Effect Of Water Stress And Nitrogen Supply On Nitrogen And Phosphorus Nutrition Of Soil And Broussonetia Papyrifera (Linn.) Vent. System In Chongqing Karst Desert Area

Karst is one of the weakest kinds of ecological environments and one of the main factors that restrict economic development in south-west China. And restoring vegetation and improving soil quality become urgent affairs of the region. In this paper, species diversity of communities with different succession times and soil property were studied in restoration processes, and simulative laboratory was done under greenhouse, One year old seedlings of Broussonetia papyrifera (Linn.)Vent. were grown in pots under greenhouse conditions with vary water contents and nitrogen treatments to study the adaptive mechanisms of Broussonetia papyrifera (Linn.)Vent. in biomass, ration of root to shoot, nitrogen and phosphorus uptake, nitrogen and phosphorus use efficiency, soil nitrogen and phosphorus contents, changes of soil organic nitrogen fractions and inorganic phosphorus fractions. The water treatments were W₁ (90% of WHC), W₂ (70% of WHC), W₃ (50% of WHC), and W₄ (30% of WHC). The nitrogen treatments were N₁ (0mgN/kg soil), N₂ (75mgN/kg soil), N₃ (150mgN/kg soil) and N₄ (300mgN/kg soil). The treatment lasted 56 days, the results obtained were as follows:1. Broussonetia papyrifera (Linn.)Vent. was the pioneer, and soil organic matter, soil nitrogen and soil water content had remarkable effect on Shannon-winner index.2. Soil water and nitrogen had remarkable effect on the biomass, and the interaction of soil moisture and nitrogen was significant. When soil moisture was 70% of WHC, biomass was significantly increased. No difference of the biomass was found when soil moisture was at 30% and 50% of WHC. The ratio of root to shoot was decreased when soil moisture increased. The biomass increased with the increase of nitrogen application rates, but even more nitrogen could not increase the biomass, N₃ treatment had the highest biomass.3. Soil moisture, nitrogen and the interaction of them had significant effect on nitrogen and phosphorus contents, nitrogen and phosphorus accumulation, net nitrogen and phosphorus uptake and use efficiency. Leaves had the highest level of nitrogen and phosphorus, and the stems had the lowest level. The contents, accumulations and net uptakes of nitrogen and phosphorus of plant were decreased with decreasing of soil water contents, while when the soil moisture was 90% of WHC, the contents, accumulations and net uptakes of nitrogen and phosphorus were lower than those of the soil moisture was 70% of WHC. The contents, accumulations and net uptakes of nitrogen and phosphorus were increased with increasing of nitrogen supply, and decreasing when nitrogen supply was 300mgN/kg soil, and there was no difference at 150mgN/kg soil and 300mgN/kg soil. Nitrogen and phosphorus use efficiency of plant were limited when soil moisture was low, nitrogen use efficiency decreased while phosphorus use efficiency increased with increasing nitrogen applied.4. Soil total nitrogen and organic nitrogen fractions were significantly influenced by soil moisture, nitrogen and the interaction of them. Contents of total nitrogen and organic nitrogen fractions were increased when soil moisture was high, while the content of (NH₄⁺ + NO₃^-) was decreased when soil moisture was high. And Contents of total nitrogen, organic nitrogen fractions and (NH₄⁺ + NO₃^-) were increased with nitrogen fertilizer supplied, and significantly higher than those of plots without nitrogen addition. The order of soil nitrogen contents was: N₄> N₃> N₂> N₁. And acid hydrolysable nitrogen> nonhydrolysable nitrogen, of the forms of acid hydrolysable nitrogen, amino acid nitrogen> hydrolysable unidentified nitrogen> ammonium nitrogen> amino sugar nitrogen.5. Soil moisture had significant effect on total phosphorus, organic phosphorus and available phosphorus, and nitrogen had significant effect on total phosphorus and available phosphorus in soil but no organic phosphorus. And the interaction of them had no significant effect on total phosphorus, organic phosphorus and available phosphorus. Total phosphorus content had almost no change, contents of organic phosphorus and available phosphorus increased with the decreasing of soil moisture. When soil moisture was at 70% of WHC, content of soil available phosphorus was the lowest. And total phosphorus and available phosphorus content significantly increase with the increasing nitrogen application rates, but organic phosphorus content had no significantly increase.Soil water content and nitrogen had significant effect on Ca₂-P, Ca₈-P, Al-P, and (Ca₈-P+ Al-P+ Fe-P), but the interaction of them had no significant effect on the fraction of inorganic phosphorus. The contents of Ca₂-P and (Ca₈-P+ Al-P+ Fe-P) decreased with the decrease of soil moisture and nitrogen application rates. The percentages of different inorganic phosphorus forms in total phosphorus in the soil were as following order: Ca₁₀-P>O-P> Fe-P>Ca₈-P>Al-P>Ca₂-P, and the percentage of (Ca₁₀-P+O-P) was 80% higher, so the soil available phosphorus was very low.6. The results showed that Broussonetia papyrifera (Linn.)Vent. seedlings had the highest biomass, nitrogen and phosphorus uptake and accumulation when soil water content was at 70% of WHC and nitrogen supply was 150mgN/kg soil. And higher soil water content and nitrogen application were beneficial to soil nitrogen and phosphorus transformation. In karst area, the frequency of provisional drought was high, but the duration was short, and precipitation occurred frequently, and this may be the main reason why Broussonetia papyrifera (Linn.)Vent. appeared in karst desert area.