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1.
Changes in soil microbial biomass, enzyme activities, microbial community structure and nitrogen (N) dynamics resulting from organic matter amendments were determined in soils with different management histories to gain better understanding of the effects of long- and short-term management practices on soil microbial properties and key soil processes. Two soils that had been under either long-term organic or conventional management and that varied in microbial biomass and enzyme activity levels but had similar fertility levels were amended with organic material (dried lupin residue, Lupinus angustifolius L.) at amounts equivalent to 0, 4 and 8 t dry matter lupin ha?1. Microbial biomass C and N, arginine deaminase activity, fluorescein diacetate hydrolysis, dehydrogenase enzyme activity and gross N mineralisation were measured in intervals over an 81-day period. The community structure of eubacteria and actinomycetes was examined using PCR–DGGE of 16S rDNA fragments. Results suggested that no direct relationships existed between microbial community structure, enzyme activities and N mineralisation. Microbial biomass and activity changed as a result of lupin amendment whereas the microbial community structure was more strongly influenced by farm management history. The addition of 4 t ha?1 of lupin was sufficient to stimulate the microbial community in both soils, resulting in microbial biomass growth and increased enzyme activities and N mineralisation regardless of past management. Amendment with 8 t lupin ha?1 did not result in an increase proportional to the extra amount added; levels of soil microbial properties were only 1.1–1.7 times higher than in the 4 t ha?1 treatment. Microbial community structure differed significantly between the two soils, while no changes were detected in response to lupin amendment at either level during the short-term incubation. Correlation analyses for each treatment separately, however, revealed differences that were inconsistent with results obtained for soil biological properties suggesting that differences might exist in the structure or physiological properties of a microbial component that was not assessed in this study.  相似文献   

2.
《Applied soil ecology》2011,48(3):210-216
Labile soil organic matter (SOM) can sensitively respond to changes in land use and management practices, and has been suggested as an early and sensitive indicator of SOM. However, knowledge of effects of forest vegetation type on labile SOM is still scarce, particularly in subtropical regions. Soil microbial biomass C and N, water-soluble soil organic C and N, and light SOM fraction in four subtropical forests were studied in subtropical China. Forest vegetation type significantly affected labile SOM. Secondary broadleaved forest (SBF) had the highest soil microbial biomass, basal respiration and water-soluble SOM, and the pure Cunninghamia lanceolata plantation (PC) the lowest. Soil microbial biomass C and N and respiration were on average 100%, 104% and 75%, respectively higher in the SBF than in the PC. The influence of vegetation on water-soluble SOM was generally larger in the 0–10 cm soil layer than in the 10–20 cm. Cold- and hot-water-soluble organic C and N were on average 33–70% higher in the SBF than in the PC. Cold- and hot-soluble soil organic C concentrations in the coniferous-broadleaved mixed plantations were on average 38.1 and 25.0% higher than in the pure coniferous plantation, and cold- and hot-soluble soil total N were 51.4 and 14.1% higher, respectively. Therefore, introducing native broadleaved trees into pure coniferous plantations increased water-soluble SOM. The light SOM fraction (free and occluded) in the 0–10 cm soil layer, which ranged from 11.7 to 29.2 g kg−1 dry weight of soil, was strongly affected by vegetation. The light fraction soil organic C, expressed as percent of total soil organic C, ranged from 18.3% in the mixed plantations of C. lanceolata and Kalopanax septemlobus to 26.3% in the SBF. In addition, there were strong correlations among soil organic C and labile fractions, suggesting that they were in close association and partly represented similar C pools in soils. Our results indicated that hot-water-soluble method could be a suitable measure for labile SOM in subtropical forest soils.  相似文献   

3.
Little is know on the impact of biosolids application on soil organic matter (SOM) stability, which contributes to soil C sequestration. Soil samples were collected in 2006 at plow layer from fields that received liquid and dry municipal biosolids application from 1972 to 2004 at the cumulative rate of 1416 Mg ha−1 in mined soil and 1072 Mg ha−1 in nonmined soil and control fields that received chemical fertilizer at Fulton County, western Illinois. The biosolids application increased the soil microbial biomass C (SMBC) by 5-fold in mined soil and 4-fold in nonmined soil. The biosolids-amended soils showed a high amount of basal respiration and N mineralization, but low metabolic quotient, and low rate of organic C and organic N mineralization. There was a remarkable increase in mineral-associated organic C from 6.9 g kg−1 (fertilizer control) to 26.6 g kg−1 (biosolids-amended) in mined soil and from 8.9 g kg−1 (fertilizer control) to 23.1 g kg−1 (biosolids-amended) in nonmined soil. The amorphous Fe and Al, which can improve SOM stability, were increased by 2–7 folds by the long-term biosolids application. It is evident from this study that the biosolids-modified SOM resists to decomposition more than that in the fertilizer treatment, thus long-term biosolids application could increase SOM stability.  相似文献   

4.
Tree species have significant effects on the availability and dynamics of soil organic matter. In the present study, the pool sizes of soil dissolved organic matter (DOM), potential mineralizable N (PMN) and bio-available carbon (C) (measured as cumulative CO2 evolution over 63 days) were compared in soils under three coniferous species — 73 year old slash (Pinus elliottii), hoop (Araucaria cunninghamii) and kauri (Agathis robusta) pines. Results have shown that dissolved organic N (DON) in hot water extracts was 1.5–1.7 times lower in soils under slash pine than under hoop and kauri pines, while soil dissolved organic C (DOC) in hot water extracts tended to be higher under slash pine than hoop and kauri pines but this was not statistically significant. This has led to the higher DOC:DON ratio in soils under slash pine (32) than under hoop and kauri pines (17). Soil DOC and DON in 2 M KCl extracts were not significantly different among the three tree species. The DOC:DON ratio (hot water extracts) was positively and significantly correlated with soil C:N (R2 = 0.886, P < 0.01) and surface litter C:N ratios (R2 = 0.768, P < 0.01), indicating that DOM was mainly derived from litter materials and soil organic matter through dissolution and decomposition. Soil pH was lower under slash pine (4.5) than under hoop (6.0) and kauri (6.2) pines, and negatively correlated with soil total C, C:N ratio, DOC and DOC:DON ratio (hot water extracts), indicating the soil acidity under slash pine favored the accumulation of soil C. Moreover, the amounts of dissolved inorganic N, PMN and bio-available C were also significantly lower in soils under slash pine than under hoop and kauri pines. It is concluded that changes in the quantity and quality of surface litters and soil pH induced by different tree species largely determined the size and quality of soil DOM, and plantations of hoop and kauri pine trees may be better in maintaining long-term soil N fertility than slash pine plantations.  相似文献   

5.
《Applied soil ecology》2007,35(2):281-290
As a result of many decades of fire suppression and atmospheric deposition the deciduous forests of eastern North America have changed significantly in stem density, basal area, tree size-frequency distribution, and community structure. Consequently, soil organic matter quality and quantity, nutrient availability, and microbial activity have likely been altered. This study evaluated the effects of four alternative forest ecosystem restoration strategies on soil microbial activity, microbial functional diversity, soil organic C, and soil N status in two mixed-oak (Quercus spp.) forests in southern Ohio, USA. The soils of these forests were sampled during the fourth growing season after application of (1) prescribed fire, (2) thinning of the understory and midstory to pre-settlement characteristics, (3) the combination of fire and thinning, and (4) an untreated control. Prescribed fire, with or without thinning, resulted in increased bacterial but not fungal activity when assessed using Biolog®. In contrast, assays of acid phosphatase and phenol oxidase activity indicated greater microbial activity in the thinning treatment than in the other three treatments. Functional diversity of both bacteria and fungi was affected by restoration treatment, with the bacterial and fungal assemblages present in the thin + burn sites and the fungal assemblage present in the thinned sites differing significantly from those of the control and burned sites. Treatments did not result in significant differences in soil organic C content among experimental sites; however, the soil C:N ratio was significantly greater in thinned sites than in sites given the other three treatments. Similarly, there were no significant differences in dissolve inorganic N, dissolved organic N, or microbial biomass N among treatments. Bacterial and fungal functional diversity was altered significantly. Based on Biolog® utilization treatments the bacterial assemblage in the thin-only treatment appeared to be relatively N-limited and the fungal assemblage relatively C-limited, whereas in the thin + burn treatment this was reversed. Although effects of restoration treatments on soil organic matter and overall microbial activity may not persist through the fourth post-treatment year, effects on microbial functional diversity are persistent.  相似文献   

6.
《Pedobiologia》2014,57(4-6):235-244
Vegetation type influences the rate of accumulation and mineralization of organic matter in forest soil, mainly through its effect on soil microorganisms. We investigated the relationships among forest types and microbial biomass C (MBC), basal respiration (RB), substrate-induced respiration (RS), N mineralization (Nmin), specific growth rate μ, microbial eco-physiology and activities of seven hydrolytic enzymes, in samples taken from 25 stands on acidic soils and one stand on limestone, covering typical types of coniferous and deciduous forests in Central Europe. Soils under deciduous trees were less acidic than soils of coniferous forests, which led to increased mineralizing activities RB and Nmin, and a higher proportion of active microbial biomass (RS/MBC) in the Of horizon. This resulted in more extractable organic C (0.5 M K2SO4) in soils of deciduous forests and a higher accumulation of soil organic matter (SOM) in coniferous forest soil. No effect of forest type on the microbial properties was detected in the Oh horizon and in the 0–10 cm layer. The microbial quotient (MBC/Corg), reflecting the quality of organic matter used for microbial growth, was higher in deciduous forests in all three layers. The metabolic quotient qCO2 (RB/MBC) and the specific growth rate μ, estimated using respiration growth curves, did not differ in soils of both forest types. Our results showed that the quality of SOM in coniferous forests supported microorganisms with higher activities of β-glucosidase, cellobiosidase and β-xylosidase, which suggested the key importance of fungi in these soils. Processes mediated by bacteria were probably more important in deciduous forest soils with higher activities of arylsulphatase and urease. The results from the stand on limestone showed that pH had a positive effect on microbial biomass and SOM mineralization.  相似文献   

7.
To understand the dynamics of soil organic matter (SOM) in the Eurasian steppe, several soil and meteorological properties were tested in order to estimate the amounts of potentially mineralizable organic carbon (PMC) and nitrogen (PMN). Total 41 surface soil samples were collected in Ukraine and Kazakhstan from cropland, forest, grassland, and desert ecosystems. The fresh soils were incubated for 133 days under constant temperature and moisture conditions, and the CO2 emissions and the mineral N from the soils were monitored. PMC and PMN were determined by fitting models to the cumulative curves of the CO2 and the mineral N. Tested soil properties included soil pH, sand, silt and clay contents, carbon and nitrogen contents of light fraction (LF, <1.6 g cm?3) and heavy fraction (HF), and C/N ratio of LF and HF. The meteorological properties considered were mean annual temperature and precipitation. Using multiple regression analysis with the stepwise method, PMC was well estimated by carbon content of LF (LFC) and clay content, compared to the simple correlation with organic carbon (OC). Similarly, PMN was better determined by nitrogen content of LF (LFN) and clay content. These results suggest the partially labile nature of clay-associating OM and of LFC and LFN. The higher PMC and PMN in the forest and grassland sites would be attributed to the higher LFC and LFN, while the lower LFC and LFN in cropland sites would suggest the relatively higher contribution of clay-associating OM to PMC and PMN.  相似文献   

8.
A 67-day incubation experiment was carried out with a soil initially devoid of any organic matter due to heating, which was amended with sugarcane sucrose (C4-sucrose with a δ13C value of ?10.5‰), inorganic N and an inoculum for recolonisation and subsequently at day 33 with C3-cellulose (δ13C value of ?23.4‰). In this soil, all organic matter is in the microbial biomass or in freshly formed residues, which makes it possible to analyse more clearly the role of microbial residues for decomposition of N-poor substrates. The average δ13C value over the whole incubation period was ?10.7‰ in soil total C in the treatments without C3-cellulose addition. In the CO2 evolved, the δ13C values decreased from ?13.4‰ to ?15.4‰ during incubation. In the microbial biomass, the δ13C values increased from ?11.5‰ to ?10.1‰ at days 33 and 38. At day 67, 36% of the C4-sucrose was left in the treatment without a second amendment. The addition of C3-cellulose resulted in a further 7% decrease, but 4% of the C3-cellulose was lost during the second incubation period. Total microbial biomass C declined from 200 μg g?1 soil at day 5 to 70 μg g?1 soil at day 67. Fungal ergosterol increased to 1.5 μg g?1 soil at day 12 and declined more or less linearly to 0.4 μg g?1 soil at day 67. Bacterial muramic acid declined from a maximum of 35 μg g?1 soil at day 5 to a constant level of around 16 μg g?1 soil. Glucosamine showed a peak value at day 12. Galactosamine remained constant throughout the incubation. The fungal C/bacterial C ratio increased more or less linearly from 0.38 at day 5 to 1.1 at day 67 indicating a shift in the microbial community from bacteria to fungi during the incubation. The addition of C3-cellulose led to a small increase in C3-derived microbial biomass C, but to a strong increase in C4-derived microbial biomass C. At days 45 and 67, the addition of N-free C3-cellulose significantly decreased the C/N ratio of the microbial residues, suggesting that this fraction did not serve as an N-source, but as an energy source.  相似文献   

9.
《Applied soil ecology》2001,16(3):229-241
Changes in the proportions of water-stable soil aggregates, organic C, total N and soil microbial biomass C and N, due to tillage reduction (conventional, minimum and zero tillage) and crop residue manipulation (retained or removed) conditions were studied in a tropical rice–barley dryland agroecosystem. The values of soil organic C and total N were the highest (11.1 and 1.33 g kg−1 soil, respectively) in the minimum tillage and residue retained (MT+R) treatment and the lowest (7.8 and 0.87 g kg−1, respectively) in conventional tillage and residue removed (CT−R) treatment. Tillage reduction from conventional to minimum and zero conditions along with residue retention (MT+R,ZT+R) increased the proportion of macroaggregates in soil (21–42% over control). The greatest increase was recorded in MT+R treatment and the smallest increase in conventional tillage and residue retained (CT+R) treatment. The lowest values of organic C and total N (7.0–8.9 and 0.82–0.88 g kg−1 soil, respectively) in macro- and microaggregates were recorded in CT−R treatment. However, the highest values of organic C and total N (8.6–12.6 and 1.22–1.36 g kg−1, respectively) were recorded in MT+R treatment. The per cent increase in the amount of organic C in macroaggregates was greater than in microaggregates. In all treatments, macroaggregates showed wider C/N ratio than in microaggregates. Soil microbial biomass C and N ranged from 235 to 427 and 23.9 to 49.7 mg kg−1 in CT−R and MT+R treatments, respectively. Soil organic C, total N, and microbial biomass C and N were strongly correlated with soil macroaggregates. Residue retention in combination with tillage reduction (MT+R) resulted in the greatest increase in microbial biomass C and N (82–104% over control). These variables showed better correlations with macroaggregates than other soil parameters. Thus, it is suggested that the organic matter addition due to residue retention along with tillage reduction accelerates the formation of macroaggregates through an increase in the microbial biomass content in soil.  相似文献   

10.
The aim of this study was to investigate the response of soil microbial biomass and organic matter fractions during the transition from conventional to organic farming in a tropical soil. Soil samples were collected from three different plots planted with Malpighia glaba: conventional plot with 10 years (CON); transitional plot with 2 years under organic farming system (TRA); organic plot with 5 years under organic farming system (ORG). A plot under native vegetation (NV) was used as a reference. Soil microbial biomass C (MBC) and N (MBN), soil organic carbon (SOC) and total N (TN), soil organic matter fractioning and microbial indices were evaluated in soil samples collected at 0–5, 5–10, 10–20 and 20–40 cm depth. SOC and fulvic acids fraction contents were higher in the ORG system at 0–5 cm and 5–10 cm depths. Soil MBC was highest in the ORG, in all depths, than in others plots. Soil MBN was similar between ORG, TRA and NV in the surface layer. The lowest values for soil MBC and MBN were observed in CON plot. Soil microbial biomass increased gradually from conventional to organic farming, leading to consistent and distinct differences from the conventional control by the end of the second year.  相似文献   

11.
The aim of this study was to determine whether tree species consistently affects soil microbial activities related to C and N cycling and to compare these activities with the characteristics of soil dissolved organic matter (DOM). Samples were taken from the mor-type organic layer (Of+Oh) underlain by podzols of six 20–72-year-old tree-species experiments on different site types in different parts of Finland. Sampling plots were dominated by silver birch (Betula pendula Roth), Norway spruce (Picea abies (L.) Karst) or Scots pine (Pinus sylvestris L., only on four sites). Amounts of C and N in the microbial biomass and rates of C mineralization (CO2 production) and net N mineralization were determined, and water extracts were analysed for concentrations of DOC and DON and characterized according to molecular size by ultrafiltration and according to chemical composition using a resin fractionation technique. In all older stands, birch, compared to spruce or pine, increased soil pH, NH4-concentration and amounts of C and N in microbial biomass and decreased the C-to-N ratio and ratio of dissolved organic N (DON)-to-mineral N. Birch had similar effects also in part of the younger stands. Birch also increased the rates of both C and net N mineralization compared to spruce or pine but only on two sites. In all soils, net nitrification was low. The distribution of DOC into different fractions based on chemical composition and molecular size was rather similar in all soils. The most abundant chemical fraction was hydrophobic acids, and the most abundant molecular size fraction was 10–100 kDa. The C-to-N ratio varied but was lowest in hydrophilic bases and in the smallest molecular size class. Mineralization of C was highly and positively correlated with concentration of DOC (Pearson's correlation coefficient r = 0.9, P < 0.01). The results indicated close interactions between microbial processes and dissolved organic matter.  相似文献   

12.
We studied the effect of no-till (disc seeder), conventional-till (tine scarifier+disc seeder) and rotary-till (rotary hoe+disc seeder) management on soil organic matter (SOM) components, rates of carbon (C) and nitrogen (N) cycling, substrate utilization and microbial community composition. We hypothesized that labile SOM fractions are sensitive to changes in tillage techniques and, in turn mediate any tillage-induced changes in microbial function and composition. A replicated field site was established in May 1998 in the semi-arid agricultural region of Western Australia and soils were collected in September 2004. We found soil pH varied between different tillage techniques as an initial lime application was mixed to deeper soil depths in rotary-till soil than no-till and conventional-till soil. Total-C was greater in surface soil and lower in subsurface soil from no-till and conventional-till plots than from rotary-till plots, but there was no effect of tillage technique on total-C when averaged across soil depths. Light (specific density <1.0 g cm?3) fraction organic matter (LFOM), dissolved organic matter (DOM) and microbial biomass (MB) C and N pools, and rates of C and N cycling all tended to decrease with soil depth. In general, LFOM-C and N, dissolved organic C (DOC) and microbial biomass carbon (MB-C), soil respiration, cellulase activity, gross immobilization rates were positively correlated (r>0.50) and were greater in no-till and conventional-till soil than rotary-till soil both within, and across soil depths. These soil variables generally increased (r>0.5) with increasing soil pH. Dissolved organic N and gross N mineralization were positively correlated (r>0.90) but neither was affected by tillage techniques. No-till soil had greater utilization of carboxylic acids and lower utilization of amino acids and carbohydrates than conventional-till and rotary-till soil; surface soil also had greater utilization of carboxylic acids than subsurface soil. In turn, substrate utilization differed between soil depths, and between no-till soil and conventional-till and rotary-till soil; these differences were correlated with soil pH, total-N, DOC, LFOM-N and microbial biomass nitrogen (MB-N). Bacterial and fungal biomasses generally decreased with soil depth and were greater in no-till and conventional-till soil than rotary-till soil. Microbial community composition differed between all tillage techniques and soil depths; these differences were correlated with soil textural classes, soil pH, and total, LFOM, DOM and microbial C and N pools. These results indicate that most tillage-induced changes to soil properties were associated with the greater soil disturbance under rotary-till than under no-till or conventional-till management. Our results indicate that tillage-induced changes to soil pH, and LFOM, DOM and microbial biomass pools are likely to be important regulators of the rates of C and N cycling, substrate utilization and microbial community composition in this coarse textured soil.  相似文献   

13.
The potential for microorganisms to affect the quantity and quality of organic and condensed forms of phosphorus (P) in soils was investigated by repeated addition of different carbon sources (glucose, starch, cellulose; 2.5 g C kg?1) with or without inorganic P (50 mg P kg?1) to acid and calcareous soils which were either natural soils or clay–sand mixtures free of organic matter. Forms of P after five amendments and subsequent incubation periods of 5 weeks each were analyzed by 31P solution nuclear magnetic resonance (NMR) spectroscopy, and the microbial community composition was assessed by selective plate counts and fatty acid methyl ester (FAME) analysis. All carbon additions induced a redistribution of P from inorganic to organic and condensed forms, which was only little affected by the addition of inorganic P. Compared to non-carbon-amended controls, the greatest increase (7–38 mg P kg?1) in organic P was observed in the monoester region. In the acid clay–sand mixture, there was a large accumulation of pyrophosphate (101 mg P kg?1) after glucose addition and smaller accumulations (6–25 mg P kg?1) after addition of starch and cellulose. Carbon additions increased the microbial biomass in all cases and except in the natural calcareous soil also the proportion of fungi. Redundancy analysis with Monte Carlo permutation tests revealed that for carbon-amended soils, the microbial community composition was more strongly influenced by soil type than by carbon source. Pyrophosphate was positively related to fungi, and diester P was positively related to soil pH. A large proportion of organic and condensed forms of P may still have been in microbial cells at the time of extraction. We have shown that soil organic P consists of some discrete and simple compounds along with some more complex forms, and that organic P recently synthesized by microbes consists almost exclusively of and thus is a likely source for the simple compounds found in natural soils.  相似文献   

14.
《Applied soil ecology》2007,35(1):247-255
Seasonally snow-covered alpine soils may be subjected to freeze/thaw cycles, particularly during years having little snow and during the late winter and early spring periods. Freeze/thaw cycles can stimulate soil mineralization and could therefore be one factor regulating nitrogen (N) and phosphorus (P) availability and cycling. In this study laboratory incubation experiments using four soils having contrasting properties have been used to characterize the change in N and P forms (microbial and soluble inorganic/organic) that occur after simulated freeze/thaw cycles.Soil samples were collected from locations representing extreme examples of either direct human management (grazed meadow (site M) and extensive grazing beneath larch (site L)) or those disturbed by more natural events (recent avalanche and colonisation by alder (site A)) and from beneath the expected forest climax vegetation beneath fir (site F). Topsoil from these sites, maintained at two different water contents (20 and 30%, w/w), were exposed to either a single (SF) or four sequential (4SF) freeze/thaw cycles. Each cycle consisted of 12 h at −9 °C and 12 h at +4 °C mimicking a diurnal pattern.A SF cycle reduced microbial N for soils from sites F and A and was accompanied by a significant increase in dissolved organic nitrogen (DON) at both moisture contents. In contrast, the microbial N of soils from M and L was not affected by the freeze/thaw cycles, suggesting a particular adaptation of soil microbes to these extremes in temperature. Freeze/thaw cycles resulted in a significant increase in the net ammonification in all soils.Extractable total dissolved N (TDN) and total dissolved P (TDP) increased in all soils after a SF cycle, however, the relative importance of the different N and P forms differed. At the lower soil moisture content, NO3− concentrations remained constant or slightly decreased in all soils, except that from site M. In all other soils DON appeared to replace NO3− as the potentially mobile N source after the freeze/thaw cycles. The relative contribution of dissolved organic P to TDP after freeze/thaw remained significant, and greater than 50% in all soils.Freeze/thaw cycles, in seasonally snow covered soils, are likely to have a selective effect on the microbial biomass. Freezing and thawing resulted in a pulse of net ammonification and DON release, which represent an important influence upon N cycling in these alpine systems.  相似文献   

15.
In the last century, conversion of native North American grasslands to Juniperus virginiana forests or woodlands has dramatically altered ecosystem structure and significantly increased ecosystem carbon (C) stocks. We compared soils under recently established J. virginiana forests and adjacent native C4-dominated grassland to assess changes in potential soil nitrogen (N) transformations and plant available N. Over a 2-year period, concentrations of extractable inorganic N were measured in soils from forest and grassland sites. Potential gross N ammonification, nitrification, and consumption rates were determined using 15N isotope-dilution under laboratory conditions, controlling for soil temperature and moisture content. Potential nitrification rates (Vmax) and microbial biomass, as well as soil physical and chemical properties were also assessed. Extractable NH4+ concentrations were significantly greater in grassland soils across the study period (P ≤ 0.01), but analysis by date indicated that differences in extractable inorganic N occurred more frequently in fall and winter, when grasses were senescent but J. virginiana was still active. Laboratory-based rates of gross N mineralization (ammonification) and nitrification were greater in grassland soils (P ≤ 0.05), but only on one of four dates. Potential nitrification rates (Vmax) were an order of magnitude greater than gross nitrification rates in both ecosystems, suggesting that nitrification is highly constrained by NH4+ availability. Differences in plant uptake of N, C inputs, and soil microclimate as forests replace grasslands may influence plant available N in the field, as evidenced by seasonal differences in soil extractable NH4+, and total soil C and N accumulation. However, we found few differences in potential soil N transformations under laboratory conditions, suggesting that this grassland-to-forest conversion caused little change in mineralizable organic N pools or potential microbial activity.  相似文献   

16.
《Applied soil ecology》2007,35(2):319-328
The effects of salinity on the size, activity and community structure of soil microorganisms in salt affected arid soils were investigated in Shuangta region of west central Anxi County, Gansu Province, China. Eleven soils were selected which had an electrical conductivity (EC) gradient of 0.32–23.05 mS cm−1. There was a significant negative exponential relationship between EC and microbial biomass C, the percentage of soil organic C present as microbial biomass C, microbial biomass N, microbial biomass N to total N ratio, basal soil respiration, fluorescein diacetate (FDA) hydrolysis rate, arginine ammonification rate and potentially mineralizable N. The exponential relationships with EC demonstrate the highly detrimental effect that soil salinity had on the microbial community. In contrast, the metabolic quotient (qCO2) was positively correlated with EC, and a quadratic relationship between qCO2 and EC was observed. There was an inverse relationship between qCO2 and microbial biomass C. These results indicate that higher salinity resulted in a smaller, more stressed microbial community which was less metabolically efficient. The biomass C to biomass N ratio tended to be lower in soils with higher salinity, reflecting the bacterial dominance in microbial biomass in saline soils. Consequently, our data suggest that salinity is a stressful environment for soil microorganisms.  相似文献   

17.
Quantitative knowledge of the amount and stability of soil organic matter (SOM) is necessary to understand and predict the role of soils in the global carbon cycle. At present little is known about the influence of soil type on the storage and stability of SOM, especially in the tropics. We compared the amount of mineral-associated SOM resistant to different chemical treatments in soils of different parent material and mineralogical composition (volcanic ashes – dominated by short-range-order aluminosilicates and marine Tertiary sediments – dominated by smectite) in the humid tropics of Northwest Ecuador. Using 13C isotope analyses we traced the origin of soil organic carbon (SOC) in mineral-associated soil fractions resistant to treatment with HCl, NaOCl, and Na4P2O7 under pasture (C4) and secondary forest (C3). Prior to chemical treatments, particulate organic matter was removed by density fractionation (cut-off: 1.6 g cm?3). Our results show that: (1) independent of soil mineralogical composition, about 45% of mineral-associated SOC was resistant to acid hydrolysis, suggesting a comparable SOM composition for the investigated soils; (2) oxidation by NaOCl isolated a SOM fraction with enhanced stability of mineral-bound SOM in soils developed from volcanic ashes; while Na4P2O7 extracted more SOC, indicating the importance of Al-humus complexes in these soils; and (3) recently incorporated SOM was not stabilized after land use change in soils developed from volcanic ashes but was partly stabilized in soils rich in smectites. Together these results show that the employed methods were not able to isolate a SOM fraction which is protected against microbial decay under field conditions and that the outcome of these methods is sensitive to soil type which makes interpretation challenging and generalisations to other soils types or climates impossible.  相似文献   

18.
This study investigates how carbon sources of soil microbial communities vary with soil depth. Microbial phospholipid fatty acids (PLFA) were extracted from 0–20, 20–40 and 40–60 cm depth intervals from agricultural soils and analysed for their stable carbon isotopes (δ13C values). The soils had been subjected to a vegetation change from C3 (δ13C≈?29.3‰) to C4 plants (δ13C≈?12.5‰) 40 years previously, which allowed us to trace the carbon flow from plant-derived input (litter, roots, and root exudates) into microbial PLFA. While bulk soil organic matter (SOM) reflected ≈12% of the C4-derived carbon in top soil (0–20 cm) and 3% in deeper soil (40–60 cm), the PLFA had a much higher contribution of C4 carbon of about 64% in 0–20 cm and 34% in 40–60 cm. This implies a much faster turnover time of carbon in the microbial biomass compared to bulk SOM. The isotopic signature of bulk SOM and PLFA from C4 cultivated soil decreases with increasing soil depth (?23.7‰ to ?25.0‰ for bulk SOM and ?18.3‰ to ?23.3‰ for PLFA), which demonstrates decreasing influence of the isotopic signature of the new C4 vegetation with soil depth. In terms of soil microbial carbon sources this clearly shows a high percentage of C4 labelled and thus young plant carbon as microbial carbon source in topsoils. With increasing soil depth this percentage decreases and SOM is increasingly used as microbial carbon source. Among all PLFA that were associated to different microbial groups it could be observed that (a) depended on availability, Gram-negative and Gram-positive bacteria prefer plant-derived carbon as carbon source, however, (b) Gram-positive bacteria use more SOM-derived carbon sources while Gram-negative bacteria use more plant biomass. This tendency was observed in all three-depth intervals. However, our results also show that microorganisms maintain their preferred carbon sources independent on soil depth with an isotopic shift of 3–4‰ from 0–20 to 40–60 cm soil depth.  相似文献   

19.
Addition of organic manure over thousands of years has resulted in the development of very fertile soils in parts of the Loess Plateau in Northwest China. This region also suffers from serious soil erosion. For that reason, afforestation of arable soils has taken place. The dynamics of soil organic matter in these soils affected by a very specific management and by land use changes is largely unknown. Therefore, we measured C mineralization in a 35-days incubation experiment and analyzed amounts and properties of water-extractable organic carbon (WEOC) in 12 topsoils of this region. The soils differed in land use (arable vs. forest) and in amounts of added organic manure. Afforestation of arable soils resulted in a distinct stabilization of organic C as indicated by the smallest C mineralization (0.48 mg C g−1 C d−1) and the highest C content (2.3%) of the studied soils. In the soils exposed to intensive crop production without regular addition of organic manure we found the largest C mineralization (0.85 mg C g−1 C d−1) and the lowest contents of organic C (0.9%). Addition of organic manure over a time scale of millennia resulted in high organic C contents (1.8%) and small C mineralization (0.55 mg C g−1 C d−1). The content of WEOC reflected differences in C mineralization between the soils quite well and the two variables correlated significantly. Water-extractable organic C decreased during C mineralization from the soil illustrating its mainly labile character. Carbon mineralization from soils was particularly large in soils with small specific UV absorbance of WEOC. We conclude that amounts and properties of WEOC reflected differences in the stability of soil organic C. Both afforestation of arable land and the long-term addition of organic manure may contribute to C accumulation and stabilization in these soils.  相似文献   

20.
《Applied soil ecology》2007,35(2-3):160-167
Soluble organic N and C were extracted from soils under long-term kikuyu grass pasture, annual ryegrass pasture and annual maize production using water, 0.5 M K2SO4 and 2 M KCl. Quantities extracted with K2SO4 were more than double those extracted with water while those extracted with KCl exceeded those using K2SO4. Differences in soluble organic C and N between land uses were much more obvious when water rather than salt solutions were used. It was suggested that water extracts give more realistic values than salt solutions. Regardless of the extractant used, the proportion of total N present as soluble N was considerably greater than the equivalent proportion of organic C present as soluble C. While the percentage of soil organic C and total N present in the light fraction and microbial biomass was lower in the kikuyu than ryegrass and maize soils, the equivalent values for water soluble C and N were, in fact, greatest in the kikuyu soil.The leaching of organic C, N and NO3− from these soils was also measured over a 6-month period in a greenhouse lysimeter study. The soils were either left undisturbed or were disturbed (broken into clods <50 mm diameter) to simulate tillage and stimulate microbial activity. Quantities of organic C and N leached were greater from the kikuyu than other treatments and tended to be greatest from the disturbed kikuyu soil. The percentage of total soil N leached as organic N was considerably greater than that of total organic C leached as soluble C. Leaching of NO3− was greatest from the disturbed kikuyu soil and least from the undisturbed kikuyu soil. The mean percentage of total soluble N present in organic form in leachates ranged from 17 to 32% confirming the importance of this form of N to leaching losses of N from agricultural soils.  相似文献   

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