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41.
北京市蔬菜价格变动的特征及影响因素 总被引:1,自引:0,他引:1
以北京市蔬菜价格的变动为研究对象,采用2007年6月~2011年1月北京市蔬菜总体或分品种的按月平均价格数据,分析了蔬菜价格变动的长期趋势、季节波动和超常波动等特征。在此基础上,运用多元回归模型对北京市蔬菜价格变动的影响因素进行了分析。结果表明,蔬菜上市量、农用生产资料价格、季节、自然灾害等因素均对北京市蔬菜价格有影响。 相似文献
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Many studies have reported impeded root growth in topsoil under reduced tillage or direct drilling, but few have quantified the effects on the least limiting water range for root growth. This study explored the effects of tillage intensity on critical soil physical conditions for root growth in the topsoil. Samples were taken from a 7-year tillage experiment on a Danish sandy loam at Foulum, Denmark (56°30′ N, 9°35′ E) in 2008. The main crop was spring barley followed by either dyer's woad (Isatis tinctoria L.) or fodder radish (Raphanus sativus L.) cover crops as subtreatment. The tillage treatments were direct drilling (D), harrowing 8-10 cm (H), and ploughing (P) to 20 cm depth. A chisel coulter drill was used in the H and D treatments and a traditional seed drill in the P treatment. Undisturbed soil cores were collected in November 2008 at soil field moisture capacity from the 4-8 and 12-16 cm depths.We estimated the critical aeration limit from either 10% air-filled porosity (εa) or relative gas diffusivity (D/D0) of 0.005 or 0.02 and found a difference between the two methods. The critical limit of soil aeration was best assessed by measuring gas diffusivity directly. Root growth was limited by a high penetration resistance in the D and H soils (below tillage depth). Poor soil aeration did not appear to be a significant limiting factor for root growth for this sandy loam soil, irrespective of tillage treatment. The soil had a high macroporosity and D/D0 exceeded 0.02 at field capacity. Fodder radish resulted in more macropores, higher gas diffusivity and lower pore tortuosity compared to dyer's woad. This was especially important for the H treatment where compaction was a significant problem at the lower depths of the arable layer (10-20 cm depth). Our results suggest that fodder radish could be a promising tool in the amelioration of soil compaction. 相似文献
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Use of organic carbon and loss-on-ignition to estimate soil organic matter in different soil types and horizons 总被引:2,自引:0,他引:2
Summary Loss-on-ignition (LOI) and the organic C content have been used to estimate soil organic matter. Organic matter is often estimated from organic C by applying a factor of 1.724. Several authors have examined the relationship between LOI, used as an estimate of organic matter, and C by simple linear regressions. In the present study, this approach was examined in relation to two sets of data. LOI overestimates organic matter in soils with significant proportions of clay minerals because of bound water, and correcting for bound water gives some LOI: C ratios of less than 1. It is concluded that differences in the nature of the organic matter in different soils and horizons make the simple regression approach unsuitable. More attention needs to be paid to studies of the nature of the organic matter. 相似文献
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Relation of soil organic matter concentration to climate and altitude in zonal soils of China 总被引:1,自引:0,他引:1
With a total of 886 data sets distributed in different regions of China, the relation of soil organic matter (SOM) concentration to climate and altitude was investigated. These data sets were obtained from the 2nd National Soil Survey of China that was completed in early 1980s. According to climate gradient and vegetation community succession, six geographical regions, including eastern, southern, northern, northeastern, northwestern and southwestern China, were divided to identify the key factors regulating surface SOM concentration in different geographical regions. Correlation analysis indicates that surface SOM concentration is in general negatively correlated with annual mean temperature (T) and positively correlated with annual mean precipitation (P) and altitude (H). A further investigation suggested that multiple regression models with different combination of T, P and H could explain 41.5%–56.2% of the variability in surface SOM concentration for different geographical regions, while the driving variables are different. Variables of T and P determined surface SOM concentration in northern, northeastern and northwestern China. In eastern and southern China, variables of P and H are key factors regulating surface SOM concentration. Surface SOM concentration in southwestern China is determined by a linear combination of T, P and H. 相似文献
47.
Methods to quantify organic carbon (OC) in soil fractions of different stabilities often involve time-consuming physical and chemical treatments. The aim of the present study was to test a more rapid alternative, which is based on the spectroscopic analysis of bulk soils in the mid-infrared region (4000-400 cm−1), combined with partial least-squares regression (PLS). One hundred eleven soil samples from arable and grassland sites across Switzerland were separated into fractions of dissolved OC, particulate organic matter (POM), sand and stable aggregates, silt and clay particles, and oxidation resistant OC. Measured contents of OC in each fraction were then correlated by PLS with infrared spectra to obtain prediction models. For every prediction model, 100 soil spectra were used in the PLS calibration and the residual 11 spectra for validation of the models. Correlation coefficients (r) between measured and PLS-predicted values ranged between 0.89 and 0.97 for OC in different fractions. By combining different fractions to one labile, one stabilized and one resistant fraction, predictions could even be improved (r=0.98, standard error of prediction=16%). Based on these statistical parameters, we conclude that mid-infrared spectroscopy in combination with PLS is an appropriate and very fast tool to quantify OC contents in different soil fractions. 相似文献
48.
T.P. Leo A.P. da Silva M.C.M. Macedo S. Imhoff V.P.B. Euclides 《Soil & Tillage Research》2006,88(1-2):279-285
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 (Dbc), 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 Dbc. 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. 相似文献
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依据云南省马龙地区2003 ~ 2012年种植季的数据资料,分别利用单一回归模型、混合模型建立预报模型,并利用历史数据资料的回代进行精度检验.结果表明:2个模型均满足精度要求,但经过改进的混合模型明显提高了拟合精度.采用混合预报模型替代传统单一预报模型,综合考虑了同时期和不同时期的因素对马铃薯晚疫病的影响,提高了模型的精度. 相似文献