Effects of rock fragments on physical degradation of cultivated soils by rainfall

To understand better the role of rock fragments in soil and water conservation processes, the effects of rock fragments in maintaining a favourable soil structure and thus also in preventing physical degradation of tilled soils was studied. Laboratory experiments were conducted to investigate the effects of rock fragment content, rock fragment size, initial soil moisture content of the fine earth and surface rock fragment cover on soil subsidence by rainfall (i.e. change in bulk density by one or more cycles of wetting and drying). A total of 15 rainfall simulations (cumulative rainfall, 192.5 mm; mean intensity, 70 mm h⁻¹) were carried out. Before and after each rainfall application the surface elevation of a 19-cm thick plough layer was measured with a laser microrelief meter. In all experiments, the bulk density of the fine earth increased with applied rainfall volume to reach a maximum value at about 200 mm of cumulative rainfall. From the experimental results it was concluded that the subsidence rate decreased sharply for soils containing more than 0.50 kg kg⁻¹ rock fragments, irrespective of rock fragment size. Fine earth bulk densities were negatively related to rock fragment content beyond a threshold value of 0.30 kg kg⁻¹ for small rock fragments (1.7–2.7 cm) and 0.50 kg kg⁻¹ for large rock fragments (7.7 cm). Initial soil moisture content influenced subsidence only in the initial stage of the experiments, when some swelling occurred in the dry soils. Surface rock fragment cover had no significant effect on subsidence of the plough layer. Therefore, subsidence of the plough layer in these experiments appears to be mainly due to changing soil strength upon drainage rather than the result of direct transfer of kinetic energy from falling drops. The relative increase in porosity of the fine earth as well as the absolute increase in macroporosity with rock fragment content will cause deeper penetration of rainfall into the soil, resulting in water conservation. Therefore, crushing of large rock fragments into smaller ones is to be preferred over removal of rock fragments from the plough layer.     

近10年《作物学报》论文及作者统计分析

西文从发文的系统分布,地区分布、经费来源等方面探讨《作物学报》发文的特点与规律,揭示作物研究领域学科研究主体、学科研究中心 和地我分布情况以及作物研究领域的学科特征,发展潜力、科研合作类型特点等。     

Improved harvesting systems for wet sites

Environmentally acceptable and economical forest operations are needed for sustainable management of forest resources. Improved methods for harvesting and transporting timber are especially needed for wet sites. As the demand for hardwood lumber continues to increase, improved and alternative methods are needed to ensure acceptance of timber harvesting for the wet site conditions that are typical of bottomland hardwoods. Some alternative technological developments include grapple saw feller-bunchers, wide tires, larger forwarders, clambunk skidders, two-stage hauling, mats, cable systems, helicopters and towed vehicles and air-cushioned vehicles. These developments have the potential to improve the performance of the harvesting system and to reduce the negative effects of conventional operations on conventional sites and on difficult sites such as wet areas. Although many of these new alternatives are now operational, others are just concepts or evolving prototypes. More research is still needed to optimize these alternative technologies and to reduce costs associated with their implementation.     

Influence of harvesting on biogeochemical exchange in sheetflow and soil processes in a eutrophic floodplain forest

Floodplain forests contribute to the maintenance of water quality as a result of various biogeochemical transformations which occur within them. In particular, they can serve as sinks for nutrient run-off from adjacent uplands or as nutrient transformers as water moves downstream. However, little is known about the potential that land management activities may have for alteration of these biogeochemical functions. This paper examines the effects of three harvesting regimes (unharvested control, clearcut, and partial cut) on the physical and chemical parameters within the Flint River floodplain located in southwestern Georgia, USA. Data presented in this paper were collected during the year following initiation of the harvesting treatments which occurred in September of 1993. Sheetflow water chemistry (total suspended solids (TSS), total dissolved solids (TDS), nitrate (NO₃⁻), phosphate (PO₄³⁻), sulfate (SO₄²⁻), calcium (Ca²⁺), potassium (K⁺), magnesium (Mg²⁺), ammonium (NH₄⁺), total phosphorous (P), total nitrogen (N), total carbon (C), dissolved organic carbon (DOC)), sedimentation rates, depth of soil oxidation after flooding, saturated hydraulic conductivity, and bulk density were measured. During the year immediately after treatment installation, alterations in some of the physical and chemical properties (TDS, NO₃⁻, total P, and K⁺) of floodwaters crossing harvest plots were detected. Soil oxidation depths, saturated hydraulic conductivity and bulk density also changed with treatment. The meaning of the changes detected is uncertain but they suggest the nature of potential changes in nutrient spiralling and non-point source cumulative effects that may occur within a managed watershed. Second-year data may offer an interesting comparison of sheetflow chemistry and sedimentation changes between vegetated and non-vegetated conditions.     

中国自然科学论文作者的简要统计分析

按国际文献计量学研究的通行做法,论文的归属按第一作者所在的地区和单位确定。通过对2004年中国自然科学论文发表作者群的统计与分析,对作者群的学科分布,机构分布,地区分布,高产、高影响作者群的分布进行了统计与分析,使人们了解中国科研队伍现状及配备情况。目前,中国作者群的学科分布不均匀,高产作者群,特别是国际论文的高产作者群主要分布在化学、物理、生物和数学学科。作者群的地区分布不均匀,特别是地域广大、跨13个省（市）的大西北论文作者数,无论是国际或国内论文的比例都不足20%。高产作者群的4/5分布在中国高等学校     

Some tools for parsimonious modelling and interpretation of within-field variation of soil and crop systems

Parsimony is a guiding principle in scientific investigation which goes back to the medieval schools. It is proposed in this paper that, in different guises, the principle of parsimony (using no more complex a model or representation of reality than absolutely necessary) is essential for agricultural research at the scale of the field or larger regions. Two examples of the principle are given, illustrated with examples from research on precision agriculture:

1. The spatio-temporal variability of a sequence of yield maps is considerable, and often defeats simple approaches to interpretation and analysis. Reducing the variability to a small number of basic temporal patterns with spatial expression allows useful information to be extracted. The normalised fuzzy partition entropy is a parsimonious criterion appropriate for identifying the number of distinct patterns in the data. It is shown in a case study how this can aid interpretation of a complex data set.

2. Models of the joint effect of different factors on crop yield can take various forms. A simple assumption is that the different factors are additive, the most complex models describe interactions. Justus von Liebig’s ‘Law of the minimum’ is a model of little complexity and may often be a parsimonious and powerful tool for modelling crop responses. A comparison of these models on some crop response data, using the Akaike information criterion to measure the parsimony of the different models, is given.

     

Divisive field-scale associations between corn yields, management, and soil information

For corn crops on soils with fine texture in eastern Canada, research has been ongoing on the relationships between corn grain yields and management practice, soil strength/compaction, and soil nutrient status. However, these variables can operate in a complex manner that may not be detectable via uni-structural models. This study used regression-tree analysis, in an exploratory context, to model corn grain yields over a relatively flat clay to silty loam field on the basis of several soil physical, soil nitrate, and management variables. It was found that corn grain yields were first subdivided by length of time under corn production (1st and 3rd year corn groups). These yield groups were further subdivided by cone penetration resistance, elevation, soil nitrate, and soil texture information. Generally, higher corn grain yields were associated with 1st year corn, lower relative soil strength/compaction, and higher post-growing season soil nitrate levels. The modeled results helped to identify equifinality, context dependent relationships, and spatial continuity in inferred formative mechanisms; issues important in many field and/or landscape studies of spatial processes.     

Development and application of landform segmentation procedures

Landscape-scale approaches to research in soil science are explicitly focused on transfers of components within and between landscapes. Despite wide-spread recognition of the importance of these transfers, the application of landscape-scale approaches has been hindered by the lack of clear, reproducible research designs. Landform segmentation is used to divide natural and human-influenced landscapes into functionally distinct units. A specific type of landform segmentation, landform element classification, was used in a comparative mensurative design to compare the effects of cultivation on soil distribution and soil organic carbon (SOC) storage and in a manipulative design to determine the relationship between N₂O emissions and fertilizer rate in a hummocky till geomorphic surface in southern Saskatchewan. Significant transfers of SOC and surface soil from convex shoulder units to lower slope positions occurred over the past 90 years, resulting in a change in the type of soils that occupy these positions at two research sites. The observed pattern is consistent with a tillage translocation dominated surface. The dominant control on N₂O emissions in the landscape are spatial differences in water-filled pore space (WFPS) that are strongly controlled by water redistribution. Emissions from drier, shoulder landform element complexes are consistently low throughout the year, whereas a strong positive relationship between N fertilizer rate and N₂O emissions occur in the wettest, level depressional elements.     

Effects of soil management practices and tillage systems on surface soil water conservation and crust formation on a sandy loam in semi-arid Kenya

The effect of different soil management practices on crust strength and thickness, soil water conservation and crop performance was investigated on a ferric lixisol in a semi-arid environment of eastern Kenya.

The study proved that manure and mulching with minimum tillage have a greater effect on the water balance of crusted soils and maize emergence. There was increase in steady infiltration rates, amount of soil water stored in the soil and better drainage. The physical effect of mulch was less important in the rehabilitation of crusted soils in the study site when it was incorporated into the soil. Manure and surface mulch with minimum tillage should therefore be taken into account in land management and water conservation in the semi-arid areas of Kenya. The response of crops to the improved water availability due to manure with minimum and with conventional tillage and surface mulch was very clear. These management practices should be recommended when considering the effectiveness of soil and water management techniques in the study area.     

Soil properties, and cotton growth, yield and fibre quality in three cotton-based cropping systems

The effects of three cotton-based cropping systems on soil properties, black root rot severity, and growth of cotton in a Vertisol were evaluated after a series of floods in eastern Australia. The experimental treatments, which had been imposed since 1985, were conventionally and minimum-tilled continuous cotton, and minimum-tilled cotton–wheat rotation. Frequent rainfall and flooding during the winter of 1998 resulted in near saturated soil at spring sowing in October. Although conventional tillage operations were completed before flooding, minimum tillage operations were not possible due to excessive moisture and cotton was sown onto the old beds with no-tillage. Soil specific volume (electrical conductivity of a 1:5 soil:water suspension) EC_1:5, exchangeable Na content, pH and organic C were determined for the top 0.6 m of the profile in summer 1998 and again in 1999. Organic C in the surface 0.10 m was also evaluated during 1998–2000. Black root rot severity and mycorrhizal fungal colonisation were evaluated at 6 weeks after sowing. Tissue nutrient concentrations were measured in mature cotton plants. Cotton lint yield and fibre quality were evaluated after picking and ginning.

In comparison with either minimum- or conventionally tilled continuous cotton, minimum-tilled cotton–wheat rotation had the lowest exchangeable Na content and severity of bacterial black root rot, best surface structure and the highest crop growth, nutrient uptake and lint yields. Subsoil structure was the best with conventionally tilled continuous cotton. The 1998 floods appear to have decreased exchangeable Na and increased soil pH in all treatments. Surface organic C also decreased between 1998 and 2000. Soil structural damage was minimised by avoiding tillage and trafficking in wet conditions. Compared with 1998, average yield decreases in 1999 were of the order of 43%. Cotton lint fibre quality was also poorer in 1999.