Using perlite as a substrate carrier for measuring microbial available phosphorus by respiration kinetics in soils

The release of CO₂ by soil microorganisms after the addition of nitrogen and glucose in excess and calibration additions of phosphorus has successfully been used to assess microbial available P, assuming the native soil P pool is then limiting respiration. However, in P-fixing soils and soils with high P content, carbon can be exhausted before the available soil P pool. It is not possible to simply increase the amount of glucose as then the glucose concentration would be lethal for microorganisms. A modified method was tested where soil is mixed with perlite. It was hypothesised that perlite, having a high water holding capacity, would dilute the concentration of glucose, while maintaining the bioavailability of added nutrients, thus avoiding carbon limitation. Factorial combinations of amount of soil and perlite (both adjusted to −25 kPa water potential) were tested to examine if perlite as such had any effect on the respiration. Five tropical soil samples with a sharp gradient in P availability and one N-limited compost material were used. The method successfully reduced the risk of carbon limitation. Microbial indices, such as basal respiration, substrate-induced respiration and maximum P-limited respiration, were directly proportional to the amount of soil in the experiments but unrelated to the amount of perlite, showing that perlite did not affect microbial measurements.     

Carbon fluxes in soil food webs of increasing complexity revealed by C labelling and C natural abundance

Soil food webs are mainly based on three primary carbon (C) sources: root exudates, litter, and recalcitrant soil organic matter (SOM). These C sources vary in their availability and accessibility to soil organisms, which could lead to different pathways in soil food webs. The presence of three C isotopes (¹²C, ¹³C and ¹⁴C) offers an unique opportunity to investigate all three C sources simultaneously. In a microcosm experiment we studied the effect of food web complexity on the utilization of the three carbon sources. We choose an incomplete three factorial design with (i) living plants, (ii) litter and (iii) food web complexity. The most complex food web consisted of autochthonous microorganisms, nematodes, collembola, predatory mites, endogeic and anecic earthworms. We traced C from all three sources in soil, in CO₂ efflux and in individual organism groups by using maize grown on soil developed under C₃ vegetation and application of ¹⁴C labelled ryegrass shoots as a litter layer. The presence of living plants had a much greater effect on C pathways than food web complexity. Litter decomposition, measured as ¹⁴CO₂ efflux, was decreased in the presence of living plants from 71% to 33%. However, living plants increased the incorporation of litter C into microbial biomass and arrested carbon in the litter layer and in the upper soil layer. The only significant effect of food web complexity was on the litter C distribution in the soil layers. In treatments with fungivorous microarthropods (Collembola) the incorporation of litter carbon into mineral soil was reduced. Root exudates as C source were passed through rhizosphere microorganisms to the predator level (at least to the third trophic level). We conclude that living plants strongly affected C flows, directly by being a source of additional C, and indirectly by modifying the existing C flows within the food web including CO₂ efflux from the soil and litter decomposition.     

重复添加植物残体后土壤微生物活性和生物量对盐度的响应特征

Microbial adaptation to salinity can be achieved through synthesis of organic osmolytes,which requires high amounts of energy;however,a single addition of plant residues can only temporarily improve energy supply to soil microbes.Therefore,a laboratory incubation experiment was conducted to evaluate the responses of soil microbes to increasing salinity with repeated additions of plant residues using a loamy sand soil with an electrical conductivity in saturated paste extract（EC_e） of 0.6 dS m^-1.The soil was kept non-saline or salinized by adding different amounts of NaCl to achieve EC_e of 12.5,25.0 and 50.0 dS m^-1.The non-saline soil and the saline soils were amended with finely ground pea residues at two rates equivalent to 3.9 and 7.8 g C kg^-1 soil on days 0,15 and29.The soils receiving no residues were included as a control.Cumulative respiration per g C added over 2 weeks after each residue addition was always greater at 3.9 than 7.8 g C kg^-1 soil and higher in the non-saline soil than in the saline soils.In the saline soils,the cumulative respiration per g C added was higher after the second and third additions than after the first addition except with3.9 g C kg^-1 at EC_e of 50 dS m^₁.Though with the same amount of C added(7.8 g C kg^-1),salinity reduced soil respiration to a lesser extent when 3.9 g C kg^-1 was added twice compared to a single addition of 7.8 g C kg^-1.After the third residue addition,the microbial biomass C concentration was significantly lower in the soils with EC_e of 25 and 50 dS m^₁ than in the non-saline soil at3.9 g C kg^-1,but only in the soil with EC_e of 50 dS m^-1 at 7.8 g C kg^-1.We concluded that repeated residue additions increased the adaptation of soil microbial community to salinity,which was likely due to high C availability providing microbes with the energy needed for synthesis of organic osmolytes.     

基于组分区分的南方红壤丘陵土壤呼吸对植被类型转换的响应

土壤呼吸是陆地生态系统碳循环的重要环节之一。长时期的水土保持生态建设导致南方红壤丘陵区植被类型转换非常普遍。探讨植被转换对土壤呼吸及其关键组分的影响,不仅对深入了解土壤呼吸与碳循环的内在机制有着重要的理论价值,还可以为科学评价水土保持生态建设在应对气候变化方面的作用提供科学依据。依托江西水土保持生态科技园对侵蚀退化裸地及其恢复承建后的百喜草地、柑橘果园和湿地松人工林进行了一年尺度的土壤呼吸速率监测。结果表明,土壤呼吸速率月变化呈夏高冬低的曲线模式,最大值出现在8月份或9月份,最小值出现在1月份,植被转换没有改变土壤呼吸的季节变化模式。土壤呼吸速率的季节变化主要受浅层土壤温度的控制,与土壤含水率没有显著关系,5 cm深度土壤温度能解释总呼吸速率和异养呼吸速率季节变异的83.3%和86.0%。随着植被类型由裸地向草地、果园和林地转换,土壤呼吸的温度敏感性系数Q10值由1.86增大到2.20、2.72和2.75,即土壤呼吸对土壤温度的变化越来越敏感。异养呼吸占土壤总呼吸的比例呈现出单峰曲线模式,平均达到72.3%,且随土壤呼吸速率的增大而增大。南方红壤丘陵区侵蚀裸地向人工草地、果园和湿地松人工林转换的过程中土壤呼吸速率有所增强,草地与裸地之间土壤总呼吸速率存在显著性差异,草地与裸地、草地与林地之间土壤异养呼吸速率差异显著。在区域或生态系统尺度上,植被类型是土壤呼吸的重要影响因子。     

Density loss and respiration rates in coarse woody debris of Pinus radiata, Eucalyptus regnans and Eucalyptus maculata

This study compared field and laboratory decomposition rates of coarse woody debris (CWD) (>10 cm diameter) from three tree species: Pinus radiata, Eucalyptus regnans, and Eucalyptus maculata. For this purpose, the density loss of logs on the ground sampled from chronosequences of sites following harvesting was determined using the water replacement technique. P. radiata logs were sampled 1, 2.5, 6, and 9 years following harvesting, and logs of E. regnans and E. maculata were collected from sites that were harvested 1, 3.5, 6.5, and 12 and 1.5, 6.5, and 11.5 years ago, respectively. In addition, the C/N ratio of wood was determined and current respiration rates of logs from these different age classes were measured through laboratory incubation. The times for loss of 95% of material (t_0.95) determined from density loss for these species were 24 years for P. radiata, 43 years for E. regnans, and 62 years for E. maculata. The decomposition rates of CWD derived from laboratory respiration were 6.1, 5.9 and 11.9 times higher than the decay rates from density loss in P. radiata, E. regnans, and E. maculata, respectively. This points to severe constraints of decomposition through adverse conditions in the field. The changes in respiration rates and C/N ratio with age of decaying logs indicated that the single component, negative exponential decay model could be applied satisfactorily only to P. radiata. In the case of the eucalypt species, substrate quality (expressed through respiration rates) declined in the oldest samples. This may be explained by the loss of rapidly decomposing sapwood and the retention of more decay-resistant heartwood. In these cases, a two-component model will be more suitable to describe the density loss of decaying wood.     

Effects of vegetation regime on denitrification potential in two tropical volcanic soils

Effects of vegetation and nutrient availability on potentail denitrification rates were studied in two volcanic, alluvial-terrace soils in lowland Costa Rica that differ greatly in weathering stage and thus in availability of P and base cations. Potential denitrification rates were significantly higher in plots where vegetation had been left undisturbed than in plots where all vegetation had been removed continuously, and were higher on the less fertile of the two soils. The potential denitrification rates were correlated strongly with respiration rates, levels of mineralizable N, microbial biomass, and moisture content, and moderately well with concentrations of extractable NH _inf4 ^sup+ , Kjeldahl N, and total C. In all plots, denitrification rates were stimulated by the removal of O₂ and by the addition of glucose but not by the addition of water or NO _inf3 ^sup- .This is Paper 2772 of the Forest Research Laboratory, Oregon State University     

Effects of long-term 2,4-D application on microbial populations and biochemical processes in cultivated soil

Summary The effects of 15 years of field applications of 2,4-dichlorophenoxy acetate (2,4-D) on soil microbial population and biochemical processes were studied in a field cropped with maize followed by potatoes. Amine or ester formulations at the rate of 0.95 kg 2,4-D per hectare applied in May and October every year. Fungal, bacterial, and actinomycete populations, and microbial biomass C and N were reduced by the 2,4-D treatment, the reduction being more marked where the ester was used. N mineralization, nitrification, and potentially mineralizable N were reduced by the 2,4-D ester only, while urease activity was depressed by both formulations. Dehydrogenase activity and soil microbial respiration tended to be temporarily increased by the amine, but were reduced substantially by the ester, indicating that the ester probably interfered with nutrient cycling.     

Water-extractable soil carbon in relation to the belowground carbon cycle

We investigated the role of water-extractable carbon (C-extr) as potential substrate for forest soil microorganisms by comparing belowground C fluxes at a plot with the forest floor removed (no-litter) and at a control plot. One-third lower soil respiration rates at the no-litter plot gave evidence that the forest floor was the source of considerable amounts of microbially degradable C. Laboratory incubation of C-extr, fractionated into neutral and acid moieties, showed that part of the C-extr was degraded rapidly, and that the high-molecular-weight acid fraction was much less degradable than the neutral C. To the extent that the degradable portion of the water-extractable C can be regenerated quickly, it may supply much of the substrate for heterotrophic soil respiration. Received: 11 December 1995     

氮输入对森林土壤有机碳截存与损耗过程的影响

大气氮沉降对受氮限制的陆地生态系统碳截存/损耗的机理尚不清楚,尤其是对土壤有机碳(SOC)的输入、转化和输出过程的认识明显不足。本文论述了外源性氮素(氮沉降、人为增氮)对凋落物分解、土壤有机碳各组分周转的影响,以及土壤呼吸各组分(根系自养呼吸、根际微生物呼吸和SOM分解)对增氮的响应等领域的最新研究进展,指出了在上述研究领域中存在的问题,并提出拟解决的途径以及未来的可能研究方向,以期为该领域的研究提供参考。     

Intersite characterization and variability of soil respiration in different arable and forest soils

Summary Soil respiration was investigated in three loamy Orthic Luvisols (two arable, one forest soil), three sandy Haplic Podzols (also two arable, one forest soil) with a modified intersite method according to Lundegardh (1924). The method allows characterization of the CO₂-flux from the soil and interpretation of the different levels with regard to temperature, nutrient and air supply. The method is sensitive to tillage and fertilization effects. In the two arable Luvisols the mean cumulative respiration rate was not uniform compared with the forest soil; in one case it was much higher and in another much lower. CO₂ evolution in the Podzol under spruce was much lower than in the two arable Podzols. In the sandy Podzols 5 replicate measurements gave adequate results, with an error probability of 10%, but in the loamy Luvisols it was necessary to use 10 replicates to specify the same degree of difference. If soil respiration is very high, immediately after fertilization with cattle slurry or dung on arable land, or after litterfall in a deciduous forest, more replicates are necessary.