The effects of N and P additions on soil microbial properties in paired stands of temperate secondary forests and adjacent larch plantations in Northeast China

The conversion of secondary forests to larch plantations in Northeast China has resulted in a significant decline in soil available nitrogen (N) and phosphorus (P), and thus affects plant productivity and ecosystem functioning. Microbes play a key role in the recycling of soil nutrients; in turn, the availability of soil N and P can constrain microbial activity. However, there is little information on the relationships between available soil N and P and the microbial biomass and activity in larch plantation soil. We studied the responses of soil microbial respiration, microbial biomass and activity to N and P additions in a 120-day laboratory incubation experiment and assessed soil microbial properties in larch plantation soil by comparing them with the soil of an adjacent secondary forest. We found that the N-containing treatments (N and N + P) increased the concentrations of soil microbial biomass N and soluble organic N, whereas the same treatments did not affect microbial respiration and the activities of β-glucosidase, N-acetyl-β-glucosaminidase and acid phosphatase in the larch plantation. In addition, the concentration of microbial biomass P decreased with N addition in larch plantation soil. In contrast, N and N + P additions decreased microbial respiration, and N addition also decreased the activity of N-acetyl-β-glucosaminidase in the secondary forest soil. The P treatment did not affect microbial respiration in either larch plantation or secondary forest soils, while this treatment increased the activities of β-glucosidase and acid phosphatase in the secondary forest soil. These results suggested that microbial respiration was not limited by available P in either secondary forest or larch plantation soils, but microbial activity may have a greater P demand in secondary forest soil than in larch plantation soil. Overall, there was no evidence, at least in the present experiment, supporting the possibility that microbes suffered from N or P deficiency in larch plantation soil.     

Organic nitrogen cycling microbial communities are abundant in a dry Australian agricultural soil

Some microbial nitrogen (N) cycling processes continue under low soil moisture levels in drought-adapted ecosystems. These processes are of particular importance in winter cropping systems, where N availability during autumn sowing informs fertilizer practices and impacts crop productivity. We evaluated the organic and inorganic N-cycling communities in a key cropping soil (Vertosol), using a controlled-environment incubation study that was designed to model the autumn break in south Australian grain growing regions. Soils from wheat, lucerne, and green manure (disced-in vetch) rotations of the Sustainable Cropping Rotations in Mediterranean Environments trial (Victoria, Australia) were collected during the summer when soil moisture was low. Microbial community structure and functional capacity were measured both before and after wetting (21, 49, and 77 days post-wetting) using terminal restriction fragment length polymorphism measures of bacterial and fungal communities, and quantitative PCR of nitrogen cycling genes. Quantified genes included those associated with organic matter decomposition (laccase, cellobiohydrolase), mineralization of N from organic matter (peptidases) and nitrification (bacterial and archaeal ammonia monooxygenase and nitrite oxidoreductase). In general, the N cycling functional capacity decreased with soil wetting, and there was an apparent shift from organic-N cycling dominance to autotrophic mineral-N cycling dominance. Soil nitrate levels were best predicted by laccase and neutral peptidase genes under drought conditions, but by neutral peptidase and bacterial ammonia monooxygenase genes under moist conditions. Rotation history affected both the structural and functional resilience of the soil microbial communities to changing soil moisture. Discing in green manure (vetch) residues promoted a resilient microbial community, with a high organic-N cycling capacity in dry soils. Although this was a small-scale microcosm study, our results suggest that management strategies could be developed to control microbial organic-N processing during the summer fallow period and thus improve crop-available N levels at sowing.     

Cattle grazing increases microbial biomass and alters soil nematode communities in subtropical pastures

This study focused on examining the impacts of cattle grazing on belowground communities and soil processes in humid grasslands. Multiple components in the soil communities were examined in heavily grazed and ungrazed areas of unimproved and improved bahiagrass (Paspalum notatum Flugge) pastures in south-central Florida. By using small (1-m×1-m) sampling plots, we were able to detect critical differences in nematode communities, microbial biomass, and mineralized C and N, resulting from the patchy grazing pattern of cattle. Soil samples were collected on three occasions between June 2002 and June 2003. Microbial C and N were greater (P?0.01) in grazed than in ungrazed plots on all sampling dates. Effects of grazing varied among nematode genera. Most genera of colonizer bacterivores were decreased (P?0.10) by grazing, but more persistent bacterivores such as Euteratocephalus and Prismatolaimus were increased, as were omnivores and predators. Higher numbers of persisters indicated that grazing resulted in a more structured nematode community. Some herbivores, particularly Criconematidae, were decreased by grazing. Abundance of omnivores, predators, and especially fungivores were strongly associated with C mineralization potential. Strong correlation of microbial C and N with nematode canonical variables composed of five trophic groups illustrates important links between nematode community structure and soil microbial resources. Including the analysis of nematode trophic groups with soil microbial responses reveals detection of grazing impact deeper into the hierarchy of the decomposition process in soil, and illustrates the complexity of responses to grazing in the soil foodweb. Although highly sensitive to grazing impacts, small-scale sampling could not be used to generalize the overall impact of cattle grazing in large-scale pastures, which might be determined by the intensity and grazing patterns of various stocking densities at the whole pasture level.     

Wetting and drying events determine soil N pools in two Mediterranean ecosystems

To improve our knowledge of how nutrient cycling in Mediterranean environments responds to climate change, we evaluated the effects of the continuous changes in soil nitrogen (N) pools during natural wetting and drying events. We measured soil N pools (microbial biomass [MB-N], dissolved organic nitrogen [DON], NH₄⁺ and NO₃⁻) and N ion exchange resins at weekly intervals for one year in two contrasting Mediterranean ecosystems. All soil N fractions in both ecosystems showed high intraseasonal and interseasonal variability that was greater in inorganic soil fractions than in organic N soil fractions. MB-N, DON and resin-NH₄⁺ showed increased concentrations during wetting events. Only the soil NO₃⁻ and resin-NO₃⁻ showed the opposite trend, suggesting a different response to water pulses compared to the other soil variables. Our results show that N pools are continuously changing, and that this high variability is not associated with the total amount of organic matter and labile soil carbon (C) and N soil fractions found in each ecosystem. The highest variability was found for inorganic N forms, which suggests that organic N forms are more buffered in soils exposed to wetting-drying cycles. Our results suggest that the changes in wetting-drying cycles expected with global climate change may have a significant impact on the availability and turnover of organic and inorganic N.     

Volatile monoterpenes in soil atmosphere under birch and conifers: Effects on soil N transformations

The aim of this study was to examine the occurrence and concentrations of volatile organic compounds (VOCs), in particular, volatile monoterpenes, in soil atmosphere under silver birch (Betula pendula L.) and two conifers, Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.), and to determine the effects of the most relevant monoterpenes on transformations of soil N. The study site was a 70-year-old tree species experiment in Kivalo, northern Finland. VOCs were collected using two methods, passive air samplers and a chamber method. In soil atmosphere under spruce and especially under pine, the concentrations of monoterpenes were high, α- and β-pinene, Δ-3-carene and myrcene being the most abundant compounds, whereas concentrations of monoterpenes in soil atmosphere under birch were negligible. Samples of humus layer from the birch stand incubated in vitro and exposed to vapors from monoterpenes typical of coniferous forest soil showed decreased rates of net N mineralization but simultaneously increased rates of C mineralization. The response of soil microbial biomass C and N to different monoterpenes varied, but some monoterpenes considerably decreased soil microbial biomass. Altogether these results suggest that these compounds have negative effects on soil N transformations, but may serve as carbon and energy source for part of soil microbes.     

Effect of phosphate rock, lime and cellulose on soil microbial biomass in acidic forest soil and its significance in carbon cycling

Phosphate rock (PR), limestone, coal combustion by-product (CCBP) high in Ca and high organic manures are potential amendments for increasing agricultural production in the acidic soils of the Appalachian region. The objective of this study was to examine effects of PR, CCBP and cellulose addition on soil microbial biomass in an acidic soil based on the measurement of soil microbial biomass P (P _mic) and on the mineralization of organic matter. Application of PR alone or in combination with CCBP increased P _mic. The P _mic was far less when the soil received PR in combination with limestone than with PR application alone or PR in combination with CCBP. Either CCBP or limestone application alone considerably decreased P _mic in the soil due to reduced P solubility. Cellulose addition alone did not increase P _mic, but P _mic was significantly increased when the soil was amended with cellulose in combination with PR. The decomposition of added cellulose was very slow in the soil without PR amendment. However, mineralization of both native organic matter and added cellulose was enhanced by PR application. Mineralization of organic matter was less when the soil was amended with PR in combination with high rates of CCBP (> 2.5%) because PR dissolution varied inversely with amount of CCBP addition. Overall, CCBP had no detrimental effect on soil microbial biomass at low application rates, although, like limestone, CCBP at a high rate may decrease P _mic in P-deficient soils through its influence on increased soil pH and decreased P bioavailability in the soil. Application of PR to an acidic soil considerably enhanced the microbial activity, thereby promoting the cycling of carbon and other nutrients. Received: 11 December 1995     

Synthesis of 15N-labelled microbial biomass in soil in situ and extraction of biomass N

Summary A Pakistani soil (Hafizabad silt loam) was incubated at 30°C with varying levels of ¹⁵N-labelled ammonium sulphate and glucose (C/N ratio of 30 at each addition rate) in order to generate different insitu levels of ¹⁵N-labelled microbial biomass. At a stage when all of the applied ¹⁵N was in organic forms, as biomass and products, the soil samples were analysed for biomass N by the chloroform (CHCl₃) fumigation-extraction method, which involves exposure of the soil to CHCl₃ vapour for 24 h followed by extraction with 500 mM K₂SO₄. A correction is made for inorganic and organic N in 500 mM K₂SO₄ extracts of the unfumigated soil. Results obtained using this approach were compared with the amounts of immobilized ¹⁵N extracted by 500 mM K₂SO₄ containing different amounts of CHCl₃. The extraction time varied from 0.5 to 4 h.The amount of N extracted ranged from 27 to 270 g g^–1, the minimum occurring at the lowest (67 g g^–1) and the maximum at the highest (333 g g^–1) N-addition rate. Extractability of biomass ¹⁵N ranged from 25% at the lowest N-addition rate to 65%a for the highest rate and increased consistently with an increase in the amount of ¹⁵N and glucose added. The amounts of both soil N and immobilized ¹⁵N extracted with 500 mM K₂SO₄ containing CHCl₃ increased with an increase in extraction time and in concentration of CHCl₃. The chloroform fumigation-extraction method gives low estimates for biomass N because some of the organic N in K₂SO₄ extracts of unfumigated soil is derived from biomass.     

Interactions between nematodes and microbial communities in a tropical soil following manipulation of the soil food web

The carrying capacity for microflora and nematofauna was manipulated (using a bactericide, a fungicide, manure or a growing millet plant) in a poor tropical soil, in order to identify relationships between the soil microbes and nematodes and to assess the influences of these organisms on nitrogen flux. The experiment was conducted for 4 months in containers under greenhouse conditions, with analyses of soil, nematofauna and microbial characteristics at regular intervals. Manure input and initial bactericide application led to a significant increase in bacterial-feeding and fungal-feeding nematodes of coloniser-persister classes 1 and 2, respectively, whereas high manure input stimulated omnivorous nematodes (i.e. Microdorylaimus rapsus) which became the dominant trophic group. Changes in abundance of the different bacterial-feeding nematode taxa between treatments seemed to be more related to changes in the structure of the microbial communities than to the total amount of micro-organisms, as suggested by the RISA fingerprint analysis of the bacterial communities. Canonical analysis of nematode feeding guilds, combined with soil microbial and mineral nitrogen parameters as well as multiple regression showed that the bacterial-feeding nematodes influenced the inorganic N content in the soil whereas microbial biomass was determined by total nematode abundance and not by any specific trophic group.     

Influence of soil properties on microbial C,N, and P in dry tropical ecosystems

Summary Relationships between soil physicochemical characteristics and soil microbial C, N, and P in Indian dry tropical ecosystems are discussed. The major ecosystem studies were on forest, savanna, cropped fields, and mine spoils. The highest microbial C, N, and P levels were recorded from the mixed forest and the lowest levels in 5-year-old mine spoil. Across the sites, microbial C ranged from 226 to 643 g g^-1, microbial N from 19 to 71 g g^-1, and microbial P from 9 to 28 g g^-1 soil. The proportion of soil organic C contained in the microbial biomass ranged from 2.2 to 5.0%. The microbial C: N ratio in these soils ranged from 7.4. to 13.1 and the microbial C: P ratio from 16.6 to 30.6. The concentrations of microbial C, N, and P were correlated with several soil properties and among themselves. The soil properties, in various linear combinations, explained 90–99% of the variability in the microbial nutrients. Grazing of the savanna had some effect on the level of microbial biomass, and as the mine spoil aged, the level of microbial C, N, and P also increased.     

Interactions between tree seedling roots and humus forms in the control of soil C and N cycling

The hypothesis that roots enhance soil-N turnover in humified soil organic matter (SOM) (mull) but not in lignified SOM (mor) was tested in a study involving the growth of eight species of tree seedlings on the two contrasting humus forms. After 12 and 22 weeks of seedling growth, soil-CO₂ efflux was measured with (1) growing seedlings, and after 22 weeks, with (2) roots only, shoots excised, and (3) with roots removed and soils amended with different rates of glucose. Indices of C-flux and of soil available-C were derived and compared to plant-N uptake, extractable soil mineral-N, anaerobically mineralized soil-N, N bioavailability to Agrostis grass following harvest of seedlings, and to seedling fine root C-chemistry. Significant soil x species interactions were found for total soil-CO₂ efflux, root-dependent CO₂, soil available-C and microbial biomass. In all cases, roots were important contributors to C-cycling in the mull soil but not in the mor soil. C was more limiting in the mor than in the mull microbial community. Plant-N uptake and the mineral-N pool was greater in the mor soil, reflecting that soil's higher specific N-supplying capacity (N-mineralized:CO₂). Seedlings decreased the mineral-N pool in both soils, but the presence of roots increased N-mineralization in the mull soil and decreased N-mineralization in the mor soil. Significant positive relationships were observed in the mull soil only between soil respiration and plant N uptake at mid-season, and between soil respiration and N-mineralization at late-season. Birch root activity in the mull soil was greater than that of all other seedlings and this observation is discussed with respect to the autecology of birch. Soil respiration correlated with the non-polar extract content but not the lignin:N ratio of fine roots. Results suggest that root-released C in mull SOM is sufficient to relieve energy limitation to soil microbes and allow them to access appreciable amounts of soil-N, whereas ligninolytic activity, which may ultimately control soil-N turnover in mor SOM, is not increased by rhizodeposition.     

Active organic matter distribution in the surface 15 cm of undisturbed and cultivated soil

Summary Organic-matter dynamics near the soil surface influence plant nutrient supplies. To evaluate the effects of cultivation on active soil organic-matter distribution in the surface 15 cm, we measured total organic C, Kjeldahl N, microbial biomass C and N (chloroform fumigation), respirable C and mineralizable N (aerobic incubations) in 1-cm layers from 0–10 cm and in 2.5-cm layers from 10–15 cm in adjacent areas of undisturbed (shortgrass steppe) and cultivated (four years wheat-fallow rotation) Ascalon sandy loam soil (Aridic Argiustoll). The active organic matter was highly concentrated in the surface centimeter of undisturbed soil. In undisturbed soil, microbial biomass C and N concentrations were more than five times greater at 0–1 cm than at 2–15 cm. Respirable-C and mineralizable-N concentrations (20-day incubations) were 8 and 18 times greater at 0–1 cm than at 2–15 cm. Below 3 cm, the concentrations were equal in both cultivated and undisturbed soil. Cultivation reduced the average concentrations at 0-15 cm of microbial biomass C and N by 62% and 32%, and of respirable C and mineralizable N by 71% and 46%, by reducing the concentrations at 0-1 cm to levels comparable to the 2- to 15-cm layers of the undisturbed soil. Decreases after cultivation ensued from disrupting the surface 1-cm layer.     

Determination of microbial phosphorus Kp factors in a spodosol: influence of extractant, water potential, and soil horizon

The acidic, sandy soils in the southeastern US are phosphorus (P) limited for forest production and are commonly fertilized with P. There is no P retention capacity in the A horizon. However, microbial biomass may immobilize and retain P fertilizer before it is leached below seedling rooting depth making P fertilization more efficient. An accurate estimation of microbial P is dependent on measuring the K_p factor in the fumigation-extraction method. The overall purpose of this study was to examine the fumigation-extraction method for microbial P in acidic, forested, sandy soils. The three objective were: to determine which extractant was the most useful extracting microbial P by comparing the standard basic extractant, 0.5 M NaHCO₃ at pH 8.5, against several acidic and oxalate extractants; to evaluate whether soil water potential influenced the K_p factor; and to test whether the K_p factor differed by soil horizon within the profile of a representative Flatwoods Spodosol. Three millimolar oxalate was determined to be the preferred extractant due to its efficient removal of microbial P and ease of analysis. The K_p factor was dependent on soil water potential and horizon. The range in K_p at different water potentials using 3 mM oxalate was from 0.31 to 0.67 in the A horizon, 0.48 to 0.91 in the E horizon, and 0.22 to 0.45 in Bh horizon. The highest K_p factors tended to be at water potentials near saturation and under the driest condition. Differences in K_p were attributed to the influence that water potential and soil horizon had on microbial assemblages and diversity. Using a literature value of K_p, instead of measuring K_p directly, caused an overestimate of −7 to 63% in the A horizon, 63-160% in the E horizon and 7-32% in the Bh horizon. The best estimate of microbial P required that K_p be evaluated for specific soil conditions.     

Effect of bamboo harvest on dynamics of nutrient pools,N mineralization,and microbial biomass in soil

The effect of harvesting bamboo savanna on the dynamics of soil nutrient pools, N mineralization, and microbial biomass was examined. In the unharvested bamboo site NO _inf3 ^sup- -N in soil ranged from 0.37 to 3.11 mg kg^-1 soil and in the harvested site from 0.43 to 3.67 mg kg^-1. NaHCO₃-extractable inorganic P ranged from 0.55 to 3.58 mg kg^-1 in the unharvested site and from 1.01 to 4.22 mg kg^-1 in the harvested site. Over two annual cycles, the N mineralization range in the unharvested and harvested sites was 0–19.28 and 0–24.0 mg kg^-1 soil month^-1, respectively. The microbial C, N, and P ranges were 278–587, 28–64, and 12–26 mg kg^-1 soil, respectively, with the harvested site exhibiting higher values. Bamboo harvesting depleted soil organic C by 13% and total N by 20%. Harvesting increased N mineralization, resulting in 10 kg ha^-1 additional mineral N in the first 1st year and 5 kg ha^-1 in the 2nd year following the harvest. Microbial biomass C, N and P increased respectively by 10, 18, and 5% as a result of bamboo harvesting.     

Impact of faunal complexity on microbial biomass and N turnover in field mesocosms from a spruce forest soil

In a field study using soil mesocosms in an acid spruce forest soil we investigated the effects of mesofauna and macrofauna on microbial biomass, dissolved organic matter, and N cycling. Intact soil monoliths were taken from the ground, defaunated by deep-freezing, and wrapped in nets of various mesh-sizes to control re-immigration of different faunal size-classes. The monoliths were then replanted in the field. Three treatments of mesocosms were prepared: (1) with only microbiota, (2) microbiota and mesofauna, and (3) microbiota, mesofauna, and macrofauna (= complex fauna). After 8 months of exposure the mesocosms and the unmanipulated control plots (treatment 4) were destructively sampled. We estimated microbial biomass by substrate-induced respiration and the chloroform fumigation-extraction method. N cycling was measured by monitoring microbial N mineralization, the NH _inf4 ^sup+ content, and selected amino acids and the activities of protease, urease, and deaminase. The results from the L/F layer showed that the pool of the microbial biomass was not changed by the activity of the mesofauna. However, the mesofauna and macrofauna together enhanced SIR. An increase in microbial N mineralization was only observed in treatment 3 (microbiota + complex fauna). Protease activity and NH _inf4 ^sup+ content increased in treatments 2 (microbiota + mesofauna) and 3 (microbiota + complex fauna). The complex fauna induced a soil pH increase in treatment 3 as opposed to treatment 1 and the control. This increase was presumably due to excretory NH _inf4 ^sup+ . Principal component analysis revealed that the complex fauna in treatment 3 caused a significantly higher N turnover per unit of microbial biomass.     

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.     

Humus macro-morphology and soil microbial community changes along a 130-yr-old Fagus sylvatica chronosequence

We aimed to characterize humus macro-morphology and the associated soil microbial community within the unmodified litter (OL), the fragmented and humified layers (FH) and the organo-mineral (A) layer along a beech (Fagus sylvatica L.) forest chronosequence with four stand age-classes (15-, 65-, 95-, 130-yr-old) in Normandy, France. Humus macro-morphology was described with 36 quantitative and semi-quantitative variables. We measured microbial biomass N (N_mic), microbial N quotient (N_mic-to-N_t), fungal ergosterol, bacterial and fungal DNA using 16S and 18S rDNA real-time qPCR and evaluated the potential metabolic profile of heterotrophic bacteria within each soil layer and stand age-class. The log-transform ergosterol/fungal DNA ratio (EFR index) was used as an indicator related to active fungal biomass and the fungal/bacterial (F/B) ratio was calculated from qPCR results. There was a shift from mull (mainly dysmull) to moder humus forms along the chronosequence. While the N_mic did not change significantly, the N_mic-to-N_t decreased along the chronosequence in the OL layer. Ergosterol content increased in FH and A layers and the F/B ratio increased in the FH layer with increasing beech forest age. The EFR index was significantly higher in the OL and A layers of the oldest stands, whereas the highest EFR index in the FH layer occurred in the 15-yr-old stands. The functional diversity of heterotrophic bacteria was greater within OL and FH layers of 130-yr-old stands, but highest in the A layer of 15-yr-old stands while the Average Well Color Development remained stable for all soil layers. We found significant correlations between macro-morphology and microbial variables, especially between FH-based morphology and fungal biomass. Our main results are that beech forest maturation is accompanied by (1) an increase in fungal biomass in the FH layers and, (2) an increase in heterotrophic bacteria functional diversity in the organic layers. We have identified key macro-morphology variables that are good predictors of the structural and functional profile of the soil microbial community during beech forest development.     

黄土丘陵区人工刺槐林恢复过程中土壤氮素与微生物活性的变化

为了探讨不同生长年限的人工刺槐(Robinnia pseudoacacia)林对土壤中氮素组成与微生物活性的影响及机理,本文采用"时空互代"法进行野外选点调查和采样,对典型黄土丘陵区陕西省安塞纸坊沟小流域不同林龄(10 a、15 a、30 a、38 a)人工刺槐林和撂荒地3个土层(0~10 cm、10~30 cm和30~60 cm)中的全氮、铵态氮、硝态氮、有机氮、微生物生物量碳和磷、基础呼吸及基本理化性质进行了研究。结果表明:人工刺槐林地土壤微生物生物量碳、磷含量和微生物熵都显著高于撂荒地(P<0.05)。随着人工刺槐林生长年限的增加,各层土壤铵态氮、硝态氮和有机氮含量均逐渐增加,其中有机氮的增加最显著;土壤微生物生物量碳、磷含量显著增加;微生物熵显著增大而呼吸熵显著减小;土壤有机碳、速效磷含量总体上显著增加(P<0.05);容重和碳氮比则呈下降趋势。随着土层深度的增加,氮素、有机碳、速效磷和微生物生物量碳、磷含量显著减小(P<0.05);容重和pH显著增加。土壤微生物生物量碳、磷和呼吸熵均与有机氮、全氮、硝态氮显著正相关(P<0.05)。分析发现,刺槐的生长促使土壤中微生物可利用碳增加,提高了碳的利用率,使土壤微生物量碳、磷含量增加;微生物活性的提高反过来促进了土壤氮素含量的提高,土壤中有机氮含量显著增加。与10 a生刺槐林相比,30 a生林地土壤表层的全氮含量明显增加,氮素肥力由7级(0.40 g.kg 1)上升为5级(0.87 g.kg 1)水平。     

Differential acquisition of amino acid and peptide enantiomers within the soil microbial community and its implications for carbon and nitrogen cycling in soil

l-isomeric amino acids and oligopeptides are thought to represent a key nitrogen (N) source for plants and soil microorganisms, bypassing the need to take up inorganic N, whilst self-cycling of d-enantiomers within peptidoglycan-containing bacteria may provide a further short circuit within the N cycle. Here we use stable isotope profiling (SIP) to identify the fate of organic N within soil microbial communities. We followed the incorporation of ¹³C-labelled d- or l-labelled amino acids/peptides into phospholipid fatty acids (PLFAs). l-alanine and its peptides were taken up more rapidly than d-enantiomers by Gram-positive bacteria with ¹³C incorporation being predominantly into anteiso- and iso-fatty acids typically associated with Gram-positive bacteria. d-enantiomer uptake was found not to differ significantly between the microbial groups, providing little support for the view that soil bacteria may self-cycle d-forms of amino acids and peptides. There was no consistent association between peptide chain length and incorporation. The concentrations of l- and d-isomeric amino acids in soil solution were 866 nM and 72 nM, respectively. We conclude that Gram-positive bacteria appear to be the primary competitors for l-enantiomeric forms of amino acids and their peptides, but that both d- and l-enantiomers are available N and C sources for bacteria and fungi.     

The effect of a perennial cover crop on net soil N mineralization and microbial biomass carbon in coconut plantations in the humid tropics

The decrease in soil fertility in agroecosystems due to continuous harvesting, loss of fine soil and oxidation of soil organic carbon (SOC) is well known. This study evaluates for a humid tropical climate in South Andaman Island, India, the impact of a 15-yr old Pueraria cover crop (CC) under a coconut plantation, with and without phosphorus (P) application, on the soil N mineralization rate (NMR), the mineral N pool (NH₄⁺-N and NO₃⁻-N), microbial biomass carbon (MB-C) and SM under four treatments, (1) no cover crop (NCC), (2) no cover crop + phosphorus (NCC+P), (3) CC, and (4) cover crop + phosphorus (CC+P) during three seasons, wet (May–October), post-wet (November–January) and dry (February–April). The NCC treatment served as a control. In addition, an ex-situ experiment was conducted to verify the effect of P application on NMR and MB-C under 100% field capacity (FC), 50 and 25% FC representing the different seasons. The NMR, mineral N pool and MB-C increased by 37, 46 and 41%, respectively under the CC compared to the control. SOC and fine soil particles were also greater under the CC by 41 and 461%, respectively, compared to the control. The application of P to the CC increased soil N mineralization, the mineral N pool and MB-C by 33, 16 and 14%, respectively. The amount of mineralized N was greater under the CC and CC+P treatments by 39 and 73%, respectively than the control. The ex-situ experiment showed that the P application increased NMR and MB-C, but the increases were highest in the 50% FC and lowest in 100% FC. It is proposed for the humid tropics that a CC could be used for enhancing SOC and increasing soil N mineralization under coconut plantations and other similar agroecosystems.     

Effects of heavy metals in soil on microbial diversity and activity as shown by the sensitivity-resistance index,an ecologically relevant parameter

A sensitivity-resistance index was developed, and proved to be a very sensitive biomonitor of soil pollution with heavy metals. The index was developed by a step-by-step approach. Ultimately, the bacterial soil microflora was divided into three groups, senstivive, tolerant, and resistant microflora. Zn and Cd sensitivity was defined as no growth occurring in the presence of 5 and 0.5 mg l^-1 of these metals, respectively, while resistance was defined as distinct growth in the presence of 50 and 16 mg l^-1, respectively. The sensitivity: resistance ratio of a referent clay soil (0.57 mg Cd kg^-1 and 140 mg Zn kg^-1) was 0.53, but for polluted (6 mg Cd kg^-1 + 670 mg Zn kg^-1) clay soil, the ratio was 0.24. For a referent (0.06 mg Cd kg^-1 + 12 mg Zn kg^-1) sandy soil the sensitivity: resistance ratio was 1.50 whereas polluted (2.3 mg Cd kg^-1 + 252 mg Zn kg^-1) sandy soil had a ratio 0.19. The ecological value of the sensitivity-resistance lies in its capacity to reflect potential deradation of aromatic compounds. It has been shown repeatedly that sensitive bacteria grow significantly better on a range of selected aromatic compounds. It has been speculated that resistance fo heavy metals may reduce the bioremediation capacity of soil towards chlorinated aromatics and polyaromatic hydrocarbons.