Influence of inorganic fertilizers and organic amendments on soil organic matter and soil microbial properties under tropical conditions

 Soil organic matter level, mineralizable C and N, microbial biomass C and dehydrogenase, urease and alkaline phosphatase activities were studied in soils from a field experiment under a pearl millet-wheat cropping sequence receiving inorganic fertilizers and a combination of inorganic fertilizers and organic amendments for the last 11 years. The amounts of soil organic matter and mineralizable C and N increased with the application of inorganic fertilizers. However, there were greater increases of these parameters when farmyard manure, wheat straw or Sesbania bispinosa green manure was applied along with inorganic fertilizers. Microbial biomass C increased from 147 mg kg^–1 soil in unfertilized soil to 423 mg kg^–1 soil in soil amended with wheat straw and inorganic fertilizers. The urease and alkaline phosphatase activities of soils increased significantly with a combination of inorganic fertilizers and organic amendments. The results indicate that soil organic matter level and soil microbial activities, vital for the nutrient turnover and long-term productivity of the soil, are enhanced by use of organic amendments along with inorganic fertilizers. Received: 6 May 1998     

Effects of a cadmium-contaminated sewage sludge compost on dynamics of organic matter and microbial activity in an arid soil

 An incubation experiment lasting 120 days was carried out to ascertain the effect on the soil microbial activity and organic matter mineralization of adding a sewage sludge compost contaminated with two different levels of Cd to an arid soil. Two composts, with a low (2 mg kg^–1) and high (815 mg kg^–1) Cd content, respectively, were used in this experiment. Both composts increased the total organic C, humic substance and water-soluble C contents, the beneficial effects still being noticeable after 120 days of incubation. The most labile C fraction (water-soluble C) was the most sensitive to the high Cd content. The high Cd concentration decreased soil microbial biomass C and stimulated the metabolic activity of the microbial biomass, the metabolic quotient (qCO₂) revealing itself to be a very sensitive index of the stress that the incorporation of a Cd-contaminated sewage sludge compost causes in a soil. The effect of Cd contamination on enzyme activities (urease, protease that hydrolyse N-α-benzoil-l-arginamide, phosphatase, and β-glucosidase) depended on the enzyme studied. Received: 10 September 1997     

Changes in microbial biomass and protease activities of soil associated with long-term sugar cane monoculture

 As part of a broader study, the aim of which is to identify soil factors that might be associated with yield decline of sugar cane, microbial biomass and protease activities were examined in soil samples collected from seven paired old and new land sites in three cane-growing districts of north Queensland. No consistent changes in soil protease activities were observed, although some sites exhibited specific effects, as a result of extensive periods of sugar cane monoculture. Soil microbial biomass, however, was significantly lower in those soils where sugar cane had been grown for extended periods. The implications of a lowering of soil microbial biomass on sugar cane yields and sustainability are discussed. Received: 24 June 1997     

Changes in soil microbial biomass, metabolic quotient, and organic matter turnover under Hieracium (H. pilosella L.)

 In New Zealand Hieracium is an opportunistic plant that invades high country sites more or less depleted of indigenous vegetation. To understand the invasive nature of this weed we assessed the changes in soil C, N and P, soil microbial biomass C, N and P contents, microbial C : N and C : P ratios, the metabolic quotient, and turnover of organic matter in soils beneath Hieracium and its adjacent herbfield resulting from the depletion of tussock vegetation. The amounts of soil organic C and total N were higher under Hieracium by 25 and 11%, respectively, compared to soil under herbfield. This change reflects an improvement in both the quantity and quality of organic matter input to mineral soil under Hieracium, with higher percentage organic C and a lower C : N ratio. The microbial biomass C, N and P contents were also higher under Hieracium. The amount of C respired during the 34-week incubation indicated differences in the nature of soil organic matter under Hieracium, the unvegetated "halo" zone surrounding Hieracium patches, and herbfield (depleted tussock grassland). Decomposition of organic matter in these zones showed that the Hieracium soil had the greatest rate of CO₂ respired, and the halo soil had the lowest. We relate the enhanced organic C turnover to the invasive nature of Hieracium. Net N mineralization was significantly lower from the Hieracium soil (57 mg N g^–1 soil N) than from herbfield and halo soils (74 and 71 mg N g^–1 soil N, respectively), confirming that the nature of organic N in Hieracium soil is different from adjoining halo and herbfield soils. It seems plausible that specific compounds such as polyphenols and lignins released by Hieracium are not only responsible for increased organic N, but also control the form and amount of N released during organic matter transformations. We conclude that the key to the success of Hieracium in the N-deficient South Island high country of New Zealand lies in its ability to control and sequester N supply through modifying the soil organic matter cycle. Received: 1 December 1998     

Relationship between rimsulfuron degradation and microbial biomass content in a clay loam soil

 The present research was conducted to determine the relationship between the degradation of rimsulfuron and soil microbial biomass C in a laboratory-incubated clay loam soil (pH=8.1; organic matter=2.1%) under different conditions and at different initial dosages (field rate, 10 and 100 times the field rate). The half-life values varied between 0.4 and 103.4 days depending on temperature, soil moisture and initial dose. Evidence suggested that rimsulfuron could pose environmental risks in cold and dry climatic conditions. Significant decreases in microbial biomass C content in rimsulfuron-treated soil, compared to untreated soil, were observed initially, especially at higher temperatures and low moisture levels, but never exceeded 20.3% of that in control soil. The microbial biomass C content then returned to initial values at varying times depending on incubation conditions. The relationship between herbicide degradation and microbial biomass C content gave parabolic curves (P<0.005 in all cases) under all conditions tested. Generally, maximum biomass C decrease coincided with the decrease in the concentration of rimsulfuron to about 50% of the initial dose, except at 10  °C and 100×, when biomass began to recover as early as 65–70% of the initial dose. The final equations could be useful to deduce the decrease of soil microbial biomass in relation to herbicide concentration. From the degradation kinetics of the herbicide, the time required to reach this decrease can also be calculated. Received: 19 July 1999     

Influence of inorganic and organic fertilization on soil microbial biomass, metabolic quotient and heavy metal bioavailability

 We studied the long-term effects (12 years) of municipal refuse compost addition on the total organic carbon (TOC), the amount and activity of the microbial biomass (soil microbial biomass C, B_C and metabolic quotient qCO₂) and heavy metal bioavaiability in soils as compared to manuring with mineral fertilizers (NPK) and farmyard manure (FYM). In addition, we studied the relationships between among the available fraction [Diethylenetriaminopentacetic acid (DTPA) extractable] of heavy metals and their total content, TOC and B_C. After 12 years of repeated treatments, the TOC and B_C of control and mineral fertilized plots did not differ. Soils treated with FYM and composts showed a significant increase in TOC and B_C in response to the increasing amounts of organic C added. Values of the B_C/TOC ratio ranged from 1.4 to 2, without any significative differences among soil treatments. The qCO₂ increased in the organic-amended soil and may have indicated microbial stress. The total amounts of metals in treated soils were lower than the levels permitted by the European Union in agricultural soils. DTPA-extractable metals increased in amended soils in response to organic C. A multiple regression analysis with stepwise selection of variables was carried out in order to discriminate between the influence exerted on DTPA-extractable metals by their total content, TOC and B_C. Results showed that each metal behaved quite differently, suggesting that different mechanisms might be involved in metal bioavailability Received: 31 October 1997     

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     

Interactions between soil organic matter status, cropping history, method of quantification and sample pretreatment and their effects on measured aggregate stability

 The effects of sample pretreatment (field-moist, air-dried or tension rewetted) on aggregate stability measured by wet sieving or turbidimetry were compared for a group of soil samples ranging in organic C content from 20 to 40 g C kg^–1. Concentrations of total N, total and hot-water-extractable carbohydrate and microbial biomass C were linearly related to those of organic C. Aggregate stability measured by wet sieving using air-dried or field-moist samples and that measured by turbidimetry, regardless of sample pretreatment, increased curvilinearly with increasing soil organic C content. However, when tension-rewetted samples were used for wet sieving, aggregate stability was essentially unaffected by soil organic C content. Measurements of aggregate stability (apart from wet sieving using rewetted soils) were closely correlated with one another and with organic C, total and extractable carbohydrate and microbial biomass C content of the soils. The short-term effects of aggregate stability were also studied. Soils from under long-term arable management and those under long-term arable followed by 1 or 3 years under pasture had similar organic C contents, but aggregate stability measured by turbidimetry and by wet sieving using air-dried or field-moist samples increased with increasing years under pasture. Light fraction C, microbial biomass and hot-water-extractable carbohydrate concentrations also increased. It was concluded that both total and labile soil organic C content are important in relation to water-stable aggregation and that the use of tension-rewetted samples to measure stability by wet sieving is unsatisfactory since little separation of values is achieved. Received: 6 January 1999     

The influence of soil compaction and the removal of organic matter on two native earthworms and soil properties in an oak-hickory forest

 Earthworms may alter the physical, chemical, and biological properties of a forest soil ecosystem. Any physical manipulation of the soil ecosystem may, in turn, affect the activities and ecology of earthworms. The effects of removing organic matter (logs and forest litter) and severely compacting the soil on native earthworm species were measured in a central USA hardwood region (oak-hickory) forest in the Missouri Ozarks (USA). Soils in this region are characterized by a cherty residuum that is primarily of the Clarksville series (Loamy-skeletal, mixed, mesic Typic Paledults). Earthworms were collected from 0–15 cm depth each spring and fall for 2 years by handsorting, and densities were determined on a per meter square basis. Two native earthworm species, Diplocardia ornata and Diplocardia smithii, were dominant on this site. Organic matter removal decreased the average individual biomass of both species. However, both species responded differently to soil compaction. Soil compaction affected D. ornata adversely and D. smithii favorably. This suggested that the degree of soil compaction was not as restrictive with respect to D. smithii (2 mm diameter) as to D. ornata (5 mm diameter). Moreover, the apparently improved soil environmental conditions resulting from the remaining organic matter in compacted soil enhanced the population and growth of D. smithii. Sampling position on the landscape affected D. ornata but not D. smithii. Soil microbial biomass C and soil microbial biomass N were decreased under soil compaction when the organic matter was removed. Other factors influencing the ecology and activity of these two species will require further study. Received: 6 January 1999     

Changes in microbial community metabolism and labile organic matter fractions as early indicators of the impact of management on soil biological quality

 Changes to the metabolic profiles of soil microbial communities could have potential for use as early indicators of the impact of management or other perturbations on soil functioning and soil quality. We compared the relative susceptibility to management of microbial community metabolism with a number of soil organic matter (OM) and microbial parameters currently used as indicators of changes in soil biological quality. Following long-term cereal cropping, plots were subjected to a 16-month treatment period consisting of either a mixed cropping sequence of vetch, spring barley and clover or a continuous grass-clover ley which was periodically mown and mulched. The treatments had no effect on soil biomass N or respiration of microbial populations inoculated into Biolog Gram negative (GN) plates. After 16 months there were no management-induced changes to total OM, light-fraction OM C and N, labile organic N or water-soluble carbohydrates. However, patterns of substrate utilization by the soil microbial population following inoculation into Biolog GN plates were found to be highly sensitive to management practice. In the mixed cropping sequence, substrate utilization changed markedly following plough-in of the vetch crop, with a smaller change occurring after harvesting of the barley. In the ley treatment, substrate utilization was not affected until the onset of mowing, when the pattern changed to become similar to that in the mixed cropping sequence. Metabolic diversity of the Biolog-culturable microbial population was increased by the ley treatment, but was not affected by the cropping sequence. We conclude that patterns of microbial substrate utilization and metabolic diversity are more sensitive to the effects of management than are OM and biomass pools, and therefore have value as early indicators of the impacts of management on soil biological properties, and hence soil quality. Received: 7 April 1999     

The impact of a low humus level in arable soils on microbial properties, soil organic matter quality and crop yield

 In arable soils in Schleswig-Holstein (Northwest Germany) nearly 30% of the total organic C (TOC) stored in former times in the soil has been mineralized in the last 20 years. Microbial biomass, enzyme activities and the soil organic matter (SOM) composition were investigated in order to elucidate if a low TOC level affects microbial parameters, SOM quality and crop yield. Microbial biomass C (C_mic) and enzyme activities decreased in soils with a low TOC level compared to soils with a typical TOC level. The decrease in the C_mic/TOC ratio suggested low-level, steady-state microbial activity. The SOM quality changed with respect to an enrichment of initial litter compounds in the top soil layers with a low TOC level. Recent management of the soils had not maintained a desirable level of humic compounds. However, we found no significant decrease in crop yield. We suggest that microbial biomass and dehydrogenase and alkaline phosphatase activities are not necessarily indicators of soil fertility in soils with a high fertilization level without forage production and manure application. Received: 12 December 1997     

Ecological study on the dynamics of soil organic matter and its related properties in shifting cultivation systems of Northern Thailand

Abstract

There is a large number of hill people in northern Thailand, who practices shifting cultivation. In order to analyze the soil ecological problems involved in the transition from traditional shifting cultivation to more intensive upland farming, the authors carried out comparative studies on the dynamics of organic matter and its related properties in soils both in the traditional shifting cultivation systems adopted by Karen people and more intensive upland farming practiced by Thai and Hmong people in the area. The contents of organic matter and available N in the surface 10 cm layers of soil from the fields continuously cultivated were lower than those in soils under prolonged fallow (more than 10 y) or natural forest. Based on the rate of soil respiration, the amount of organic matter decomposed within 1 y was estimated to reach nearly 10% of that stored in the upper 50 cm layers of the soil profile in the upland crop fields. These results indicate that the organic matter-related resources markedly decreased under continuous cropping. The contents of C, N, and P in the microbial biomass of the surface 10 cm layers of soil ranged from 0.37 to 2.09 mg C g^?l soil, from 22.7 to 188 µg N g^?l soil, and from 6.1 to 65.7 µg P g^?l soil, respectively. Since the contents of microbial C, N, and P in the surface soils were generally higher under prolonged fallow and natural forests than in the fields continuously cultivated, the microbial activity and/or the amounts of C, N, and P available for biological activity seemed to have declined under continuous upland farming. The incubation experiment to assess the N mineralization pattern showed two remarkable characteristics: 1) there was an initial time lag until active mineralization of N occurred in the soils from young fallow forest and 2) the soil burning effect was observed after burning in the fields under prolonged fallow. The active process of nitrification after N mineralization was always associated with a sharp fall in soil pH, suggesting that soil acidification was promoted and basic cations were lost from the soils. In conclusion, rapid deterioration of the soil organic matter-related properties in cropping fields can be considered to be one of the ecological reasons why upland fields must be returned to fallow again a few years after forest reclamation in traditional shifting cultivation systems. Therefore, in alternative farming systems with more intensive land use, it is essential to apply organic materials into soils to decrease the rate of soil degradation, or to improve the soil fertility, in avoiding soil acidification along with nitrification.     

Fatty acid patterns of phospholipids and lipopolysaccharides in the characterisation of microbial communities in soil: a review

 This review discusses the analysis of whole-community phospholipid fatty acid (PLFA) profiles and the composition of lipopolysaccharides in order to assess the microbial biomass and the community structure in soils. For the determination of soil microbial biomass a good correlation was obtained between the total amount of PLFAs and the microbial biomass measured with methods commonly used for determinations such as total adenylate content and substrate-induced respiration. Generally, after the application of multivariate statistical analyses, whole-community fatty acid profiles indicate which communities are similar or different. However, in most cases, the organisms accounting for similarity or difference cannot be determined, and therefore artefacts could not be excluded. The fatty acids used to determine the biomass vary from those which determine the community structure. Specific attention has to be paid when choosing extraction methods in order to avoid the liberation of fatty acids from non-living organic material and deposits, and to exclude the non-target selection of lipids from living organisms, as well. By excluding the fatty acids which were presumed to be common and widespread prior to multivariate statistical analysis, estimates were improved considerably. Results from principal component analysis showed that determining the levels of fatty acids present in both low and high concentrations is essential in order to correctly identify microorganisms and accurately classify them into taxonomically defined groups. The PLFA technique has been used to elucidate different strategies employed by microorganisms to adapt to changed environmental conditions under wide ranges of soil types, management practices, climatic origins and different perturbations. It has been proposed that the classification of PLFAs into a number of chemically different subgroups should simplify the evaluating procedure and improve the assessment of soil microbial communities, since then only the subgroups assumed to be involved in key processes would be investigated. Received: 24 August 1998