| The Karst landform, distributed largely in the subtropical region, affected easily by the change of environmental factors because of its fragile ecological environment and poor stability. The biggest continuous belt of Karst landscape in southwest China, has the basic characteristics of high exposure of rocks, thin soil layer, and poor water retention capacity, the plants often suffer drought, and will be alleviated by rainfall. On the other hand, its distributed area has abundant precipitation, and the frequency-intensity rainfall will increase during the 21st century because of global climate chances. Thus the plants in natural environment of Karst are suffering or will suffer from repeated drying-wetting alternation habitat. N, one of the most important elements to plant, affects plant growth, development and reproduction, and lead to the impact on productivety of ecosystem finally. However. The N content in the atmosphere is increasing due to human activities in recent years leading to the soil N content increase to the terrestrial ecosystem. So it will pose what effects to annual herbs of fragile Karst area. The study about the effects of N addition on plants under the background of global climate change is one of the research hotspots. This research selected Xanthium sibirium and Bidens pilosa, wich are the annal herbs in Karst, exploring the growth and physiology of annual herbs growing different water habitat on N addition in Karst regions, according to nitrogen and water control experiment. There were two nitrogen levels (N-and N+) and five water treatments (the control CK, drying one week and re-watering DW-1, drying two weeks and re-watering DW-2, drying three weeks and re-watering DW-3 and the drought D). The main conclusions of study are as follows:(1) The above-ground bio mass accumulation and leaf area reduced, root-shoot ratio increased slightly but not significantly, efficient absorption area and root extension were limited under drought stress to limit growth of X. sibirium and B.pilosa. DW (drying-wetting alternation) group had a certain degree of compensation effect compared with drought, and the drought limiting effects increased with prolonged drought. N addition promoted above-ground and under-ground growth of X.sibirium and B.pilosa under CK, DW and D water treatments, such as increased plant height, larger leaf area, wider root extension and increased efficient absorption area. Overall, the promoting effect of N addition in drying-wetting alternation (DW) was greater than in drought treatment (D), which in above-ground part was greater than in underground part.(2) Drought limited the net photo synthetic rate (Pn) of X.sibirium and B.pilosa due to significant lower stomatal conductance (gs) compared with the control (CK), and the influence to water use efficiency of B.pilosa was larger than X.sibirium. Drought stress did not largely damage the leaf photosynthetic structure, and Fv/Fm, ΦPS Ⅱ and ETR significantly reduced, but heat dissipation creased. The stomatal conductance (gs) of treated plants under drying-wetting alternation was not recovered but lowers than plants under drought stress, without a significant increase in ΦPSⅡ and ETR, so the Pn was lowest in water treatments, and as the drought last the Pn decreased. N addition significantly improved the stomatal conductance and water use efficiency of X.sibirium and B.pilosa under drying-wetting alternation DW-2 and DW-3, and the promoting effect was greater than the drought. Each parameter index of X.sibirium and B.pilosa under drying-wetting alternation recovered to a certain extent, and the promoting effects reduced with prolonged drought, the biggest promoted degree in water treatments of X.sibirium and B.pilosa were DW-2 and DW-3 respectively.(3) Drought reduced C, N, P content of roots, stems and leaves of X.sibirium and B.pilosa (the extent of impact N, P>C). Plant total C, N and P accumulation reduced. The distributed patterns of C, N and P in root, stem and leaf were different:N and P allocation percentage in root increased, C, N, P allocation percentage in stem increased, but C, N, P allocation percentages in leaf decreased relatively. Plant total C, N, P accumulation of X.sibirium and B.pilosa under drying-wetting alternation group increased compared with drought, and the differences of C, N, P accumulation under drying-wetting alternation group DW-1, DW-2 and DW-3 was not significant. N addition increased plant total C, N and P accumulation under different water treatments, but the promoting effect differ from soil moisture conditions to soil moisture conditions, and the distributed patterns of roots, stems and leaves were different: drought reduced the P percentage in roots and C, N and P percentage in the leaves, but increased the C, N, P percentage in stems. The promoting effects of N addition on C, N and P accumulation and different components under different water treatments were increased in varied degrees:CK>DW>D, root(stem)>leaf, but there was no significant difference between the drying-wetting alternate treatments. On the whole, the influences of element content under different water treatments of X.sibirium were smaller than B.pilosa. N addition improved plant total C, N, P accumulation, and the biggest impact showed on leaf, and improved the absorption efficiency of P.Results showed that soil water deficit in karst reduced photosynthesis and element absorption efficiency, and limited growth and above-ground biomass accumulation, and reduced plant total C, N, P accumulation to change the allocation of limited resources. Drying-wetting alternate treatments had compensatory effect, but with the extension of drought time, the compensation effect weakened. N addition promoted growth, physiological, element absorption, element accumulation and distribution of Xsibirium and B.pilosa, but their strength of promoting effects was different because of different soil moisture content (CK>DW>D). Therefore, N addition enhanced the annual herb adaptability to drought and drying-wetting alternation in Karst, and promoted growth and physiology of the annual herb under drying-wetting alternation and drought habitat, the promoting effect was larger in B.pilosa than in X.sibirium. |
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