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排序方式： 共有55条查询结果，搜索用时 781 毫秒
1.
人工林林分密度最优控制策略的数学模型 总被引：7，自引：0，他引：7
采用变分法及最优控制理论就如何从空间和时间上合理安排与调控森林资源做了具体研究,得出了人工林的间伐强度和蓄积密度的最优控制策略及其连续变化的数学模型,其优点在于突破了以往仅给出离散状态下模型的局限性,从而为人工林的持续经营管理提供了理论依据。 相似文献
2.
木材硅石改性剂的制备工艺 总被引：1，自引：0，他引：1
为了降低木材硅化改性成本促其应用,直接将固态硅石粉高效液态活化,通过正交试验和极差分析优化制备了木材硅石改性剂,测试分析了浸渍改性杨木的性能。结果表明:①反应时间和原料n(SiO2)∶n(NaOH)对活化溶液的pH值、黏度和密度影响不显著,碱料种类和反应温度对活化溶液的黏度、密度、SiO2转化率、固体质量分数和生产成本影响显著,对溶液性能综合影响最大的是反应温度和碱料种类,其次是n(SiO2)∶n(NaOH)和反应时间;②pH值随反应时间、温度和n(SiO2)∶n(NaOH)增大而减小,密度、黏度、固体质量分数、SiO2转化率和模数随反应时间、温度和n(SiO2)∶n(NaOH)增大而增大,KOH活化效果更好但价格较高,综合平衡优化工艺为:反应温度200℃、碱料为NaOH、n(SiO2)∶n(NaOH)=1.715∶1、反应时间4 h;③硅石改性剂能充分渗入杨木,使其吸药量达252%,密度提高81%,抗弯强度提高69.6%,炭化温度减小89℃,残炭率提高41%,改性效果优于硅溶胶。 相似文献
3.
鲁西平原毛白杨造林地土壤全氮空间变异性研究 总被引：1，自引：0，他引：1
以位于鲁西平原冠县毛白杨6.0hm2造林地为研究区,将土壤在水平方向按50m×50m分为24块样地,每块样地分为0～20cm,20～40cm,40～60cm三层取样,进行三维空间采样和室内土壤全氮含量测定,对土壤全氮空间分异规律进行研究。结果表明:研究区土壤全氮在林地尺度存在一定的空间变异;从研究区土壤总体分异状况看,土壤全氮含量范围是0.02～0.46gkg-1,总体均值为0.17gkg-1,三个层次土壤全氮含量分别为0.33±0.06gkg-1,0.14±0.04gkg-1,0.05±0.02gkg-1,全氮含量属于最低水平。三个层次土壤全氮含量变异系数分别为19%、32%、39%,变异程度属于中等变异水平,土壤全氮随着土层加深分异程度明显增大。土壤全氮含量在不同土壤层次水平变化复杂,0～20cm层西北高,东南部低,20～40cm和40～60cm两层变化恰好与之相反。研究结果可为毛白杨造林试验地布设、养分精准管理、确定合理采样数提供依据。 相似文献
4.
5.
P.L. Mafongoya R. Chintu T.S. Chirwa J. Matibini S. Chikale 《Agroforestry Systems》2003,59(3):279-288
Seeking an alternative to Sesbania spp. tree fallows, a Tephrosia species and provenance trial was conducted at Msekera Research Station, Chipata (Zambia) to evaluate eleven Tephrosia vogelii and three Tephrosia candida provenances. They were tested for biomass production, quality of biomass, resistance to root-knot nematodes, nitrogen release,
and for their effects on soil nitrogen dynamics. At the end of 1.5 years, the T. candida provenances 02970, 02971 and 02972 from Madagascar produced two times greater amount of aboveground biomass than the T. vogelii provenances. There was little variability among the T. vogelii provenances in terms of litter and biomass production. Weed growth was significantly greater under T. vogelii than T. candida provenances. While Tephrosia vogelii provenance 98/02 from Zambia and T. candida 02972 were highly tolerant to the Meloidogyne incognita nematodes, T. vogelii provenances 02977, 98/03, 02973 from Kenya, Zambia and Malawi, respectively, were highly susceptible to the nematodes. The
Tephrosia species and provenances showed a wide variability in terms of N, lignin and polyphenol concentration in their foliage. Mineralization
of N in the foliage of T. candida provenances 02970 and 02971 and T. vogelii provenances 98/04 and 02974 from Malawi occurred rapidly within 14 weeks of incubation. At the end of the 2-year growth period,
there was significantly greater total inorganic N under T. candida provenance 02972 (12.5 mg kg−1) than T. vogelii (5 mg kg−1) provenance Mungwi 98/02. Maize (Zea mays L.) yields after T. candida provenances were greater than those after T. vogelii provenances. Further testing of the most promising provenances is needed for their effects on subsequent maize yields under
a range of farm conditions.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
6.
Sesbania [Sesbania sesban (L.) Merr.] fallows are being promoted as a means for replenishing soil fertility in N-depleted soils of small-scale, resource-poor
farmers in southern Africa. Knowledge of soil water distribution in the soil profile and water balance under proposed systems
is important for knowing the long-term implications of the systems at plot, field and watershed levels. Soil water balance
was quantified for maize (Zea mays L.) following 2-year sesbania fallow and in continuous maize with and without fertilizer during 1998–1999 and 1999–2000 at
Chipata in eastern Zambia. Sesbania fallow increased grain yield and dry matter production of subsequent maize per unit amount
of water used. Average maize grain yields following sesbania fallow, and in continuous maize with and without fertilizer were
3, 6 and 1 Mg ha−1 with corresponding water use efficiencies of 4.3, 8.8 and 1.7 kg mm−1 ha−1, respectively. Sesbania fallow increased the soil-water storage in the soil profile and drainage below the maximum crop root
zone compared with the conventionally tilled non-fertilized maize. However, sesbania fallow did not significantly affect the
seasonal crop water use, mainly because rainfall during both the years of the study was above the normal seasonal water requirements
of maize (400 to 600 mm). Besides improving grain yields of maize in rotation, sesbania fallows have the potential to recharge
the subsoil water through increased subsurface drainage and increase nitrate leaching below the crop root zone in excess rainfall
seasons.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
7.
8.
In the subsistence-agricultural region of eastern Zambia, less than 10% of the households have adequate supply of maize (Zea mays L.), the staple food, throughout the year. A major constraint to increasing crop production in the region is poor fertility
status of the soil. In order to address this problem, improved fallow has been introduced as a technology for improving soil
fertility within a short span of two to three years. Farmers have been testing the technology and a number of empirical studies
have been undertaken over the years to identify the factors influencing farmers' decision to adopt the technology. This paper
presents a synthesis of the results of adoption studies and highlights generic issues on the adoption of improved fallows
in Zambia. The synthesis indicates that farmers' decision on technology adoption does not have a simple directed relationship
of some technological characteristics only, but constitutes a matrix of factors including household characteristics, community
level factors, socioeconomic constraints and incentives that farmers face, access to information, local institutional arrangements
and macro policies on agriculture. The adoption of improved fallows is not strictly speaking a binary choice problem but a
continuous process in which farmers occupy a position along a continuum in the adoption path. Further, adoption of improved
fallows may not take place in a policy vacuum but needs to be facilitated by appropriate and conducive policy and institutional
incentives. Several questions and issues that require further study emerge from the synthesis. These include determination
of the relative importance of the factors in the adoption matrix, identification of the conditions under which farmers use
a combination of inputs and their profitability under changing price scenarios, exact definition to delineate between `non-adopters',
`testers' and `adopters' of agroforestry technologies, and understanding the impact of cash crop farming in farmers' adoption
decisions of improved fallows (where off farm opportunities exist). Further, there is a need to determine the inter-relationship
between household poverty, labor availability and the adoption of improved fallows and, to evaluate a combination of policy
interventions at both national and local level to promote the adoption of agroforestry-based soil fertility management.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
9.
金沙江头塘小流域5种类型人工林林下植物多样性研究 总被引：1，自引：0，他引：1
采用标准样地调查法,对金沙江头塘小流域退耕还林地5种不同的人工林林下的灌木和草本层植物群落数量特征,以及物种丰富度、多样性、均匀度等特征进行研究。结果表明,该区域的人工林林下共有植物59种,隶属31科,蔷薇科和菊科植物占优势。各种人工林林下植被的优势种不同;物种丰富度以华山松林最高,圣诞树林最低;云南松林、旱冬瓜林和云南松-华山松混交林的灌木层和草本层分布均匀,群落多样性高;华山松林下缺乏灌木层,但草本层物种丰富。通过比较,发现金沙江头塘小流域人工林的灌木物种多样性显著低于云南松原始林的物种多样性,而草本物种的多样性则相反,即人工林高于云南松原始林。 相似文献
10.