Mineralization rate of C-labelled dissolved organic matter from leaf litter in soils of a weathering chronosequence

During the processes of primary succession and soil development, large stocks of organic C with very long residence times accumulate in many soils. Soluble organic C adsorbed by soils may contribute to the stock of organic C accumulating during soil development. We determined whether the mineralization rate of water-soluble organic C and the insoluble residue from ¹⁴C-labelled leaf litter added to soils from a weathering chronosequence decrease as soil age and adsorption capacity increase. The soils were formed on mudflows of andesitic material deposited about 75, 255, 616 y ago, and another older but undetermined time before this study. The percentage of the DOC adsorbed by the soils increased with age. After 1 year of incubation there were no significant differences in the mineralization rates of DOC added to soils of different ages. The DOC appeared to be comprised of two fractions, one that comprises about 32% of the total that mineralized with a half decay time of 0.02 y (7 d) and a second fraction comprising 68% with a half decay time of about 1.6 y. Consequently, the slowly mineralized fraction of the soluble C contributed to the accumulation of slowly mineralized C in the soil. Both the slowly and rapidly mineralized fractions of the insoluble residue decomposed more slowly than the corresponding fractions in DOC. We found no support for the idea that increased adsorption capacity due to weathering resulted in protection of soluble organic C from microbial mineralization.     

Functional diversity of microbial communities in the mixed boreal plain forest of central Canada

This study was designed to examine whether or not specific tree species (Picea glauca, Picea mariana, Pinus banksiana, Populus tremuloides), their post-fire stand age, or their position in a successional pathway had any significant effect on the functional diversity of associated soil microbial communities in a typical mixed boreal forest ecosystem (Duck Mountain Provincial Forest, Manitoba, Canada). Multivariate analyses designed to identify significant biotic and/or abiotic variables associated with patterns of organic substrate utilization (assessed using the BIOLOG™ System) revealed the overall similarity in substrate utilization by the soil microbial communities. The five clusters identified differed mainly by their substrate-utilization value rather than by specific substrate utilization. Variability in community functional diversity was not strongly associated to tree species or post-fire stand age; however, redundancy analysis indicated a stronger association between substrate utilization and successional pathway and soil pH. For example, microbial communities associated with the relatively high pH soils of the P. tremuloides-P. glauca successional pathway, exhibited a greater degree of substrate utilization than those associated with the P. banksiana-P. mariana successional pathway and more acidic soils. Differences in functional diversity specific to tree species were not observed and this may have reflected the mixed nature of the forest stands and of their heterogeneous forest floor. In a densely treed, mixed boreal forest ecosystem, great overlap in tree and understory species occur making it difficult to assign a definitive microbial community to any particular tree species. The presence of P. tremuloides in all stand types and post fire stand ages has probably contributed to the large amount of overlap in utilization profiles among soil samples.     

Microbial activity and bacterial composition of H2-treated soils with net CO2 fixation

The effects of H₂ gas treatment of an agricultural soil cultivated previously with a mixture of clover (Trifolium pratense) and alfalfa (Medicago sativa) on CO₂ dynamics and microbial activity and composition were analyzed. The H₂ emission rate of 250 nmol H₂ g⁻¹ soil h⁻¹ was similar to the upper limit of estimated H₂ amounts emitted from N₂ fixing nodules into the surrounding soil ([Dong, Z., Layzell, D.B., 2001. H₂ oxidation, O₂ uptake and CO₂ fixation in hydrogen treated soil. Plant and Soil 229, 1-12.]). After 1 week of H₂ supply to soil samples simultaneously with H₂ uptake net CO₂ production declined continuously and this finally led to a net CO₂ fixation rate in the H₂-treated soil of 8 nmol CO₂ g⁻¹ soil h⁻¹. The time course of H₂ uptake and CO₂ fixation in the soils corresponded with an increase in microbial activity and biomass of the H₂-treated soil determined by microcalorimetric measurements, fluorescence in situ hybridization analysis (FISH) and DNA staining (DAPI). Shifts in the bacterial community structure caused by the supply of H₂ were recorded. While the H₂ treatment stimulated β-and γ-subclasses of Proteobacteria, it had no significant effect on α-Proteobacteria. In addition, FISH-detectable bacteria of the Cytophaga-Flavobacterium-Bacteroides phylum increased in numbers.     

Surface cast production by the earthworm Aporrectodea nocturna in a pre-alpine meadow in Switzerland

Field and laboratory experiments were carried out to describe the effects of Aporrectodea nocturna on soil characteristics in a pre-alpine meadow and to support the development of a model of cast production. In the prealpine meadow, increased cast production, first observed about 20 years ago around a newly planted hedge, was recorded to a distance of maximal 170 m from the hedge. Numbers of A. nocturna between 130 and 165 m from the hedge decreased from 164 to 16 individuals m^-2. In the same area cast production steadily decreased from about 1.5 kg m^-2 week^-1 to nil, the plant community structure changed and the microbial biomass decreased, but the root biomass and the moisture content did not change. Laboratory experiments demonstrated that high cast production was not a specific feature of the A. nocturna population nor of the soil in the meadow. Diapause of A. nocturna was terminated in the laboratory during September. A model of cast production potential by the earthworm A. nocturna was established using laboratory determinations of the relationships with body weight, temperature, and water potential. The model was shown to predict cast production in the field given the assumption that the water potential was 0 MPa. According to the model, 81% of surface cast production was by juveniles, and 19% by adults of A. nocturna.     

Soil aggregation: Influence on microbial biomass and implications for biological processes

Our 1988 paper, describing the effects of cultivation on microbial biomass and activity in different aggregate size classes, brought together the ‘aggregate hierarchy theory’ and the ‘microbial biomass concept’. This enabled us to identify the relationships between microbial and microhabitat (aggregate) properties and organic matter distribution and explain some of their responses to disturbance. By combining biochemical and direct microscopy based quantification of microbial abundance with enzyme activities and process measurements, this study provided evidence for the role of microbial biomass (especially fungi) in macroaggregate dynamics and carbon and nutrient flush following cultivation. In the last ten years environmental genomic techniques have provided much new knowledge on bacterial composition in aggregate size fractions yet detailed information about other microbial groups (e.g. fungi, archaea and protozoa) is lacking.We now know that soil aggregates are dynamic entities – constantly changing with regard to their biological, chemical and physical properties and, in particular, their influences on plant nutrition and health. As a consequence, elucidation of the many mechanisms regulating soil C and nutrient dynamics demands a better understanding of the role of specific members of microbial communities and their metabolic capabilities as well as their location within the soil matrix (e.g. aggregates, pore spaces) and their reciprocal relationship with plant roots. In addition, the impacts of environment and soil type needs to be quantified at the microscale using, wherever possible, non-destructive ‘in situ’ techniques to predict and quantify the impacts of anthropogenic activities on soil microbial diversity and ecosystem level functions.     

The effect of surface growth of blue-green algae and bryophytes on some microbiological,biochemical, and physical soil properties

Summary The influence of surface growth of inoculated cyanobacteria (blue-green algae) on subsurface properties of a brown earth, silt loam soil was studied in reconstituted flooded soil columns. One blue-green algae species, Nostoc muscorum, become dominant within the first 7 days of inoculation. In light control columns (not inoculated) a bryophyte, Barbula recurvirostra, was dominant although significant growth of indigenous blue-green algae occurred. The blue-green algae counts were in the range of 1×10⁶ g^-1 dry soil in the surface layer (0–0.7 cm) in both columns. Any effect of surface phototrophic growth on soil properties was restricted to the surface layer. In inoculated columns there was a twofold increase in microbial biomass and an eightfold increase in bacterial numbers by week 13. However, bacterial numbers declined so that there was only a 2.8-fold increase by week 21. Dehydrogenase (x2.1), urease (x2.8) and phosphatase (x3.1) activities and polysaccharides (+69%) increased by week 21 as a result of the blue-green algae inoculation along with a significant improvement in soil aggregation. However, similar increases occurred in the light control columns, indicating that given appropriate conditions of light and moisture indigenous species may be ultimately as effective as introduced species in bringing about biochemical and microbiological changes to soil.     

Amino acids in grassland soils: Climatic effects on concentrations and chirality

The response of soil organic nitrogen (SON) dynamics to climate may partly be deduced from changes in the concentration and origin of the major N constituents in soil, such as amino acids. In this study, we determined the enantiomers of bound amino acids in 18 native grassland soils (0–10 cm) that were sampled along a transect from central Saskatchewan, Canada, to Southern Texas, USA. Mean annual temperature (MAT) ranged from 0.9 to 23.4 °C and mean annual precipitation (MAP) from 300 to 1308 mm. d-alanine and d-glutamic acid served as markers for the bacterial origin of SON. The d-content of lysine, phenylalanine, and aspartic acid indicated an ageing of the respective SON forms. Deuterium labeling was applied to account for hydrolysis-induced racemization reactions. We found that the concentration of the bacterial biomarkers was weakly but significantly parabolically related to MAT, as previously reported for microbial-derived amino sugars. The age markers d-lysine, d-phenylalanine, and d-aspartic acid comprised 2–15% of the respective l-form. The presence of these compounds demonstrated that the structures that contained these d-enantiomers had survived microbial attack, i.e., these hydrolyzable SON forms were conserved in soil despite a living environment. First estimates indicate that the mean residence time of the lysine-containing organic matter forms extend beyond a century. Within this time-scale we did not find that climate significantly affects the degree of ageing of SON constituents in the studied topsoils.     

不同肥料配施对酸性土钙素活化及花生产量和品质的影响

为了揭示增施钙肥和不同肥料配施对沿海地区酸性土钙素增量及活化效应,于2015—2016年设置单施无机肥(CK_1)、无机肥/熟石灰配施(T_1)、无机肥—有机肥配施(CK_2)、无机肥—有机肥—熟石灰配施(T_2)、无机肥—有机肥—微生物肥配施(CK_3)、无机肥—有机肥—微生物肥—熟石灰配施(T_3)6个处理,研究了不同肥料配施对酸性土有效钙含量、花生植株性状、荚果产量和籽仁品质的影响。结果表明:CK_2和CK_3均在一定程度上提高酸性土中水溶性钙和交换性钙含量,0—20cm水溶性钙含量各生育期平均分别比CK_1提高48.13%和66.50%,交换性钙含量分别提高39.12%和60.88%。增施钙肥处理(T_1、T_2和T_3)能显著增加水溶性钙和交换性钙含量,但_T1处理对有效钙含量增加较少,而T_2和T_3处理增加幅度较大,其中T_3处理0—20cm水溶性钙和交换性钙含量各生育期平均比CK_3提高1.63,1.74倍,说明钙肥与有机肥和微生物肥配施能显著提高酸性土钙素活化度。不同肥料配施均提高了酸性土花生的主茎高、分枝数、主茎绿叶数、单株干物质重、叶面积指数和实收株数,增加了单株结果数和荚果饱满度,显著提高了荚果产量,并提高了蛋白质、总氨基酸和脂肪含量以及O/L值。其中T_3处理分别比CK_3、T_2、CK_2、T_1和CK_1增产14.38%,4.99%,18.31%,25.65%和52.52%,增产幅度最大,其籽仁品质也最优。增施钙肥并与有机肥和微生物肥配施能显著提高酸性土钙素活化度、有效钙含量、荚果产量和籽仁品质。     

Influence of agricultural traffic and crop management on collembola and microbial biomass in arable soil

Collembola and microbial biomass C were investigated in a field experiment with controlled agricultural traffic and crop rotation over a period of 27 months. The wheel-induced compactive efforts were applied according to management practices within the crop rotation of sugar beet, winter wheat, and winter barley. Increasing wheel traffic produced increasing soil compaction, mainly due to a reduction in surface soil porosity. Increasing soil compaction was accompanied by a decrease in microbial biomass C and the density of collembola. The influence of soil compaction on microbial biomass C was smaller than that of the standing crop. However, for collembola, especially euedaphic species, a reduction in pore space appeared to be of more importance than the effects of a standing crop. Within the crop rotation, microbial biomass C and the density of collembola increased in the order sugar beet, winter wheat, and winter barley.     

水土流失防治措施对马尾松林土壤微生物群落分子生态网络的影响

水土流失防治措施可以提高侵蚀退化林地的土壤质量。为了探究典型水土流失防治措施对土壤微生物分子生态网络的影响,在南方红壤区马尾松林下建立了水土流失防治措施小区,设置3种不同处理,分别为挖设鱼鳞坑加种植草本,挖设鱼鳞坑加种植草本与灌木,以及对坡面土壤不设置任何措施的对照。利用16S rRNA和18S rRNA基因 Illumina MiSeq 高通量测序技术,测定3种处理下的土壤微生物群落组成,基于随机矩阵的方法构建微生物网络。结果表明,措施实施后,绿弯菌门相对丰度显著降低,变形菌门与酸杆菌门的相对丰度显著升高（P < 0.05）。2种措施处理下微生物网络总节点数、总连接数、平均连通度以及模块性均较高,微生物网络的规模增大,微生物间互相作用更复杂。3个微生物网络均以负互相作用（60.59%～67.49%）为主,措施实施后物种间竞争作用进一步加强。绿弯菌门、放线菌门和变形菌门中的部分菌群在本研究区的微生物网络中起着重要的连接作用,此外,3个微生物网络的部分关键节点所属的菌群相对丰度较低（< 1%）,对于构建微生物网络也具有关键作用。2种措施处理下微生物网络平均路径距离较长,微生物作用的响应速度慢,群落结构稳定性提高,其中挖设鱼鳞坑并种植草本与灌木的措施较挖设鱼鳞坑并只种植草本的措施效果更优。冗余分析表明,土壤容重（R2 = 0.465,P < 0.05）、pH（R2 = 0.377,P < 0.05）、有机质（R2 = 0.383,P < 0.05）、全氮（R2 = 0.545,P < 0.01）、对细菌群落结构有显著影响,土壤含水量（R2 = 0.485,P < 0.05）对真菌群落结构有显著影响。上述研究结果表明,水土流失防治措施实施后土壤微生物群落结构发生明显变化,网络规模增大、物种间互作强度以及微生物群落结构的稳定性提高。