Ryegrass straw component decomposition during mesophilic and thermophilic incubations

The decomposition of perennial ryegrass straw was examined under mesophilic and thermophilic temperatures. Thermophilic conditions were used to define the composting process. The change in lipids, sugars, soluble polysaccharides, cellulose, and lignin was determined during a 45-day incubation. C, H, O, and N steadily decreased in both temperature treatments. The lignin content, as measured by the Klason or 72% H₂SO₄ method, decreased by 10% under mesophilic and 29% under thermophilic conditions. The Klason lignin C loss was 25 and 39% under mesophilic and thermophilic incubations, respectively. The changes in element (C, N, H, and O) ratios indicated that 94% of the lignin fraction was altered during both low- and high-temperature incubations. The changes in the lignin-like fraction as shown by elemental ratios were more extensive than those indicated by the Klason method, showing that this lignin determination has limited value in describing plant residue decomposition. The decomposition of the straw components and the concomitant degradation of the lignin fraction represent an important decomposition process that facilitates the composting of ryegrass straw with a high C:N ratio.     

Biodegradation of sunflower hulls with different nitrogen sources under mesophilic and thermophilic incubations

The decomposition of sunflower hulls and mixtures of sunflower hulls and other crop residues was examined under mesophilic (25°C) and thermophilic (50°C) temperatures during a 45-day incubation. Four treatments were set up: sunflower hull, sunflower hull+5% alfalfa, sunflower hull+5% vetch, sunflower hull+0.1% urea, to asses the efficiency of composting. Changes in total organic C (TOC), oxidizable C, N, pH, electrical conductivity (EC), and different fibre fractions—cellulose, hemicellulose and lignin—were determined. TOC decreased by 13–21% at 25°C and 25–41% at 50°C and the largest loss of C was for hulls amended with vetch residues and urea. Loss of oxidizable C was not affected by either the temperature or the treatments. The largest loss of cellulose occurred at 50°C in all the treatments. The hemicellulose content of the sunflower hulls alone and in the treatment with urea was significantly different with respect to the other treatments, whereas lignin content showed either a small increase (1.6% in sunflower hulls amended with alfalfa at 50°C and 1.8% in sunflower hulls with urea at 25°C) or a decrease of between 2.5% and 19% in the other treatments. The C/N ratio never fell below 50:1 and the highest decrease was for the thermophilic incubation. Increases in EC and pH values accounted for 3 and 1.5 units, respectively, and occurred after a 45-day incubation for mesophilic and thermophilic conditions. In general the incubation at 50°C facilitated the decomposition of sunflower hulls with high C/N ratios and little N addition.     

Microbial carbon dynamics in nitrogen amended Arctic tundra soil: Measurement and model testing

We examined the responses of grazers (protozoa and nematodes) and their main food sources to low levels of nitrogen (N) fertilisation and applied carbon (C) flux models to our data. Replicate plots of tundra soil adjacent to the Kongsfjorden (Svalbard 78°N) were amended with ammonium and nitrate at concentrations of 1 and 5 kg N ha⁻¹ to assess the impact of anthropogenic N deposition over three summers. Bacterial abundance as determined using the fluorochrome SYBR Green and epifluorescence microscopy ranged between 9.73×10⁸ and 102.49×10⁸ cells/g dry wt of soil, with a significant response to N addition occurring only during the second sampling in 2001. Despite little change in bacterial biomass, bacterial production (measured by the incorporation of ³H thymidine into DNA) during the second sampling in 2002, increased in NH₄ enriched plots compared to control and NO₃ amended plots, indicating that NH₄ was the preferred source of inorganic N. The main bacterial predators were heterotrophic flagellates (HNAN) and naked amoebae, which showed no significant response to the N addition. HNAN showed a correlation with bacterial abundance suggesting a dependence on bacteria as a food source. The inability of a microbial C flux model to fit our data (RWSS/data=18.6, r²=0.088) was at least partly due to insufficient bacterial production to meet the C demands of predator taxa, and high variability in the data over time. This is reflected in the performance statistics for model variants where select microbial taxa and data were removed. The optimal model in terms of predictive utility was a model with data from 2002 only, minus naked amoebae (RWSS/data=2.45, r²=0.806).     

Microbial immobilisation and turnover of N labelled substrates in two arable soils under field and laboratory conditions

Microbial biomass N dynamics were studied under field and laboratory conditions in soils of high yield (HY) and low yield (LY) areas in an agricultural field. The objective of the study was to determine the size and activity of soil microbial biomass in the soils of the different yield areas and to compare these data obtained under field and laboratory conditions. Soils were amended with ¹⁵N labelled mustard (Sinapis alba) residues (both experiments) and labelled nitrate (laboratory only) at 30 μg N g⁻¹ dry soil. Soil microbial biomass (SMB) N, mineral N (N_min) and total N content was monitored both in the field and in the laboratory. N₂O efflux was additionally measured in laboratory treatments. Isotope ratios were determined for SMB in both experiments, for all other parameters only in the laboratory treatments. In the laboratory less amounts of added substrate N were immobilised by the SMB in HY soils compared to LY soils, whereas in the field immobilisation of added N by SMB was higher in HY soils initially and slightly lower after 40 days of incubation. Calculated turnover times in the laboratory nitrate, laboratory mustard and field mustard amendments were 0.18, 0.27 and 0.74 years (HY) and 0.22, 0.61 and 1.01 years (LY), respectively. The turnover times of added substrate N always showed the trend to be faster in HY soils compared to LY soils. A faster turnover of nutrients in the HY soils may involve a better nutrient supply of the plants, which coincides with the higher agricultural yield observed in these areas.     

Microbial activity and nutrient dynamics in earthworm casts (Lumbricidae)

Summary Microbial respiration, microbial biomass and nutrient requirements of the microflora (C, N, P) were studied in the food substrate (soil taken from the upper 3 cm of the mineral soil of a beech wood on limestone), the burrow walls and the casts of the earthworm Aporrectodea caliginosa (Savigny). The passage of the soil through the gut caused an increase in soil microbial respiration of about 90% over a 4-week period. Microbial biomass was increased only in freshly deposited casts and decreased in aging faeces to a level about 10% lower than in soil. Microbial respiration of the burrow walls was only increased over a shorter period (about 2 weeks). The microflora of the soil and the burrow walls was limited by P, whereas in earthworm casts, microbial growth was limited by the amount of available C. In aging faeces the P requirement of the microflora increased and approached that of the soil. Immobilization of phosphate in earthworm casts is probably caused by mainly abiotic processes. C mineralization by soil microflora fertilized with glucose and P was limited by N, except in freshly deposited casts. Ammonium, not nitrate, was responsible for this process. N dynamics in earthworm casts are discussed.     

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     

Dynamics of soil biomass C,N, and P in a dry tropical forest in India

Summary Three dry tropical forest soils along a topographic sequence were examined to determine the seasonal dynamics of microbial C, N, and P. The lowest microbial biomass was found in forest soils at the foot of the hill followed by midslope forest soils. The hilltop soil, which had the most fine particles, water-holding capacity, organic C, and total N, reflected the presence of greater amounts of microbial C, N, and P. Mean annual microbial C, N, and P ranges were 466–662, 48–72 to 21–30 g g^-1, respectively. The seasonal pattern of microbial biomass, C, N, and P was similar at all sites, the values being greatest during the dry season and lowest during the wet season. The seasonal values for microbial biomass C, N, and P were positively correlated with each other and a negative correlation was found between microbial biomass and the fine root mass in these forest soils.     

Microbial biomass and C mineralization in agricultural soils as affected by atrazine addition

This study examines the effects of atrazine on both microbial biomass C and C mineralization dynamics in two contrasting agricultural soils (organic C, texture, and atrazine application history) located at Galicia (NW Spain). Atrazine was added to soils, a Humic Cambisol (H) and a Gleyic Cambisol (G), at a recommended agronomic dose and C mineralization (CO₂ evolved), and microbial biomass measurements were made in non-treated and atrazine-treated samples at different time intervals during a 12-week aerobic incubation. The cumulative curves of CO₂–C evolved over time fit the simple first-order kinetic model [Ct = Co (1 − e ^−kt )], whose kinetic parameters were quantified. Differences in these parameters were observed between the two soils studied; the G soil, with a higher content in organic matter and microbial biomass C and lower atrazine application history, exhibited higher values of the total C mineralization and the potentially mineralizable labile C pool than those for the H soil. The addition of atrazine modified the kinetic parameters and increased notably the C mineralized; by the end of the incubation the cumulative CO₂–C values were 33–41% higher than those in the corresponding non-added soils. In contrast, a variable effect or even no effect was observed on the soil microbial biomass following atrazine addition. The data clearly showed that atrazine application at normal agricultural rates may have important implications in the C cycling of these two contrasting acid soils.     

Microbial biomass and C and N transformations in forest floors under European beech, sessile oak, Norway spruce and Douglas-fir at four temperate forest sites

The purpose of this research was to compare soil chemistry, microbially mediated carbon (C) and nitrogen (N) transformations and microbial biomass in forest floors under European beech (Fagus sylvatica L.), sessile oak (Quercus petraea (Mattuschka) Lieblein), Norway spruce (Picea abies (L.) Karst) and Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) at four study sites. We measured soil chemical characteristics, net N mineralization, potential and relative nitrification, basal respiration, microbial and metabolic quotient and microbial biomass C and N under monoculture stands at all sites (one mixed stand). Tree species affected soil chemistry, microbial activities and biomass, but these effects varied between sites. Our results indicated that the effect of tree species on net N mineralization was likely to be mediated through their effect on soil microbial biomass, reflecting their influence on organic matter content and carbon availability. Differences in potential nitrification and relative nitrification might be related to the presence of ground vegetation through its influence on soil NH₄ and labile C availability. Our findings highlight the need to study the effects of tree species on microbial activities at several sites to elucidate complex N cycle interactions between tree species, ground vegetation, soil characteristics and microbial processes.     

Soil microbial biomass C and N along a climatic transect in the Mongolian steppe

The relationships of soil microbial biomass C (C_mic) or N (N_mic) with mean annual precipitation and temperature were studied along a climatic transect in the Mongolian steppe. Soil organic C (C_org) and total N (N_t), respiration rate, C_mic and N_mic at depths of 0–5 and 5–10 cm decreased with increasing aridity. The contents of C_org and N_t in the 0- to 5-cm soil layers decreased linearly with precipitation reduction along the transect. C_mic and N_mic changes with precipitation were not linear, with higher changes between 330 and 128 mm mean annual precipitation. C_mic/C_org and N_mic/N_t increased with increasing aridity. The metabolic quotient qCO₂ of 0- to 5-cm soil layers was low between 330 and 273 mm precipitation. The relationship between the qCO₂ of the 0- to 5-cm soil layers and the mean annual precipitation was well fitted with a quadratic function y =0.0006x² –0.40x +86.0, where y is the qCO₂ (µmol CO₂-C mmol^–1 C_mic) and x is the mean annual precipitation (mm). C_org, N_t, C_mic, N_mic and respiration rate decreased exponentially with increasing mean annual temperature in both the 0- to 5- and 5- to 10-cm soil layers, and change rate was lower when the mean annual temperature was higher than 2.6°C. The close relationships of the mean annual precipitation or temperature with soil C_org, N_t, C_mic, N_mic, C_mic/C_org and qCO₂ indicate that each parameter can be calculated by determining the other parameters in this specific climatic range.     

Microbial characteristics of ectomycorrhizal mat communities in Oregon and California

Summary Specialized ectomycorrhizal fungi form dense mats in forest soils that have different enzyme levels, higher respiration rates, more biomass, different soil fauna, and different soil chemistry compared with adjacent soils not obviously colonized by these mats. In this study, mats formed by two genera of fungi collected in three locations were compared with a wide range of measurements. Per cent moisture, pH, chloroform fumigation-flush C, anaerobic N mineralization, exchangeable ammonium, and respiration, N₂ fixation, and denitrification rates were compared between soils or litter colonized by ectomycorrhizal mat-forming fungi and adjacent non-mat material. Significant differences were observed between the two genera of mat-forming fungi and also between mats formed primarily in mineral soil and those formed in litter. These differences suggest that different mat-forming fungi perform different functions in forest soils and that these fungi function differently in mineral soil compared with litter.Published as Technical Paper 9496, Oregon Agricultural Experiment Station     

营林措施对沙地樟子松人工林土壤养分、酶活性及微生物量碳的影响

在科尔沁沙地南缘章古台地区,对采用3种营林措施的樟子松人工林土壤养分、酶活性和微生物量碳进行测定分析,结果表明:2001围封处理区除速效钾外其它测定指标都低于对照区和同龄2007年围封处理区,2001围封+间伐处理区与其它处理相比,除全磷、速效钾和中性磷酸酶外,其它指标显著提高;32年生林分已修枝7年的樟子松人工林,随修枝强度的增大,土壤脲酶活性和微生物量碳增加,土壤过氧化氢酶和中性磷酸酶活性及土壤养分含量下降;40年生左右的樟子松人工林,通过卫生伐改造成疏林处理区与对照区相比,土壤养分含量、酶的活性和微生物量碳显著提高。研究认为沙地樟子松人工林地力衰退可通过营林措施加以控制,关键要合理经营。     

Why does temperature affect relative uptake rates of nitrate, ammonium and glycine: A test with Eucalyptus pauciflora

Few studies have examined how temperature affects uptake of nitrate, ammonium and amino acids from soil. This study tests the hypothesis that cool temperatures favour uptake of the amino acid glycine while warm temperatures favour uptake of inorganic forms of N such as nitrate. We used glasshouse-grown ectomycorrhizal seedlings of the sub-alpine tree species Eucalyptus pauciflora Sieber ex Spreng. Seedlings were grown in soil (humic umbrosol, from species' habitat) that was dominated by amino acids and ammonium with only small amounts of nitrate. To examine if root physiology affects temperature responses of N uptake, we measured uptake from ¹⁵N-labelled hydrosolutions containing equimolar 100 μmol L⁻¹ mixtures of ammonium, nitrate and glycine at temperatures from 5 to 35 °C. We also examined if the effect of temperature on uptake of N forms was due to plant-microbe competition by following the fate of equimolar amounts of labelled ammonium, nitrate and glycine injected into the soil at temperatures of 5 °C and 25 °C. Hydrosolution experiments showed that uptake of glycine was favoured by warm temperatures and inorganic N by cool temperatures. In contrast, when ¹⁵N was injected into soil the uptake of glycine was favoured by low temperatures and nitrate by warm temperatures. At 25 °C, glycine was 17% of the N taken up from soil and nitrate was 51%; whereas at 5 °C glycine was 30% of the N taken up from soil and nitrate was 23%. Microbes were better competitors than seedlings for all forms of N, but temperature did not affect microbial preference for the different N forms. Hence, while microbes limit N available for plant uptake, they do not seem to be the cause of the greater plant uptake of glycine at cool temperatures and nitrate at warm temperatures. Intact uptake of glycine by plants was suggested by the positive relationship between uptake of ¹³C and ¹⁵N and detection by GC-MS of intact , ¹⁵N glycine molecules in roots. In conclusion, uptake of glycine is favoured by cool temperatures and nitrate by warm temperatures, but this is apparently not a function of root physiology or competition with soil microbes.     

Litter, warming and plants affect respiration and allocation of soil microbial and plant C, N and P in arctic mesocosms

In a mesocosm experiment, we studied decomposition rates as CO₂ efflux and changes in plant mass, nutrient accumulation and soil pools of nitrogen (N) and phosphorus (P), in soils from a sub-arctic heath. The soil was incubated at 10 °C and 12 °C, with or without leaf litter and with or without plants present. The purpose of the experiment was to analyse decomposition and nutrient transformations under simulated, realistic conditions in a future warmer Arctic.Both temperature enhancement and litter addition increased respiration rates. Temperature enhancement and surprisingly also litter addition decreased microbial biomass carbon (C) content, resulting in a pronounced increase of specific respiration. Microbial P content increased progressively with temperature enhancement and litter addition, concomitant with increasing P mineralisation, whereas microbial N increased only in the litter treatment, at the same time as net N mineralisation decreased. In contrast, microbial biomass N decreased as temperature increased, resulting in a high mobilisation of inorganic N.Plant responses were closely coupled to the balance of microbial mineralisation and immobilisation. Plant growth and N accumulation was low after litter addition because of high N immobilisation in microbes and low net mineralisation, resulting in plant N limitation. Growth increased in the temperature-enhanced treatments, but was eventually limited by low supply of P, reflected in a low plant P concentration and high N-to-P ratio. Hence, the different microbial responses caused plant N limitation after litter addition and P limitation after temperature enhancement. Although microbial processes determined the main responses in plants, the plants themselves influenced nutrient turnover. With plants present, P mobilisation to the plant plus soil inorganic pools increased significantly, and N mobilisation non-significantly, when litter was added. This was presumably due to increased mineralisation in the rhizosphere, or because the nutrients in addition to being immobilised by microbes also could be absorbed by plants. This suggests that the common method of measuring nutrient mineralisation in soils incubated without plants may underestimate the rates of nutrient mobilisation, which probably contributes to a commonly observed discrepancy of measured lower rates of net nutrient mineralisation than uptake rates in arctic soils.     

Soil properties and N application effects on microbial activities in two winter wheat cropping systems

The application of mineral N fertilizers may influence biologically mediated processes that are important in nutrient transformations and availability. This study was conducted to assess the effect of N application on microbial activities in irrigated and non-irrigated winter wheat systems. Carbon decomposition and microbial biomass C in soils with three N application rates (0, 150, and 300 kg N ha^–1 as urea) were measured over 40 days in a laboratory incubation experiment. Carbon, N, and P contents in the soil under the irrigated wheat were higher than those in the soil under the non-irrigated wheat. The reverse trend was observed for soil pH and Ca and Mg contents. However, soils from the two systems had similar C/N ratios. Carbon decomposition and microbial biomass C in the soil under the irrigated wheat increased significantly (p <0.05). Increasing rates of N fertilizer resulted in higher C decomposition and microbial biomass C levels in both soil systems. Results indicate that different wheat cropping systems affect soil properties that will then have an impact on C turnover in the soil. Moreover, the irrigated wheat system favors soil conditions required for a faster C turnover. In conclusion, it is likely that due to positive effects on microbial activity, N fertilization will increase nutrient cycling and, subsequently, crop productivity will improve in N-poor soils.     

Relationships between C and N availability, substrate age, and natural abundance C and N signatures of soil microbial biomass in a semiarid climate

Soil microbial organisms are central to carbon (C) and nitrogen (N) transformations in soils, yet not much is known about the stable isotope composition of these essential regulators of element cycles. We investigated the relationship between C and N availability and stable C and N isotope composition of soil microbial biomass across a three million year old semiarid substrate age gradient in northern Arizona. The δ¹⁵N of soil microbial biomass was on average 7.2‰ higher than that of soil total N for all substrate ages and 1.6‰ higher than that of extractable N, but not significantly different for the youngest and oldest sites. Microbial ¹⁵N enrichment relative to soil extractable and total N was low at the youngest site, increased to a maximum after 55,000 years, and then decreased slightly with age. The degree of ¹⁵N enrichment of microbial biomass correlated negatively with the C:N mass ratio of the soil extractable pool. The δ¹³C signature of soil microbial biomass was 1.4‰ and 4.6‰ enriched relative to that of soil total and extractable pools respectively and showed significant differences between sites. However, microbial ¹³C enrichment was unrelated to measures of C and N availability. Our results confirm that ¹⁵N, but not ¹³C enrichment of soil microbial biomass reflects changes in C and N availability and N processing during long-term ecosystem development.     

Microbial degradation of hydrolysable and condensed tannin polyphenol-protein complexes in soils from different land-use histories

Polyphenols are capable of binding to proteins and form polyphenol-protein complexes thus reducing the release of N from decomposing plant materials. The objective of this work was to test if under polyphenol-rich vegetations adapted microbial communities had developed capable of breaking down recalcitrant polyphenol-protein complexes. Soils used for this investigation were from different 10-year-old tropical agricultural systems (maize, sugarcane plots and Gliricidia sepium or Peltophorum dasyrrachis woodlots) and natural systems (secondary forest and Imperata cylindrica grassland). TA (tannic acid, hydrolysable tannin), QUE (quebracho, condensed tannin), BSA (bovine serum albumin, protein) or TA/BSA and QUE/BSA polyphenol-protein complexes were incubated at 28 °C in these soils. CO₂-C and ¹³C evolution were periodically monitored and mineral N release, microbial biomass N and phospholipid fatty acid (PLFA) profiles measured at the end.QUE was able to bind about 25% more protein than TA. In all systems the individual uncomplexed substrates were more easily degraded than the complexes. On average, net cumulative CO₂-C evolution from TA/BSA complexes was more than 5 times higher than from QUE/BSA complexes, indicating higher C availability and/or lower protection capability of TA compared to QUE. However, net N release was higher from QUE/BSA than from TA/BSA probably due to their higher protein-binding capacity and associated larger degradation of partly unprotected protein as suggested by ¹³C-CO₂ signatures. Microbial respiration patterns indicated that polyphenol complexes were initially degraded more quickly in the maize cropping system than in soils from under polyphenol-rich communities (Peltophorum and natural forest) but this pattern reversed with time. Long-term incubation of QUE/BSA complexes even caused a negative effect on microbial respiration in agricultural soils with low polyphenol contents (e.g. maize and sugarcane).Incubation of polyphenol complexes in soil depressed microbial biomass N in maize, sugarcane, Imperata and forest systems and led to reduced soil pH. However, microbial biomass was increased under the polyphenol-rich vegetation of Peltophorum. The PLFA group 18:2w6,9 was highly enhanced by condensed tannin-protein complexes additions as compared to control and hydrolysable polyphenol-protein complexes in soils with high polyphenol contents. Polyphenol complexes increased the fungi:bacteria ratio in systems with a high polyphenol content, particularly with condensed tannin complexes. The results indicated that systems with a high polyphenol content favoured development of fungal communities that are highly adaptable to phenol-rich soil conditions and high acidity, particularly with regards to the more recalcitrant condensed tannin-protein complexes.     

添加硝化抑制剂DMPP对红壤水稻土硝化作用及微生物群落功能多样性的影响

通过室内培育试验,研究了不同施氮水平下添加硝化抑制剂(DMPP)处理对红壤水稻土NH4+-N、NO3–-N含量、微生物生物量碳及微生物群落功能多样性的影响。结果表明:56天培养期内,不同处理的NH4+-N含量总体呈下降趋势,而NO3–-N含量呈上升趋势。随施氮水平提高,培养期内NH4+-N平均含量从0 mg/kg处理的24.10 mg/kg增加到400 mg/kg处理的412.10 mg/kg,NO3–-N平均含量从0 mg/kg处理的41.88 mg/kg增加到400 mg/kg处理的99.83 mg/kg。添加DMPP显著抑制硝化作用进行,抑制效果随施氮量增加而提高,400 mg/kg施氮水平下,添加DMPP硝化率和硝化速率比不添加DMPP处理分别下降了29.0%和44.3%,下降幅度远大于其他施氮水平处理。施氮水平也影响土壤微生物生物量碳和微生物群落功能多样性。施氮量从0 mg/kg增加到400 mg/kg,土壤微生物生物量碳下降了12.5%,AWCD值下降了78.4%,Shannon指数下降了22.3%;与不添加DMPP处理相比,添加DMPP处理的土壤微生物生物量、AWCD值、Shannon指数分别提高了2.1%、23.9%、7.8%,尤其在400 mg/kg施氮水平下,提高的幅度更加明显。     

Microbial responses and nitrous oxide emissions during wetting and drying of organically and conventionally managed soil under tomatoes

The types and amounts of carbon (C) and nitrogen (N) inputs, as well as irrigation management are likely to influence gaseous emissions and microbial ecology of agricultural soil. Carbon dioxide (CO₂) and nitrous oxide (N₂O) efflux, with and without acetylene inhibition, inorganic N, and microbial biomass C were measured after irrigation or simulated rainfall in two agricultural fields under tomatoes (Lycopersicon esculentum). The two fields, located in the California Central Valley, had either a history of high organic matter (OM) inputs (“organic” management) or one of low OM and inorganic fertilizer inputs (“conventional” management). In microcosms, where short-term microbial responses to wetting and drying were studied, the highest CO₂ efflux took place at about 60% water-filled pore space (WFPS). At this moisture level, phospholipid fatty acids (PLFA) indicative of microbial nutrient availability were elevated and a PLFA stress indicator was depressed, suggesting peak microbial activity. The highest N₂O efflux in the organically managed soil (0.94 mg N₂O-N m⁻² h⁻¹) occurred after manure and legume cover crop incorporation, and in the conventionally managed soil (2.12 mg N₂O-N m⁻² h⁻¹) after inorganic N fertilizer inputs. Elevated N₂O emissions occurred at a WFPS >60% and lasted <2 days after wetting, probably because the top layer (0–150 mm) of this silt loam soil dried quickly. Therefore, in these cropping systems, irrigation management might control the duration of elevated N₂O efflux, even when C and inorganic N availability are high, whereas inorganic N concentrations should be kept low during times when soil moisture cannot be controlled.     

Microbial biomass dynamics during the decomposition of leaf litter of poplar and eucalyptus in a sandy loam

Soil microbiological properties during decomposition of leaf litter of poplar (Populus deltoides) and eucalyptus (Eucalyptus tereticornis) were studied under laboratory conditions. Microbial biomass C and ninhydrin-N were measured at different intervals up to 90 days following incorporation of poplar and eucalyptus leaves separately @ 20 and 100t ha^-1. In general, the net increase in total biomass C or ninhydrin N following amendment was larger in the soils which received poplar leaves than in the soils that received eucalyptus leaves. The amounts of biomass C, at day 90, in the soils which received eucalyptus leaves @ 20 and 100 t ha^-1 was about half and one-third, respectively, that of the soils that received poplar leaves at the same rates. Similarly, the field soils naturally receiving eucalyptus leaf litter contained about half the amounts of biomass C or ninhydrin N of the soils that received poplar leaf litter. In contrast, the amounts of organic C and total N were more in soils which received eucalyptus leaves both in the laboratory experiment and under field conditions than in the soils that received poplar leaves, indicating that the decomposition of eucalyptus leaves in soils was slower than that of poplar leaves. The ratio of biomass C/soil organic C in soils receiving eucalyptus leaves was about 2–4 times lower than those in soils with no admendment or soils receiving poplar leaves. These results, therefore, suggest that the allelochemicals released into soil during decomposition of eucalyptus leaves had a toxic effect on soil microorganisms and may thus affect the nutrient cycling and hence soil fertility.