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1.
Sci Total Environ ; 705: 135899, 2020 Feb 25.
Artigo em Inglês | MEDLINE | ID: mdl-31864167

RESUMO

Precipitation is known to have legacy effects on plant diversity and production of many terrestrial ecosystems. Precipitation regimes are expected to become more variable with increasing extreme precipitation events. However, how previous-year precipitation regimes affect the current-year aboveground biomass (AGB) remains largely unknown. Here we measured long-term (2004-2017) AGB in a semi-arid grassland of the Chinese Loess Plateau to evaluate the impact of previous-year precipitation amount on current-year AGB. Furthermore, to assess the response of current-year AGB to previous-year precipitation regimes, we conducted a field manipulation experiment that included three precipitation regimes during 2014-2017: (i) ambient precipitation, (ii) monthly added four 5 mm rain events, and (iii) monthly added one 20 mm event. Both the long-term (2004-2017) observations under ambient precipitation and short-term (2014-2017) measurements under manipulative treatments showed significant positive effects of previous-year precipitation on current-year AGB. Our path analysis suggested that previous-year precipitation frequency had negative effects on the current-year density and mean height of grass (Leymus secalinus) while had positive effects on forb (Artemisia capillaris). The forb had much smaller height and AGB (65% and 53% less, respectively) than the grass. Consequently, the AGB reduced in the weekly small events treatment, causing the sensitivity of AGB to precipitation to decrease. Therefore, our findings indicated that the impacts of precipitation regimes on plant community dynamics should be taken into consideration while assessing the precipitation legacy effect on ecosystem production.

2.
Ying Yong Sheng Tai Xue Bao ; 27(1): 125-32, 2016 Jan.
Artigo em Chinês | MEDLINE | ID: mdl-27228601

RESUMO

To reveal the effects of different fertilization regimes on weed communities in wheat fields under a rice-wheat rotation system, a survey was conducted before wheat harvest in 2014 after a 4-year long-term recurrent fertilization scheme. Weed species types, density, height and diversity index under different fertilization and straw-returning schemes in wheat fields were studied and complemented with a canonical correspondence analysis on weed community distribution and soil nutrient factors. Twenty weed species were recorded among 36 wheat fields belonging to 19 genera and 11 families. Beckmannia syzigachne, Hemistepta lyrata, Malachium aquaticum and Cnidium monnieri were widely distributed throughout the sampled area. Long-term fertilization appeared to reduce weed species richness and density, particularly for broadleaf weeds, but increased weed height. Diversity and evenness indices of weed communities were lower and dominance indices were higher in fields where chemical fertilizers were applied alone or combined with organic fertilizers, especially, where organic-inorganic compound fertilizer was used, in which it readily caused the outbreak of a dominant species and severe damage. Conversely, diversity and evenness indices of weed communities were higher and dominance indices were lower when the straw was returned to the field combined with chemical or organic fertilizers, in which weed community structures were complex and stable with lower weed density. Under these conditions weeds only caused slight reduction of wheat growth.


Assuntos
Agricultura/métodos , Fertilizantes , Oryza , Plantas Daninhas/classificação , Triticum , Plantas Daninhas/crescimento & desenvolvimento , Solo/química
3.
Zhongguo Yi Liao Qi Xie Za Zhi ; 29(4): 273-6, 2005 Jul.
Artigo em Chinês | MEDLINE | ID: mdl-16268356

RESUMO

The applications of PET-CT have developed from qualitative analysis to quantitative analysis. Target volume is important for tumor biological volume defining, tumor isotope therapy, organ function evaluation, acceptor affinity calculation, and pharmaceutical metabolic kinetics. Many factors work on the target volume calculation, such as PET image acquisition mode, scatter correction, attenuation correction, reconstruction method, image display mode, positron pharmacy. The commonly-used methods of target volume calculation are background-threshold, max threshold, and background-max threshold. In this article we will discuss about the methods of target volume calculation.


Assuntos
Algoritmos , Tomografia por Emissão de Pósitrons/métodos , Tomografia Computadorizada por Raios X/métodos , Planejamento da Radioterapia Assistida por Computador/métodos
4.
Zhongguo Yi Liao Qi Xie Za Zhi ; 29(5): 360-1, 367, 2005 Jul.
Artigo em Chinês | MEDLINE | ID: mdl-16419945

RESUMO

This article is to introduce the dramatic improvements in efficiency, sensitivity and bandwidth of the ultrasound transducers--PureWave Crystal Technology.


Assuntos
Transdutores , Ultrassom/instrumentação , Processamento de Sinais Assistido por Computador
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