Levels Of Soil Microbial Biomass Research Articles

Hydrogen stable isotope probing of lipids demonstrates slow rates of microbial growth in soil

The rate at which microorganisms grow and reproduce is fundamental to our understanding of microbial physiology and ecology. While soil microbiologists routinely quantify soil microbial biomass levels and the growth rates of individual taxa in culture, there is a limited understanding of how quickly microbes actually grow in soil. For this work, we posed the simple question: what are the growth rates of soil microorganisms? In this study, we measure these rates in three distinct soil environments using hydrogen-stable isotope probing of lipids with 2H-enriched water. This technique provides a taxa-agnostic quantification of in situ microbial growth from the degree of 2H enrichment of intact polar lipid compounds ascribed to bacteria and fungi. We find that growth rates in soil are quite slow and correspond to average generation times of 14 to 45 d but are also highly variable at the compound-specific level (4 to 402 d), suggesting differential growth rates among community subsets. We observe that low-biomass microbial communities exhibit more rapid growth rates than high-biomass communities, highlighting that biomass quantity alone does not predict microbial productivity in soil. Furthermore, within a given soil, the rates at which specific lipids are being synthesized do not relate to their quantity, suggesting a general decoupling of microbial abundance and growth in soil microbiomes. More generally, we demonstrate the utility of lipid-stable isotope probing for measuring microbial growth rates in soil and highlight the importance of measuring growth rates to complement more standard analyses of soil microbial communities.

Mixture of N2-fixing tree species promotes organic phosphorus accumulation and transformation in topsoil aggregates in a degraded karst region of subtropical China

Many studies have shown that introducing N2-fixing tree species into plantations can increase soil nitrogen (N) availability, via biological N2 fixation and faster N cycling. However, the effects and regulatory mechanisms of N2-fixing tree species in planting single species vs a combination of two species on phosphorus (P) accumulation and transformation in degraded karst soils remain poorly understood and appear site-dependent. This study aimed to determine the effects of introducing N2-fixing tree species (via single species vs a combination of two species) into a degraded karst region on organic phosphorus (Po) accumulation and transformation in topsoil aggregates. A comparative experiment was performed involving 8-year-old pure plantations of Dalbergia odorifera (PD) and Acrocarpus fraxinifolius (PA), a mixed plantation of Dalbergia odorifera and Acrocarpus fraxinifolius (MP), and an adjacent natural weed field (no trees, CK) in Mashan, Guangxi, subtropical China. The results showed that the contents of soil organic carbon (SOC), nitrate nitrogen (NO3−-N), available phosphorus (AP) and C:N ratios in bulk soil and aggregates, increased significantly (P < 0.05) in MP compared to CK. The levels of soil microbial biomass N (MBN) and microbial biomass P (MBP) in bulk and aggregate soils increased significantly (P < 0.05), except for micro-aggregates (<0.25 mm) in MP samples. In contrast, no significant differences of MBN or MBP were found among PD, PA and CK plots in bulk and most aggregate soils. The phospholipid fatty acid (PLFA) contents of all microbes, bacteria, fungi, arbuscular mycorrhizal (AM) fungi and actinomycetes were also significantly higher (P < 0.05) in MP than those in CK and PD in bulk and most aggregate soils. Labile Po and highly resistant Po, and the activities of all tested N and P hydrolytic enzymes, improved in bulk soil and most aggregate size classes in all forest types, especially in MP samples. Finally, redundancy analysis (RDA) indicated that Po fractions were primarily driven by alkaline phosphatase (ALP), MBN and AM. Our findings suggest that introducing N2-fixing tree species (especially MP) may be an effective method for increasing N availability and AM colonization of roots, and thereby promoting Po accumulation and transformation in degraded karst regions of southwest China.

Characteristics of Paddy Soil Organic Carbon Mineralization and Influencing Factors Under Different Water Conditions and Microbial Biomass Levels

Paddy soil often undergoes frequent dry-wet alternation. The change in water status not only affects the physical and chemical properties of the soil, but also changes the structure and diversity of the soil microbial communities, which in turn determines the rate of soil organic carbon mineralization. However, the effects of different water conditions and soil microbial biomass levels on the process of soil organic carbon mineralization and its mechanisms are still unclear. Therefore, this study took typical subtropical paddy soil as the research object, applied a laboratory incubation experiment with two water treatments of dry-wet and continuous flooding, and reduced the soil microbial biomass through chloroform fumigation, thereby obtaining high and low soil microbial biomass carbon contents, to elucidate the influencing mechanisms of microbial biomass and water conditions on organic carbon mineralization in paddy soil. The results showed that during the first 30 d of incubation, the dry-wet treatment was in a non-flooded stage and its cumulative CO2 emissions were significantly lower than those of the continuous flooded treatment. After 30 d, the dry-wet treatment entered the flooded stage. The difference in the cumulative CO2 emissions of the soils with a high microbial biomass carbon content between the dry-wet and continuous flooding treatments gradually decreased, and there was no significant difference on day 78. In the soil with a low microbial biomass carbon content, the cumulative CO2 emissions of the dry-wet treatment on day 78 was still significantly lower than that of the continuous flooded treatment. The soils with a low microbial biomass carbon content showed a faster CO2 emission rate at the beginning of the incubation period (first 20 d), which was 1.1-6.1 times greater than that of the high microbial biomass carbon soils owing to their high soil dissolved organic carbon (DOC) content, and the CO2 emission rate then gradually decreased until it was below that of the soil with a high microbial biomass carbon content. The soil organic carbon mineralization rate became stable later in the incubation period (days 45-78). The stable mineralization rate of the high microbial biomass carbon soil was 20%-30% higher than that of the low microbial biomass carbon soil. The multiple regression analysis results showed that the decrease in the soil DOC content (ΔDOC) and the increase in the Fe2+ content (ΔFe2+) significantly affected the change in cumulative CO2 emissions (ΔCO2) under continuous flooding conditions, but had no effect on ΔCO2 during the flooding stage of the dry-wet treatment. The correlation analysis showed that the daily CO2 emission rate of soils with high microbial biomass carbon was significantly positively correlated with glucosidase activity under dry-wet treatment and significantly negatively correlated with acetylglucosaminidase (NAG) and peroxidase activities under continuous flooding treatment. In the low microbial biomass carbon soils, the daily CO2 emission rate of the continuous flooding treatment was negatively correlated with the NAG activity, but showed no correlation with enzyme activities under dry-wet management. In summary, the cumulative CO2 emissions of dry-wet treatment were lower than those of continuous flooding treatment, and the difference was significant in soils with low microbial biomass carbon. The size of the soil microbial biomass determined the level of the stable soil organic carbon mineralization rate. The amount of soluble organic carbon and iron reduction affected the soil CO2 emissions under continuous flooding conditions, and the soil water conditions affected the daily CO2 emission rate and its key influencing enzymes. This study provides data and theoretical support for the carbon cycle and carbon sequestration potential in paddy soil.

Land use change: A key ecological disturbance declines soil microbial biomass in dry tropical uplands

Land use changes such as transformation of natural landscapes, forest degradation and increase in croplands due to human activities are considered amongst the most influential ecological disturbances affecting soil, ecosystems and environmental sustainability. The previous works from India are limited to show that soil disturbances influence abiotic and biotic factors along a rural–urban gradient. However, variations in soil microbial biomass (SMB) –C, –N and –P quantity due to land use changes at different soil depths across different land use types remain poorly understood on comparative ground. We investigated the impact of land use types on soil properties and SMB –C, –N and –P levels across different soil depths (0–10, 10–20 and 20–30 cm) in dry tropical uplands. Four land use types/covers (natural forest, mixed forest, savanna and agriculture land) were selected. The present study is based on two hypotheses: i) different land use types affect SMB levels in top surface soil (0–10 cm), but have less effects in deeper soil profiles (20–30 cm); and ii) SMB levels in top surface soil are highest in natural forest, followed by mixed forest and then savanna and agriculture lands. ANOVA showed significant differences in SMB values due to land use covers (P < 0.001), soil depths (P < 0.001) and land use types × soil depths interaction (P < 0.001). Although, there had no effect of land use types on SMB levels in deeper soil profiles (20–30 cm) but soil parameters (soil pH, soil moisture, soil temperature, total-N, C/N ratio and organic-C) significantly affect SMB levels in top surface (0–10 cm) soil. The study suggests that SMB may be considered as a key indicator of soil fertility index, while land use practices are a major cause for loss of microbial community composition/biomass in dry tropical upland soil.

A meta-analysis of soil microbial biomass levels from established tree plantations over various land uses, climates and plant communities

Uncertainties remain as to the potential for tree plantations to affect soil microbial biomass. Our aim was to determine the factors accountable for the maintenance and the increase of soil microbial biomass following tree plantation. Based on mixed effect models, we conducted a meta-analysis with three fixed and two random factors to test the impact of tree plantation on soil microbial biomass. Previous land use was more important than climate or plant species in its effect on soil microbial biomass after tree plantation. There was a positive impact on soil microbial biomass for tree plantations on bare land but a negative impact for which on previously forested land. Climate and plant species were found to be not as important in their effects on soil microbial biomass. Our meta-analysis gives a general pattern that previous land use type is the major controlling factor of soil microbial biomass following tree plantations and promotes our understanding of the effects of rehabilitation of degraded sites on vegetation recruitment.

Biochemical properties of highly mineralised and infertile soil modified by acacia and spinifex plants in northwest Queensland, Australia

Root zone soil properties can significantly influence the establishment of revegetated plant communities and alter their development trajectories in mined landscapes, due to closely coupled biogeochemical linkages between soil and plant systems. The present study aimed to characterise physicochemical and biochemical conditions in soil colonised by slow-growing native plant species: Acacia chisholmii (C3, native leguminous shrub) and Triodia pungens (spinifex C4 grass) in Mt Isa, North-west Queensland, Australia. This is to provide the basis for engineering growth media and root zones suitable for supporting target native plant communities to be revegetated in mined landscapes under subtropical and semiarid climatic conditions. Litter chemistry, soil physicochemical properties, and microbial community structure based on phospholipid fatty acids (PLFAs) biomarker method and activities (basal respiration, net mineralisation, dehydrogenase, invertase, urease and neutral phosphatase activities) were characterised in the surface soils beneath the keystone native plant species. Results showed that soils sampled were generally infertile with low levels of total organic carbon (TOC), available nutrients and slow cycling processes with bacteria dominant microbial communities supporting the native plant species. Surface soils underneath acacia and spinifex were modified by in situ litter return, in terms of TOC, and structure and functions of microbial communities. The levels of soil microbial biomass C and N, basal respiration rate and net mineralisation rate in the acacia soil were twice as much as those in the spinifex. Microbial communities in the acacia soil had a greater fungal:bacterial ratio than in the spinifex. On this basis, growth media and root zones for revegetating native acacia-spinifex communities at local mined landscapes may be engineered by using plant organic matter remediation to supply available nutrients and to rehabilitate suitable microbial communities for in situ litter decomposition and nutrient cycling.

Effects of soil management regimes on biochemical properties of a loess soil

A long-term field experiment was conducted over a twenty year period to examine the effects of three different soil management regimes (Abandonment, Fallow and Cropping) and eight nutrient management regimes under Cropping on soil organic carbon (SOC), N and P levels, microbial biomass, and enzymatic activities related to C, N, and P cycling in a loess soil. The nutrient management regimes examined involved treatment with various combinations of inorganic nitrogen, phosphorus, and potassium fertilizers (N, NP, PK, and NPK), as well as combinations of NPK fertilizers with either residual plant material (SNPK) or manure (MN1PK and MN2PK). Abandonment resulted in greater levels of soil microbial biomass than did Cropping but similar levels of enzyme activity were observed under both regimes. The Fallow regime gave significantly lower soil organic carbon levels and enzyme activities than did Cropping. Within the Cropping system, the treatments containning nitrogen and phophorus significantly improved SOC, N and P levels and also increased microbial biomass and enzyme activity relative to the control. In general, the highest values of the tested soil parameters were observed under the M2NPK treatment. With the exception of invertase, the activity of all soil enzymes tested correlated significantly with SOC and microbial biomass. It was concluded that the use of fertilization regimes involving applying organic material in conjunction with NPK fertilizers should be encouraged in order to maintain or improve the chemical and biological properties of the tested loess soil and to thereby increase its productivity.

Organic amendment application influence soil organism abundance in saline alkali soil

A 6-month study was carried out at the China Agricultural University greenhouse to determine the effects of organic amendments (fertilizer) on the abundance and community composition of soil organisms in saline alkali soils. Treatments with two different organic amendments (cattle dung and vermicompost) were compared with treatments with a chemical fertilizer. Results showed increased soil nutrient content with both organic amendments and chemical fertilizers without significant alteration of soil pH or electrical conductivity. Compared with the chemical fertilizer treatment, organic amendments led to higher levels of soil microbial biomass, nematode abundance, and mite abundance. The increased level of soil organisms could be attributed to the increased level of soil organic matter, although no significant effect was observed on plant cultivation. Soil organism community composition varied among the different organic amendment treatments. The findings of this study suggest that the application of an organic amendment to saline alkali soil may directly or indirectly improve the growth of salt-tolerant plants by elevating the soil nutrient content and the abundance of soil organisms.

Structural and functional diversity of soil bacterial and fungal communities following woody plant encroachment in the southern Great Plains

In the southern Great Plains (USA), encroachment of grassland ecosystems by Prosopis glandulosa (honey mesquite) is widespread. Mesquite encroachment alters net primary productivity, enhances stores of C and N in plants and soil, and leads to increased levels of soil microbial biomass and activity. While mesquite’s impact on the biogeochemistry of the region is well established, it effects on soil microbial diversity and function are unknown. In this study, soils associated with four plant types (C3 perennial grasses, C4 midgrasses, C4 shortgrasses, and mesquite) from a mesquite-encroached mixed grass prairie were surveyed to in an attempt to characterize the structure, diversity, and functional capacity of their soil microbial communities. rRNA gene cloning and sequencing were used in conjunction with the GeoChip functional gene array to evaluate these potential differences. Mesquite soil supported increased bacterial and fungal diversity and harbored a distinct fungal community relative to other plant types. Despite differences in composition and diversity, few significant differences were detected with respect to the potential functional capacity of the soil microbial communities. These results may suggest that a high level of functional redundancy exists within the bacterial portion of the soil communities; however, given the bias of the GeoChip toward bacterial functional genes, potential functional differences among soil fungi could not be addressed. The results of this study illustrate the linkages shared between above- and belowground communities and demonstrate that soil microbial communities, and in particular soil fungi, may be altered by the process of woody plant encroachment.

Carbon dynamics of sodic and saline soils following gypsum and organic material additions: A laboratory incubation

Carbon fluxes in sodic and saline soils were investigated by measuring the soil microbial biomass (SMB) and soil respiration rates under controlled conditions over 12 weeks. Gypsum (10 t/ha) and organic material, as kangaroo grass (10 t/ha), were incorporated in an acidic and an alkaline saline–sodic soils. Cumulative soil respiration rates were lowest in the sodic and saline soils without amendment, while the highest rates were found in those soils that had organic material addition. The addition of gypsum decreased the cumulative respiration rates in the 0–5 cm layer compared to the addition of organic material and the addition of organic material and gypsum. Similarly, the SMB was lowest in the sodic and saline soils without amendment and highest in the soils which had organic material addition, while the effects of gypsum addition were not significant. The low levels of respiration and SMB were attributed to the low soil organic carbon (SOC) levels that result from little or no C input into the soils of these highly degraded landscapes as the high salinity and high sodicity levels have resulted in scarcity or absence of vegetation. Following the addition of organic material to the sodic and saline soils, SMB levels and respiration rates increased despite adverse soil environmental conditions. This suggests that a dormant population of salt-tolerant SMB is present in these soils, which has become adapted to such environmental conditions over time and multiplies rapidly when substrate is available.

Variations in soil microbial biomass and crop roots due to differing resource quality inputs in a tropical dryland agroecosystem

The influence of exogenous organic inputs on soil microbial biomass dynamics and crop root biomass was studied through two annual cycles in rice–barley rotation in a tropical dryland agroecosystem. The treatments involved addition of equivalent amount of N (80 kg N ha −1) through chemical fertilizer and three organic inputs at the beginning of each annual cycle: Sesbania shoot (high-quality resource, C:N 16, lignin:N 3.2, polyphenol+lignin:N 4.2), wheat straw (low-quality resource, C:N 82, lignin:N 34.8, polyphenol+lignin:N 36.8) and Sesbania+wheat straw (high-and low-quality resources combined), besides control. The decomposition rates of various inputs and crop roots were determined in field conditions by mass loss method. Sesbania (decay constant, k=0.028) decomposed much faster than wheat straw ( k=0.0025); decomposition rate of Sesbania+wheat straw was twice as fast compared to wheat straw. On average, soil microbial biomass levels were: rice period, Sesbania⩾ Sesbania+wheat straw>wheat straw⩾fertilizer; barley period, Sesbania+wheat straw> Sesbania⩾wheat straw⩾fertilizer; summer fallow, Sesbania+wheat straw> Sesbania>wheat straw⩾fertilizer. Soil microbial biomass increased through rice and barley crop periods to summer fallow; however, in Sesbania shoot application a strong peak was obtained during rice crop period. In both crops soil microbial biomass C and N decreased distinctly from seedling to grain-forming stages, and then increased to the maximum at crop maturity. Crop roots, however, showed reverse trend through the cropping period, suggesting strong competition between microbial biomass and crop roots for available nutrients. It is concluded that both resource quality and crop roots had distinct effect on soil microbial biomass and combined application of Sesbania shoot and wheat straw was most effective in sustained build up of microbial biomass through the annual cycle.

Soil microbial biomass influence on growth and biocontrol efficacy of Trichoderma harzianum

Abstract Hyphal growth and biocontrol efficacy of Trichoderma harzianum may depend on its interactions with biotic components of the soil environment. Effects of soil microbial biomass on growth and biocontrol efficacy of the green fluorescent protein transformant T. harzianum ThzID1-M3 were investigated using different levels of soil microbial biomass (153, 328, or 517 μg biomass carbon/g of dry soil). Hyphal growth of T. harzianum was significantly inhibited in soil containing 328 or 517 μg biomass carbon/g of dry soil compared with soil containing 153 μg biomass carbon/g. However, when ThzID1-M3 was added to soil as an alginate pellet formulation, recoverable populations of ThzID1-M3 varied, with the highest populations in soil containing 517 μg biomass carbon/g. When sclerotia of Sclerotinia sclerotiorum were added to soils (10 sclerotia per 150 g soil) with ThzID1-M3 (20 pellets per 150 g soil), colonization of sclerotia by ThzID1-M3 was significantly lower in the soil containing the highest level of biomass. Addition of alginate pellets of ThzID1-M3 to soils (10 pellets per 50 g) resulted in increased indigenous microbial populations (total fungi, bacteria, fluorescent Pseudomonas spp., and actinomycetes). Our results suggest that higher levels of microbial soil biomass result in increased interactions between introduced T. harzianum and soil microorganisms, and further that microbial competition in soil favors a shift from hyphal growth to sporulation in T. harzianum , potentially reducing its biocontrol efficacy.

Influence of Soil Microbial Biomass on Growth and Biocontrol Efficac of Trichoderma harzianum

The hyphal growth and biocontrol efficacy of Trichodemo harzianum in soil may depend on its interactions with biotic components of the soil environment. The effect of soil microbial biomass on growth and biocontrol efficacy of T. hanianum isolate ThzIDl-M3 (green fluorescent protein transformant) was investigated using artificially prepared different levels of soil microbial biomass (153,328, or 517ug biomass carbon per g of dry soil; BC). The hyphal growth of T. harzanum was significantly inhibited in the soil with 328 or 517 <TEX>$\mu$</TEX>g BC compared with 153 ug BC. When ThzIDl-M3 was added to the soils as an alginate pellet formulation, the recoverable population of ThzIDl-M3 varied, but the highest population occurred in 517ug BC. Addition of alginate pellets of ThzIDl-M3 to the soils (10 per 50 g) resulted in increased indigenous microbial populations (total fungi, bacterial fluorescent Pseudomonas app., and actinomycetes). Furthermore, colonizing ability of ThzIDl-M3 on sclerotia of Sclerotinia sclerotiorum was significantly reduced in the soil with high revel of BC. These results suggest that increased soil microbial biomass contributes to increased interactions between introduced T. harzianum and soil microorganisms, consequently reducing the biocontrol efficacy of 1T. harzianum.

Long-term effects of intercropping and bio-litter recycling on soil biological activity and fertility status of sub-tropical soils

On-farm field experiments were carried out at two sites having 38- and 10-year-old orchard cropping systems under sub-tropical climatic regions to evaluate changes in organic carbon accumulation and chemical and microbiological properties of the soils. Under a system of different intercropped fruit trees, the cultivation of coconut ( Cocos nucifera L.) intercropped with guava ( Psidium guajava L.) enhanced the soil microbial activity approximately 2-fold after 38 yrs over 10 yrs of the same intercropped system. Soil organic carbon increased from 3.4 to 7.8 and 2.4 to 6.2 g kg −1 after 38 and 10 yrs, respectively, following the establishment of orchards. The increase was attributed to greater recycling of bio-litters. Levels of dehydrogenase, phosphatase and soil microbial biomass under field conditions generally depended more on the nature of the cropping system than on soil types. Similarly, average carbon inputs of bio-litter to the soil in monocrop (0.98 Mg ha −1 yr −1) was less than intercropped fruit trees (2.07 Mg ha −1 yr −1). The average level of soil microbial biomass carbon was 1158 kg ha −1 (0–0.15 m depth) and the organic carbon turnover rate was 8.5 yr −1 after 38 yrs of intercropped fruit trees, which resulted in a lower ratio (1.81) of carbon inputs to soil microbial biomass carbon.

Variations in soil microbial biomass and N availability due to residue and tillage management in a dryland rice agroecosystem

Seasonal changes in the levels of soil microbial biomass C (MBC) and N (MBN), N-mineralization rate and available-N concentration were studied in rice–barley supporting tropical dryland (rainfed) agroecosystem under six combinations of tillage (conventional, minimum and zero tillage) and crop residue manipulation (retained or removed) conditions. Highest levels of soil MBC and MBN (368–503 and 38.2–59.7 μg g −1, respectively) were obtained in minimum tillage residue retained (MT+R) treatment and lowest levels (214–264 and 20.3–27.1 μg g −1, respectively) in conventional tillage residue removed (CT−R, control) treatment. Along with residue retention tillage reduction from conventional to zero increased the levels of MBC and MBN (36–82 and 29–104% over control, respectively). The proportion of MBC and MBN in soil organic C and total N contents increased significantly in all treatments compared to control. This increase (28% in case of C and 33% N) was maximum in MT+R and minimum (10% for C and N both) in minimum tillage residue removed (MT−R) treatment. In all treatments concentrations of N in microbial biomass were greater at seedling stage, thereafter these concentrations decreased drastically (21–38%) at grain-forming stage of both crops. In residue removed treatments, N-mineralization rates were maximum during the seedling stage of crops and then decreased through the crop maturity. In residue retained treatments, however, N-mineralization rates were lower than in residue removed treatments at seedling stage of both crops. At grain-forming stage in all instances the N-mineralization rates in residue retained treatments considerably exceeded the rates in corresponding residue removed treatments. Tillage reduction and residue retention both increased the proportion of organic C and total N present in soil organic matter as microbial biomass. Microbial immobilization of available-N during the early phase of crops and its pulsed release later during the period of greater N demand of crops enhanced the degree of synchronization between crop demand and N supply. The maximum enhancement effects were recorded in the minimum tillage along with residue retained treatment. In the dryland agroecosystem studied, two management practices in combination proved more advantageous than either practice alone in maintaining soil fertility levels. For soil fertility amelioration in dryland agroecosystems with least dependence upon chemical fertilizer input, post-harvest retention of about 20 cm shoot biomass (accounting for 25–40% aboveground biomass) of previous crop and its incorporation in soil through minimum tillage in the succeeding crop is suggested, especially in the case of cereal.

Effets de deux incorporations d'engrais verts sur le rendement et la nutrition en azote du blé (Triticum aestivum L.), ainsi que sur les propriétés physiques et biologiques du sol

The effects of two applications of green manures (1993 and 1995) on soil physical and biological properties, on wheat (Triticum aestivum L.) yields and N uptake were investigated in 1996 in a Le Bras loam (Humic Gleysol). The green manures as main factor were clover (Trifolium pratense L.), buckwheat (Fagapyrum esculentum L.), millet (Sorghum sudanensis L.), mustard (Brassica hirta Moench), colza (Brassica campestris L.) and a control without green manure. The sub-factors consisted of four N fertilizer rates for subsequent wheat: 0, 30, 60 and 90 kg N ha−1. Green manure application significantly increased the soil water stable aggregates (MWD), and the &gt; 0,25 mm fractions of water-stable aggregates (P &lt; 0,05). Levels of soil microbial biomass, alcaline phosphatase and urease activities, and the N mineralization potential were also significantly increased by green manure treatments compared to the control. A 200 to 300% increase in wheat yields and N uptake were obtained, depending on green manure species, compared to the control. The results of this study provide quantitative evidence that wheat yields and N uptake increases were mainly due to N addition into soil and the improvement in soil physical and biological properties by green manure application. Key words: Green manure, wheat yields and N uptake, water stable aggregates, microbial biomass, N mineralization potential, soil enzymes

Temporal variation of C and N turnover in soil after oilseed rape straw incorporation in the field: simulations with the soil-plant-atmosphere model DAISY

The Soil Organic Matter submodel of the soil-plant-atmosphere model DAISY was evaluated using data from one year field trials with and without incorporation of 8 t ha −1 rape straw into the soil. Periodic measurements of soil microbial biomass (C and N), mineral N and light particulate soil organic matter in the top 15 cm of the soil, and of soil CO 2-evolution were made. The simulation of the temporary pattern of soil microbial biomass and mineral N was improved markedly by systematic modification of the default turnover rate coefficients, the substrate utilization efficiencies and the initial levels of soil microbial biomass. Metabolic quotients ( qCO 2) turnover rates of microbial biomass and substrate utilization efficiencies derived from the parametrization of the model were evaluated against literature data. The soil microbial biomass seems to be associated with the production of temporarily protected microbial residual products. The production of these residuals might be responsible for the nitrogen immobilization observed after incorporation of rape straw. In the early stage of rape straw decomposition, measured carbon in light particulate soil organic matter seemed to be represented by one of the added organic matter pools simulated in the DAISY model. We propose that turnover rate coefficients of microbial biomass and added organic matter obtained by fitting a model to measured values may be used as a tool to characterize the physiological state of microbial populations in their natural environment.

Microbial biomass, and C and N mineralization, in litter and mineral soil of adjacent montane ecosystems in a southern beech ( Nothofagus) forest and a tussock grassland

Comparisons were made of total and microbial C and N pools and C and N metabolism in litter and mineral soil of a mountain beech ( Nothofagus solandri var. cliffortioides) forest, ca. 100 m below timberline, and an adjacent tussock grassland (dominated by Chionochloa pallens), ca. 100 m above timberline, in Canterbury, New Zealand. Mean annual precipitation at the sites is ca. 1600 mm and mean annual air temperature ca. 6°C. The silt loam soils are Andic Dystrochrepts. Total C and N and microbial C and N contents in litter and mineral soil (0–50 cm depth) differed appreciably in the two ecosystems and were 1.41, 2.00, 1.71 and 1.98 times, respectively, greater on an area basis at the grassland than at the forest site. Ratios of microbial C-to-total C, microbial N-to-total N and microbial C-to-N generally declined with profile depth. CO 2C production, per unit of total C, and metabolic quotients (qCO 2 values) tended to be greater in mineral soil from the forest than from the grassland. CO 2C production, calculated on an area basis to 50 cm depth of mineral soil, was similar in both ecosystems. Net N mineralization (during 0–56 days at 25°C) was appreciable in the forest litter, but absent in tussock litter; it was similar in both systems at 0–10 cm and 20–50 cm depths of mineral soil, but was lower in the forest than in the grassland at 10–20 cm depth. Nitrification was not detected in the litter samples and was either absent or very low in the samples of mineral soil. Results, overall, show that marked differences in soil and microbial properties can occur in adjacent, indigenous ecosystems in almost the same climatic environment. Although soil microbial biomass levels were lower in the forest than in the grassland, the potential metabolic activity of the component microorganisms tended to be greater in the forest. The relevance of these results to the turnover of organic matter in these ecosystems is briefly discussed.

Mineralization of carbon and nitrogen from cowpea leaves decomposing in soils with different levels of microbial biomass

Soils with greater levels of microbial biomass may be able to release nutrients more rapidly from applied plant material. We tested the hypothesis that the indigenous soil microbial biomass affects the rate of decomposition of added green manure. Cowpea (Vigna unguiculata L.) Walp.] leaves were added to four soils with widely differing microbial biomass C levels. C and N mineralization of the added plant material was followed during incubation at 30°C for 60 days. Low levels of soil microbial biomass resulted in an initially slower rate of decomposition of soil-incorporated green manure. The microbial biomass appeared to adjust rapidly to the new substrate, so that at 60 days of incubation the cumulative C loss and net N mineralization from decomposing cowpea leaves were not significantly affected by the level of the indigenous soil microbial biomass.

Seasonal changes in microbial biomass in soils cropped with palmarosa (Cymbopogon martinii L.) and Japanese mint (Mentha arvensis L.) in subtropical India

Changes in microbial C, N, and P were investigated for 1 year in two soils with similar physicochemical properties but supporting different crops under subtropical conditions. One was cropped with palmarosa (Cymbopogon martinii L.) and the other with Japanese mint (Mentha arvensis L.). Both the season and the type of cropping had a significant influence on changes in the soil microbial biomass. In general, soil microbial biomass C, N, and P were highest in summer months and lowest in midwinter. Soil microbial biomass levels and microbial C:N and C:P ratios were higher and N:P ratios lower under palmarosa soil than under mint.