泥炭土壤上与胞外酶相关的土壤微生物体与Marsh orchid的交互作用

The nature of the interactions between microbes and roots of plants in a peaty soil were studied in a laboratorybased experiment by measuring activities of β-glucosidase, phosphatase, N-acetylglucosaminidase, and arylsulphatase. The experiment was based on control （autoclaved）, bacteria-inoculated, and plant （transplanted with Dactylorhiza） treatments, and samples were collected over 4 sampling intervals. Higher enzyme activities were associated with the bacteria-inoculated treatment, suggesting that soil enzyme activities are mainly of microbial origin. For example,β-glucosidase activity varied between 25-30μmol g^-1 min^-1 in the bacteria-inoculated samples whilst the activity of the control ranged between 4-12 μmol g^-1 min^-1. A similar pattern was found for all other enzymes. At the end of the incubation, the microcosms were destructively sampled and the enzyme activities determined in bulk soil, rhizospheric soil, and on the root surface. Detailed measurement in different fractions of the peat indicated that higher activities were found in rhizosphere. However, the higher activities of β-glucosidase, N-acetylglucosaminidase, and arylsulphatase appeared to be associated with bacterial proliferation on the root surface, whilst a larger proportion of phosphatase appeared to be released from root surface.     

Influence of varied soil microbial and inorganic nutrient conditions on the growth and vesicular-arbuscular mycorrhizal colonization of little bluestem [Schizachyrium scoparium (Michx.) Nash]

Summary A difference in biomass production between plants grown in autoclaved soil and non-autoclaved soil under N and base (Ca + Mg) treatments was probably caused by soil microbes other than vesicular-arbuscular mycorrhizal fungi. The plants were grown for 70 days in autoclaved soil, autoclaved soil with a vesicular-arbuscular mycorrhizal-free filtrate of non-autoclaved soil added, and non-autoclaved soil. The plants in each substrate received additional N, P, or Ca + Mg (base treatment) weekly. Control plants received no additional nutrients. The plant response to various substrates was a function of nutrient treatment. Colonization of roots by vesicular-arbuscular mycorrhizal fungi in non-autoclaved soil was lowest with the N and P treatments. There were significant negative correlations between vesicular-arbuscular mycorrhizal colonization and all plant growth variates. For all nutrient treatments, there were no differences in total biomass between plants grown in non-autoclaved soil and in the autoclaved-plus-filtrate substrate.     

Cowpea rhizobia: Comparison of plant infection and plate counts

Enumeration of Rhizobium in soil is usually accomplished by the most probable number (MPN) plant infection method. The accuracy of MPN counts as compared to plate counts was determined for five strains of cowpea rhizobia. Host plants included cowpea (Vigna unguiculata (L.) Walp.), siratro (Macroptilium atropurpureum (DC.) Urb.) and peanut (Arachis hypogaea L.). Plastic growth pouches were used primarily for cowpea and siratro while plastic cups containing vermiculite were used for peanut. The number of rhizobia determined by the MPN method using cowpea, siratro and peanut underestimated the population from 10- to 100-fold. A control experiment using soybeans (Glycine max (L.) Merrill) indicated that the MPN method was accurate for R. japonicum. Experimentors using the MPN method should be aware of its accuracy for particular Rhizobiutn-legume combinations.     

Effects of soil compaction on N-mineralization and microbial-C and -N. I. Field measurements

Soil biological parameters, such as soil respiration or N-mineralization, may be more sensitive to soil compaction than physical parameters. Therefore we studied the effects of soil compaction on net N-mineralization and microbial biomass dynamics in the field. The soils were silty clay loams (Typic Endoaquepts) in either a well-structured permanent pasture with high organic-C content (46 mg g⁻¹) or a site which had been continuously cropped with cereals for 28 years with low organic-C content (21 mg g⁻¹) and a very poor structure. Compaction treatments were applied by five passes of a tractor (total weight 4880 kg, speed 2.2 m s⁻¹). An energy flux of either 2712 J m⁻² (assuming deflecting tyres) or 6056 J m⁻² (assuming rigid tyres) per pass of the rear tyres was estimated. Soil dry bulk densities were initially 1.00 and 1.30 Mg m⁻³ in the pasture and cropped sites, respectively, and increased significantly only in the less dense pasture site. However, soil surface CO₂-fluxes decreased substantially after compaction on both sites (57–69%) because of the highly reduced air permeability of the topsoil. At the cropped site this was also accompanied by a significant decrease in oxygen-diffusion rate (45%). Using the in situ core technique with covered cores the apparent net N-mineralization rate was less in compacted than in non-compacted areas of the pasture ((0.27 and 0.38 μg N g⁻¹ day⁻¹, respectively), but did not differ at the cropped site (average 0.15 μg N g⁻¹ day⁻¹). However, N-mineralization measurements by the in situ core technique were found to be problematic as denitrification possibly occurred and concealed actual net N-mineralization. Microbial biomass did not change significantly as a result of the compaction treatment, but was shown to either decrease or increase over time depending on the methodology used to estimate microbial biomass.     

Compression of agricultural soils from Quebec

Static uniaxial compression tests were performed on 26 agricultural soils from Quebec. Compression lines (bulk density vs. applied load) were obtained at different water contents for each soil previously sieved to 6 mm. For soils with clay contents less than 35%, the compression index (slope of the compression line) was best correlated with the mineral fraction of the soil (r = 0.75^** with clay and r =−0.78^** with sand). For clay-rich soils, the compression index was best correlated with organic carbon content (r = −0.75^**). The bulk density under standard compression conditions (100 kPa load and 50% water saturation) was related to both clay (r = −0.80^**) and organic carbon (r = −0.77^**). This parameter was also highly correlated with the soil lower plastic limit (r = −0.95^**) which corroborates the observation that the consistency limits can be good predictors of other mechanical properties which are more difficult to determine. Results suggested that both the mineral and the organic fractions have much influence on the compressive behaviour of Quebec agricultural soils.     

Protozoa from aboveground and ground soils of a tropical rain forest in Puerto Rico

Decomposition occurs in the aboveground and ground litter and soils of tropical rain forests, but little is known about the protozoa that stimulate bacterial activity and turnover. I examined litter and ground soils, epiphytic bryophyte soils on tree trunks and branches, and adventitious roots of lianas attached to tree trunks, within 2 m above ground in the Luquillo Experimental Forest, within the Caribbean National Forest, Puerto Rico. Amoebae numbered 69,000–170,000, ciliates 1000–25,000, and testate amoebae 58,000–190,000 g⁻¹ dry wt. of litter, but were reduced by 0.25–0.5 of these abundances in the underlying soils. In the aboveground soils, amoebae numbered 64,000–145,000, ciliates 1000–8000, and testate amoebae 84,000–367,000 g⁻¹ dry wt. of soil. Eighty species of ciliates and 104 species of testate amoebae were found. About 50% of the individuals in ciliate and 33% in testate amoebae populations were small r-selected species, illustrating that functional differences between species determine community composition. Although protozoan numbers are best described as “protozoan potential” because many individuals may be dormant, the high moisture content of tropical rain forest litter and soils suggest almost continually connected soil water films (necessary for protozoan transport), and together with the large numbers and biodiversity of protozoa, suggest that a major proportion of these protozoa contribute to the bacterial decomposition channel of organic matter.     

Effects of mechanical energy inputs on soil respiration at the aggregate and field scales

Cultivation machinery applies large amounts of mechanical energy to the soil and often brings about a decrease in soil organic carbon (SOC). New experiments on the effects of mechanical energy inputs on soil respiration are reported and the results discussed. In the laboratory, a specific energy, K, of 150 J kg⁻¹, similar to that experienced during typical cultivation operations, was applied to soil aggregates using a falling weight. Respiration (carbon dioxide, CO₂ emission) of the samples was then measured by an electrical conductimetric method. Basal respiration (when K=0) measured on Chromic Luvisol aggregates, was found to increase with increasing SOC, from 1.88 μg CO₂ g⁻¹ h⁻¹ for a permanent fallow soil (SOC=11 g kg⁻¹) to 8.25 μg CO₂ g⁻¹ h⁻¹ for a permanent grassland soil (SOC=32 g kg⁻¹). Basal respiration of a Calcic Cambisol, more than doubled (2.0–5.2 μg CO₂ g⁻¹ h⁻¹) with increasing gravimetric soil water contents. Mechanical energy inputs caused an initial burst of increased respiration, which lasted up to 4 h. Over the following 4–24 h period, arable soils with lower SOC contents, (11–21 g kg⁻¹), respiration rates dropped back to a level, approximately 1.14 times higher than the basal value. However, grassland soils with higher SOC contents (28–32 g kg⁻¹), increases in this longer-term respiration rate following 150 J kg⁻¹ of energy, were negligible. A field experiment, in which CO₂ was measured by infra-red absorption, also showed that tillage stimulated increased levels of soil respiration for periods ranging from 12 h to more than one week. The highest respiration rates, 80 mg CO₂ m⁻² h⁻¹ were associated with high energy, powered tillage on clay soils. On the same soil, low energy draught tillage resulted in a respiration rate of approximately half this value. The results of these experiments are discussed in relation to equilibrium levels of soil organic matter. The application of known quantities of mechanical energy to soil aggregates under laboratory conditions, in order to simulate the effect of different cultivation practices, when combined with the subsequent measurement of soil respiration, can provide useful indication of the likely consequences of soil management on SOC.     

Microbial biomass and metabolic activity in four acid soils

The fumigation method was used to estimate microbial biomass C in four Haplumbrepts developed over different kinds of rock. In order to investigate the relationship between metabolic activity and microbial biomass and population density, CO₂ release from the glucose-enriched and unenriched soils was measured during 28 days of incubation.

Biomass C levels lay between 36 and 112 mg 100 g⁻¹ of dry soil, and made up only a small proportion of total soil C (0.77–1.38%). Only a small fraction of this biomass was detected by counting viables, but the microbial population was nevertheless significantly correlated with the biomass determined by fumigation. Among the physico-chemical properties of the soils, microbial biomass and population size were both chiefly affected (favourably) by humidity, total C and N and Al gel content. Metabolic activity was slight, either because part of the micro-organisms are inactive or because of a limited supply of substrate (the organic matter present may be unsuitable as a substrate or protected from microbial attack). Percentage C mineralization was inversely related to organic matter, silt and Al gel contents, and likewise failed to exhibit positive correlation with respiration, the biomass determined by fumigation or the counted population. The metabolic activity of the biomass appeared to depend upon the quality and nature of soil organic matter rather than its quantity, which nevertheless controlled microbial population size.

Neither microbial biomass estimates nor viable population counts faithfully reflected metabolic activity in the soils.     

Root–soil adhesion as affected by crop species in a volcanic sandy soil of Mexico

Field observations have shown that root residues maintain root-adhering soil for several months after harvest. The aim of this work was to compare post-harvest effect of Amaranthus hypochondriacus (amaranth), Phaseolus vulgaris (common bean) and Zea mays (maize) roots on root-adhering soil, aggregation and organic carbon content. The experimental site was located on a volcanic sandy soil (Typic Ustifluvent) in the Valley of Mexico. In 1999 and 2000, maize had the highest root mass (92 and 94 g m⁻²) and the highest root-adhering soil (9051 and 5876 g m⁻²) when a root–soil monolith of 0.20 m × 0.20 m × 0.30 m was excavated after harvest. In contrast, bean roots (2 and 5 g m⁻²) had only 347 and 23 g m⁻² of adhering soil per monolith in each year. Amaranth had intermediate values between maize and bean. Dry soil aggregate classes (<0.25, 0.5, 1, 2, 5 and >5 mm) were similarly distributed among the three species. The sum of the three soil macro-aggregates classes >1 mm was 0.1 g g⁻¹ in both years. Neither water stability of the 2–5 mm aggregates (0.05–0.09 g g⁻¹) nor soil organic C (SOC) in three aggregate classes (<0.25, 1–2 and >5 mm; mean 14.6 mg g⁻¹) was affected by species (P < 0.05) in either year. Observations of thin sections (10× and 40×) revealed absence of macro-aggregates under maize. Soil compaction was attributed to high mass of maize roots in the sampled soil volume. Root systems sampled after harvest had the capacity to maintain a well structured soil mass, which was proportional to root mass. Root-adhering soil measured in the field could be used to select species promoting soil adhesion by roots.     

A proposal for re-evaluating the most probable number procedure for estimating numbers of Bradyrhizobium spp.

The reliability of the most probable number (MPN) method for estimating bradyrhizobial numbers was evaluated by comparison with the plate count procedure. MPN estimates increased with time of nodulation scoring after seedling inoculation through 6 weeks of incubation. Ratios of MPN to plate counts increased as the bradyrhizobial cell suspension concentration increased. The MPN method could not detect Bradyrhizobium japonicum numbers at concentrations of 10³ colony forming units (CFU) ml^-1 and below. A proposal for re-evaluating MPN estimates is discussed.     

Populations of predatory protozoa in field soils after 5 years of elemental s fertilizer application

The effect of 5 yr of repeated application of elemental S (S°) fertilizer on predatory protozoa in soil was investigated. Protozoa that feed on the bacteria Arthrobacter globiformis and Enterobacter aerogenes or the fungi Fusarium solani and Neurospora crassa were enumerated by most probable number (MPN) methods. The application of S° fertilizer reduced the microbial biomass and its activity in soil. Soils treated with 44kg S° ha⁻¹ yr⁻¹ for 5 yr exhibited a 30–71% decline in MPN of protozoa feeding on bacteria and more than a 84% decline in the population of mycophagous amoebae. This decline in protozoa populations parallelled changes in microbial biomass, especially in the case of mycophagous amoebae and fungal biomass. The adverse effect of repeated S° applications on microbial biomass and predatory protozoa was long lasting. Since nutrient transformations (e.g. mineralization) in soil are influenced by microbial interactions, our results suggest reduced nutrient turnover via microbial predation in S° treated soils.     

盐碱地小麦根际联合固氮菌数量分布研究

结合气相色谱仪和乙炔还原(ARA),采用最可能计数法(MPN)和荧光抗体(FA)染色法对盐碱地小麦根际联合固氮菌及优良固氮菌株Pseudomonassp.ChW1和Zoogloeasp.ChW6数量分布进行了测定。结果表明:小麦根际各部位均存在联合固氮菌类群,但数量相对较小、差异较大(102～106个g-1干土或根),分布趋势呈RP>RS>NRS>HP;菌株Pseudomonassp.ChW1是一种相对广谱的联合固氮菌株,在小麦根际不同部位均有分布,数量除在根系表面(RP)较多(105个g-1干根)外,其它部位较少(102～103个g-1干土或根);Zoogloeasp.ChW6菌株是一种分布谱相对较窄的联合固氮菌株,只分布于根表土壤(RS)和根系表面(RP),数量102～104个g-1干土或根。自然状况下,小麦根际固氮菌数量较少,有必要通过使用联合固氮菌接种剂提高小麦根际固氮菌种群数量,增强种群竞争力,增加根际微生物固氮总量。     

Identification and Characterization of High Acid Tolerant and Aluminum Resistant Yeasts Isolated from Tea Soils

From acidic tea soils of Kagoshima Prefecture in Japan, some soil properties were determined and 38 strains of acid tolerant microorganisms were isolated. Different Al³⁺ concentrations were applied to YG media to estimate Al resistance. Selected microbial strains could grow strongly in the liquid media in the presence of 100 mM Al³⁺ and survive even in 300 mM Al³⁺ at pH 3.0. Their base sequences of 28S rDNA-D1/D2 were determined and sequence data were searched using the Basic Local Alignment Search Tool (BLAST) system. The results of sequencing revealed that the isolates belong to two different species, Cryptococcus sp. and Candida palmioleophila. When cultivated with various Al³⁺ concentrations, the yeast growth was inhibited at a concentration of 200 mM. Pre-cultivation of these strains with 0–30 mM Al³⁺ did not promote the growth response caused by Al³⁺. Inductively-Coupled Plasma-Mass Spectrometry (ICP-MS) was used to assess the elimination of Al. The amount of Al remaining in culture media was decreased considerably after cultivation. Due to a capacity for resistance to significant Al concentrations as well as high Al elimination, these acid tolerant and Al resistant yeasts may have potential applications in the bio- and phyto-remediation of Al and acid-contaminated soils.     

Rapid multipoint method for quantification of various physiological groups of bacteria in soil

A modified multipoint method for enumeration of bacteria capable of various biochemical degradation reactions in soil is described. The method is a combination of multipoint inoculation of a dilution series and the most probable number (MPN) technique. Five serial soil dilutions are prepared and inoculated directly on plates containing different substrates. Five replicates of a constant amount of each dilution are transferred with a multipoint inoculator to plates suitable for visualization of the physiological groups involved. At least five replicate plates of each substrate are recommended. Degradation zones at the inoculation sites are scored, and a MPN method is used for enumeration of the bacterial groups in question. A MPN table is provided for the dilution series used. The inoculation method is rapid, allowing the same soil sample to be plated on many different substrates in a short time. The use of plates instead of tubes simplifies the method and allows long incubation periods, and only one plate is required for 25 inoculation sites. For comparison, a general isolation technique for evaluation of various physiological groups of bacteria in soil was conducted.     

Long-term impact of reduced tillage and residue management on soil carbon stabilization: Implications for conservation agriculture on contrasting soils

Residue retention and reduced tillage are both conservation agricultural management options that may enhance soil organic carbon (SOC) stabilization in tropical soils. Therefore, we evaluated the effects of long-term tillage and residue management on SOC dynamics in a Chromic Luvisol (red clay soil) and Areni-Gleyic Luvisol (sandy soil) in Zimbabwe. At the time of sampling the soils had been under conventional tillage (CT), mulch ripping (MR), clean ripping (CR) and tied ridging (TR) for 9 years. Soil was fully dispersed and separated into 212–2000 μm (coarse sand), 53–212 μm (fine sand), 20–53 μm (coarse silt), 5–20 μm (fine silt) and 0–5 μm (clay) size fractions. The whole soil and size fractions were analyzed for C content. Conventional tillage treatments had the least amount of SOC, with 14.9 mg C g⁻¹ soil and 4.2 mg C g⁻¹ soil for the red clay and sandy soils, respectively. The highest SOC content was 6.8 mg C g⁻¹ soil in the sandy soil under MR, whereas for the red clay soil, TR had the highest SOC content of 20.4 mg C g⁻¹ soil. Organic C in the size fractions increased with decreasing size of the fractions. In both soils, the smallest response to management was observed in the clay size fractions, confirming that this size fraction is the most stable. The coarse sand-size fraction was most responsive to management in the sandy soil where MR had 42% more organic C than CR, suggesting that SOC contents of this fraction are predominantly controlled by amounts of C input. In contrast, the fine sand fraction was the most responsive fraction in the red clay soil with a 66% greater C content in the TR than CT. This result suggests that tillage disturbance is the dominant factor reducing C stabilization in a clayey soil, probably by reducing C stabilization within microaggregates. In conclusion, developing viable conservation agriculture practices to optimize SOC contents and long-term agroecosystem sustainability should prioritize the maintenance of C inputs (e.g. residue retention) to coarse textured soils, but should focus on the reduction of SOC decomposition (e.g. through reduced tillage) in fine textured soils.     

Organic matter addition, N, and residue burning effects on infiltration, biological, and physical properties of an intensively tilled silt-loam soil

Seventy years of different management treatments have produced significant differences in runoff, erosion, and ponded infiltration rate in a winter wheat (Triticum aestivum L.)–summer fallow experiment in OR, USA. We tested the hypothesis that differences in infiltration are due to changes in soil structure related to treatment-induced biological changes. All plots received the same tillage (plow and summer rod-weeding). Manure (containing 111 kg N ha⁻¹), pea (Pisum sativum L.), vine (containing 34 kg N ha⁻¹), or N additions of 0, 45 and 90 kg ha⁻¹ were treatment variables with burning of residue as an additional factor within N-treatments. We measured soil organic C and N, water stability of whole soil, water stable aggregates, percolation through soil columns, glomalin, soil-aggregating basidiomycetes, earthworm populations, and dry sieve aggregate fractions. Infiltration was correlated (r = 0.67–0.95) to C, N, stability of whole soil, percolation, and glomalin. Basidiomycete extracellular carbohydrate assay values and earthworm populations did not follow soil C concentration, but appeared to be more sensitive to residue burning and to the addition of pea vine residue and manure. Dry sieve fractions were not well correlated to the other variables. Burning reduced (p < 0.05) water stability of whole soil, total glomalin, basidiomycetes, and earthworm counts. It also reduced dry aggregates of 0.5–2.0 mm size, but neither burning nor N fertilizer affected total C or total N or ponded infiltration rate. Water stability of whole soil and of 1–2-mm aggregates was greater at 45 kg N ha⁻¹ than in the 0 and 90 kg N ha⁻¹ treatments. Zero N fertilizer produced significantly greater 0.5–2.0 mm dry aggregate fractions. We conclude that differences in infiltration measured in the field are related to relatively small differences in aggregate stability, but not closely related to N or residue burning treatments. The lack of an effect of N fertilizer or residue burning on total C and N, along with the excellent correlation between glomalin and total C (r = 0.99) and total N (r = 0.98), indicates that the major pool of soil carbon may be dependent on arbuscular mycorrhizal fungi.     

严重退化红壤植被恢复后有机质富集和团聚体稳定性

Three types of soils： an eroded barren soil under continuous fallow, an eroded soil transplanted with Lespedeza shrubs （Lespedeza bieolor）, and an eroded soil transplanted with camphor tree （Cinnaraomum camphora） were investigated to quantify organic matter pools and aggregates in reforested soils using physical fractionation techniques and to determine aggregate stability in relation to the enrichment of soil organic carbon （SOC）. Soil organic matter （SOM） was physically fractionalized into free particulate organic matter （fPOM）, occluded particulate organic matter （oPOM）, and mineralassociated organic matter （mOM）. The SOM was concentrated on the surface soil （0 5 cm）, with an average C sequestration rate of 20-25 g C m^-2 year^-1 over 14 years. As compared to the eroded barren land, organic C content of fPOM, oPOM, and mOM fractions of the soil under Lespedeza and under camphor tree increased 12-15, 45-54, and 3.1-3.5 times, respectively. A linear relationship was found between aggregate stability and organic C （r^2 = 0.45, P 〈 0.01）, oPOM （r^2 = 0.34, P 〈 0.05）, and roOM （r^2 = 0.46, P 〈 0.01） of aggregates. The enrichment of organic C improved aggregate stability of the soil under Lespedeza but not that under camphor tree. However, further research is needed on the physical and biological processes involved in the interaction of soil aggregation and SOC sequestration in ecosystem.     

Short-term changes in nitrogen availability, gas fluxes (CO2, NO, N2O) and microbial biomass after tillage during pasture re-establishment in Rondônia, Brazil

Anthropogenic conversion of primary forest to pasture for cattle production is still frequent in the Amazon Basin. Practices adopted by ranchers to restore productivity to degraded pasture have the potential to alter soil N availability and N gas losses from soils. We examined short-term (35 days) effects of tillage prior to pasture re-establishment on soil N availability, CO₂, NO and N₂O fluxes and microbial biomass C and N under degraded pasture at Nova Vida ranch, Rondônia, Brazilian Amazon. We collected soil samples and measured gas fluxes in tilled and control (non tilled pasture) 12 times at equally spaced intervals during October 2001 to quantify the effect of tillage. Maximum soil NH₄⁺ and NO₃⁻ pools were 13.2 and 6.3 kg N ha⁻¹ respectively after tillage compared to 0.24 and 6.3 kg N ha⁻¹ in the control. Carbon dioxide flux ranged from 118 to 181 mg C–CO₂ m² h⁻¹ in the control (non-tilled) and from 110 to 235 mg C–CO₂ m² h⁻¹ when tilled. Microbial biomass C varied from 365 to 461 μg g⁻¹ in the control and from 248 to 535 μg g⁻¹ when tilled. The values for N₂O fluxes ranged from 1.22 to 96.9 μg N m⁻² h⁻¹ in the tilled plots with a maximum 3 days after the second tilling. Variability in NO flux in the control and when tilled was consistent with previous measures of NO emissions from pasture at Nova Vida. When tilled, the NO/N₂O ratio remained <1 after the first tilling suggesting that denitrification dominated N cycling. The effects of tilling on microbial parameters were less clear, except for a decrease in qCO₂ and an increase in microbial biomass C/N immediately after tilling. Our results suggest that restoration of degraded pastures with tillage will lead to less C matter, at least initially. Further long-term research is needed.     

In situ and potential CO2 evolution from a Fluventic Ustochrept in southcentral Texas as affected by tillage and cropping intensity

Quality of agricultural soils is largely a function of soil organic matter. Tillage and crop management impact soil organic matter dynamics by modification of the soil environment and quantity and quality of C input. We investigated changes in pools and fluxes of soil organic C (SOC) during the ninth and tenth year of cropping with various intensities under conventional disk-and-bed tillage (CT) and no tillage (NT). Soil organic C to a depth of 0.2 m increased with cropping intensity as a result of greater C input and was 10% to 30% greater under NT than under CT. Sequestration of crop-derived C input into SOC was 22±2% under NT and 9±4% under CT (mean of cropping intensities ± standard deviation of cropping systems). Greater sequestration of SOC under NT was due to a lower rate of in situ soil CO₂ evolution than under CT (0.22±0.03 vs. 0.27±0.06 g CO₂–C g⁻¹ SOC yr⁻¹). Despite a similar labile pool of SOC under NT than under CT (1.1±0.1 vs. 1.0±0.1 g mineralizable C kg⁻¹ SOC d⁻¹), the ratio of in situ to potential CO₂ evolution was less under NT (0.56±0.03) than under CT (0.73±0.08), suggesting strong environmental controls on SOC turnover, such as temperature, moisture, and residue placement. Both increased C sequestration and a greater labile SOC pool were achieved in this low-SOC soil using NT and high-intensity cropping.     

Denitrification in a cultivated soil: Optimal glucose and nitrate concentrations

An experiment was conducted in the laboratory on a cultivated soil incubated in serum bottles with a range of C-to-nitrate concentrations. C was added in form of glucose and nitrate in form of Ca(NO₃)₂. It was shown that an C-N concentration of respectively 500 μg C (glucose-equivalent, Glc-Eq.) and 36 μg N g⁻ dry soil was optimal for denitrification. Results obtained either in the laboratory, in soil columns or in the field were in good agreement with one another. In particular, the root zone was shown to be favorable for denitrifying activity because the water-soluble C (Glc-Eq.) and N concentrations were more favorable than in bare soil. Furthermore, the water-soluble extractable Glc-Eq. appeared to be closely related to the denitrification rate and is thus likely to represent the energetic C pool supporting denitrification.

This was related to an inhibiting effect of increasing NO₃⁻ and NO₂⁻ concentrations on NO₃⁻ loss and N₂O production. Such inhibition can affect short-term measurements of denitrification in the field.