Influence of biochemical quality on C and N mineralisation from a broad variety of plant materials in soil

We studied C and N mineralisation patterns from a large number of plant materials (76 samples, covering 37 species and several plant parts), and quantified how these patterns related to biological origin and selected indicators of chemical composition. We determined C and N contents of whole plant material, in water soluble material and in fractions (neutral detergent soluble material, cellulose, hemicellulose and lignin) obtained by stepwise chemical digestion (modified van Soest method). Plant materials were incubated in a sandy soil under standardised conditions (15 °C, optimal water content, no N limitation) for 217days, and CO₂ evolution and soil mineral N contents were monitored regularly. The chemical composition of the plant materials was very diverse, as indicated by total N ranging from 2 to 59 mg N g^–1, (i.e. C/N-ratios between 7 and 227). Few materials were lignified (median lignin=4% of total C). A large proportion of plant N was found in the neutral detergent soluble (NDS) fraction (average 84%) but less of the plant C (average 46%). Over the entire incubation period, holocellulose C content was the single factor that best explained the variability of C mineralisation (r=–0.73 to –0.82). Overall, lignin C explained only a small proportion of the variability in C mineralisation (r=–0.44 to –0.51), but the higher the lignin content, the narrower the range of cumulative C mineralisation. Initial net N mineralisation rate was most closely correlated (r=0.76) to water soluble N content of the plant materials, but from Day 22, net N mineralisation was most closely correlated to total plant N and NDS-N contents (r varied between 0.90 and 0.94). The NDS-N content could thus be used to roughly categorise the net N mineralisation patterns into (i) sustained net N immobilisation for several months; (ii) initial net N immobilisation, followed by some re-mineralisation; and (iii) initially rapid and substantial net N mineralisation. Contrary to other studies, we did not find plant residue C/N or lignin/N-ratio to be closely correlated to decomposition and N mineralisation.     

The effect of woodland soil translocation on carbon and nitrogen mineralisation processes

Two investigations into the translocation of temperate deciduous woodland soil were carried out in Kent, S. E. England, to study the effects on C and N mineralisation. In the field experiment, two translocation methods were compared: (i) placement, moving soil as an intact surface profile and (ii) loose-tipping in which the surface profile was mixed. These were implemented in winter both in situ (under the woodland canopy) and ex situ (soil moved to a receptor site outside woodland). In a second experiment, intact soil cores from the woodland site were subjected to different levels of disturbance in a polythene tunnel environment. Measurements of soil CO₂ evolution and N mineralisation in both experiments showed a clear seasonal pattern, strongly influenced by temperature. Over a 7-month period, cumulative net N mineralisation in the field was greater in the woodland controls and placement treatments than loose-tipping treatments. Soil CO₂ emissions were also greater in woodland control plots in the winter compared with ex situ treatments. Similarly, in the polythene tunnel environment, CO₂ emissions were highest in the undisturbed soil cores, while N mineralisation varied with soil depth but, across the whole profile, was also greater in the controls. We conclude that the mixing of organic rich topsoil with mineral subsoil in clayey soil may have protected the organic residues on the clay-silt surfaces, resulting in overall lower mineralisation rates in the disturbed soil. These results indicate that N mineralisation does not necessarily increase when soil translocation operations are carried out on clayey soils in winter. Placement methods appeared the most likely to conserve soil mineralisation processes close to those in undisturbed woodland soil, but depend greatly on the success of maintaining the soil profile intact. It appears that, on clayey soils, the development of vegetation at the receptor site is more likely to be determined by alterations in the light, soil temperature and moisture regime that will occur in open conditions after woodland translocation than from increased soil N supply.     

Dynamics of mineral N availability in grassland ecosystems under increased [CO2]: hypotheses evaluated using the Hurley Pasture Model

The following arguments are outlined and then illustrated by the response of the Hurley Pasture Model to [CO₂] doubling in the climate of southern Britain. 1. The growth of N-limited vegetation is determined by the concentration of N in the soil mineral N pools and high turnover rates of these pools (i.e., large input and output fluxes) contribute positively to growth. 2. The size and turnover rates of the soil mineral N pools are determined overwhelmingly by N cycling into all forms of organic matter (plants, animals, soil biomass and soil organic matter — `immobilisation' in a broad sense) and back again by mineralisation. Annual system N gains (by N₂ fixation and atmospheric deposition) and losses (by leaching, volatilisation, nitrification and denitrification) are small by comparison. 3. Elevated [CO₂] enriches the organic matter in plants and soils with C, which leads directly to increased removal of N from the soil mineral N pools into plant biomass, soil biomass and soil organic matter (SOM). ‘Immobilisation’ in the broad sense then exceeds mineralisation. This is a transient state and as long as it exists the soil mineral N pools are depleted, N gaseous and leaching losses are reduced and the ecosystem gains N. Thus, net immobilisation gradually increases the N status of the ecosystem. 4. At the same time, elevated [CO₂] increases symbiotic and non-symbiotic N₂ fixation. Thus, more N is gained each year as well as less lost. Effectively, the extra C fixed in elevated [CO₂] is used to capture and retain more N and so the N cycle tracks the C cycle. 5. However, the amount of extra N fixed and retained by the ecosystem each year will always be small (ca. 5–10 kg N ha^-1 yr^-1) compared with amount of N in the immobilisation-mineralisation cycle (ca. 1000 kg N ha^-1 yr^-1). Consequently, the ecosystem can take decades to centuries to gear up to a new equilibrium higher-N state. 6. The extent and timescale of the depletion of the mineral N pools in elevated [CO₂] depends on the N status of the system and the magnitude of the overall system N gains and losses. Small changes in the large immobilisation—mineralisation cycle have large effects on the small mineral N pools. Consequently, it is possible to obtain a variety of growth responses within 1–10 year experiments. Ironically, ecosystem models — artificial constructs — may be the best or only way of determining what is happening in the real world. This revised version was published online in June 2006 with corrections to the Cover Date.     

CO2 efflux from a Mediterranean semi-arid forest soil. I. Seasonality and effects of stoniness

We studied the seasonality of total soil CO₂efflux and labeled C-CO₂ released from ¹⁴Clabeled straw incubated in the H horizon of asemi-arid Mediterranean forest soil. Fieldmeasurements were carried out over 520 days in aseries of reconstructed soil profiles with and withouta gravel layer below the H horizon. We monitored soilclimate and related this to soil CO₂ efflux.Seasonal variations in soil CO₂ efflux in asemiarid Mediterranean forest were mainly related tochanges in soil temperature. In spite of drought, highrespiration rates were observed in mid summer. Highsoil CO₂ efflux in hot and dry episodes wasattributed to increases in soil biological activity.The minimum soil CO₂ efflux occurred in latesummer also under dry conditions, probably related toa decrease in soil biological activity in deephorizons. Biological activity in organic layers waslimited by water potential () in summer and bytemperature in winter. Rewetting a dry soil resultedin large increases in soil CO₂ efflux only at hightemperatures. These large increases represented asignificant contribution to the decomposition oforganic matter in the uppermost horizons. Soilbiological activity in the uppermost horizons was moresensitive to changes in soil and hence tosummer rainstorms than the bulk soil microbialactivity. The presence of a layer of gravel improvedboth moisture and temperature conditions for thedecomposition of organic matter. As a result, soilCO₂ efflux increased in soils containing rockfragments. These effects were especially large for theorganic layers.     

Development of nitrogen mineralisation gradients through surface soil depth and their influence on surface soil pH

Following mixing of the surface soil to about 7.5 cm depth in the field, soil layers (0–2.5, 2.5–5, 5–10 and 10–15 cm) were separately incubated in the laboratory to determine the rate of development of net N mineralisation gradients through surface soil depth under fallow, wheat and subterranean clover plots. Gradients in net N mineralisation were compared with those observed in the field, and their contribution to the observed pH changes was investigated.Heterotrophic activity, and thus net N mineralisation, decreased only slightly with depth immediately after soil mixing. This pattern persisted over time in soil layers sampled from fallow plots. In contrast, within 1 growing season after soil mixing, heterotrophic activity and net N mineralisation decreased significantly with depth in soil sampled from wheat and clover plots. In 0–15 cm soil sampled from under senescing plants, 32–38% of CO₂-C produced and net N mineralised originated from the surface 2.5 cm, while 52–56% originated from the surface 5 cm of soil. This resulted from an increase of pH and organic substrate concentration within the surface 2.5 cm of soil following plant residue return. Limitations of the in situ measurement of net N mineralisation in fallow soil was identified.Laboratory incubation studies showed that since most net N mineralisation occurred within the surface 2.5 cm of soil under senescing plants, nitrification and acidification were also concentrated at this depth. Despite this, compared to fallow soil, high potential acidification rates of 0–2.5 cm soil under senescing plants were not realised in the field due to the exposure to prolonged dry periods and moist-dry cycles. As a consequence, in the field the large magnitude of surface soil pH gradient which resulted from the return of alkaline plant residues was maintained over summer and autumn.     

Simulation of C and N mineralisation during crop residue decomposition: A simple dynamic model based on the C:N ratio of the residues

C and N mineralisation kinetics obtained in laboratory incubations during decomposition of crop residues under non-limiting nitrogen conditions were simulated using a simple dynamic model. This model includes three compartments: the residues, microbial biomass and humified organic matter. Seven parameters are used to describe the C and N fluxes. The decomposed C is either mineralised as CO₂ or assimilated by the soil microflora, microbial decay producing both C humification and secondary C mineralisation. The N dynamics are governed by the C rates and the C:N ratio of the compartments which remain constant in the absence of nitrogen limitation. The model was parameterised using apparent C and N mineralisation kinetics obtained for 27 different residues (organs of oilseed rape plants) that exhibited very wide variations in chemical composition and nitrogen content. Except for the C:N ratio of the residues and the soil organic matter, the other five parameters of the model were obtained by non-linear fitting and by minimising the differences between observed and simulated values of CO₂ and mineral N. Three parameters, namely the decomposition rate constant of the residues, the biomass C:N ratio and humification rate, were strongly correlated with the residues C:N ratio. Hyperbolic relationships were established between these parameters and the residues C:N ratio. In contrast, the other two parameters, i.e. the decay rate of the microbial biomass and the assimilation yield of residue-C by the microbial biomass, were not correlated to the residues C:N ratio and were, therefore, fixed in the model. The model thus parameterised against the residue C:N ratio as a unique criterion, was then evaluated on a set of 48 residues. An independent validation was obtained by taking into account 21 residues which had not been used for the parameterisation. The kinetics of apparent C and N mineralisation were reasonably well simulated by the model. The model tended to over-estimate carbon mineralisation which could limit its use for C predictions, but the kinetics of N immobilisation or mineralisation due to decomposition of residues in soil were well predicted. The model indicated that the C:N ratio of decomposers increased with the residue C:N ratio. Higher humification was predicted for substrates with lower C:N ratios. This simple dynamic model effectively predicts N evolution during crop residue decomposition in soil.     

Responses of N fluxes and pools to elevated atmospheric CO2 in model forest ecosystems with acidic and calcareous soils

The objectives of this study were to estimate how soil type, elevated N deposition (0.7 vs. 7 g N m^–2y^–1) and tree species influence the potential effects of elevated CO₂ (370 vs. 570 mol CO₂ mol^–1) on N pools and fluxes in forest soils. Model spruce-beech forest ecosystems were established on a nutrient-rich calcareous sand and on a nutrient-poor acidic loam in large open-top chambers. In the fourth year of treatment, we measured N concentrations in the soil solution at different depths, estimated N accumulation by ion exchange resin (IER) bags, and quantified N export in drainage water, denitrification, and net N uptake by trees. Under elevated CO₂, concentrations of N in the soil solution were significantly reduced. In the nutrient-rich calcareous sand, CO₂ enrichment decreased N concentrations in the soil solution at all depths (–45 to –100%). In the nutrient-poor acidic loam, the negative CO₂ effect was restricted to the uppermost 5 cm of the soil. Increasing the N deposition stimulated the negative impact of CO₂ enrichment on soil solution N in the acidic loam at 5 cm depth from –20% at low N inputs to –70% at high N inputs. In the nutrient-rich calcareous sand, N additions did not influence the CO₂ effect on soil solution N. Accumulation of N by IER bags, which were installed under individual trees, was decreased at high CO₂ levels under spruce in both soil types. Under beech, this decrease occurred only in the calcareous sand. N accumulation by IER bags was negatively correlated with current-years foliage biomass, suggesting that the reduction of soil N availability indices was related to a CO₂-induced growth enhancement. However, the net N uptake by trees was not significantly increased by elevated CO₂. Thus, we suppose that the reduced N concentrations in the soil solution at elevated CO₂ concentrations were rather caused by an increased N immobilisation in the soil. Denitrification was not influenced by atmospheric CO₂ concentrations. CO₂ enrichment decreased nitrate leaching in drainage by 65%, which suggests that rising atmospheric CO₂ potentially increases the N retention capacity of forest ecosystems.     

Six years of in situ CO2 enrichment evoke changes in soil structure and soil biota of nutrient-poor grassland

Nutrient‐poor grassland on a silty clay loam overlying calcareous debris was exposed to elevated CO₂ for six growing seasons. The CO₂ exchange and productivity were persistently increased throughout the experiment, suggesting increases in soil C inputs. At the same time, elevated CO₂ lead to increased soil moisture due to reduced evapotransporation. Measurements related to soil microflora did not indicate increased soil C fluxes under elevated CO₂. Microbial biomass, soil basal respiration, and the metabolic quotient for CO₂ (qCO₂) were not altered significantly. PLFA analysis indicated no significant shift in the ratio of fungi to bacteria. 0.5 m KCl extractable organic C and N, indicators of changed DOC and DON concentrations, also remained unaltered. Microbial grazer populations (protozoa, bacterivorous and fungivorous nematodes, acari and collembola) and root feeding nematodes were not affected by elevated CO₂. However, total nematode numbers averaged slightly lower under elevated CO₂ (?16%, ns) and nematode mass was significantly reduced (?43%, P = 0.06). This reduction reflected a reduction in large‐diameter nematodes classified as omnivorous and predacious. Elevated CO₂ resulted in a shift towards smaller aggregate sizes at both micro‐ and macro‐aggregate scales; this was caused by higher soil moisture under elevated CO₂. Reduced aggregate sizes result in reduced pore neck diameters. Locomotion of large‐diameter nematodes depends on the presence of large enough pores; the reduction in aggregate sizes under elevated CO₂ may therefore account for the decrease in large nematodes. These animals are relatively high up the soil food web; this decline could therefore trigger top‐down effects on the soil food web. The CO₂ enrichment also affected the nitrogen cycle. The N stocks in living plants and surface litter increased at elevated CO₂, but N in soil organic matter and microbes remained unaltered. Nitrogen mineralization increased markedly, but microbial N did not differ between CO₂ treatments, indicating that net N immobilization rates were unaltered. In summary, this study did not provide evidence that soils and soil microbial communities are affected by increased soil C inputs under elevated CO₂. On the contrary, available data (¹³C tracer data, minirhizotron observations, root ingrowth cores) suggests that soil C inputs did not increase substantially. However, we provide first evidence that elevated CO₂ can reduce soil aggregation at the scale from µ m to mm scale, and that this can affect soil microfaunal populations.     

Microbial degradation of geogenic organic C and N in mine spoils

Many mine spoils present at the surface of reclamation sites in the Lower Lusatian mining district are carboniferous substrates, i.e. contain geogenic organic matter. Depending on its susceptibility to microbial degradation, geogenic organic matter might influence the establishment of a carbon requiring microflora in mine spoils. As geogenic organic matter contains substantial amounts of organic nitrogen it is also a potential source for plant available N. The objective of the present study was to quantify C and N mineralisation and microbial biomass in geogenic organic matter present at reclamation sites in Lower Lusatia. We also studied, whether these properties can be influenced by raising the originally low pH to near neutral conditions. In laboratory incubation studies, the rates of CO₂ evolution and net N mineralisation were determined in geogenic organic matter and carboniferous mine spoil with and without addition of lime. At the same time, microbial biomass carbon was estimated. As a reference, soil organic matter originating from the humus layer of a 60-year-old Pinus sylvestris stand was used. As indicated by the initial rates of C mineralisation, geogenic carbon was microbially available but to a lower extent than soil organic carbon. During incubation, C mineralisation remained constant or tended to increase with time, depending on the origin of the sample, while it decreased in soil organic matter. Unlike in soil organic matter, in geogenic organic matter and carboniferous mine spoil, C mineralisation was not consistently promoted by lime addition. Prior to incubation, microbial biomass in geogenic organic matter and carboniferous mine spoil was about 10-fold lower than in soil organic matter and tended to increase with incubation time while it decreased in soil organic matter. Similar to C mineralisation, microbial biomass in geogenic organic matter increased after liming, while it declined in carboniferous mine spoil immediately after lime addition. Rates of net N mineralisation were very low in geogenic organic matter and carboniferous mine spoil regardless of the length of incubation and could not be enhanced by raising the pH. It was concluded, that in mine spoils where accumulation of soil organic matter has not yet occurred, geogenic organic matter can be favourable for the establishment of a heterotrophic microflora. However, in the short term, geogenic matter is no source for plant available N in mine spoils. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Soil respiration response to three years of elevated CO2 and N fertilization in ponderosa pine (Pinus ponderosa Doug. ex Laws.)

We measured growing season soil CO₂ evolution under elevated atmospheric [CO₂] and soil nitrogen (N) additions. Our objectives were to determine treatment effects, quantify seasonal variation, and compare two measurement techniques. Elevated [CO₂] treatments were applied in open-top chambers containing ponderosa pine (Pinus ponderosa L.) seedlings. N applications were made annually in early spring. The experimental design was a replicated factorial combination of CO₂ (ambient, + 175, and +350 L L^–1 CO₂) and N (0, 10, and 20 g m^–2 N as ammonium sulphate). Soils were irrigated to maintain soil moisture at > 25 percent. Soil CO₂ evolution was measured over diurnal periods (20–22 hours) in October 1992, and April, June, and October 1993 and 1994 using a flow-through, infrared gas analyzer measurement system and corresponding pCO₂ measurements were made with gas wells. Significantly higher soil CO₂ evolution was observed in the elevated CO₂ treatments; N effects were not significant. Averaged across all measurement periods, fluxes, were 4.8, 8.0, and 6.5 for ambient + 175 CO₂, and +350 CO₂ respectively).Treatment variation was linearly related to fungal occurrence as observed in minirhizotron tubes. Seasonal variation in soil CO₂ evolution was non-linearly related to soil temperature; i.e., fluxes increased up to approximately soil temperature (10cm soil depth) and decreased dramatically at temperatures > 18°C. These patterns indicate exceeding optimal temperatures for biological activity. The dynamic, flow-through measurement system was weakly correlated (r = 0.57; p < 0.0001; n = 56) with the pCO₂ measurement method.     

Living roots magnify the response of soil organic carbon decomposition to temperature in temperate grassland

Increasing atmospheric carbon dioxide (CO₂) concentration is both a strong driver of primary productivity and widely believed to be the principal cause of recent increases in global temperature. Soils are the largest store of the world's terrestrial C. Consequently, many investigations have attempted to mechanistically understand how microbial mineralisation of soil organic carbon (SOC) to CO₂ will be affected by projected increases in temperature. Most have attempted this in the absence of plants as the flux of CO₂ from root and rhizomicrobial respiration in intact plant‐soil systems confounds interpretation of measurements. We compared the effect of a small increase in temperature on respiration from soils without recent plant C with the effect on intact grass swards. We found that for 48 weeks, before acclimation occurred, an experimental 3 °C increase in sward temperature gave rise to a 50% increase in below ground respiration (ca. 0.4 kg C m^?2; Q₁₀ = 3.5), whereas mineralisation of older SOC without plants increased with a Q₁₀ of only 1.7 when subject to increases in ambient soil temperature. Subsequent ¹⁴C dating of respired CO₂ indicated that the presence of plants in swards more than doubled the effect of warming on the rate of mineralisation of SOC with an estimated mean C age of ca. 8 years or older relative to incubated soils without recent plant inputs. These results not only illustrate the formidable complexity of mechanisms controlling C fluxes in soils but also suggest that the dual biological and physical effects of CO₂ on primary productivity and global temperature have the potential to synergistically increase the mineralisation of existing soil C.     

Comparison of methodologies for field measurement of net nitrogen mineralisation in arable soils

This study evaluated the suitability of a soil core incubation technique (with acetylene added to inhibit loss of N by denitrification; CIT) and a resin-core incubation technique (RCT) for measurement of net N mineralisation under arable cropping conditions. A conventional N balance (BAL) approach to the measurement of N mineralisation was used for comparison. In a sandy soil during winter 1996/97, CIT estimates of net N mineralisation were approximately 3 times greater than RCT and BAL estimates, which were in close agreement. Soil disturbance (with the consequent exposure of physically protected organic matter) did not enhance the rate of net N mineralisation measured by CIT on the sandy, low-organic-matter soil studied, although an increase in soil aeration may have enhanced rates above those measured by RCT and BAL. Overall, RCT was considered to be the more favourable technique for estimation of net N mineralisation. It also provided a measure of nitrate leaching which was comparable to that obtained by porous ceramic water samplers. However, separate estimates of the likely loss of N by denitrification should be obtained with soils which are particularly vulnerable (eg. poor aeration and high clay or water content). Spatial variability was a particular problem with all three techniques which can be overcome by taking a large number of soil cores to increase sample replication. This revised version was published online in June 2006 with corrections to the Cover Date.     

Elevated CO2 stimulates grassland soil respiration by increasing carbon inputs rather than by enhancing soil moisture

It is not clear whether the consistent positive effect of elevated CO₂ on soil respiration (soil carbon flux, SCF) results from increased plant and microbial activity due to (i) greater C availability through CO₂‐induced increases in C inputs or (ii) enhanced soil moisture via CO₂‐induced declines in stomatal conductance and plant water use. Global changes such as biodiversity loss or nitrogen (N) deposition may also affect these drivers, interacting with CO₂ to affect SCF. To determine the effects of these factors on SCF and elucidate the mechanism(s) behind the effect of elevated CO₂ on SCF, we measured SCF and soil moisture throughout a growing season in the Biodiversity, CO₂, and N (BioCON) experiment. Increasing diversity and N caused small declines in soil moisture. Diversity had inconsistent small effects on SCF through its effects on abiotic conditions, while N had a small positive effect that was unrelated to soil moisture. Elevated CO₂ had large consistent effects, increasing soil moisture by 26% and SCF by 45%. However, CO₂‐induced changes in soil moisture were weak drivers of SCF: CO₂ effects on SCF and soil moisture were uncorrelated, CO₂ effect size did not change with soil moisture, within‐day CO₂ effects via soil moisture were neutral or weakly negative, and the estimated effect of increased C availability was 14 times larger than that of increased soil moisture. Combined with previous BioCON results indicating elevated CO₂ increases C availability to plants and microbes, our results suggest that increased SCF is driven by CO₂‐induced increases in substrate availability. Our results provide further support for increased rates of belowground C cycling at elevated CO₂ and evidence that, unlike the response of productivity to elevated CO₂ in BioCON, the response of SCF is not strongly N limited. Thus, N limited grasslands are unlikely to act as a N sink under elevated CO₂.     

森林土壤融化期异养呼吸和微生物碳变化特征

采用室内土柱培养的方法,研究在不同湿度(55%和80%WFPS,土壤充水孔隙度)和不同氮素供给(NH_4Cl和KNO_3,4.5 g N/m~2)条件下,外源碳添加(葡萄糖,6.4 g C/m~2)对温带成熟阔叶红松混交林和次生白桦林土壤融化过程微生物呼吸和微生物碳的激发效应。结果表明:在整个融化培养期间,次生白桦林土壤对照CO_2累积排放量显著高于阔叶红松混交林土壤。随着土壤湿度的增加,次生白桦林土壤对照CO_2累积排放量和微生物代谢熵(q_(CO_2))显著降低,而阔叶红松混交林土壤两者显著地增加(P0.05)。两种林分土壤由葡萄糖(Glu)引起的CO_2累积排放量(9.61—13.49 g C/m~2)显著大于实验施加的葡萄糖含碳量(6.4g C/m~2),同时由Glu引起的土壤微生物碳增量为3.65—27.18 g C/m~2,而施加Glu对土壤DOC含量影响较小。因此,这种由施加Glu引起的额外碳释放可能来源于土壤固有有机碳分解。融化培养结束时,阔叶红松混交林土壤未施氮处理由Glu引起的CO_2累积排放量在两种湿度条件下均显著大于次生白桦林土壤(P0.001);随着湿度的增加,两种林分土壤Glu引起的CO_2累积排放量显著增大(P0.001)。单施KNO_3显著地增加两种湿度的次生白桦林土壤Glu引起的CO_2累积排放量(P0.01)。单施KNO_3显著地增加了两种湿度次生白桦林土壤Glu引起的微生物碳(P0.001),单施NH_4Cl显著地增加低湿度阔叶红松混交林土壤Glu引起的微生物碳(P0.001)。结合前期报道的未冻结实验结果,发现冻结过程显著地影响外源Glu对温带森林土壤微生物呼吸和微生物碳的刺激效应(P0.05),并且无论冻结与否,温带森林土壤微生物呼吸和微生物碳对外源Glu的响应均与植被类型、土壤湿度、外源氮供给及其形态存在显著的相关性。     

Soil Carbon Dioxide Flux in Antarctic Dry Valley Ecosystems

The Antarctic dry valleys of southern Victoria Land are extreme desert environments where abiotic factors, such as temperature gradients, parent material, and soil water dynamics, may have a significant influence on soil carbon dioxide (CO₂) flux. Previous measurements of soil respiration have demonstrated very low rates of CO₂ efflux, barely above detection limits. We employed a modified infrared gas-analyzer system that enabled detection of smaller changes in CO₂ concentration in the field than previously possible. We measured diel CO₂ fluxes and monitored soil microclimate at three sites in Taylor Valley. Soil CO₂ flux ranged from –0.1 to 0.15 mol m^–2 s^–1. At two of the three sites, we detected a physically driven flux associated with diel variability in soil temperature. At these sites, CO₂ uptake (negative flux) was associated with dropping soil temperatures, whereas CO₂ evolution (positive flux) was associated with increases in soil temperature. These observations are corroborated by laboratory experiments that suggest that CO₂ flux is influenced by physically driven processes. We discuss four potential mechanisms that may contribute to physically driven gas exchange. Our results suggest there are strong interactions between biological and abiotic controls over soil CO₂ flux in terrestrial ecosystems of the Antarctic dry valleys, and that the magnitude of either may dominate depending on the soil environment and biological activity.     

Soil respiration in a poor upland site of Scots pine stand subjected to elevated temperatures and atmospheric carbon concentration

Soil respiration rates under elevated temperature and atmospheric CO₂ concentrations were studied in eastern Finland (62° 47N, 30° 58E, 144 m.a.s.1.) around naturally regenerated 20 – 30 years old Scots pine trees, enclosed in open top chambers. The production of CO₂ varied spatially and temporally, but clearly followed the changes in temperature measured at the soil surface. However, soil respiration in the open control was higher than that in chambers; i.e. the chamber itself changed the conditions by increasing the temperature, altering the movement of water, and thereby soil moisture. Nevertheless, an elevation in the concentration of atmospheric CO₂ raised soil respiration and brought it nearer to the level in the open control. An increase in temperature seemed to inhibit this rise, possibly because of an imbalance between temperature and moisture.     

Net mineralization and nitrification rates in a clay soil measured and predicted in permanent grassland from soil temperature and moisture content

Summary Net mineralization of N and net nitrification in field-moist clay soils (Evesham-Kingston series) from arable and grassland sites were measured in laboratory incubation experiments at 4, 10 and 20°C. Three depth fractions to 30 cm were used. Nitrate accumulated at all temperatures except when the soil was very dry (=0.13 cm³ cm^–3). Exchangeable NH₄-ions declined during the first 24 h and thereafter remained low. Net mineralization and net nitrification approximated to zero-order reactions after 24 h, with Q₁₀ values generally <1.6. The effect of temperature on both processes was linear although some results conformed to an Arrhenius-type relationship. The dependence of net mineralization and net nitrification in the field soil on soil temperature (10 cm depth) and moisture (0–15, 15–25, 25–35 cm depths) was modelled using the laboratory incubation data. An annual net mineralization of 350 kg N ha^–1 and net nitrification of 346 kg N ha^–1 were predicted between September 1980 and August 1981. The model probably overstressed the effect of soil moisture relative to soil temperature.     

Effect of elevated CO2 on carbon and nitrogen distribution within a tree (Castanea sativa Mill.) — soil system

Two-year-old sweet chestnut trees were grown outside in normal or double CO₂ atmospheric concentration. In spring and in autumn of two growing seasons, a six day labelling pulse of¹⁴C labelled CO₂ was used to follow the carbon assimilation and distribution in the plant-soil system. Doubling atmospheric CO₂ had a significant effect on the tree net carbon uptake. A large proportion of the additional C uptake was lost through the root system. This suggests that increased C uptake under elevated CO₂ conditions increases C cycling without necessarily increasing C storage in the plant. Total root derived material represented a significant amount of the extra-assimilated carbon due to the CO₂ treatment and was strongly correlated with the phenological stage of the tree. Increasing root rhizodeposition led to a stimulation of microbial activity, particularly near the end of the growing season. When plant rhizodeposition was expressed as a function of the root dry weight, the effect of increasing CO₂ resulted in a higher root activity. The C to N ratios were significantly higher for trees grown under elevated CO₂ except for the fine root compartment. An evaluation of the plant-soil system nitrogen dynamics showed, during the second season of CO₂ treatment, a decrease of soil N mineralization rate and total N uptake for trees grown at elevated CO₂ levels.     

Estimation of carbon allocation to the roots from soil respiration measurements of oil palm

CO₂ flux from the soil was measured in situ under oil palms in southern Benin. The experimental design took into account the spatial variability of the root density, the organic matter in the soil-palm agrosystem and the effect of factors such as the soil temperature and moisture.Measurements of CO₂ release in situ, and a comparison with the results obtained in the laboratory from the same soil free of roots, provided an estimation of the roots contribution to the total CO₂ flux. The instantaneous values for total release in situ were between 3.2 and 10.0 mol CO₂ m^-2 s^-1. For frond pile zones rich in organic matter, and around oil palm trunks, root respiration accounted for 30% of the efflux when the soil was at field capacity and 80% when the soil was dry with a pF close to 4.2. This proportion remained constant in interrow zones at around 75%, irrespective of soil moisture.Subsequently carbon allocation to the roots was determined. Total CO₂ release over a year was 57 Mg of CO₂ ha^-1 yr^-1 (around 1610 g of C per m² per year), and carbon allocation to the roots was approximately 53 Mg of CO₂ ha^-1 yr^-1 of which approximately 13 Mg CO₂ ha^-1 yr^-1 (25%) was devoted to turn-over and 40 Mg CO₂ ha^-1 yr^-1 (75%) to respiration.     

Effects of atmospheric CO2 enrichment, water status and applied nitrogen on water- and nitrogen-use efficiencies of wheat

Atmospheric CO₂ levels are expected to exceed 700 mol mol^–1 by the end of the 21st century. The influence of increased CO₂ concentration on crop plants is of major concern. This study investigated water- and nitrogen-use efficiency (WUE and NUE, respectively, were defined by the amount of biomass accumulated per unit water or N uptake) of spring wheat (Triticum aestivumL.) grown under two atmospheric CO₂ concentrations (350 and 700 mol mol^–1), two soil moisture treatments (well-watered and drought) and five nitrogen amendment treatments. Results showed that enriched CO₂ concentration increased canopy WUE, and more N supply led to higher WUE under the increased CO₂. Canopy WUE was significantly lower in well-watered treatments than in drought treatment, but increased with the increased N supply. Elevated CO₂ reduced the apparent recovery fraction of applied N by the plant root system (N_r, defined as the ratio of the increased N uptake to N applied), but increased the NUE and agronomic N efficiency (NAE, defined as the ratio of the increased biomass to N applied). Water limitation and high N application reduced the N_r, NUE and NAE, indicating a poor N efficiency. In addition, there was a close relationship between the root mass ratio and NUE. Canopy WUE was negatively related to the root mass ratio and NUE. Our results indicated that CO₂ enrichment enhanced WUE more at high N application, but increased NUE more when N application was less.