农业土壤中频繁施用有机质提高土壤抑菌作用

Soil-borne plant pathogens are among the most important limiting factors for the productivity of agro-ecosystems.Fungistasis is the natural capability of soils to inhibit the germination and growth of soil-borne fungi in the presence of optimal abiotic conditions.The objective of this study was to assess the effects of different soil managements,in terms of soil amendment types and frequency of application,on fungistasis.For this purpose,a microcosm experiment was performed by conditioning a soil with frequent applications of organic matter with contrasting biochemical quality (i.e.,glucose,alfalfa straw and wheat straw).Thereafter,the fungistasis response was assessed on four fungi (Aspergillus niger,Botrytis cinerea,Pyrenochaeta lycopersici and Trichoderma harzianum).Conditioned soils were characterized by measuring microbial activity (soil respiration) and functional diversity using the BIOLOG EcoPlatesTM method.Results showed that irrespective of the fungal species and amendment types,frequent applications of organic matter reduced fungistasis relief and shortened the time required for fungistasis restoration.The frequent addition of easily decomposable organic compounds enhanced soil respiration and its specific catabolic capabilities.This study demonstrated that frequent applications of organic matter affected soil fungistasis likely as a result of higher microbial activity and functional diversity.     

Effects of soil management history on the rate of organic matter decomposition

In a sustainable agriculture farming systems experiment, soils managed under organic farming practices had greater microbial abundance and activity, and higher numbers of bacterial-feeding nematodes during crop growth, than those managed under conventional farming practices. We tested rates of organic matter decomposition in the two soils and monitored the abundance and activity of soil biota during the decomposition process. Differences in soil biology between soils from organic and conventional farming systems did not persist when soils were amended with organic matter and maintained under similar conditions. Microbial communities in soil from the conventional system were sufficient and active enough to respond to organic inputs. There were minimal differences in the ability of the microbial communities of the two soils to decompose organic residues. However, when soils were removed from the field at different times, cover crop decomposition rates were more consistent in the organic soils, suggesting a greater abundance and diversity of the microbial community in those soils. Microbial activity was most suppressed when field soils were dry but responded to organic matter amendment very rapidly when favorable moisture contents were restored. The pattern of microbial activity in both organic and conventional soils following organic matter incorporation consisted of a 100 h activity phase and then a gradual decline to a relatively constant stasis phase.     

不同土地利用方式下土壤微生物群落水平生理图谱(CLP)与土壤碳、氮有效性之间的相互关系

The goal of this work was to assess soil microbial respiration, determined by the assay of community-level physiological profiling in an oxygen-sensitive microplate (O₂-CLPP), in response to endogenous C and several individual C substrates in the soils with different organic C contents (as a function of soil type and management practice). We also used the O₂-CLPP to determine the respiratory response of these soils to endogenous C and amended C substrates with N addition. A respiratory quotient (RQ) was calculated based on the ratio of the response to endogenous soil C vs. each C-only substrate, and was related to total organic carbon (TOC). For assessing N availability for microbial activity, the effect of N supplementation on soil respiration, expressed as N_ratio, was calculated based on the response of several substrates to N addition relative to the response without N. Soils clustered in 4 groups after a principal component analysis (PCA), based on TOC and their respiratory responses to substrates and endogenous C. These groups reflected differences among soils in their geographic origin, land use and C content. Calculated RQ values were significantly lower in natural forest soils than in managed soils for most C-only substrates. TOC was negatively correlated with RQ (r = - 0.65), indicating that the soils with higher organic matter content increased respiratory efficiency. The N addition in the assay in the absence of C amendment (i.e., only endogenous soil C present) had no effect on microbial respiration in any soil, indicating that these soils were not intrinsically N-limited, but substrate-dependent variation in N_ratio within soil groups was observed.     

黄河三角洲退化湿地微生物群落特性研究

Five different sites with a soluble salt gradient of 3.0--17.7 g kg-1 dry soil from the coast to the inland were selected, and the microbial population size, activity and diversity in the rhizospheres of five common plant species and the adjacent bulk soils (non-rhizosphere) were compared in a degraded wetland of the Yellow River Delta, Shandong Province, China to study the effects of soil environment (salinity, seasonality, depth, and rhizosphere) on microbial communities and the wetland’s ecological function, thus providing basic data for the bioremediation of degraded wetlands. There was a significant negative linear relationship between the salinity and the total number of microorganisms, overall microbial activity, or culturable microbial diversity. Salinity adversely affected the microbial community, and higher salinity levels resulted in smaller and less active microbial communities. Seasonal changes were observed in microbial activity but did not occur in the size and diversity. The microbial size, activity and diversity decreased with increasing soil depth. The size, activity and diversity of culturable microorganisms increased in the rhizospheres. All rhizospheres had positive effects on the microbial communities, and common seepweed had the highest rhizosphere effect. Three halophilic bacteria (Pseudomonas mendocina, Burkholderia glumae, and Acinetobacter johnsonii) were separated through BIOLOG identification, and common seepweed could be recommended for bioremediation of degraded wetlands in the Yellow River Delta.     

Short-term effects of defoliation on the soil microbial community associated with two contrasting Lolium perenne cultivars

Intra-species variation in response to defoliation and soil amendment has been largely neglected in terms of the soil microbial community (SMC). The influence of defoliation and soil fertiliser amendment on the structure of the SMC was assessed with two Lolium perenne cultivars contrasting in ability to accumulate storage reserves. Plant response to defoliation was cultivar specific and depended on the nutrient amendment of the soil. Results suggested a greater ability to alter plant biomass allocation in the low carbohydrate accumulating cultivar (S23) compared to the high carbohydrate cultivar (AberDove) when grown in improved (IMP), but not in unimproved (UNI), soil. Although differences in plant growth parameters were evident, no treatment effects were detected in the size of the active microbial biomass (total phospholipid fatty acid (PLFA) 313.8 nmol g⁻¹ soil±33.9) or proportions of PLFA signature groups. A lower average well colour development (AWCD) of Biolog sole carbon source utilisation profiles (SCSUPs) in defoliated (D) compared to non-defoliated (ND) treatments may be indicative of lower root exudation 1 week following defoliation, as a consequence of lower root non-structural carbohydrate (NSC) concentrations. Within the bacterial community the lower cyclopropyl-to-precursor ratio of PLFAs, and the trans/cis ratio of 16:1w7, in UNI relative to IMP soil treatments indicates lower physiological stress in UNI soils regardless of L. perenne cultivar. Discrimination of broad scale SMC structure, measured by PLFA analysis, revealed that soil treatment interacted strongly with cultivar and defoliation. In IMP soils the SMCs discriminated between cultivars while defoliation had little effect. Conversely, in UNI soils defoliation caused a common shift in the SMC associated with both cultivars, causing convergence of overall community structure. Separation of SMC structure along the primary canonical axis correlated most strongly (P<0.001) with root:shoot ratio (47.6%), confirming that differences in cultivar C-partitioning between treatments were influential in defining the rhizosphere microbial community.     

Effects of Ant Mounds on the Plant and Soil Microbial Community in an Alpine Meadow of Qinghai–Tibet Plateau

Ants are important soil engineers, affecting the structure and function of ecosystems. To address the impacts of ants (Camponotus herculeanus ) on the properties of an alpine meadow ecosystem of Qinghai–Tibet Plateau, we investigated the effects of ant mounds on plant biomass, soil physicochemical properties, microbial diversity, and functions. We found that the total biomass of plant community was significantly greater in ant mound periphery. Plant species richness in ant mounds was reduced compared with that of control plots without ant mounds. Significant changes in physicochemical properties of soil were also observed. Soil organic matter, total nitrogen, available phosphorous, total potassium, and available potassium increased in ant mound soil due to the excavation activities by ants as well as the accumulation of organic matter and other nutrients during mound construction. For example, roots/soil contents (g/g) and soil moisture in ant mound soils were lower than those in controls. Microbial community composition and microbial biomass were clearly changed in ant mound soils. BIOLOG analysis further indicated that the functional diversity of the microbial community of ant mound soil increased and differed from that of controls. This study indicates that ant‐induced modification of soil properties indirectly influences plant biomass and species composition, and ant mounds have different microbial communities from those of control soil. Copyright © 2016 John Wiley & Sons, Ltd.     

不同植物秸秆腐解特性与土壤微生物功能多样性研究

采用网袋法探讨不同秸秆在3个长期试验地的腐解特征,结合Biolog微平板技术,对不同长期试验地土壤微生物群落多样性进行了研究。结果表明,随着腐解时间的增加,秸秆腐解的变化趋势为烘干秸秆新鲜秸秆。葡萄园土壤微生物活性高,稳定性好,其次为桃园和农田。农田土壤微生物活性低、稳定性差,不同处理的秸秆在腐解过程中残留率变化较大,而果园土壤微生物活性相对较高、稳定性好,不同处理的秸秆在腐解过程中残留率变化较小。土壤碱解氮、速效磷和土壤温度与土壤微生物群落的三大指数呈极显著相关,且不同秸秆处理的腐解残留率与土壤微生物群落的优势度呈显著负相关,微生物群落在一定程度上影响了秸秆分解的速率。     

Decomposition and distribution of residual activity of some 13C-microbial polysaccharides and cells,glucose, cellulose and wheat straw in soil

Studies were made to determine the rate of decomposition of some ¹⁴C-labeled microbial polysaccharides, microbial cells, glucose, cellulose and wheat straw in soil, the distribution of the residual ¹⁴C in various humic fractions and the influence of the microbial products on the decomposition of plant residues in soil. During 16 weeks from 32 to 86 per cent of the C of added bacterial polysaccharides had evolved as ¹⁴CO₂. Chromobacterium violaceum polysaccharide was most resistant and Leuconostoc dextranicus polysaccharide least resistant. In general the polysaccharides, microbial cells, and glucose exerted little effect on the decomposition of the plant products. Upon incubation the ¹⁴C-activity was quickly distributed in the humic. fulvic and extracted soil fractions. The pattern of distribution depended upon the amendment and the degree of decomposition. The distribution was most uniform in the highly decomposed amendments. After 16 weeks the bulk of the residual activity from Azotobacter indicus polysaccharide remained in the NaOH extracted soil. From C. violaceum polysaccharide both the extracted soil and the humic acid fraction contained high activity. About 50–80 per cent of the residual activity from the ¹⁴C-glucose, cellulose and wheat straw amended soils could be removed by hydrolysis with 6 n HCl. The greater part of this activity in the humic acid fraction was associated with the amino acids and that from the fulvic acids and residual soils after NaOH extraction with the carbohydrates. About 8 16 per cent of the activity of the humic acid fraction was present in substances (probably aromatic) extracted by ether after reductive or oxidative degradation.     

Microbial function in adjacent subtropical forest and agricultural soil

Soil microbial communities and their activities are altered by land use change; however impacts and extent of these alterations are often unclear. We investigated the functional responses of soil microbes in agricultural soil under sugarcane and corresponding native soil under Eucalyptus forest to additions of contrasting plant litter derived from soybean, sugarcane and Eucalyptus in a microcosm system, using a suite of complimentary techniques including enzyme assays and community level physiological profiles (CLPP). Initially agricultural soil had 50% less microbial biomass and lower enzyme activities than forest soil, but significantly higher nitrification rates. In response to litter addition, microbial biomass increased up to 11-fold in agricultural soil, but only 1.8-fold in forest soil, suggesting a prevalence of rapidly proliferating ‘r’ and slower growing ‘K’ strategists in the respective soils. Litter-driven change in microbial biomass and activities were short lived, largely returning to pre-litter addition levels by day 150. Decomposition rates of sugarcane and soybean litter as estimated via CO₂ production were lower in agricultural than in forest soil, but decomposition of more recalcitrant Eucalyptus litter was similar in both soils, contradicting the notion that microbial communities specialise in decomposing litter of the dominant local plant species. Enzyme activities and community level physiological profiles (CLPP) were closely correlated to microbial biomass and overall CO₂ production in the agricultural soil but not the forest soil, suggesting contrasting relationships between microbial population dynamics and activity in the two soils. Activities of enzymes that break down complex biopolymers, such as protease, cellulase and phenol oxidase were similar or higher in the agricultural soil, which suggests that the production of extracellular biopolymer-degrading enzymes was not a factor limiting litter decomposition. Enzyme and CLPP analyses produced contrasting profiles of microbial activity in the two soils; however the combination of both analyses offers additional insights into the changes in microbial function and community dynamics that occur after conversion of forest to agricultural land.     

Effect of biochar application on mineral and microbial properties of soils growing different plant species

Biochar is widely used as a soil amendment to increase crop yields. However, the details of its impact on soil properties have not been fully understood. A pot experiment was conducted using soybean (Glycine max (L.) Merr. cv. Toyoharuka) and sorghum (Sorghum bicolor (L.) Moench cv. Hybrid Sorgo) under four soil treatment combinations (cattle farmyard manure with or without biochar and rapeseed cake with or without biochar) to elucidate the mechanisms of its beneficial effects on plant growth in terms of the microbial community structure and mineral availability in soils with different types of organic manure application. The application of biochar significantly increased the growth of both species, particularly sorghum with rapeseed cake application by 1.48 times higher than that without biochar. Microbial activity in soil was also enhanced by biochar application in both species with rapeseed cake application, particularly in sorghum. Principal component analysis using Biolog EcoPlate^TM data indicated that biochar application changed the microbial community structure in soil, particularly sorghum-grown soil. The changes in microbial community structure in sorghum were considered to be at least partly affected by changes in soil pH due to interaction between plant and biochar under organic manure application. Biochar application had little effect on the profile of ammonium-acetate-extractable mineral elements in soil including calcium, potassium, magnesium, sodium and sulfur with both types of manure application under soybean. Under sorghum, however, biochar with rapeseed cake manure application altered the profile. This alteration is attributable to an increase in the extractable concentration of certain metals in the soil including aluminum, cadmium and zinc, possibly caused by enhanced organic matter decomposition producing metal-chelating organic compounds. These different changes in the soil properties by biochar application may be directly or indirectly related to the different growth responses of different plant species to biochar application under organic manure application.     

Effects of straw and its biochar applications on the abundance and community structure of CO2-fixing bacteria in a sandy agricultural soil

Purpose

Crop straw and biochar application can potentially increase carbon sequestration and lead to changes in the microbial community in agricultural soils. Sequestration of CO₂ by autotrophic microorganisms is key to biogeochemical carbon cycling in soil ecosystems. The effects of straw and its biochar, derived from slow pyrolysis, on CO₂ fixation bacteria in sandy soils, remain unclear. Therefore, this study compared the response of abundance and community of CO₂ fixation bacteria to the two straw application methods in a sandy agricultural soil. The overall aim of the study was to achieve an efficient use of straw residues for the soil sustainablility.

Materials and methods

We investigated the soil organic carbon content and autotrophic bacteria over four consecutive years (2014–2018) in a field experiment, including the following four treatments: whole maize straw amendment (S), whole maize straw translated biochar amendment (B), half biochar and half straw amendment (BS), and control (CK) without straw or biochar amendment. The autotrophic bacterial abundance and community structure were measured using molecular methods of real-time PCR, terminal restriction fragment length polymorphisms (T-RFLP), and a clone library targeting the large subunit gene (cbbL) of ribulose-1,5-bisphosphate carboxylase/oxygenase.

Results and discussion

The results showed that the content of soil total organic carbon (TOC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC) in B, S, and BS treatments was significantly increased compared with the CK treatment. Soil TOC and available potassium (AK) in the B treatment significantly increased by 15.4% and 23.3%, respectively, but soil bulk density, DOC, and MBC significantly decreased by 8.5%, 10.6%, and 14.5%, respectively, compared with the S treatment. The abundance of the cbbL gene as well as of the bacterial 16S rRNA gene increased significantly in straw or biochar application treatments as compared to the CK treatment. The B treatment, but not the BS treatment, significantly increased the cbbL gene abundance when compared to the S treatment. No significant differences were observed in the bacterial 16S rRNA gene abundance among the three straw or biochar applications. The application of straw biochar could increase the diversity of the autotrophic bacteria, which also altered the overall microbial composition. Physicochemical properties of the soil, such as soil pH, SOC, and bulk density, can help explain the shift in soil microbial composition observed in the study.

Conclusions

Taken together, our results suggest that straw biochar, rather than straw application, leads to an increase in the abundance and diversity of CO₂-fixing bacteria, which would be advantageous for soil autotrophic CO₂ fixation.

     

Short-term effects of organic municipal wastes on wheat yield,microbial biomass,microbial activity,and chemical properties of soil

The objectives of this work were to (a) investigate the short-term effects of applications of mineral fertilizer, municipal solid waste (MSW) compost, and two sewage sludges (SSs) subjected to different treatments (composting and thermal drying) on microbial biomass and activity of soil by measuring microbial biomass C, adenosine 5′-triphosphate content, basal respiration, and dehydrogenase, catalase, urease, phosphatase, β-glucosidase, and N-α-benzoyl-l-argininamide-hydrolyzing activities and (b) explore the relationships between soil microbiological, biochemical, and chemical properties and wheat yields under semiarid field conditions by principal component analysis. The additions of MSW compost, SS compost, and thermally dried SS did not affect significantly soil microbial biomass, as compared to mineral fertilization and no amendment. However, microbial activity increased in organically amended soils, probably due to the stimulating effect of the added decomposing organic matter. Changes in soil microbiological and biochemical properties showed no significant relationships with wheat yields, probably because plant growth was primarily water-limited, as typically occurs in semiarid regions.     

Pasture and forest soil microbial communities show distinct patterns in their catabolic respiration responses at a landscape scale

Catabolic responses to specific substrates can be used to differentiate soil microbial communities. We hypothesized that the catabolic respiration responses of microbial communities from pastures would differ from those of forest soils, and that the differences would be consistent at a landscape scale, due to inherent differences in litter quality and management regimes. We analysed respiration responses to 25 different substrates of 20 pasture soils (dominated by rye grass and white clover) and 20 forest soils (indigenous forest species or the plantation species Pinus radiata) over a wide geographical range in New Zealand.Within each pasture or forest category, the catabolic responses showed a similar pattern, suggesting similarities in functional catabolic capability and microbial community Indigenous forests and pine forests microbial communities did not differ in their responses. Pasture soil communities had significantly higher relative responses to carbohydrate and amino-acid substrates and significantly lower relative response to carboxylic acid substrates, than microbial communities from forest soils. Forest soils had relatively greater responses to carboxylic acids as a group, as well as citric acid, α-ketobutyric acid, α-ketoglutaric acid, and α-ketovaleric acid, than did the pasture soils. A subset of 6 substrates was equally as effective at differentiating the microbial catabolic response of pasture soils from forest soils as the entire set of 25 substrates. The results demonstrated distinct differences in the respiration responses of the soil microbial communities of pastures and forests, but showed strong similarities within each vegetation class, despite the wide geographical spread, different soils and plant species.     

Effects of plant species on microbial biomass phosphorus and phosphatase activity in a range of grassland soils

Soil P transformations are primarily mediated by plant root and soil microbial activity. A short-term (40 weeks) glasshouse experiment with 15 grassland soils collected from around New Zealand was conducted to examine the impacts of ryegrass (Lolium perenne) and radiata pine (Pinus radiata) on soil microbial properties and microbiological processes involved in P dynamics. Results showed that the effect of plant species on soil microbial parameters varied greatly with soil type. Concentrations of microbial biomass C and soil respiration were significantly greater in six out of 15 soils under radiata pine compared with ryegrass, while there were no significant effects of plant species on these parameters in the remaining soils. However, microbial biomass P (MBP) was significantly lower in six soils under radiata pine, while there were no significant effects of plant species on MBP in the remaining soils. The latter indicated that P was released from the microbial biomass in response to greater P demand by radiata pine. Levels of water soluble organic C were significantly greater in most soils under radiata pine, compared with ryegrass, which suggested that greater root exudation might have occurred under radiata pine. Activities of acid and alkaline phosphatase and phosphodiesterase were generally lower in most soils under radiata pine, compared with ryegrass. The findings of this study indicate that root exudation plays an important role in increased soil microbial activities, solubility of organic P and mineralization of organic P in soils under radiata pine.     

Temperature sensitivity of soil organic matter decomposition in southern and northern areas of the boreal forest zone

Much effort has been made to improve understanding of factors controlling the temperature dependence of soil organic matter (SOM) decomposition. The question of how soils formed in different geographical locations and conditions respond to temperature changes is still open. In addition to climate, residence times of soil organic matter are controlled by its decomposability and microbial community. In this work we hypothesized that the decomposition of SOM is adapted to the prevailing SOM quality and climatic conditions. This should result in different temperature vs. decomposition curves for northern and southern soils. We studied short-term temperature dependence of SOM decomposition near the northern and southern borders of the boreal forest zone using a Gaussian model. As carbon mineralization rate is driven by microbial activity, we focused on organic carbon fractions available to microbes and the size, composition and functioning of microbial communities in the soil. Despite differences in microbial community structure and behavior, similar amounts and qualities of the microbially available carbon led to similar temperature dependences of carbon mineralization in the north and south. The overall soil respiration rate level was higher in spruce forest sites than in pine forest sites irrespective of climate conditions. Our results do not mean that there is no risk of carbon losses from northern soils due to warming climate conditions. As temperature sensitivity of the decomposition increases with decreasing temperature regime, the proportional increase in the decomposition rate in northern latitudes might lead to significant carbon losses from the soils.     

Arbuscular mycorrhizal fungus enhances crop yield and P-uptake of maize (Zea mays L.): A field case study on a sandy loam soil as affected by long-term P-deficiency fertilization

The P efficiency, crop yield, and response of maize to arbuscular mycorrhizal fungus (AMF) Glomus caledonium were tested in an experimental field with long-term (18-year) fertilizer management. The experiment included five fertilizer treatments: organic amendment (OA), half organic amendment plus half mineral fertilizer (1/2 OM), mineral fertilizer NPK, mineral fertilizer NK, and the control (without fertilization). AMF inoculation responsiveness (MIRs) of plant growth and P-uptake of maize were estimated by comparing plants grown in unsterilized soil inoculated with G. caledonium and in untreated soil containing indigenous AMF. Soil total P, available P, microbial biomass P, alkaline phosphatase activity, plant biomass, crop yield and total P-uptake of maize were all significantly increased (P < 0.05) by the application of OA, 1/2 OM, and NPK, but not by the application of NK. Specifically, the individual crop yield of maize approached zero in the NK-fertilized soils, as well as in the control soils. All maize plants were colonized by indigenous AMF, and the root colonization at harvest time was not significantly influenced by fertilization. G. caledonium inoculation increased mycorrhizal colonization significantly (P < 0.05) only with the NK treatment, and produced low but demiurgic crop yield in the control and NK-fertilized soils. Compared to the inoculation in balanced-fertilized soils, G. caledonium inoculation in either the NK-fertilized soils or the control soils had significantly greater (P < 0.05) impacts on soil alkaline phosphatase activity, stem length, plant biomass, and total P-uptake of maize, indicating that AMF inoculation was likely more efficient in extremely P-limited soils. These results also showed that balanced mineral fertilizers and organic amendments did not differ significantly in their effects on MIRs in these soils.     

Examining N-limited soil microbial activity using community-level physiological profiling based on O2 consumption

Nitrogen-limited soil microbial activity has important implications for soil carbon storage and nutrient availability, but previous methods for assessing resource limitation have been restricted, due to enrichment criteria (i.e., long incubation periods, high substrate amendments) and/or logistical constraints (e.g. use of radioisotopes). A microtiter-based assay of basal and substrate induced soil respiration based on O₂ consumption may be a rapid, ecologically relevant means of assessing N limitation. The present study evaluated this approach by examining 1) the extent and duration of N limitation on soil respiratory activity following different levels of N fertilization in the field, and 2) the relationship between N-limited activities and growth under the assay conditions. Fertilization rate and the time since fertilization had significant impacts on the degree of N limitation of soil microbial activity. The highest fertilization rate showed the earliest and most persistent reduction in N limitation, as would be predicted from the higher concentration of extractable inorganic soil N observed with this treatment. Bacterial growth under the assay conditions, as estimated by quantitative-PCR of 16S rRNA genes, was less than twofold in soils demonstrating a rapid respiratory response (i.e. peak within 6–8 h of initiating incubation) to up to fourfold in soils demonstrating a slower respiratory response (i.e., peak response after ∼14 h of incubation). Increased respiratory response with N amendment was usually associated with increased cell growth, although for rapidly responding soils some C sources showed N-limited use without growth. This was likely due to exhaustion of the relatively low levels of available C amendment before growth was detected. The method appears useful for assessing N-limited microbial growth, and it may be effective as a rapid indicator of bioavailable soil N. It may also be a tool to evaluate the complexity of N limitation among various metabolic pathways found in soil microbial communities, particularly if linked to dynamics in community structure and gene activation.     

Soil structure and microbial activity dynamics in 20‐month field‐incubated organic‐amended soils

Soil structure formation is essential to all soil ecosystem functions and services. This study aims to quantify changes in soil structure and microbial activity during and after field incubation and examine the effect of carbon, organic amendment and clay on aggregate characteristics. Five soils dominated by illites, one kaolinitic soil and one smectitic soil were sieved to 2 mm, and each soil was divided into two parts and one part amended with ground rape shoots (7.5 t ha^?1) as an organic amendment. Samples were incubated in the field for 20 months with periodic sampling to measure water‐dispersible clay (WDC) and fluorescein diacetate activity (FDA). After incubation, WDC and FDA were measured on air‐dried 1–2‐mm aggregates. Tensile strength was measured on four aggregate classes (1–2, 1–4, 4–8 and 8–16 mm) and results used to assess soil friability and workability. Intact cores were also sampled to determine compressive strength. During incubation, the amount of WDC depended on soil carbon content while the trends correlated with moisture content. Organic amendment only yielded modest decreases (mean of 14% across all sampling times and soils) in WDC, but it was sufficient to stimulate the microbial community (65–100% increase in FDA). Incubation led to significant macroaggregate formation (>2 mm) for all soils. Friability and strength of newly‐formed aggregates were negatively correlated with clay content and carbon content, respectively. Soil workability was best for the kaolinite‐rich soil and poorest for the smectite‐rich soil; for illitic soils, workability increased with increasing organic carbon content. Organic amendment decreased the compression susceptibility of intact, incubated samples at smaller stress values (<200 kPa).     

The influence of two agricultural biostimulants on nitrogen transformations, microbial activity, and plant growth in soil microcosms

The influence of two experimental soil treatments, Z93 and W91, on nitrogen transformations, microbial activity and plant growth was investigated in soil microcosms. These compounds are commercially marketed fermentation products (Agspectrum) that are sold to be added to field soils in small amounts to promote nitrogen and other nutrient uptake by crops in USA. In laboratory microcosm experiments, soils were amended with finely ground alfalfa-leaves or wheat straw, or left unamended, in an attempt to alter patterns of soil nitrogen mineralization and immobilization. Soils were treated in the microcosms with Z93 and W91 at rates equivalent to the recommended field application rates, that range from 0.2 to 1.1 l ha⁻¹, (0.005-0.03 μl g⁻¹ soil). We measured their effects on soil microbial activity (substrate-induced respiration (SIR), dehydrogenase activity (DHA) and acid phosphatase activity (PHOS)), soil nitrogen pools (microbial biomass N, mineral N, dissolved organic N), and transformations (net N mineralization and nitrification, ¹⁵N dilution of the mineral N pool, and accumulation of mineral N on ion-exchange resins), and on wheat plant germination and growth (shoot and root biomass, shoot length, N uptake and ¹⁵N enrichment of shoot tissues), for up to 56 days after treatment. To follow the movement of nitrogen from inorganic fertilizer into plant biomass we used a ¹⁵N isotopic tracer. Most of the soil and plant responses to treatment with Z93 or W91 differed according to the type of organic amendment that was used. Soil treatment with either Z93 or W91 influenced phosphatase activity strongly but did not have much effect on SIR or DHA. Both chemicals altered the rates of decomposition and mineralization of organic materials in the soil, which was evidenced by significant increases in the rates of the decomposition of buried wheat straw, and by the acceleration of net, rates of N mineralization, relative to those of the controls. Soil nitrate availability increased at the end of the experiment in response to both chemical treatments. In alfalfa-amended soils, the final plant biomass was decreased significantly by treatment with W91. Increased plant growth and N-use efficiency in straw-amended soil, resulting from treatments with Z93 or W91, was linked to increased rates of N mineralization from indigenous soil organic materials. This supports the marketing of these compounds as promoters of N uptake at these low dosage inputs.     

Spring dynamics of soil carbon, nitrogen, and microbial activity in earthworm middens in a no-till cornfield

Earthworm activity may be an important cause of spatial and temporal heterogeneity of soil properties in agroecosystems. Structures known as “earthworm middens,” formed at the soil surface by the feeding and casting activities of some earthworms, may contribute significantly to this heterogeneity. We compared the temporal dynamics of carbon (C), nitrogen (N), and microbial acitivity in Lumbricus terrestris middens and in surrounding non-midden (bulk) soil during the spring, when seasonal earthworm activity was high. We sampled soil from middens and bulk soil in a no-till cornfield on four dates during May and June 1995. Soil water content and the weight of coarse organic litter (>2mm) were consistently higher in middens than in bulk soil. Total C and N concentrations, C:N ratios, and microbial activity also were greatest in midden soil. Concentrations of ammonium-nitrogen and dissolved organic N were greater in middens than in bulk soil on most dates, suggesting accelerated decomposition and mineralization in middens. However, concentrations of nitrate were usually lower in middens, indicating reduced nitrification or increased leaching and denitrification losses from middens, relative to bulk soil. Fungal activity, as well as total microbial activity, was consistently greater in middens. The contribution of fungae to overall microbial activity differed significantly between middens and bulk soil only on one date when both soils were very dry; the contribution of fungae to microbial activity was lower in the middens on this date. We conclude that the midden-forming activity of L. terrestris can be a major determinant of spatial heterogeneity in some agricultural soils, and that this can potentially affect overall rates of soil processes such as organic matter decomposition, N mineralization, denitrification, and leaching. Received: 4 April 1997