重庆市土壤有机碳库的估算及其空间分布特征

基于重庆市第二次土壤普查的1411个土壤剖面数据，结合重庆市土壤图、土地利用现状图和行政区划图，在地理信息系统技术的支持下，对重庆市土壤有机碳密度及储量进行了估算、同时引入有机碳丰度指数这一指标，对有机碳在不同土壤、不同区域以及不同景观中的分布特征进行了分析。结果表明：重庆市20cm和100cm深度的土壤有机碳储量分别为0．27Tg和1．0Tg；20cm深度的土壤有机碳密度介于0．33～30．36kg／m^2之间．100cm深度的土壤有机碳密度介于1．27～72．69kg／m^2之间；重庆市土壤有机碳库在不同土壤、不同区域以及不同景观的分布具有高度的空间变异性，100cm深度的土壤、区域和景观有机碳丰度指数分别为0．58～1．95，0．55～1．39和0．46～1．58．与气候、植被、人类活动等因素密切相关。     

A new approach for modelling vertical stress distribution at the soil/tyre interface to predict the compaction of cultivated soils by using the PLAXIS code

Soil compaction by agricultural machines can have adverse effects on crop production and the environment. Different models based on the Finite Element Method have been proposed to calculate soil compaction intensity as a function of vehicle and soil properties. One problem when modelling soil compaction due to traffic is the estimation of vertical stress distribution at the soil surface, as the vertical stress is inhomogeneous (non-uniform) and depends on soil and tyre properties. However, uniform stress distribution at the soil/tyre interface is used to predict the compaction of cultivated soils in most FEM compaction models. We propose a new approach to numerically model vertical stress distribution perpendicular to the driving direction at the soil/tyre interface, employing the FEM models of PLAXIS code. The approach consists of a beam (characterised by its geometric dimensions and flexural rigidity) introduced at the soil surface and loaded with a uniform stress with the aim to simulate the action of a wheel at the soil surface. Different shapes of stress distribution are then obtained numerically at the soil surface by varying the flexural rigidity of the beam and the mechanical parameters of the soil. PLAXIS simulations show that the soil type (soil texture) modifies the shape of the stress distribution at the edges of the contact interface: a parabolic form is obtained for sand, whereas a U-shaped is obtained for clay. The flexural rigidity of the beam changes the shape of distribution which varies from a homogenous (uniform) to an inhomogeneous distribution (parabolic or U-shaped distribution). These results agree with the measurements of stress distributions for different soils in the literature. We compared simulations of bulk density using PLAXIS to measurement data from compaction tests on a loamy soil. The results show that simulations are improved when using a U-shaped vertical stress distribution which replaces a homogenous one. Therefore, the use of a beam (cylinder) with various flexural rigidities at the soil surface can be used to generate the appropriate distribution of vertical stress for soil compaction modelling during traffic.     

Bulk density by Proctor test as a function of texture for agricultural soils in Maputo province of Mozambique

Soil degradation processes may be of various kinds, including soil compaction. The present study was carried out with the objective of assessing the sensitivity of agricultural or recently abandoned soils in Maputo province of Mozambique to compaction. The assessment is based on the maximum of bulk density attained using the Proctor test (MBD).

In this study the soil texture is expressed by silt plus clay (S + C) or clay (C). The relations between the soil texture and MBD, and between soil texture and critical water content (CWC—soil water at which MBD is attained) were determined. Selected soils range from 10 to 74% of S + C and 9 to 60% of C.

The results suggest there is a relationship between the considered parameters, being that between S + C and MBD or CWC, the best. For MBD the relationship is represented by two quadratic equations with the boundary in between these being a S + C value of 25% and C value of 20%.

Based on the obtained results, one can conclude that the selected parameters may be a useful basis for estimation of the sensitivity to compaction of the Maputo province's soils. It is recommended that similar studies be carried out for soils under forest land and for soil of other provinces to establish the national physical degradation hazard as a function of soil parameters determined routinely and at low cost. The suggested parameters are texture and soil organic matter (SOM).     

A comparison of two methods for the isolation of free and occluded particulate organic matter

Various methods exist for the isolation of particulate organic matter (POM), one of the soil‐organic‐matter (SOM) fractions reacting most sensitive on land‐use or soil‐management changes. A combination of density separation and ultrasonic treatment allows to isolate two types of POM: (1) free POM and (2) POM occluded in soil aggregates. POM fractions are closely linked to their biochemical function for the formation and stabilization of aggregates, therefore methods using different aggregate sizes may result in different POM fractions isolated. We evaluated two physical fractionation procedures to reveal whether they yield different POM fractions with respect to amount and composition, using grassland and arable soils with sandy‐loam to sandy–clay‐loam texture and thus low macroaggregate stability. Method I used air‐dried aggregates of <2.0 mm size and a low‐energy sonication for aggregate disruption, method II used field‐moist aggregates <6.3 mm and a high‐energy–sonication procedure for aggregate disruption. POM fractions were analyzed by elemental analysis (C, N) and CPMAS ¹³C‐NMR spectroscopy. With both methods, about similar proportions of the SOM are isolated as free or occluded POM, respectively. The free‐ and occluded‐POM fractions obtained with method I are also rather similar in C and N concentration and composition as shown by ¹³C‐NMR spectroscopy. Method II isolates a free‐ and occluded‐POM fraction with significantly different C and N concentrations. NMR spectra revealed significant differences in the chemical composition of both fractions from method II, with the occluded POM having lower amounts of O‐alkyl C and higher amounts of aryl C and alkyl C than the free POM. Due to the use of larger, field‐moist aggregates with minimized sample pretreatment, two distinctly different POM fractions are isolated with method II, likely to be more closely linked to their biochemical function for the formation and stabilization of aggregates. High‐energy sonication as in method II also disrupts small microaggregates <63 µm and releases fine intraaggregate POM. This fraction seems to be a significant component of occluded POM, that allows a differentiation between free and occluded POM in sandy soils with significant microaggregation. It can be concluded, that microaggregation in arable soils with sandy texture is responsible for the storage of a more degraded occluded POM, that conversely supports the stabilization of fine microaggregates.     

Increased plant density with reduced nitrogen input can improve nitrogen use efficiency in winter wheat while maintaining grain yield

ABSTRACT

Plant density and nitrogen (N) input level have notable effects on root development, distribution in the soil profile, and in turn, N-uptake of winter wheat. Our study objectives were to identify whether a high yield can be maintained with a reduced N input by increasing plant density. Field studies were conducted during four successive seasons (2014–2015, 2015–2016, 2016–2017, and 2017–2018) using a widely planted cultivar, Tainong18. Two regimes of N fertilization (180 kg ha^?1 and 240 kg ha^?1) and three planting densities (135, 270, and 405 plants per m²) were used. Higher plant density led to increased root length density (RLD) and enhanced N uptake from the whole soil profile. The RLD in the soil profile at 0–1.2 m, 0–0.4 m, and 0.4–0.8 m decreased while in the 0.8–1.2 m layer it increased in response to reduced N input. The combined effects of higher plant density and lower N input resulted in reduced N uptake, a lower nitrogen nutrition index (NNI), unchanged grain yield, and improved N use efficiency. In conclusion, it is possible and sustainable to maintain a high wheat yield with reduced N input by increasing plant density.     

Least limiting water range: A potential indicator of changes in near-surface soil physical quality after the conversion of Brazilian Savanna into pasture

The Brazilian savanna, or “Cerrado”, is an ecosystem that originally covered more than 200 Mha in Brazil. It is estimated that about 49.5 Mha in the Cerrado are now covered with cultivated pastures, which are responsible for half of Brazilian beef production. However, soil and pasture degradation represent a threat to this productive system and to the Cerrado ecosystem itself. Thus, the objective of this research was to evaluate the least limiting water range (LLWR) as an index of near-surface soil physical quality after conversion of Brazilian savanna to continuous and short-duration grazing systems. Three sites were evaluated: native Cerrado (NC), continuous grazing (CG), and short-duration grazing (SG). Thirty soil cores (5 cm height, 5 cm diameter) were collected at each site, and used for soil bulk density, soil water retention curve, and soil penetration resistance curve determinations. The results were used for quantification of LLWR and critical bulk density (D_bc), in which LLWR equals zero. The near-surface soil physical quality, as evaluated by the LLWR, was most restrictive for potential root growth in SG. In CG, potential restriction was moderate; however, the entire soil bulk density range was below the D_bc. In NC, potential restriction was minimum. The soil structural degradation process was primarily related to the increase in stocking rates in the grazing systems. The LLWR proved to be a useful indicator of Cerrado soil physical quality, being sensitive to alterations in near-surface physical properties.     

Correlates of extinction risk and hunting pressure in gamebirds (Galliformes)

The 284 species of Galliformes are a highly threatened group of birds subject to direct exploitation for food, sport and cultural practices. The impact of hunting is often assumed to contribute to the high percentage of species (26.4%) listed as threatened with extinction in the IUCN Red List. We take a macroecological approach to examine the anthropogenic and ecological correlates of extinction risk and hunting pressure using linear and stepwise regression. Independent contrasts are analysed, as well as raw species data, to control for the potential confounding influence of phylogenetic trends. Extinction risk is found to be predicted both by the ecological factors considered (e.g., latitudinal range, body mass, elevational range, habitat use) and secondarily by human factors (e.g., human population density, total calorie intake, composition of diet). Hunting pressure itself is also predicted well for the threatened species by several of the anthropogenic and ecological variables. The study demonstrates that human variables can be used successfully to predict extinction risk, and represent an improvement upon methods which examine ecological variables alone. Furthermore, we show that individual threats can be explored using similar techniques, providing a more detailed insight into the processes leading to extinction. As applied to the Galliformes, both approaches provide evidence supporting the hypothesis that hunting pressure has contributed to the high proportion of threatened species in this group, and thus reinforce the case for urgent measures to reduce the impacts of direct exploitation upon these birds.     

若干阔叶树树冠各层叶气孔密度及光照条件对气孔密度的影响

４０种阔叶树树冠的外、中、内层同龄叶中，均以外层叶的气孔密度最高，其次为中层叶，内层叶气孔密度最低。新银合欢和马尖相思幼苗在高光照强度下所形成的叶气孔密度最高，中等光照者次之，低光照者最低。在蓝光照射下形成的叶气孔密度最高，其次为红光照射者，自然漫射光下最低。这说明光照强度和光波长对气孔密度有深刻影响。     

线纹海马渊H ipp ocamp us erectus 冤不同养殖密度下 体理化因子和细菌数量的动态变化

研究线纹海马(Hippocampus erectus)在室内水泥池不同养殖密度下,水体环境因子和细菌数量的动态变化情况.结果表明,在一个倒池换水周期中,养殖组1和养殖组2的磷酸磷(pO434-p)、硝酸氮(NO3--N)、亚硝酸氮(NO2--N)和氨氮(NH4+-N)等离子物质的浓度和细菌、弧菌和异养菌数量均随着养殖时间的推移不断上升,通常在换水后9d或12d达到最高,再次换水后降到极低值.单因素方差分析结果表明,在不同养殖密度下,PO43--p、NO3--N、NO2--N和NH4+-N等离子浓度随着养殖密度的升高而升高;而低密度养殖池中细菌数比高密度养殖池的细菌数显著更高.养殖水体中的pO43--p浓度与异养细菌数呈负相关关系,NO2--N浓度与细菌数量和弧菌数呈正相关关系,NO3--N浓度与细菌数、异养菌数和弧菌数呈负相关关系,NH4+-N浓度与细菌数、异养菌数和弧菌数均呈正相关关系.研究结果可为科学开展海马室内规模化养殖提供理论参考.     

玉米不同品种根系分布和干物质

对四个不同类型的玉米品种的根系分布动态及其干物质积累进行了研究。结果表明:玉米的根系分布具有相似的规律,在土壤中的垂直分布:在0～40cm耕层占总根量的50%～60%,41～70cm占25%～30%,71cm以下深层相对较少;在土壤中的水平分布:以株为中心由里向外逐渐减少,1/4行距处为40%～45%,1/2株距处为25%～30%,1/2行距处为20%～25%;但不同类型玉米品种也各有特点,掖单22的根系相对比较集中,可以减少行距和株距,增加密度;豫玉22根系分布比较分散,利于充分利用水肥条件,具有较强的抗旱性;登海9号和农大108根系垂直分布比较集中,水平分布均匀,根系在后期比较强壮,必须提供足够的水肥,并适当延长生长期,才能发挥其潜力。