Net N mineralization and nitrification were determined using the closed_top PVC tube in situ incubation method in a subalpine meadow (Saussurea iodostegia Hance + Carex capillaris L.)→shagspine peashrub (Caragana jub...
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Net N mineralization and nitrification were determined using the closed_top PVC tube in situ incubation method in a subalpine meadow (Saussurea iodostegia Hance + Carex capillaris L.)→shagspine peashrub (Caragana jubata (Pall.) Poir) shrubland→ribbed birch (Betula costata Trautv.) successional sere. The ability of the three communities to supply available N was comparatively studied. The results showed that there were apparent seasonal changes in the inorganic N pools (including NH + 4_N and NO - 3_N) and net N mineralization and nitrification rates in the three sites. There were generally no significant differences in the inorganic N pools among sampling events. But the NH + 4_N concentration in both birch (P<0.01) and meadow (P<0.01) sites was significantly higher than that in peashrub site in June 1996, and the NO - 3_N concentration in peashrub site was significantly higher than that of meadow site (P<0.05) in August 1996. The annual net N mineralization and nitrification rates in peashrub site (16.01 kg·hm -2 ) were higher than in birch (12.05 kg·hm -2 ) and meadow sites (1.64 kg·hm -2 ). The annual net nitrification rate in peashrub site (11.37 kg·hm -2 ) was higher than in meadow site (10.90 kg·hm -2 ) and much lower than in birch site (14.36 kg·hm -2 ). We conclude that the ability of peashrub shrubland soil to supply available N for plant uptake and the ability to prevent available N from denitrification and leaching were higher than that of the other two sites. The leguminous peashrub might play a potential role in supplying more N, which in turn facilitate the invasion of birch saplings during the successional processes.
Sulphur (S) cycling and implications on S fertilizer requirement in sheep-grazed pastures receiving three levels of S fertilizer at rates of 0, 30 and 60 kg S/hm 2 in 1995 and 1996 were studied using a mass-balance a...
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Sulphur (S) cycling and implications on S fertilizer requirement in sheep-grazed pastures receiving three levels of S fertilizer at rates of 0, 30 and 60 kg S/hm 2 in 1995 and 1996 were studied using a mass-balance approach which accounted both for S inputs to and outputs from the soil-plant-animal system. Results indicated that S fertilizer increased by 50% of herbage S uptake and 15% or more of S bio-cycling rate in grazing system. The total S recovery of 30 and 60 kg S/hm 2 treatments from 1995 to 1996 was 74.0% and 37.6% respectively. Mineralization of organic S may be an important source of S for pasture when other S sources are low, accounted for 70% of the total S inputs without S fertilizer. The grazing sheep has an important ecological function, the feature of S cycling in grazed pastures was that 90% of herbage S ingested by grazing sheep was returned to soils as excreta. The amount of S released by sheep excreta accounted about 30% of the total released S through S bio-recycling. Loss from soil S leaching beyond the major plant rooting zone was the main S output from grazing system. However, urinary and fecal S losses, including transfer and leaching losses, also affected S budgets of grazing system. It is therefore important to investigate the recycling rate of both urinary and fecal S under field conditions. Based on the mass-balance principle, at least 10 kg S/hm 2 per year may be necessary for sustainable development of grazing system.
On the sunny days, there were little diurnal changes in both azimuth and directional angle for either sun_ or shade_leaflet. However, there existed a significant diurnal change in midrib angle that reflected movements...
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On the sunny days, there were little diurnal changes in both azimuth and directional angle for either sun_ or shade_leaflet. However, there existed a significant diurnal change in midrib angle that reflected movements regarding evasion of light stress around noon hours. On the cloudy day, a very little diurnal difference was found in azimuth, directional and midrib angle. It is suggested that changing light environment is the main factor for inducing leaflet movement of Robinia pseudoacacia , and the rhythmical movement does not attribute to the leaflet movement. Leaf orientation control test showed that the photosynthetic rate ( Pn ), stomatal conductance ( g s ) and transpiration ( Tr ) of the artificial fixed_leaflets were significantly lower than that of control_leaflets. And the leaf temperature ( Tl ) of the fixed leaflets significantly exceeded that of control_leaflets, which may attribute to the significant difference of light interception between the fixed and control_leaflets. The light_dependent leaflet movement is the morphological adjustment of maintaining optimal physiological status.
Water is usually considered to be a key limiting factor for the growth and reproduction of steppe plants in the Xilin River Basin, Nei Mongol. Foliar delta C-13 values, an indicator of long-term intercellular carbon d...
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Water is usually considered to be a key limiting factor for the growth and reproduction of steppe plants in the Xilin River Basin, Nei Mongol. Foliar delta C-13 values, an indicator of long-term intercellular carbon dioxide concentration and thus of water-use efficiency (WUE) in plants, were measured on Leymus chinensis (Trin.) Tzvel. and Cleistogenes squarrosa (Trin.) Keng. in six communities of different habitats in die Xilin River Basin. The foliar delta C-13 values of both species tended to increase with decreasing soil water content (SWC) and a significant negative correlation was found between foliar delta C-13 Values and SWC in different soil layers, indicating that the two species could change WUE according to water availability. We also found relatively constant leaf water contents (LWC) of the two species in different habitats. Our results implied that the two steppe species might have adapted to different soil water regimes either through adjusting stomatal conductance to get a proper WUE, or through enhancing the osmosis-regulating ability to keep a relatively stable LWC. Our findings could partially explain why the two plant species have a wide distribution range and become dominant in the Xilin River Basin.
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