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81.
82.
Godswill Makombe Johannes M. Makadho Rajan K. Sampath 《Irrigation and Drainage Systems》1998,12(3):253-263
This paper analyses the water management performance of small holder irrigation systems in Zimbabwe. The government and farmer managed systems are compared in terms of their ability to match desired with actual water supply. Desired supply is defined as crop water requirements adjusted downwards by rainfall where relevant. The Theil measure of accuracy of forecasts is used to calculate the error committed by each system in trying to match water supply and demand. The analysis shows that, everything else being equal, the farmer managed system performs better than the government system in matching supply and demand. This means that the farmer managed systems should be encouraged for future small holder irrigation development in Zimbabwe. 相似文献
83.
Soil microbial biomass and nitrogen supply in an irrigated lowland rice soil as affected by crop rotation and residue management 总被引:5,自引:0,他引:5
C. Witt Kenneth G. Cassman Johannes C. G. Ottow Ulrich Biker 《Biology and Fertility of Soils》1998,28(1):71-80
Processes that govern the soil nitrogen (N) supply in irrigated lowland rice systems are poorly understood. The objectives
of this paper were to investigate the effects of crop rotation and management on soil N dynamics, microbial biomass C (CBIO) and microbial biomass N (NBIO) in relation to rice N uptake and yield. A maize-rice (M-R) rotation was compared with a rice-rice (R-R) double-cropping
system over a 2-year period with four cropping seasons. In the M-R system, maize (Zea mays L.) was grown in aerated soil during the dry season (DS) followed by rice (Oryza sativa L.) grown in flooded soil during the wet season (WS). In the R-R system, rice was grown in flooded soil in both the DS and
WS. Three fertilizer N rates (0, 50 or 100 kg urea-N ha–1 in WS) were assigned to subplots within the cropping system main plots. Early versus late crop residue incorporation following
DS maize or rice were established as additional treatments in sub-subplots in the second year. In the R-R system, the time
of residue incorporation had a large effect on NO3
–-N accumulation during the fallow period and also on extractable NH4
+-N, rice N uptake and yield in the subsequent cropping period. In contrast, time of residue incorporation had little influence
on extractable N in both the fallow and rice-cropping periods of the M-R system, and no detectable effects on rice N uptake
or yield. In both cropping systems, CBIO and NBIO were not sensitive to residue incorporation despite differences of 2- to 3-fold increase in the amount of incorporated residue
C and N, and were relatively insensitive to N fertilizer application. Extractable organic N was consistently greater after
mid-tillering in M-R compared to the R-R system across N rate and residue incorporation treatments, and much of this organic
N was α-amino N. We conclude that N mineralization-immobilization dynamics in lowland rice systems are sensitive to soil aeration
as influenced by residue management in the fallow period and crop rotation, and that these factors have agronomically significant
effects on rice N uptake and yield. Microbial biomass measurements, however, were a poor indicator of these dynamics.
Received: 31 October 1997 相似文献
84.
Physical and chemical soil degradation hinder the introduction of mixed cropping systems on former tea lands in the Central Highlands of Sri Lanka. For instance on two typical soil catenas soil properties such as texture, pH, acidity parameters, pedogenic oxides and available plant nutrients indicating fertility are presented. Obviously Al3+ occupies a larger part of the exchange complex and accounts for more soil acidity than H+. Al toxicity is hence a major restricting factor for plant growth in the area. The investigated data also permit an evaluation of soil erodibility and a soil fertility assessment according to the Fertility Capability Classification System (Sanchez et al., 1982). Agricultural measures to reduce soil acidity and cover the lime requirements of the soils are advised. Additional soil conservation measures should diminish soil erosion in order to improve the sustainability of the cropping systems. 相似文献
85.
P. F. A. M. Römkens Johannes van der Plicht Jan Hassink 《Biology and Fertility of Soils》1999,28(3):277-284
Conversion of arable land (maize) to pasture will affect the soil organic matter (SOM) content. Changes in the SOM content
were studied using a size- and density-fractionation method and 13C analysis. Twenty-six years of maize cropping had resulted in a depletion of carbon stored in the macro-organic fractions
(>150 μm) and an increase in the <20 μm fraction. Maize-derived carbon in the upper 20 cm increased from 10% in the finest
fraction (<20 μm) to 91% in the coarse (>250 μm), light (b.d. <1.13 g cm–3) fractions. Pasture installation resulted in a rapid recovery of the total SOM content. Up to 90% of the pasture-derived
carbon that was mineralized during maize cropping was replaced within 9 years. Especially the medium and coarse size (>150
μm) and light (b.d. <1.13 g cm–3) fractions were almost completely regenerated by input of root-derived SOM. The amount of medium-weight and heavy macro-organic
fractions (>150 μm; b.d. >1.13 g cm–3) in the 0- to 20-cm layer was still 40–50% lower than in the continuous pasture plots. Average half-life times calculated
from 13C analyses ranged from 7 years in the light fractions to 56 years in heavy fractions. Fractionation results and 13C data indicated that mechanical disturbance (plowing) during maize cropping had resulted in vertical displacement of dispersed
soil carbon from the 0- to 20-cm layer down to 60–80 cm. Conversion of arable land to pasture, therefore, not only causes
a regeneration of the soil carbon content, it also reduces the risk of contaminant transport by dispersed soil carbon.
Received: 10 March 1998 相似文献
86.
87.
Effect of an intensive mechanical removal effort on a population of non‐native rainbow trout Oncorhynchus mykiss in a South African headwater stream
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Jeremy Shelton Olaf Weyl Johannes Van Der Walt Sean Marr Dean Impson Kristine Maciejewski Donovan Tye Helen Dallas Karen Esler 《水产资源保护:海洋与淡水生态系统》2017,27(5):1051-1055
88.
Conservation implications of establishment success of the Critically Endangered Twee River redfin ‘Pseudobarbus’ erubescens (Skelton, ) in an artificial impoundment in South Africa
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Martine S. Jordaan Johannes A. van der Walt Zanné Brink Sonja Erasmus Olaf L. F. Weyl 《水产资源保护:海洋与淡水生态系统》2017,27(4):886-889
89.
Pedohydrological properties were investigated on a piped slope in the Bergisches Land in the Rhenish Slate Mountains (Nordrhein‐Westfalen, Germany). The study confirmed that genesis and development of pipes decisively depend on hydrological conditions in the soils. Vertical water permeability of saturated samples was very high. This promoted fast seepage. Many macropores produced by earthworms also caused high transport capacity for soil water. Even more efficient were the burrows of moles and mice, enabling immediate infiltration and direct vertical and lateral water movement. On the contrary, the horizontal saturated permeability was low indicating no correlation with piping. Porosity of piped soils was not different to that of other soils of the region without pipes. 相似文献
90.
Canopy-held organic matter develops into a distinct soil system separate from the forest floor in wet temperate coniferous forests, creating a natural microcosm. We distinguished between fungal and bacterial components of the decomposer community in one site with Maple (Acer macrophyllum) and one site with Alder (Alnus rubra) by using direct measurements of growth; acetate incorporation into ergosterol, and leucine incorporation for fungi and bacteria, respectively. The higher organic matter content of the canopy soils correlated with higher fungal growth. The relative importance of fungi, indicated by fungal:bacterial growth ratio, was higher in the canopy soil of the Maple site, while there was no difference in the Alder site. The high C:N ratio of the Maple canopy soil likely contributed to this difference. These results demonstrate a divergence between canopy and forest floor that should be explored to gain insights in decomposer ecology using the natural microcosms that the canopy soils provide. 相似文献