Characterizing the relationships between soil organic matter components and microbial function and composition along a tillage disturbance gradient

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.     

Impact of long-term N fertilization on the structural composition of spruce litter and mor humus

The aims of this study were to determine the degree of lignin degradation and to investigate changes in the chemical composition of the organic matter in the forest floor in an N fertilized Norway spruce forest soil. Needle litter and mor humus were collected from the field experiment at Skogaby in southern Sweden (56°33′N; 13°13′E). The spruce stand had been fertilized for 11 years with 100 kg N ha⁻¹ yr⁻¹ as (NH₄)₂SO₄. The degree of lignin degradation was determined with alkaline CuO oxidation followed by HPLC analysis. The chemical composition of the organic matter was characterized by CPMAS ¹³C NMR. Tannin was specifically analyzed using dipolar dephasing CPMAS ¹³C NMR and the N distribution was studied by CPMAS ¹⁵N NMR.The C-to-N ratios in the fertilized Oi and Oe layers were significantly lower than in the unfertilized layers (24 compared to 34 and 23 compared to 27, respectively). Neither the sum of the CuO oxidation products (Vanillyls+Syringyls+Cinnamyls expressed as VSC) nor the acid-to-aldehyde ratio ((Ac/Al)_V) showed any significant treatment effects. The content of aromatic C (including phenolic C) was significantly lower in the unfertilized than in the fertilized Oi layer (18 versus 21%). In the unfertilized soil, VSC was positively correlated (r=+0.63, p<0.05) with the C-to-N ratio, whereas the phenolic C content was negatively correlated (r=−0.61, p<0.05). The tannin index showed a tendency of increasing from Oi to Oe layers in both treatments. Most of the organic N was found as amide-N, whereas no heterocyclic N was detected. We have not been able to show any major C structural changes due to N fertilization. We suggest that the significantly higher content of aromatic and phenolic C in the fertilized Oi layer is due to an initial stimulation of the microbial community.     

Controls on soil nitrogen cycling and microbial community composition across land use and incubation temperature

We conducted a laboratory incubation of forest (Scots pine (Pinus sylvestris) or beech (Fagus sylvatica)), grassland (Trifolium repens/Lolium perenne) and arable (organic and conventional) soils at 5 and 25 °C. We aimed to clarify the mechanisms of short-term (2-weeks) nitrogen (N) cycling processes and microbial community composition in relation to dissolved organic carbon (DOC) and N (DON) availability and selected soil properties. N cycling was measured by ¹⁵N pool dilution and microbial community composition by denaturing gradient gel electrophoresis (DGGE), phospholipid fatty acid (PLFA) and community level physiological profiles (CLPP). Soil DOC increased in the order of arable<grassland<forest soil while DON and gross N fluxes increased in the order of forest<arable<grassland soil; land use had no affect on respiration rate. Soil DOC was lower, while respiration, DON and gross N fluxes were higher at 25 than 5 °C. Gross N fluxes, respiration and bacterial biomass were all positively correlated with each other. Gross N fluxes were positively correlated with pH and DON, and negatively correlated with organic matter, fungal biomass, DOC and DOC/DON ratio. Respiration rate was positively correlated with bacterial biomass, DON and DOC/DON ratio. Multiple linear modelling indicated that soil pH, organic matter, bacterial biomass, DON and DOC/DON ratio were important in predicting gross N mineralization. Incubation temperature, pH and total-C were important in predicting gross nitrification, while gross N mineralization, gross nitrification and pH were important in predicting gross N immobilization. Permutation multivariate analysis of variance indicated that DGGE, CLPP and PLFA profiles were all significantly (P<0.05) affected by land use and incubation temperature. Multivariate regressions indicated that incubation temperature, pH and organic matter content were important in predicting DGGE, CLPP and PLFA profiles. PLFA and CLPP were also related to DON, DOC, ammonium and nitrate contents. Canonical correlation analysis showed that PLFA and CLPP were related to differences in the rates of gross N mineralization, gross nitrification and soil respiration. Our study indicates that vegetation type and/or management practices which control soil pH and mediate dissolved organic matter availability were important predictors of gross N fluxes and microbial composition in this short-term experiment.     

Soil microbial populations and activities as influenced by legume green fallow in a semiarid climate

Tilled fallow-wheat (Triticum aestivum L.) (F-W) is the most commonly used cropping system in the semiarid Canadian prairie. However, because frequent fallowing degrades soil, a partial fallow with annual legumes as green manure [i.e. ‘green fallow’ (GF)] has been proposed as a soil conserving and more bio-resource efficient alternative to bare fallow. We conducted a 6-year study to compare the influence of four GF-W, a F-W and a continuous W (Cont. W) system, on soil microbial communities, microbial biomass (MB) and activities in an Orthic Brown Chernozem, silt loam (Aridic Haploboroll) at Swift Current, Sask., Canada. The four GF legumes used were black lentil (Lens culinaris Medikus), Tangier flatpea (Lathyrus tingitanus L.), chickling vetch (Lathyrus sativus L.) and feedpea (Pisum sativum L.). They were grown to full bloom and then incorporated with a tandem disk. Analysis of soil taken from 0 to 10 cm depth after growing wheat in the sixth year of the experiment (i.e. 15 months after the most recent legume GF had been turned under) showed that most of the soil biochemical and microbiological attributes assessed were significantly improved (compared to F-W) by increasing cropping intensity (Cont. W), and even more by using the GF systems. The average improvement gained from the four GF legumes relative to F-W, was 385% for number of bacteria, 210% for filamentous fungi, 170% for MB-C, 191% for MB-N, 205% for cumulative C mineralization in 30 days at 21 °C, 202% for dehydrogenase, 171% for phosphatase, and 287% for arylsulfatase activity. The biologically active C and N, when expressed as ratios of MB-C or MB-N to total soil C or N, increased from 1.6 and 2.0% in F-W to 1.9 and 2.6% in Cont. W and to an average of 2.4 and 3.5% in GF-W. A sensitivity analysis (ratio of other treatment values to value for F-W) showed that Cont. W, and the GF-W systems even more, increased all major soil biological attributes tested. Among the GF systems, lentil-W consistently increased sensitivity the most, while Tangier flatpea-W usually increased it the least. The dynamic direct and indirect microbiological attributes were more sensitive indicators of changes in soil productivity than total organic C or N. The sensitivity of the attributes decreased in the sequence: Bacteria>Arylsulfatase>Filamentous fungi≈Cumulative C mineralization≈Dehydrogenase>MB-N>Phosphatase≈MB-C. Compared to the earlier published sensitivities of select physical and chemical attributes of soil quality to these same agronomic treatments, the microbiological attributes proved to be far more sensitive and more responsive to the beneficial influence of legume green fallowing in this semiarid loam.     

Gross nitrogen mineralisation rates in pastural soils and their relationships with organic nitrogen fractions, microbial biomass and protease activity under glasshouse conditions

In this study, gross nitrogen (N) mineralisation rates were determined in six pasture soils (Fleming, Kairanga, Karapoti, Lismore, Templeton and Waikoikoi) from three different regions of New Zealand. The soils were kept under controlled soil water potential (–10 to –30 kPa) and temperature (12–20°C) conditions in a glasshouse. The gross N mineralisation rates ranged from 0.76 to 5.87 g N g^–1 soil day^–1 in the six soils and were positively correlated with the amount of amino acid-N (AA-N), ammonia-N (NH₃-N), total hydrolysable-N (TH-N), microbial biomass-carbon (MB-C), microbial biomass-N (MB-N), protease activity and organic C and N. A stepwise regression was used to generate equations that could best describe gross N mineralisation rates. Microbial biomass-carbon and AA-N were included in the equation that best described the gross N mineralisation rate:

C and N turnover in structurally intact soils of different texture

The turnover of native and applied C and N in undisturbed soil samples of different texture but similar mineralogical composition, origin and cropping history was evaluated at −10 kPa water potential. Cores of structurally intact soil with 108, 224 and 337 g clay kg⁻¹ were horizontially sliced and ¹⁵N-labelled sheep faeces was placed between the two halves of the intact core. The cores together with unamended treatments were incubated in the dark at 20 °C and the evolution of CO₂-C determined continuously for 177 d. Inorganic and microbial biomass N and ¹⁵N were determined periodically. Net nitrification was less in soil amended with faeces compared with unamended soil. When adjusted for the NO₃-N present in soil before faeces was applied, net nitrification became negative indicating that NO₃-N had been immobilized or denitrified. The soil most rich in clay nitrified least N and ¹⁵N. The amounts of N retained in the microbial biomass in unamended soils increased with clay content. A maximum of 13% of the faeces ¹⁵N was recovered in the microbial biomass in the amended soils. CO₂-C evolution increased with clay content in amended and unamended soils. CO₂-C evolution from the most sandy soil was reduced due to a low content of potentially mineralizable native soil C whereas the rate constant of C mineralization rate peaked in this soil. When the pool of potentially mineralizable native soil C was assumed proportional to volumetric water content, the three soils contained similar proportions of potentially mineralizable native soil C but the rate constant of C mineralization remained highest in the soil with least clay. Thus although a similar availability of water in the three soils was ensured by their identical matric potential, the actual volume of water seemed to determine the proportion of total C that was potentially mineralizable. The proportion of mineralizable C in the faeces was similar in the three soils (70% of total C), again with a higher rate constant of C mineralization in the soil with least clay. It is hypothesized that the pool of potentially mineralizable C and C rate constants fluctuate with the soil water content.     

Gross N mineralization rates after application of composted grape marc to soil

Grape marc is a common waste product of the wine production industry. When partially composted and applied to soil it may contain enough N to affect vine growth and hence wine quality. Yet little is known about the quantity and timing of N release from composted grape marc. A laboratory incubation was conducted where composted grape marc amended and non-amended soils were periodically sampled over 148 days at 15 °C for gross N mineralization rates, C mineralization and microbial biomass-C. Gross N mineralization rates were determined by ¹⁵N pool dilution using both analytical equations and the numerical model FLUAZ (Mary, B., Recous, S., Robin, D., 1998. A model for calculating nitrogen fluxes in soil using 15N tracing. Soil Biology & Biochemistry 30, 1963-1979.). Both analytical and FLUAZ determined gross N mineralization rates were in close agreement in the control soil. However, in composted grape marc amended soils there was a discrepancy between the two solutions. Findings indicate that composted grape marc caused a net immobilization of N for the first 50-days of incubation, after which enough N was released to require consideration in fertilizer-N strategies.     

Dynamics of maize (Zea mays L.) leaf straw mineralization as affected by the presence of soil and the availability of nitrogen

An incubation experiment was carried out with maize (Zea mays L.) leaf straw to analyze the effects of mixing the residues with soil and N amendment on the decomposition process. In order to distinguish between soil effects and nitrogen effects for both the phyllospheric microorganisms already present on the surface of maize straw and soil microorganisms the N amendment was applied in two different placements: directly to the straw or to the soil. The experiment was performed in dynamic, automated microcosms for 22 days at 15 °C with 7 treatments: (1) untreated soil, (2) non-amended maize leaf straw without soil, (3) N amended maize leaf straw without soil, (4) soil mixed with maize leaf straw, (5) N amended soil, (6) N amended soil mixed with maize leaf straw, and (7) soil mixed with N amended maize leaf straw. ¹⁵NH₄¹⁵NO₃ (5 at%) was added. Gas emissions (CO₂, ¹³CO₂ and N₂O) were continuously recorded throughout the experiment. Microbial biomass C, biomass N, ergosterol, δ¹³C of soil organic C and of microbial biomass C as well as ¹⁵N in soil total N, mineral N and microbial biomass N were determined in soil samples at the end of the incubation. The CO₂ evolution rate showed a lag-phase of two days in the non-amended maize leaf straw treatment without soil, which was completely eliminated when mineral N was added. The addition of N generally increased the CO₂ evolution rate during the initial stages of maize leaf straw decomposition, but not the cumulative CO₂ production. The presence of soil caused roughly a 50% increase in cumulative CO₂ production within 22 days in the maize straw treatments due to a slower decrease of CO₂ evolution after the initial activity peak. Since there are no limitations of water or N, we suggest that soil provides a microbial community ensuring an effective succession of straw decomposing microorganisms. In the treatments where maize and soil was mixed, 75% of microbial biomass C was derived from maize. We concluded that this high contribution of maize using microbiota indicates a strong influence of organisms of phyllospheric origin to the microbial community in the soil after plant residues enter the soil.     

Determination of the fate of C labelled maize and wheat rhizodeposit-C in two agricultural soils in a greenhouse experiment under C-CO2-enriched atmosphere

A deeper understanding of the contribution of carbon (C) released by plant roots (rhizodeposition) to soil organic matter (SOM) can help to increase our knowledge of global C-cycling. These insights can eventually lead to sustainable management of SOM especially in agricultural systems. This study was conducted to determine the fate of ¹³C labelled rhizodeposit-C of maize and wheat plants. They were grown in a greenhouse in permeable nylon bags filled with upper soil material from two agricultural soils of the same location, but with different crop yields. The bags were placed into pots, which were also filled with soil surrounding the bags. Soil inside the bags was considered as rhizosphere soil, wheras the one outside the bags represented bulk soil. The contributions of rhizodeposits to water extractable organic carbon (WEOC), microbial biomass-C (MB-C), CO₂-C evolution, and total organic carbon (C_org) were investigated during a 7-week growing period. The WEOC, MB-C, CO₂-C, C_org contents and the respective δ¹³C values were determined regularly, and a newly developed method for determining δ¹³C values in soil extracts was applied.In both soils, regardless of crop yield potential, significant incorporation of rhizodeposition-derived C was observed in the MB-C, CO₂-C, and C_org pool, but not in the WEOC. The pattern of C incorporation into the different pools was the same for both soils with both plants, and rhizodeposit-derived C was recovered in the order MB-C<C_org<CO₂-C. This showed that rhizodeposits were mainly respired, but since C_org was the second largest pool of the overall balances, they were also stabilized in the soils at least in the short term. It is suggested that the increased SOM mineralization observed in this study (positive priming effects) was probably induced by C exchange processes between the soil matrix and soluble rhizodeposits. Moreover, soluble rhizodeposit-C was detected in MB-C and CO₂-C evolved outside the direct root zone, showing the availability of these C-components in the bulk soil.     

Estimates of Gross Transformation Rates of Dairy Manure N Using 15N Pool Dilution

Abstract

Most measurements of dairy manure nitrogen (N) availability depend on net changes in soil inorganic N concentration over time, which overlooks the cycling of manure N in the soil. Gross transformations of manure N, including mineralization (m), immobilization (i), and nitrification (n), can be quantified using ¹⁵N pool dilution methods. This research measures gross m, n, and i resulting from application of four freeze‐dried dairy manures that had distinctly different patterns of N availability. A sandy loam soil (coarse‐loamy, mixed, frigid Typic Haplorthod) was amended with four different freeze‐dried dairy manures and incubated at 25°C with optimal soil water content. The dilution of ¹⁵ammonium (NH4⁺) during a 48‐h interval (7–9 d and 56–58 d after manure application) was used to estimate m, whereas the dilution of ¹⁵nitrate (NO₃ ^?) was used to estimate n. Gross immobilization was calculated as gross minus net mineralization. Gross mineralization in the unamended soil was similar at 7‐ to 9‐d and 56‐ to 58‐d intervals and was significantly increased by the application of manures. For both amended and unamended soil, m was much greater (i.e., three‐ to nine‐fold) than estimated net mineralization, illustrating the degree to which manure N can be cycled in soil. At the early interval, both m and i were directly related to the manure C input, demonstrating the linkage between substrate C availability and N utilization by soil microbes. This research clearly shows that the application of dairy manures stimulates gross N transformation rates in the soil, improving our understanding of the impact of manure application on soil N cycling.     

Effect of extracellular-enzyme activities on solubilization rate of soil organic nitrogen

In a sandy soil containing ¹⁵N-labeled active (soluble and easily degradable) and non-labelled passive (recalcitrant) fractions of soil organic matter, the rate of net N mineralization (solubilization) was determined during a 55-day incubation at 25°C, 63% water-holding capacity and different levels of soil extracellular-enzyme activities. The active fraction of soil N was labelled by preincubation (at 5°C and 74% water-holding capacity for 6 months) of soil amended with ¹⁵N-labeled plant material. Increases in the activity of extracellular-enzymes in soil were induced by the addition of glucose and KH₂PO₄ at the beginning of the incubation. The results show that the contents of total soluble N (NO ₃ ^– –N+NH ₄ ⁺ –N + soluble organic N) were significantly higher in glucose-amended soil compared to the unamended soil. The increases in soluble N in soil amended with 1 and 2 mg glucose g^-1 dry soil corresponded to a mean rate of net solubilization of 7.9±1.4 and 18.8±0.7 nmol N g^-1 dry soil day^-1, respectively. The mean rate of net N solubilization (3.6±1.0 nmol N g^-1 dry soil day^-1) in unamended soil was significantly lower than those of glucose amended soils. The content of ¹⁵N in total soluble N in soil amended with 2 mg glucose, for example, was diluted from 3.11±0.08 atom% before the incubation to 2.77±0.03 atom% after 55 days. This indicates that 89% of soluble-N accumulated in soil by the end of the incubation originated from the active fraction of soil N and the rest, estimated at 11%, originated from the passive fraction. The activities of soluble and total proteases as well as the rate of N solubilization in the soil increased with the application of glucose. The activity of these extracellular enzymes was highly correlated with the rates of net N solubilization. Thus, increases in extracellular-enzyme activities in glucose-amended soils had a priming effect on the solubilization of ¹⁵N-labeled active and non-labeled passive fractions of soil organic N. It seems that the activity of extracellular-enzymes expressed in terms of total and soluble protease activities could be a rate-limiting factor in the processes of soil organic N solubilization.     

Tracing the source and fate of dissolved organic matter in soil after incorporation of a C labelled residue: A batch incubation study

While dissolved organic matter (DOM) in soil solution is a small but reactive fraction of soil organic matter, its source and dynamics are unclear. A laboratory incubation experiment was set up with an agricultural topsoil amended with ¹³C labelled maize straw. The dissolved organic carbon (DOC) concentration in soil solution increased sharply from 25 to 186 mg C L⁻¹ 4 h after maize amendment, but rapidly decreased to 42 mg C L⁻¹ and reached control values at and beyond 2 months. About 65% of DOM was straw derived after 4 h, decreasing to 29% after one day and only 1.3% after 240 days. A significant priming effect of the straw on the release of autochthonous DOM was found. The DOM fractionation with DAX-8 resin revealed that 98% of the straw derived DOM was hydrophilic in the initial pulse while this hydrophilic fraction was 20-30% in control samples. This was in line with the specific UV absorbance of the DOM which was significantly lower in the samples amended with maize residues than in the control samples. The δ¹³C of the respired CO₂ matched that of DOC in the first day after amendment but exceeded it in following days. The straw derived C fractions in respired CO₂ and in microbial biomass were similar between 57 and 240 days after amendment but were 3-10 fold above those in the DOM. This suggests that the solubilisation of C from the straw is in steady state with the DOM degradation or that part of the straw is directly mineralised without going into solution. This study shows that residue application releases a pulse of hydrophilic DOM that temporarily (<3 days) dominates the soil DOM pool and the degradable C. However, beyond that pulse the majority of DOM is derived from soil organic matter and its isotope signature differs from microbial biomass and respired C, casting doubt that the DOM pool in the soil solution is the major bioaccessible C pool in soil.     

Natural N abundances in a tallgrass prairie ecosystem exposed to 8-y of elevated atmospheric CO2

After 8-y of elevated CO₂, we previously detected greater amounts of total soil nitrogen, suggesting that rates of ecosystem N flux into or out of tallgrass prairie had been altered. Denitrification and associative N fixation rates are the two primary biological processes that are known to control N loss and accumulation in tallgrass prairie soil. Therefore, our objective was to assess the natural abundance of plant and soil ¹⁵N isotopes as a cumulative index of potential change in efflux or influx of N into and out of the tallgrass prairie after 8-y of exposure to elevated CO₂. Aboveground plant delta ¹⁵N values of Andropogon gerardii were close to zero and more positive as a result of elevated CO₂, but whole-soil values at the 5-30 cm depth were significantly reduced (6.8 vs 7.3; P<0.05) under elevated CO₂-chamber (EC) relative to ambient CO₂- chamber (AC). Total, aboveground plant biomass, root-in-growth, extractable N, microbial biomass N, and soil pools collectively exhibited a range of delta ¹⁵N values from −2.8 to 7.3. Measurements of surface soil ¹⁵N indicate that a change in N inputs and outputs has occurred as a result of elevated atmospheric CO₂. In addition to possible changes in denitrification and N₂ fixation, other sources of N such as the re-translocation of N to the surface from deeper soil layers are needed to explain how soil N accrues in surface soils as a consequence of elevated CO₂. Our results support the notion that C accrual may promote N accrual, possibly driven by high plant and microbial N demand amplified by soil N limitation.     

Nitrifier dominance of Arctic soil nitrous oxide emissions arises due to fungal competition with denitrifiers for nitrate

Arctic soils emit nitrous oxide, which is a potent greenhouse gas and also represents an important loss of nitrogen to oligotrophic Arctic ecosystems. However, little is known about the temperature sensitivity of nitrous oxide release in Arctic soils or the organisms mainly responsible for it. We investigated controls on nitrous oxide emissions in an Arctic soil across a typical temperature range (between 4 and 13 °C) on Truelove Lowland, Devon Island, Canada (75°40′N 84°35′W) at two different moisture contents. When fertilized with ammonia or nitrate, nitrous oxide emissions and temperature dependence of nitrous oxide emissions were insensitive to soil moisture content but linked to nitrification rates. Stable isotope analysis revealed that nitrous oxide was predominantly released by nitrifiers. However, nitrous oxide emissions were not linked to nitrifier prevalence with an insignificant (P < 0.219) increase in amoA genes and a (P < 0.01) decrease in archaeal nitrifiers. In contrast, denitrifier nosZ prevalence was 10,000 times greater than that of nitrifiers and was related to nitrous oxide emission potential when soils were fertilized with nitrate. Manipulating water-filled pore space should have changed the pattern of N₂O emissions. We used selective inhibitors to further explore why denitrification did not occur under field conditions when we manipulated water-filled pore space or when we used ¹⁵N analysis. When fungi were inhibited in the soil, nitrous oxide emissions from denitrifiers increased with no change in nitrous oxide released by nitrifiers. When fungi were active in the soil, there was little available nitrate but when fungi were inhibited, available soil nitrate increased over the incubation period. The dominance of nitrifiers in nitrous oxide emissions from Arctic soils under field conditions is linked to the competition for nitrate between fungi and denitrifiers.     

Soil microbial activity and crop sustainability in a long-term experiment with three soil-tillage and two crop-rotation systems

Reduction in soil disturbance can stimulate soil microbial biomass and improve its metabolic efficiency, resulting in better soil quality, which in turn, can increase crop productivity. In this study we evaluated microbial biomass of C (MB-C) by the fumigation-extraction (FE) or fumigation-incubation (FI) method; microbial biomass of N (MB-N); basal respiration (BR) induced or not with sucrose; metabolic quotient (obtained by the ratio BR/MB-C) induced (qCO₂(S)), or not with sucrose (qCO₂); and crop productivity in a 14-year experiment in the state of Paraná, southern Brazil. The experiment consisted of three soil-tillage systems [no-tillage (NT), conventional tillage (CT) and no-tillage using a field cultivator every 3 years (FC)] and two cropping systems [a soybean–wheat-crop sequence (CS), and a soybean–wheat–white lupin–maize–black oat–radish crop rotation (CR)]. There were six samplings in the 14th year, starting at the end of the winter crop (wheat in the CS and lupin in the CR plots) and finishing at full flowering of the summer crop (soybean in the CS and maize in the CR). Differences in microbiological parameters were greater than those detected in the total C (TCS) and total N (TNS) contents of the soil organic matter (SOM). Major differences were attributed to tillage, and on average NT was higher than the CT in the following parameters: TCS (19%), TNS (21%), MB-C evaluated by FE (74%) and FI (107%), and MB-N (142%). The sensibility of the microbial community and processes to soil disturbance in the tropics was highlighted, as even a moderate soil disturbance every 3 years (FC) affected microbial parameters but not SOM. The BR was the parameter that most promptly responded to soil disturbance, and strong differences were perceived by the ratio of qCO₂ evaluated with samples induced and non-induced with sucrose. At plowing, the qCO₂(S):qCO₂ was five times higher under CT, indicating a C-starving low-effective microbial population in the C-usage. In general, crop rotation had no effect on microbial parameters or SOM. Grain yield was affected by tillage and N was identified as a limiting nutrient. Linear regressions between grain yields and microbial parameters showed that soybean was benefited from improvements in the microbial biomass and metabolic efficiency, but with no significant effects observed for the maize crop. The results also indicate that the turnover of C and N in microbial communities in tropical soils is rapid, reinforcing the need to minimize soil disturbance and to balance inputs of N and C.     

洞庭湖区不同土地利用方式耕作土壤氮素含量与循环

通过对洞庭湖典型地区的密集采样分析和农户调查,研究了4种利用方式耕作土壤全N、微生物生物量氮(MB-N)含量、两者关系和N素循环特征。结果表明:耕作土壤全N、MB-N含量平均值为3.00±0.48g/kg和101.4±49.2mg/kg。双季稻、一季稻、水田旱作和旱地全N平均含量依次为3.12±0.40g/kg、3.03±0.39g/kg、2.79±0.43g/kg2、.10±0.46g/kg。4种利用方式的MB-N含量分别为124.0±56.6mg/kg、96.4±39.2mg/kg、108.0±48.6mg/kg、75.2±30.5mg/kg。除水田旱作外,MB-N与全N之间存在极显著的正相关关系(P<0.01)。土壤N素盈余量依次为双季稻(105.0kg/hm2.a)>一季稻(75.1kg/hm2.a)>水田旱作油菜(64.5kg/hm2.a)>旱地苎麻(51.9kg/hm2.a)。     

Nitrogen mineralization and microbial activity in permanent pastures amended with nitrogen fertilizer or dung

 Gross rates of soil processes and microbial activity were measured in two grazed permanent pasture soils which had recently been amended with N fertilizer or dung. ¹⁵N studies of rates of soil organic matter turnover showed gross N mineralization was higher, and gross N immobilization was lower, in a long-term fertilized soil than in a soil which had never received fertilizer N. Net mineralization was also found to be higher in the fertilized soil: a consequence of the difference between the opposing N turnover processes of N mineralization and immobilization. In both soils without amendments the soil microbial biomass contents were similar, but biomass activity (specific respiration) was higher in the fertilized soil. Short-term manipulation of fertilizer N input, i.e. adding N to unfertilized soil, or witholding N from previously fertilized soil, for one growing season, did not affect gross mineralization, immobilization or biomass size and activity. Amendments of dung had little effect on gross mineralization, but there was an increase in immobilization in both soils. Total biomass also increased under dung in the unfertilized soil, but specific respiration was reduced, suggesting changes in the composition of the biomass. Dung had a direct effect on the microbial biomass by temporarily increasing available soil C. Prolonged input of fertilizer N increases soil C indirectly as a result of enhanced plant growth, the effect of which may not become evident within one seasonal cycle. Received: 18 December 1998     

Rhizodeposition of C and N in peas and oats after C-N double labelling under field conditions

Compounds released by plant roots during growth can make up a high proportion of below-ground plant (BGP) carbon and nitrogen, and therefore influence soil organic matter turnover and plant nutrient availability by stimulating the soil microorganisms. The present study was conducted to examine the amount and fate of C (CdfR) and N rhizodeposits (NdfR), in this study defined as root-derived C or N present in the soil after removal of roots and root fragments, released during reproductive growth. BGP biomass of peas (Pisum sativum L.) and oats (Avena sativa L.) was successfully labelled in situ with a ¹³C-glucose-¹⁵N-urea mixture under field conditions using a stem feeding method. Pea plants were labelled at the beginning of flowering and harvested 36 and 52 days after labelling at pod filling (P_P) and maturity (P_M), respectively. Oat plants were labelled at grain filling and harvested 42 days after labelling at maturity (O_M). CdfR was 24.2% (P_P), 29.6% (P_M) and 30.8% (O_M) of total recovered plant C. NdfR was 32.1% (P_P), 36.4% (P_M) and 30.0% (O_M) of total plant N. Due to higher N assimilation, amounts of NdfR were four times higher in peas in comparison with oats. The results for NdfR in peas were higher than results from other studies. The C-to-N ratio of rhizodeposits was lower under peas (17.3) than under oats (41.9) at maturity. At maturity, microbial CdfR at 0-30 cm soil depth was 37% of the microbial biomass C in peas and 59% in oats. Microbial NdfR was 15% of microbial N in peas and 5% in oats. Furthermore, inorganic NdfR was 34% in peas and 9% in oats at 0-30 cm at maturity. These results show that rhizodeposits of peas provide a more easily available substrate to soil microorganisms, which are incorporated to a greater extent and turned over faster in comparison with oats. Beside the higher amounts of N released from pea roots, this process contributes to the higher N-availability for subsequent crops.     

Long-term nitrogen additions increased surface soil carbon concentration in a forest plantation despite elevated decomposition

Forests cover one-third of the Earth’s land surface and account for 30-40% of soil carbon (C). Despite numerous studies, questions still remain about the factors controlling forest soil C turnover. Present understanding of global C cycle is limited by considerable uncertainty over the potential response of soil C dynamics to rapid nitrogen (N) enrichment of ecosystems, mainly from fuel combustion and fertilizer application. Here, we present a 15-year-long field study and show an average increase of 14.6% in soil C concentration in the 0-5 cm mineral soil layer in N fertilized (defined as N+ hereafter) sub-plots of a second-rotation Pinus radiata plantation in New Zealand compared to control sub-plots. The results of ¹⁴C and lignin analyses of soil C indicate that N additions significantly accelerate decomposition of labile and recalcitrant soil C. Using an annual-time step model, we estimated the soil C turnover time. In the N+ sub-plots, soil C in the light (a density < 1.70 g cm⁻³) and heavy fractions had the mean residence times of 23 and 67 yr, respectively, which are lower than those in the control sub-plots (36 and 133 yr in the light and heavy fractions, respectively). The commonly used lignin oxidation indices (vanillic acid to vanillin and syringic acid to syringaldehyde ratios) were significantly greater in the N+ sub-plots than in the control sub-plots, suggesting increased lignin decomposition due to fertilization. The estimation of C inputs to forest floor and δ¹³C analysis of soil C fractions indicate that the observed buildup of surface soil C concentrations in the N+ sub-plots can be attributed to increased inputs of C mass from forest debris. We conclude that long-term N additions in productive forests may increase C storage in both living tree biomass and soils despite elevated decomposition of soil organic matter.     

Atmospheric nitrate deposition and the microbial degradation of cellobiose and vanillin in a northern hardwood forest

Human activity has increased the amount of N entering terrestrial ecosystems from atmospheric NO₃⁻ deposition. High levels of inorganic N are known to suppress the expression of phenol oxidase, an important lignin-degrading enzyme produced by white-rot fungi. We hypothesized that chronic NO₃⁻ additions would decrease the flow of C through the heterotrophic soil food web by inhibiting phenol oxidase and the depolymerization of lignocellulose. This would likely reduce the availability of C from lignocellulose for metabolism by the microbial community. We tested this hypothesis in a mature northern hardwood forest in northern Michigan, which has received experimental atmospheric N deposition (30 kg NO₃⁻-N ha⁻¹ y⁻¹) for nine years. In a laboratory study, we amended soils with ¹³C-labeled vanillin, a monophenolic product of lignin depolymerization, and ¹³C-labeled cellobiose, a disaccharide product of cellulose degradation. We then traced the flow of ¹³C through the microbial community and into soil organic carbon (SOC), dissolved organic carbon (DOC), and microbial respiration. We simultaneously measured the activity of enzymes responsible for lignin (phenol oxidase and peroxidase) and cellobiose (β-glucosidase) degradation. Nitrogen deposition reduced phenol oxidase activity by 83% and peroxidase activity by 74% when compared to control soils. In addition, soil C increased by 76%, whereas microbial biomass decreased by 68% in NO₃⁻ amended soils. ¹³C cellobiose in bacterial or fungal PLFAs was unaffected by NO₃⁻ deposition; however, the incorporation of ¹³C vanillin in fungal PLFAs extracted from NO₃⁻ amended soil was 82% higher than in the control treatment. The recovery of ¹³C vanillin and ¹³C cellobiose in SOC, DOC, microbial biomass, and respiration was not different between control and NO₃⁻ amended treatments. Chronic NO₃⁻ deposition has stemmed the flow of C through the heterotrophic soil food web by inhibiting the activity of ligninolytic enzymes, but it increased the assimilation of vanillin into fungal PLFAs.