Indigenous vegetation in the foreland of river oases at the extremely arid southern margin of the Taklimakan Desert in Xinjiang,NW China,is dominated by a few perennial phreatophytes,primarily Alhagi sparsifolia Shap.,Tamarix ramosissima Ledeb.and Karelinia caspica Less.Water is well known as a limiting factor for plant growth in deserts,availability of nutrients has also been determined as a critical factor limiting plant growth in arid regions.Water source for plants is groundwater at the southern fringe of the Taklimakan Desert.So nutrients are the most important resource for plant growth in the same groundwater depth.When the soil N content or the nitrogen validity are low,plants with the ability to fix N2 might have an advantage and be capable of taking in more N than plants without this ability.Leaf N content correlates strongly with photosynthesis as most of the leaf N is allocated to photosynthesis.So some people considered that the nitrogen fixing plants might have higher Pmax than the nitrogen non-fixing ones living in the same habitat.However,in our previous research,we found that nitrogen non-fixing species had higher photosynthetica capacity.In order to answer this question,nitrogen fixing species A.sparsifolia and nitrogen non-fixing species K.caspica and T.ramosissima were compared in their typical habitat at the southern fringe of the Taklimakan Desert.The diurnal variations of net photosynthetic rate,indexs of light and CO2 to net photosynthetic rate(Pn),Chl content,leaf nitrogen content of nitrogen fixing and non-fixing species were investigated in June,July and August 2008.The results showed that from 08∶00 to 20∶00 the diurnal variations of Pn showed double peak curve both nitrogen fixing and non-fixing species in June and July.In August,the curve changed into a single peak.The order of average photosynthetic rate of three months: K.caspicaT.ramosissimaA.sparsifolia.It also showed that nitrogen non-fixing species had higher Pn than nitrogen fixing species.The maximum net photosynthetic rate(Pmax),apparent quantum requirement(AQY),CO2 saturation point(CSP),dark respiration rate(Rday) of nitrogen non-fixing species were higher than nitrogen fixing species,the difference was significant(P0.05),respectively.Carboxylation efficiency(Vcmax) and electron transport rate(Jmax) were not significantly different between nitrogen fixing and non-fixing species(P0.05).The nitrogen fixing species had significantly higher light compensation point(LCP),light saturation point(LSP),CO2 compensation point(CCP),Chl content and leaf nitrogen content than nitrogen non-fixing species(P0.05).Through correlative analysis,we found that there was no obviously relationship between Pmax and Chl,leaf nitrogen content in A.sparsifolia.There was no obviously relationship between Pmax and Chl in K.caspica and T.ramosissima.Pmax of T.ramosissima was negatively correlated with leaf nitrogen content.But Pmax of K.caspica was positively correlated with leaf nitrogen content.In despite of higher leaf nitrogen content in nitrogen fixing species,nitrogen non-fixing species had higher photosynthetic capacity.The higher leaf nitrogen content not always bring higher photosynthetic rate in nitrogen fixing and non-fixing species.
Abstract. Changes in precipitation variability are known to influence grassland growth. Field measurements of aboveground net primary productivity (ANPP) in temperate grasslands suggest that both positive and negative asymmetric responses to changes in precipitation may occur. Under normally variable precipitation regimes, wet years typically result in ANPP gains being larger than ANPP declines in dry years (positive asymmetry), whereas increases in ANPP are lower in magnitude in extreme wet years compared to reductions during extreme drought (negative asymmetry). Whether ecosystem models that couple carbon-water system in grasslands are capable of simulating these non-symmetrical ANPP responses is an unresolved question. In this study, we evaluated the simulated responses of temperate grassland primary productivity to scenarios of altered precipitation with fourteen ecosystem models at three sites, Shortgrass Steppe (SGS), Konza Prairie (KNZ) and Stubai Valley meadow (STU), spanning a rainfall gradient from dry to moist. We found that: (1) Gross primary productivity (GPP), NPP, ANPP and belowground NPP (BNPP) showed concave-down nonlinear response curves to altered precipitation in all the models, but with different curvatures and mean values. (2) The slopes of spatial relationships (across sites) between modeled primary productivity and precipitation were steeper than the temporal slopes obtained from inter-annual variations, consistent with empirical data. (3) The asymmetry of the responses of modeled primary productivity under normal inter-annual precipitation variability differed among models, and the median of the model-ensemble suggested a negative asymmetry across the three sites, in contrast to empirical studies. (4) The median sensitivity of modeled productivity to rainfall consistently suggested greater negative impacts with reduced precipitation than positive effects with increased precipitation under extreme conditions. This study indicates that most models overestimate the extent of negative drought effects and/or underestimate the impacts of increased precipitation on primary productivity under normal climate conditions, highlighting the need for improving eco-hydrological processes in models.
Abstract A two‐fold enhancement in the sensitivity of atmospheric CO 2 growth rate (CGR) to tropical temperature interannual variability () till early 2000s has been reported, which suggests a drought‐induced shift in terrestrial carbon cycle responding temperature fluctuations, thereby accelerating global warming. However, using six decades long atmospheric CO 2 observations, we show that has significantly declined in the last two decades, to the level during the 1960s. The decline begs the question of whether the sensitivity of ecosystem carbon cycle to temperature variations at local scale has largely decreased. With state‐of‐the‐art dynamic global vegetation models, we further find that the recent decline is barely attributed to ecosystem carbon cycle response to temperature fluctuations at local scale, which instead results from a decrease in spatial coherence in tropical temperature variability and land use change. Our results suggest that the recently reported loss of rainforest resilience has not shown marked influence on the temperature sensitivity of ecosystem carbon cycle. Nevertheless, the increasing extent of land use change as well as more frequent and intensive drought events are likely to modulate the responses of ecosystem carbon cycle to temperature variations in the future. Therefore, our study highlights the priority to continuously monitor the temperature sensitivity of CGR variability and improve Earth system model representation on land use change, in order to predict the carbon–climate feedback.
The coupled contrast tests and the random constast tests were conducted to find the corrosion behaviour difference between A3 steel and 16Mn steel in soil. The tests consisted of over 60 in situ buried test spots in a nearly 2×10 4 km 2 area. By means of mathematical statistics and pattern recognition mathods, a conclusion was drawn that the average corrosion rates, pitting rates and rust features of two kinds of steels were almost the smae. At a few test spots (less than 10% of total), the small differences were observed. In these scarce cases, 16Mn steel was always in preference to A3 steel in their anticorrosion properties, especially, in the soils with high content of chloride and heavy capability weight. Further, a mathematical model covering four soil parameters was established to judge in which type of soils 16Mn might have better anticorrosion properties than A3 steel.
Current standards in petroleun industry prescribe two ispecting indexes for estimation of DC interference corrosion of buried pipelines,namely:excursion of pipe/soil potential and magnitude of potential gradient in surface.These indexes are not well defined and lack of quantitative relationship with interference corrosion,so that the standards are difficult to perform in practices.In present paper,three new inspecting indexes for DC interference corrosion are proposed and the corresponding estimating systems are discussed and created.
A method of measuring and assessing the soil homogeneity was proposed to replace the method of Standard DIN 50929. The index of homogeneity,ΔE(Fe) was defined as the maximum, difference of potentials between two adjacent electrodes, which were inserted in soils being studied. The method was used in corrosion investigation in TaLim Area and the data were treated by correlative analyses. It was indicated that the ΔE(Fe) had no correlation with other soil corrosion factors, but had important contribution to soil corrosivity. It will be beneficial to choose ΔE(Fe) as a key parameter in study of corrosion modeling. This conclusion was confirmed by practice in soil corrosion researches on Lun Ku Pipelines.
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