The Responses Of Steppe Plants And Soil Phosphorus Pool To Fertilization And Grazing Exclusion

Phosphorus is one of the essential nutrient elements for plants, playing an important role in plant physiological and biochemical processes. It has been reported that the soils of natural grassland inInner Mongolia are short ofphosphorus, especially the deteriorated grassland. This limits the sustainable development of animal husbandry and steppe plants growth. Up to date, grazing exclusionand grassland fertilizationare the two main methods in improving grassland productivity. However, few studies reported the changes of soil phosphorus under the two improving methods. Therefore, in this study, we conducted two trials on steppes of Inner Mongolia,fertilization and grazing exclusion.In fertilization experiment, continuously grazed deteriorated typical steppe was fenced in June 2014, and fertilized in July 2014 and June 2015. There were two kinds of fertilizer, phosphate fertilizer(calcium superphosphate) and organic fertilizer(fermented sheep manure). The phosphate rates were 0 kg/ hm2, 187.5 kg/ hm2, 375 kg/ hm2 and 562.5 kg/ hm2. The organic fertilizer rates were 4000 kg/ hm2 and 8000 kg/ hm2. We analyzed the effects of soil fertilization onplant composition,contents of plant phosphorus, the aboveground biomass, belowground biomass, soil phosphorusfractions and soil phosphatase activity. In grazing eclusion experiment, soils from one continuously grazed deteriorated steppe and one fenced steppe(12 years) nearby were sampled in July 2014, to analyze the rhizosphere and non-rhizosphere soil phosphorus characteristics and their responses to grazing exclusion. The experiment preliminary results are as follows.1. Steppe fertilization with phosphate fertilizer and organic fertilizer increased the concentrations of soil inorganic phosphorus fractions(Ca2-P, Ca8-P, Al-P, Fe-P, O-P and Ca10-P), which increased with the increase of fertilizing rates. The effects of phosphate fertilizer was greater than organic fertilizer on the concentrations of soil phosphorus fractions. Within a short time after fertilization, concentrations of Ca2-P, Ca8-P, Al-P and Fe-P increased obviously, and then reduced before next fertilization. However, the concentrations of O-P and Ca10-P steadly increased after fertilization. After the application of phosphate fertilizer and organic fertilizer, the concentrations of soil organic phosphorus fractions increased to some extents. In a short period, the concentrations of soil labile organic phosphorus increased obviously. In a long period, the concentrations of moderate resistant organic phosphorus and resistant organic phosphorus increased obviously, whereas the moderate labile organic phosphorus increased weakly. The influence of organic fertilizer was greater than phosphate fertilizer on organic phosphorus fractions.2. After fertilization, the concentrations of soil available phosphorus and total phosphorus increased to some extent, which also increased with the increase of fertilization rates. The influence of phosphate fertilizer was greater than organic fertilizer on the concentrations of soil available and total phosphorus.Correlation analysis indicated that the Ca2-P concentration was significantly correlated withthe available phosphorus concentration, whereas the concentrations of O-P and Ca10-P were significantly correlated with the total phosphorus concentrations.3. Overall, the concentrations of soil phosphorus fractions, available phosphorus and total phosphorus in 0-10 cm soil depth were higher than those in 10-20 cm soil depth. A weak increase of soil phosphorus concentrations in 10-20 cm soil layer was also abserved after two years’ fertilization.4. Two years after fertilization, the activities of soil acid, alkaline, and neutral phosphatase were insignificantly improved. Soil phosphatase activity was also associated with soil depth. Acid and neutral phosphatase activities in 0-10 cm soil depth were higher than those in 10-20 cm soil depth, whereas the alkaline phosphatase activity in 10-20 cm soil depth was higher than that in 0-10 cm soil depth.5. No significant effects of fertilization on the aboveground biomass, belowground biomass and root shoot ratio were observed(P>0.05) in two years.. However, phosphate fertilizer was better in improving belowground biomass than organic fertilizer. Fertilization had a trend in increasing plant root to shoot ratio. The plant phosphorus concentrations significantly increased with increasing fertilizer rates. A lower concentration of plant phosphorus at late growth stage was also observed than at vigorous growth stage.6. The soil phosphorusfractions in rhizosphere and non rhizospheresoils were compared between continuously grazed and and grazing exclusion steppes.The results showed that grazing exclusion significantly affected the soil pH, water soluble phosphorus concentration and phosphatase activity, whereas had no significant effects on other phosphorus fractions(except Al-P). The rhizosphere soil pH valuesin three plants pecies(Stipa krylovii, Leymus chinensis, Cleistoenes squarrosa)were lower than those in non-rhizosphere soil. Overall, the concentrations of water soluble phosphorus, Ca2-P, C a8-P, Al-P, Fe-P and Ca10-P in rhizosphere soil were higher than those in non rhizosphere soil,whereas the concentration of O-P was the opposite. The concentrations of soil organic phosphorus fractions in rhizosphere soil were higher than those in non-rhizosphere soil. The concentrations of soil phosphorus fractions from high to low was O-P ≈ Ca10-P>Fe-P>Al-P>Ca8-P >Ca2-P, moderate labile organic phosphorus>moderate resistant organic phosphorus>resistant organic phosphorus>labile organic phosphorus.7. Fertilization and grazing exclusion were two effective measures in improving the deteriorated steppes. However, compared to grazing exclusion, the responses of soil phosphorus fractions were quicker to fertilization in deteriorated grassland. In the future, it was suggested that the combination of these two methods might be more efficient in improving deteriorated typical steppe.