M. Díaz-Raviña

Seasonal changes in microbial biomass and nutrient flush in forest soils

Microbial biomass and N, P, K, and Mg flushes were estimated in spring, summer, autumn, and winter samples of different forest soils. The microbial biomass showed significant seasonal fluctuations with an average distribution of 880±270 μg C g-1 soil in spring, 787±356 μg C g-1 soil in winter, 589±295 μg C g-1 soil in summer, and 560±318 μg C g-1 soil in autumn. The average annual concentrations of C, N, P, K, and Ca in the microbial biomass were 704, 106, 82, 69 and 10 μg g-1 soil, respectively. Microbial C represented between 0.5 and 2% of the organic soil C whereas the percentage of microbial N with respect to the total soil N was two-to threefold higher than that of C; the annual fluctuations in these percentages followed a similar trend to that of the microbial biomass. Microbial biomass was positively correlated with soil pH, moisture, organic C, and total N. The mean nutrient flush was 31, 15, 7, and 4 μg g-1 soil for N, K, P, and Mg, respectively, and except for K, the seasonal distribution was autumn ≥spring ≫ winter ≥summer. The average increase in available nutrient due to the mineralization of dead microbial cells was 240% for N, and 30, 26, and 14% for P, K, and Mg, respectively. There was a positive relationship between microbial biomass and the N, P, K, and Mg flushes. All the variables studied were significantly affected by the season, the type of soil, and the interaction between type of soil and season, but soil type often explained most of the variance.

Microbial Biomass, Community Structure and Metal Tolerance of a Naturally Pb-Enriched Forest Soil

The effect of long-term elevated soil Pb levels on soil microbiota was studied at a forest site in Norway, where the soil has been severely contaminated with Pb since the last period of glaciation (several thousand years). Up to 10% Pb (total amount, w/w) has been found in the top layer. The microbial community was drastically affected, as judged from changes in the phospholipid fatty acid (PLFA) pattern. Specific PLFAs that were high in Pb-enriched soil were branched (especially br17:0 and br18:0), whereas PLFAs common in eukaryotic organisms such as fungi (18:2ω6,9 and 20:4) were low compared with levels at adjacent, uncontaminated sites. Congruent changes in the PLFA pattern were found upon analyzing the culturable part of the bacterial community. The high Pb concentrations in the soil resulted in increased tolerance to Pb of the bacterial community, measured using both thymidine incorporation and plate counts. Furthermore, changes in tolerance were correlated to changes in the community structure. The bacterial community of the most contaminated soils showed higher specific activity (thymidine and leucine incorporation rates) and higher culturability than that of control soils. Fungal colony forming units (CFUs) were 10 times lower in the most Pb-enriched soils, the species composition was widely different from that in control soils, and the isolated fungi had high Pb tolerance. The most commonly isolated fungus in Pb-enriched soils was Tolypocladium inflatum. Comparison of isolates from Pb-enriched soil and isolates from unpolluted soils showed that T. inflatum was intrinsically Pb-tolerant, and that the prolonged conditions with high Pb had not selected for any increased tolerance.

Microbial biomass and metabolic activity in four acid soils

Abstract The fumigation method was used to estimate microbial biomass C in four Haplumbrepts developed over different kinds of rock. In order to investigate the relationship between metabolic activity and microbial biomass and population density, CO 2 release from the glucose-enriched and unenriched soils was measured during 28 days of incubation. Biomass C levels lay between 36 and 112 mg 100 g −1 of dry soil, and made up only a small proportion of total soil C (0.77–1.38%). Only a small fraction of this biomass was detected by counting viables, but the microbial population was nevertheless significantly correlated with the biomass determined by fumigation. Among the physico-chemical properties of the soils, microbial biomass and population size were both chiefly affected (favourably) by humidity, total C and N and Al gel content. Metabolic activity was slight, either because part of the micro-organisms are inactive or because of a limited supply of substrate (the organic matter present may be unsuitable as a substrate or protected from microbial attack). Percentage C mineralization was inversely related to organic matter, silt and Al gel contents, and likewise failed to exhibit positive correlation with respiration, the biomass determined by fumigation or the counted population. The metabolic activity of the biomass appeared to depend upon the quality and nature of soil organic matter rather than its quantity, which nevertheless controlled microbial population size. Neither microbial biomass estimates nor viable population counts faithfully reflected metabolic activity in the soils.

Bacterial activity in a forest soil after soil heating and organic amendments measured by the thymidine and leucine incorporation techniques

Abstract The effects of soil heating (200°C, 1 h) and organic amendments (straw and poultry manure) on bacterial activity in a forest soil were studied in a laboratory experiment. Measurements were made over a 15-week incubation using the thymidine (TdR) and leucine (Leu) incorporation techniques. Soil heating initially reduced TdR and Leu incorporation rates by 96–98% within 2 weeks of incubation. Thereafter the bacterial activity recovered, but by the end of the incubation the values were still lower than in the corresponding unheated soil. Reinoculation of heated soil with fresh soil improved the reestablishment of bacterial activity after a long lag, since the values were comparable to those observed for the unheated soil only after 13 weeks of incubation. The organic amendments affected bacterial activity in both unheated and heated soils. An enhanced bacterial activity was found within 2 weeks after the addition of straw or poultry manure to the unheated soils, the increase being greater and maintained longer for the latter treatment. In the heated soil no effect of straw addition was detected, while a large increase in bacterial activity following poultry manure treatment was observed after a long lag (up to 8 weeks). The presence of a toxic substance appeared to be the cause of the prolonged reduced bacterial activity in the heated soils, since an inhibitory effect of water extract from heated soil was found on TdR and Leu incorporation rates of bacteria from both unheated and heated soils. Furthermore, the inhibition was less pronounced for bacteria from the heated soil, indicating that a bacterial community tolerant to the toxic substances had developed.

Microbial biomass and its contribution to nutrient concentrations in forest soils

Abstract Soil microbial biomass and its contribution to the available Ca, Mg, Na, K and P concentrations were studied in a wide range of forest soils. Biomass C was measured using the chloroform fumigation-incubation technique; N, P, K and Ca in biomass were estimated from biomass C values. Soils were incubated at 25°C for 10 days and the differences between the available nutrient contents in the fumigated and the unfumigated soils (the flush of nutrients) were used to estimate the contribution of the microorganisms to the available nutrient concentrations. Microbial biomass C and N ranged from 282 to 1614 μg C g −1 dry soil (d.s.) and from 42 to 242 μg N g −1 d.s. The average quantities of P, K and Ca in microbial biomass were about 86, 73 and 10 μg g −1 d.s., respectively. The death of microorganisms by CHCl 3 fumigation caused an immediate flush (flush during day 0) of N, K, Na, P, and Mg with mean values of 10, 21, 8, 7 and 4 μg g −1 d.s., respectively. After 10 days of incubation the flush of nutrients (flush during day 10) showed mean values of 36 μg g −1 d.s. for N, and 20, 9 and 6 μg g −1 d.s. for the K, P. and Mg respectively, whereas the Na flush values were not significant in most soils. As for Ca the variability between the soil replicates was higher than the variability caused by the fumigation or incubation. Biomass C showed a significant relationship with the N flush measured in day 10 and with the Mg, K and P flush measured in both days 0 and 10. The results showed that soil microorganisms contained substantial amounts of both C and inorganic nutrients and that their contribution to the pool of available nutrients was large for N, important in the cases of P, K and Mg and not significant for Na.

Response of soil bacterial communities pre-exposed to different metals and reinoculated in an unpolluted soil

Changes in tolerance levels of soil bacterial communities pre-exposed to different metal concentrations and then reestablished in an unpolluted soil were examined during a one year incubation period using the thymidine incorporation technique. The study was performed with a sterilized agricultural soil, which was reinoculated with bacteria extracted from soils previously contaminated with high doses of Zn, Cu or Cd, and from an unpolluted soil. Bacteria pre-exposed to metal addition initially exhibited a greater tolerance than those non-preexposed. The microbial communities responded to the absence of metals in the reinoculated soil with a rapid decrease in community tolerance. losing most of the acquired tolerance (70-90%) within the first week. After that no changes in community tolerance were detected. Thus, a long-lasting effect of the original community tolerance of the inoculum was detected even 12 months after the metal stress was removed. At this time there was still a dose-response effect left since higher tolerance levels were usually found in soils with higher tolerant inoculum. Changes in tolerance levels over time showed similar trends, irrespective of which metal the bacterial communities were initially tolerant

PHYSICAL AND CHEMICAL CHARACTERIZATION OF FOUR COMPOSTED URBAN REFUSES

Abstract The physical and chemical characteristics of four composted urban refuses (one of them amended with CaCO 3 during the composting process) from Spanish industrial composting plants were studied from the point of view of their use as organic fertilizers. The four composts were very fine in texture with low bulk density and high salinity. The pH was close to neutrality; the organic matter content ranged from 42 to 60% and the C/N ratio from 16 to 22 (7 for the amended urban refuse). Most of the total N was in organic forms; NH 4 + -N largely predominatedNO 3 − -N in the composts, except for the amended composted refuse where the predominant inorganic form was NO 3 − -N. Calcium was the most abundantnutrient followed by K, Na, Mg and P. Most of the Ca and Na were in available forms; available K and Mg were lower and available P very small. Total Al and Fe were extremely abundant followed by Zn, Mn, Pb, Cu, Cr, Ni and Cd. The percentage of available Mn was very high, followed by available Cu and Pb in two of the composts, and available Zn and Cd. Available Al, Fe, Ni and Cr were very low or negligible. Most of the total Zn, important percentages of total Pb, Mn, Al and Fe, but very low proportions of total Cr and Ni and only traces of Cd, were complexed with organic matter; these compounds seemed to be soluble organo-metallic complexes, except part of those formed by Al and Cd that could be stable complexes. Although the four composted refuses were unbalanced with regard to the main nutrients they all had potential agronomic value. Total C contents and C/N ratios in the three non-amended composts were in the range for stabilized composts; however, the NH 4 + -N content seemed to point to the presence of non-stabilized substances.

Fumigation-extraction method to estimate microbial biomass in heated soils

Abstract A fumigation-extraction method that measures the fraction of the biomass rendered extractable to K2SO4 after microbial cells lysis by chloroform was used to estimate microbial biomass C and N in forest soil samples unheated or heated at 160, 350 or 600°C for 30 min, 2 or 4 h. The biomass C and N of the unheated samples were 1011 μg C g−1 and 124 μg N g−1 dry soil. Chloroform fumigation of the heated samples did not solubilize non-microbial soil organic matter. The microbial C and N in the heated samples were reduced to undetectable amounts regardless of the temperature and duration of heating. After restoring the microbial population in the heated soils by inoculation with fresh unheated soil and incubation, microbial biomass C and N were detected in the samples heated at 160°C (441 μg C g−1 and 42 μg N g−1 dry soil) and 350°C (63 μg C g−1 and 11 μg N g−1 dry soil) whereas the values in samples heated at 600°C were below the limits of detection. The decrease in biomass values were in accordance with a decrease in the number of microorganisms and hyphal length the recovery of which was inversely related to the temperature of heating. The results suggest that the fumigation-extraction technique can be used to estimate microbial biomass C and N in heated soils.

Microbiological characterization of four composted urban refuses

The microbial populations of four composed urban refuses, one of them amended with CaCO3, have been studied. The physiological groups with greatest influence upon the evolution and fertility of the soil were determined. The results were compared with the common numbers in soils and organic wastes. Variations in soil microbial population and nitrite oxidizers attributable to the action of the added compost were also recorded. Composted urban refuses had a high microbial population. Bacteria, a small proportion of which were spore-forming, predominated, but actinomycetes and fungi were also in high densities; on the contrary, algae were absent, except in CaCO3-amended compost. Most of the population had preteolytic, ammonificant, amylolytic and aerobic cellulolytic capacities, followed by denitrifiers, sulphate reducers and elementary sulphur oxidizers. The densities of aerobic and anaerobic free-nitrogen fixers, anaerobic cellulolytics, pectolytics, anaerobic mineralizers of sulphur and sulphide oxidizers were low. An important feature was the lack of nitrifiers in the wastes studied, the only exception being the compost amended with CaCO3 which had a relatively high population of nitrifiers. The urban refuse composts did not substantially affect the soil microbial population and had a short-term, slightly-positive, effect over nitrite oxidizers. Composting with CaCO3 seemed to notably improve the nitrite oxidizer population of the soil.

Seasonal fluctuations in microbial populations and available nutrients in forest soils

Viable microorganisms, soil respiration, and available N, Ca, Mg, Na, K, and P contents were determined in samples of five different forest soils collected in spring, summer, autumn, and winter. Viable microorganisms and soil respiration were positively correlated and showed a clear seasonal trend. The soils exhibited high microbial population values in spring and autumn and low values in summer and winter; total respiration values were largely higher in autumn than in the other seasons. Seasonal variations in available Ca, Na, and K contents were much more marked than those found for available N, Mg, and P. Available N and K and the microbial population showed similar seasonal trends whereas available Ca, Mg, Na, and P did not exhibit a distinguishable and uniform seasonal pattern. The quantities of available nutrients in soils followed the order Ca>K=Na>Mg>P>N. Soils developed over basic rocks showed higher values of both microbial density and microbial activity than those in soils developed over acid rocks. All the variables analysed were clearly related to the type of soil but varied with the date of sampling; a significant seasonal effect on the microbial population, microbial activity and available nutrients was detected in all the soils studied.