Significant and persistent impact of timber harvesting on soil microbial communities in Northern coniferous forests

Forest ecosystems have integral roles in climate stability, biodiversity and economic development. Soil stewardship is essential for sustainable forest management. Organic matter (OM) removal and soil compaction are key disturbances associated with forest harvesting, but their impacts on forest ecosystems are not well understood. Because microbiological processes regulate soil ecology and biogeochemistry, microbial community structure might serve as indicator of forest ecosystem status, revealing changes in nutrient and energy flow patterns before they have irreversible effects on long-term soil productivity. We applied massively parallel pyrosequencing of over 4.6 million ribosomal marker sequences to assess the impact of OM removal and soil compaction on bacterial and fungal communities in a field experiment replicated at six forest sites in British Columbia, Canada. More than a decade after harvesting, diversity and structure of soil bacterial and fungal communities remained significantly altered by harvesting disturbances, with individual taxonomic groups responding differentially to varied levels of the disturbances. Plant symbionts, like ectomycorrhizal fungi, and saprobic taxa, such as ascomycetes and actinomycetes, were among the most sensitive to harvesting disturbances. Given their significant ecological roles in forest development, the fate of these taxa might be critical for sustainability of forest ecosystems. Although abundant bacterial populations were ubiquitous, abundant fungal populations often revealed a patchy distribution, consistent with their higher sensitivity to the examined soil disturbances. These results establish a comprehensive inventory of bacterial and fungal community composition in northern coniferous forests and demonstrate the long-term response of their structure to key disturbances associated with forest harvesting.     

土壤紧实度变化对小麦籽粒产量和品质的影响

以济南17(强筋品种)、烟农15(中筋品种)、鲁麦22(弱筋品种)为供试品种,设置人为碾压和不碾压2种处理,研究了土壤紧实度(以土壤容重表示)变化对不同类型小麦品种的籽粒产量和加工品质的影响。结果表明,随着土壤紧实度提高,3个品种的分蘖成穗率均显著降低,从而导致单位面积穗数和籽粒产量降低。3个品种相比较分蘖成穗率低的鲁麦22籽粒产量降幅最大。相关品质指标测定结果显示,提高土壤紧实度,对3个品种的蛋白质含量、湿面筋含量、沉淀值和吸水率均无显著影响,但济南17的面筋指数明显降低,面团断裂时间和面团稳定时间显著缩短,单位重量面粉烘焙所得的面包体积变小,而烟农15和鲁麦22受影响较小。其原因可能与土壤紧实度提高条件下济南17籽粒中谷蛋白／醇溶蛋白比例和谷蛋白大聚体含量降低有关。将济南17面团流变学特性年际间变化幅度与紧实度变化的处理效应相比较发现,土壤紧实度是影响强筋小麦品种品质性状稳定性的重要因素之一。     

Effects of soil compaction and mechanical damage at harvest on growth and biomass production of short rotation coppice willow

The effects of soil compaction and mechanical damage to stools at harvesting on the growth and biomass production of short rotation coppice (SRC) of willow (Salix viminalis L.) were monitored on clay loam (CL) and sandy loam (SL) soils. Moderate compaction, more typical of current harvesting situations did not reduce biomass yields significantly. Even heavy compaction only reduced stem biomass production by about 12% overall; effects were statistically significant only in the first year of the experiment on sandy loam. Heavy compaction increased soil strength and bulk density down to 0.4 m depth and reduced soil available water and root growth locally. Soil loosening treatments designed to alleviate the effects of heavy compaction did not markedly improve the growth of willow on compacted plots. Hence the focus fell on harvesting. Extensive mechanical damage to stools caused a 9% and 21% reduction in stem dry mass on the clay loam and sandy loam soils as a result of fewer stems being produced. The particularly severe effect on the sandy loam soil probably resulted from a combination of dry conditions in the year of treatment, root damage and soil compaction under stools and might have been aggravated by the young age of the plants (1 year) at the time of treatment.     

Shifting abundances of communities associated with nitrogen cycling in soils promoted by sugarcane harvest systems

Sugarcane cultivation supports Brazil as one of the largest world sugar and ethanol producer. In order to understand the impact of changing sugarcane harvest from manual to mechanized harvest, we studied the effect of machinery traffic on soil and consequently soil compaction upon soil microbial communities involved in nitrogen cycling. The impact of sugarcane harvest was dependent on soil depth and texture. At deeper soil layers, mechanized harvesting increases the abundance of nitrogen fixers and denitrifying communities (specifically nosZ clade I and II) while manual harvesting increases the abundance of ammonia oxidizers (specifically AOA) and increases denitrifying communities (nosZ clade I and II) on top and at intermediate depth. The effect of change on the harvest system is more evident on sandy soil than on clay soil, where soil indicators of compaction (bulk density and penetration resistance) were negatively correlated with soil microorganisms associated with the nitrogen cycle. Our results point to connections between soil compaction and N transformations in sugarcane fields, besides naming biological variables to be used as proxies for alterations in soil structure.     

BAP1 enhances Polycomb repression by counteracting widespread H2AK119ub1 deposition and chromatin condensation

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A biophysical analysis of root growth under mechanical stress

The factors controlling root growth in hard soils are reviewed alongside summarised results from our recent studies. The turgor in cells in the elongation zone of roots pushes the apex forward, resisted by the external pressure of the soil and the tension in the cell walls. The external pressure of the soil consists of the pressure required to deform the soil, plus a component of frictional resistance between the root and soil. This frictional component is probably small due to the continuous sloughing of root cap cells forming a low-friction lining surrounding the root. Mechanically impeded roots are not only thicker, but are differently shaped, continuing to increase in diameter for a greater distance behind the root tip than in unimpeded roots. The osmotic potential decreases in mechanically impeded roots, possibly due to accumulation of solutes as a result of the slower root extension rate. This more negative osmotic potential is not always translated into increased turgor pressure, and the reasons for this require further investigation. The persistent effect of mechanical impedance on root growth is associated with both a stiffening of cell walls in the axial direction, and with a slowing of the rate of cell production.     

Physiological markers of stress susceptibility in maize and triticale under different soil compactions and/or soil water contents

Differences between two maize and two triticale genotypes grown in low soil compaction (LSC), moderate soil compaction (MSC) and severe soil compaction (SSC) and with a limited (D) or excess (W) soil water content were observed as a decrease in shoot (S) and root (R) biomass, leaf greening (SPAD) and increase in membrane injury (LI), root and leaf water potential (ψ), photosynthesis (Pn), transpiration (E) and stomata conductance (g_S). Close correlations between ψ_L and ψ_R, and between differences ψ_L and ψ_R (Δψ) were found. Drought or waterlogging with LSC conditions in both maize genotypes resulted in higher WUE than in control plants (LSC C), but under the SSC WUE declined. However, for triticale differences in WUE, between treatments were small and insignificant. In general, changes in markers were greater for genotypes sensitive to the soil compaction (Ankora, CHD-12) than in resistant ones (Tina, CHD-247) and were higher in seedlings grown under SSC conditions.

Abbreviations: ψ_R, ψ_L: root and leaf water potential; C: control; D: drought; E: transpiration rate; FWC: field water capacity; g_S: stomatal conductance; LSC, MSC, SSC: low, moderate and severe soil compaction; P_n: photosynthesis rate; W: waterlogging     

西双版纳热带季节雨林的粗死木质残体及其养分元素

对西双版纳热带季节雨林的粗死木质残体及养分元素贮量进行了研究。结果表明,西双版纳热带季节雨林粗死木质残体的贮量为16.2 t·hm^-2,约占地上活体生物量的4%左右。其中枯立木占32.5%,倒木占33.2%,倒木残体占12.6%,大枝占21.7%。本研究的粗死木质残体贮量低于巴西和东南亚的热带雨林,处于全球热带雨林的下限。粗死木质残体的氮、磷、钾、钙、镁5种元素的贮量分别为61.8、3.4、30.4、118.3和16.5 kg·hm^-2。其中倒木所占比例最大,为31.9%～44.8%;枯立木次之（21.5%～28.7%）;倒木残体占10.3%～25.5%;3个径级的大枝占14.4%～28.9%。枯立木的C/N、C/P和N/P要高于倒木、倒木残体和大枝。