratio trend from the surface horizon to the deeper mineral horizons of the soil profiles. Received: 30 March 2000