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     

长期施肥对红壤性水稻土微生物生物量与活性的影响

土壤微生物及其活性是指示土壤增肥过程和土壤环境变化的灵敏指标.本文研究了红壤荒地开垦为水田后不同施肥制度定位施肥 16 年后水稻土的微生物生物量与活性特征,结果表明:经 16 年水稻耕植,不同施肥措施下土壤的微生物生物量和活性还处于较低水平.施肥制度显著影响了水稻土的微生物生物量 C 和基质诱导呼吸,但对基础呼吸的影响还不明显.只施用 N、K 肥对提高土壤微生物生物量和活性没有显著效果,在不施肥或施用化肥的基础上配合有机循环可以显著提高土壤微生物的生物量、代谢活性和微生物呼吸的温度敏感性,N、P、K 肥配合有机循环的施肥制度对提高土壤微生物生物量和代谢活性的作用最好.     

Biological and chemical properties of arable soils affected by long-term organic and inorganic fertilizer applications

 Using soils from field plots in four different arable crop experiments that have received combinations of manure, lime and inorganic N, P and K for up to 20 years, the effects of these fertilizers on soil chemical properties and estimates of soil microbial community size and activity were studied. The soil pH was increased or unaffected by the addition of organic manure plus inorganic fertilizers applied in conjunction with lime, but decreased in the absence of liming. The soil C and N contents were greater for all fertilized treatments compared to the control, yet in all cases the soil samples from fertilized plots had smaller C:N ratios than soil from the unfertilized plots. The soil concentrations of all the other inorganic nutrients measured were greater following fertilizer applications compared with the unfertilized plots, and this effect was most marked for P and K in soils from plots that had received the largest amounts of these nutrients as fertilizers. Both biomass C determined by chloroform fumigation and glucose-induced respiration tended to increase as a result of manure and inorganic fertilizer applications, although soils which received the largest additions of inorganic fertilizers in the absence of lime contained less biomass C than those to which lime had been added. Dehydrogenase activity was lower in soils that had received the largest amounts of fertilizers, and was further decreased in the absence of lime. This suggests that dehydrogenase activity was highly sensitive to the inhibitory effects associated with large fertilizer additions. Potential denitrification and anaerobic respiration determined in one soil were increased by fertilizer application but, as with both the microbial biomass and dehydrogenase activity, there were significant reductions in both N₂O and CO₂ production in soils which received the largest additions of inorganic fertilizers in the absence of lime. In contrast, the size of the denitrifying component of the soil microbial community, as indicated by denitrifying enzyme activity, was unaffected by the absence of lime at the largest rate of inorganic fertilizer applications. The results indicated differences in the composition or function of microbial communities in the soils in response to long-term organic and inorganic fertilization, especially when the soils were not limited. Received: 10 March 1998     

Assessment of the influence of phosphate fertilizers on the microbial activity of pasture soils

The objective of the present work was to examine the effects of phosphate fertilizers on the microbial activity of pasture soils. Various microbial characteristics were measured using soils from an existing long-term phosphate fertilizer field trial and a short-term incubation experiment. The measurements included basal respiration, substrate induced respiration, inhibition of substrate-induced respiration by streptomycin sulphate (fungal activity) and actidione (bacterial activity) and microbial biomass C. The long-term field trials was initiated during 1985 to examine the effectiveness of different sources of phosphate fertilizers (single superphosphate, North Carolina phosphate rock, partially acidulated North Carolina phosphate rock, and diammonium phosphate) on pasture yield. The incubation experiment was conducted for 8 weeks using the same soil and the sources of phosphate fertilizers used in the field trial. In the incubation experiment the fertilizer addition caused an initial decrease in basal and substrate-induced respiration but had no effect on total microbial biomass. The initial decline in basal and substrate-induced respiration with the fertilizer addition was restored within 8 weeks after incubation. In the field experiment the fertilizer addtion had no significant effect on basal respiration but increased substrate-induced respiration and microbial biomass C. The short-term and the long-term effects of phosphate fertilizer addition on the microbial characteristies of the soils are discussed in relation to its effects on pH, salt concentration, and the nutrient status of the soils.     

Interactions between microclimatic variables and the soil microbial biomass

Summary Soil moisture, temperature, microbial substrate-induced respiration and basal respiration were monitored in two plots in an agricultural field from April 30 to September 25, 1987, and in a further two plots from May 26 to August 27, 1988. An attempt to relate biological variables to microclimatic variables was made through the use of correlation analysis. The microbial substrate-induced and basal respiration were both strongly positively correlated with the soil moisture content, and to a lesser extent positively related to soil temperature, especially when partial correlation was used to control for variation in soil moisture. Short-term changes in substrate-induced and basal respiration were correlated with changes in soil moisture but were largely independent of soil temperature. The ratio of basal to substrate-induced respiration (indicating the respiration: biomass ratio and therefore ecosystem stability or persistence) was negatively associated with the soil moisture content and in some instances with soil temperature when partial correlation analysis (correcting for soil moisture variation) was used. This suggests that the climatic conditions which contributed to the lowest ecosystem stability were low temperature, low moisture conditions.     

Influence of soil properties on microbial populations, activity and biomass in humid subtropical mountainous ecosystems of India

 Microbial populations, biomass, soil respiration and enzyme activities were determined in slightly acid organic soils of major mountainous humid subtropical terrestrial ecosystems, along a soil fertility gradient, in order to evaluate the influence of soil properties on microbial populations, activity and biomass and to understand the dynamics of the microbial biomass in degraded ecosystems and mature forest. Although the population of fungi was highest in the undisturbed forest (Sacred Grove), soil respiration was lowest in the 7-year-old regrowth and in natural grassland (approximately 373 μg g^–1 h^–1). Dehydrogenase and urease activities were high in "jhum" fallow, and among the forest stands they were highest in the 7-year-old regrowth. Microbial biomass C (MBC) depended mainly on the organic C status of the soil. The MBC values were generally higher in mature forest than in natural grassland, 1-year-old jhum fallow and the 4-year-old alder plantation. The MBC values obtained by the chloroform-fumigation-incubation technique (330–1656 μg g^–1) did not vary significantly from those obtained by the chloroform-fumigation-extraction technique (408–1684 μg g^–1), however, the values correlated positively (P<0.001). The enzyme activities, soil respiration, bacterial and fungal populations and microbial biomass was greatly influenced by several soil properties, particularly the levels of nutrients. The soil nutrient status, microbial populations, soil respiration and dehydrogenase activity were greater in Sacred Grove, while urease activity was greater in grassland. Received: 14 October 1998     

Effects of increasing periods under intensive arable vegetable production on biological, chemical and physical indices of soil quality

 The effects on soil condition of increasing periods under intensive cultivation for vegetable production on a Typic Haplohumult were compared with those of pastoral management using soil biological, physical and chemical indices of soil quality. The majority of the soils studied had reasonably high pH, exchangeable cation and extractable P levels reflecting the high fertilizer rates applied to dairy pasture and more particularly vegetable-producing soils. Soil organic C (C_org) content under long-term pasture (>60 years) was in the range of 55 g C kg^–1 to 65 g C kg^–1. With increasing periods under vegetable production soil organic matter declined until a new equilibrium level was attained at about 15–20 g C kg^–1 after 60–80 years. The loss of soil organic matter resulted in a linear decline in microbial biomass C (C_mic) and basal respiratory rate. The microbial quotient (C_mic/C_org) decreased from 2.3% to 1.1% as soil organic matter content declined from 65 g C kg^–1 to 15 g C kg^–1 but the microbial metabolic quotient (basal respiration/C_mic ratio) remained unaffected. With decreasing soil organic matter content, the decline in arginine ammonification rate, fluorescein diacetate hydrolytic activity, earthworm numbers, soil aggregate stability and total clod porosity was curvilinear and little affected until soil organic C content fell below about 45 g C kg^–1. Soils with an organic C content above 45 g C kg^–1 had been under pasture for at least 30 years. At the same C_org content, soil biological activity and soil physical conditions were markedly improved when soils were under grass rather than vegetables. It was concluded that for soils under continuous vegetable production, practices that add organic residues to the soil should be promoted and that extending routine soil testing procedures to include key physical and biological properties will be an important future step in promoting sustainable management practices in the area. Received: 18 November 1997     

Influence of beech litter fragmentation and glucose concentration on the microbial biomass in three different litter layers of a beechwood

In February 1993 samples of litter from three different litter layers (upper, intermediate, and lower) were taken from a beechwood growing on basalt soil. Using the substrate-induced respiration method, we investigated the influence of fragmentation and glucose concentration on the maximum initial respiratory response. Glucose concentrations ranged between 0 and 160000 g g^-1 dry weight. The initial respiratory response reached a maximum at 80000 g glucose g^-1 dry weight. The addition of higher concentrations of glucose resulted in negligible changes in respiration. Litter materials of four different size classes (intact leaves, fragmented <100 mm², <25 mm², and <5 mm²) were amended with 80000 g glucose g^-1 dry weight. Substrate-induced respiration was at a maximum in the size class <25 mm². The addition of glucose to intact litter did not result in microbial growth. It is concluded that C is not the primary limiting element for the microflora in litter layers of the study site. Fragmentation of beech litter enabled the microorganisms to grow. Presumably, nutrients that limited microbial growth in intact litter were mobilized by the fragmentation procedure and enabled microorganisms to grow in fragmented litter materials.     

长期施用化肥对红壤性水稻土中微生物生物量、活性及群落结构的影响

利用化肥长期定位试验,研究了施肥对土壤微生物生物量、活性及其群落结构的影响.结果表明:与不施肥相比,长期施用化肥不仅增加了土壤微生物生物量,而且导致了土壤微生物群落结构的分异.其中,有机无机配施处理和2倍NPK配施处理显著提高了土壤有机质含量、全N含量、土壤微生物生物量和土壤微生物活性.NPK均衡施肥处理对土壤有机质、土壤微生物生物量及其活性的影响小于非均衡施肥的处理(NP、NK、N、P、K),适当增施K肥有利于提高土壤微生物中真菌的比例.     

Effects of long-term waste water irrigation on soil organic matter, soil microbial biomass and its activities in central Mexico

 The effect of long-term waste water irrigation (up to 80 years) on soil organic matter, soil microbial biomass and its activities was studied in two agricultural soils (Vertisols and Leptosols) irrigated for 25, 65 and 80 years respectively at Irrigation District 03 in the Valley of Mezquital near Mexico City. In the Vertisols, where larger amounts of water have been applied than in the Leptosols, total organic C (TOC) contents increased 2.5-fold after 80 years of irrigation. In the Leptosols, however, the degradability of the organic matter tended to increase with irrigation time. It appears that soil organic matter accumulation was not due to pollutants nor did microbial biomass:TOC ratios and qCO₂ values indicate a pollutant effect. Increases in soil microbial biomass C and activities were presumably due to the larger application of organic matter. However, changes in soil microbial communities occurred, as denitrification capacities increased greatly and adenylate energy charge (AEC) ratios were reduced after long-term irrigation. These changes were supposed to be due to the addition of surfactants, especially alkylbenzene sulfonates (effect on denitrification capacity) and the addition of sodium and salts (effect on AEC) through waste water irrigation. Heavy metals contained in the sewage do not appear to be affecting soil processes yet, due to their low availability. Detrimental effects on soil microbial communities can be expected, however, from further increases in pollutant concentrations due to prolonged application of untreated waste water or an increase in mobility due to higher mineralization rates. Received: 28 April 1999     

Soil organisms and carbon, nitrogen and phosphorus mineralisation in Norway spruce and mixed Norway spruce – Birch stands

 We examined how soil organisms and C, N and P mineralisation are affected by admixing deciduous tree species, silver birch (Betula pendula) and woollen birch (B. pubescens), in managed Norway spruce (Picea abies) stands. Pure spruce and mixed spruce–birch stands were examined at four sites in southern and central Sweden. Soil macroarthropods and enchytraeids were sampled in litter and soil. In the uppermost 5 cm of soil humus we determined microbial biomass and microbial respiration; we estimated the rate of C, N and P mineralisation under laboratory conditions. The densities of Coleoptera, Diptera and Collembola were larger in mixed stands than in spruce stands. Soil fauna composition differed between mixed and spruce stands (as revealed by redundancy analysis). Staphyliniidae, Elateridae, Cecidiomyidae larvae and Onychiuridae were the families that increased most strongly in mixed stands. There were no differences in microbial biomass and microbial respiration, nor in the C, N and P mineralisation rates, between mixed and spruce stands. However, within mixed stands microbial biomass, microbial activity and C mineralisation were approximately 15% higher under birch trees than under spruce trees. We propose that the presence of birch leaf litter was likely to be the most important factor causing differences in soil fauna composition. Birch may also influence the quality and the decomposition rate of humus in mixed stands. However, when the proportion of birch trees is low, the short-term (decades) effect of this species on decomposition is likely to be small in mixed stands on acid forest soils. Received: 20 February 1998     

Soil organic carbon stocks, storage profile and microbial biomass under different crop management systems in a tropical agricultural ecosystem

 We investigated the soil organic C and N stocks, storage profiles and microbial biomass as influenced by different crop management systems in a tropical agricultural ecosystem. The different crop management systems significantly affected the C and N stocks and microbial biomass C and N at different soil depths. Amongst the systems evaluated, the rice-wheat system maintained a higher soil organic C content. Inclusion of legumes in the system improved the soil organic matter level and also soil microbial biomass activity, vital for the nutrient turnover and long-term productivity of the soil. Irrespective of the cropping system, approximately 58.4%, 25.7% and 15.9% of the C was distributed in 0–15, 15–30 and 30–60 cm depths, respectively. Received: 10 October 1999     

Soil microbial biomass and activity under a potato crop fertilised with N with and without C

Summary A range of soil microbiological parameters were measured at intervals throughout the growing season of a potato crop. Treatments applied to the soil at sowing were zero N fertilisation of N fertilisation at 120 kg N ha^–1, either alone or supplemented with straw or sucrose at 1200 kg C ha^–1. C and N flushes determined by fumigation-incubation and fumigation-extraction, and substrate-induced respiration, were measured as indicators of microbial biomass. Microbial activity was measured as respiration (CO₂ production) and dehydrogenase activity (formazan production). The greatest effects were obtained from the addition of N plus sucrose. Both biomass size and activity were significantly stimulated for up to 25 days after incorporation, with the magnitude of the effects consistently diminishing over time. By 125 days after planting, there was no detectable legacy from any of the treatmentson any of the biomass parameters that were measured, and all values had reverted to those prevalent at planting. There was no consistent effect from adding N, either alone or supplemented with straw, on any of the biomass parameters. There was no evidence for crop-induced stimulation of the biomass. The experiment demonstrates that biomass is only influenced where the quantity, quality, and rate of incorporation of C into the soil is appropriate, in this case, only by adding C as a pulse of sucrose.     

Changes in 15N abundance and amounts of biologically active soil nitrogen

 Estimation of the capacity of soils to supply N for crop growth requires estimates of the complex interactions among organic and inorganic N components as a function of soil properties. Identification and measurement of active soil N forms could help to quantify estimates of N supply to crops. Isotopic dilution during incubation of soils with added ¹⁵NH₄ ⁺ compounds could identify active N components. Dilution of ¹⁵N in KCl extracts of mineral and total N, non-exchangeable NH4₄ ⁺, and N in K₂SO₄ extracts of fumigated and non-fumigated soil was measured during 7-week incubation. Samples from four soils varying in clay content from 60 to 710 g kg^–1 were used. A constant level of ¹⁵N enrichment within KCl and K₂SO₄ extracted components was found at the end of the incubation period. Total N, microbial biomass C and non-exchangeable NH₄ ⁺ contents of the soils were positively related to the clay contents. The mineralized N was positively related to the silt plus clay contents. The active soil N (ASN) contained 28–36% mineral N, 29–44% microbial biomass N, 0.3–5% non-exchangeable NH₄ ⁺ with approximately one third of the ASN unidentified. Assuming that absolute amounts of active N are related to N availability, increasing clay content was related to increased N reserve for crop production but a slower turnover. Received: 7 July 1998