Researches To Improve Cotton Yield And Nutrient Efficiency By Manupulating Phosphorus Bioavailability In Root Layer In Saline Field

Saline desert lands are considered as reserved resources for arable land and one of the feasible approaches to protect cultivated lands is to rationally exploit and utilize saline wasteland without environmental risk. Deficiency of freshwater resources limits cultivation and utilization of saline desert land in Xinjiang province. Salinity soils distributed in most of cotton field in Xinjiang, and improvement and utilization of saline lands is restricted by many limiting factors, including the soil phosphorus availability in rhizosphere, soil salinity, pH, soil temperature in root layer and soil microbial activity. Eliminating these limiting factors to improve root growth environment and soil phosphorus bioavailability in rhizosphere is an effective way to promote cotton nutrient uptake efficiency and yield. Based on this porpose, soil salt concentration, nutrient content and microbial community in the processes of converting saline desert land to arable land were qualitatively and quantitatively investigated with specific compounds on micrbial cell membrane or wall. The contribution of indigenous AM fungi in salinity soil to cotton growth, nutrition uptake and salt tolerance was quantified in situ with a in-growth core constructed with PVC core and nylon mesh. Effects of optimized cultivating measures and fertilizer application on cotton growth and nutrient uptake were studied using mulched drip irrigation technique. The main results were listed as follows:(1) Soil physicochemical characteristics and soil microbial community in saline field were significantly varied by land use type and cotton cultivated years. Soil salt concentration and pH were significantly decreased when saline desert lands were converted to arable land, while soil organic C and nutrients contents were remarkly increased. The transformation of land use type inhibited soil microbial reproduction. After one-year’s cultivation, biomass of bacteria (muramic acid) and fungi (glucosamine) were all declined by almost30%compared with saline desert land. Similar results were found in PLFA analysis. As cultivated years increased, two stages (rapidly improvement stage and stable shift stage) were found in change of soil salt concentration, nutrients content and soil microbial biomass. In rapidly improvement stage, salt concentration significantly decreased in first3years after cultivation, while cotton yield, soil nutrient content and microbial biomass were rapidly increased. Ratios of fungi to microbial biomass and Gram-negative to Gram-positive significantly increased. Salt concentration was dominant limited factor for cotton growth. In stable shift stage, salt concentration, nutrient content and microbial biomass reached a new equilibrium after5years of cultivation. Exploration and improvement of dominant limited factor in this stage was a possible method to promote cotton yield. Results of principal component analysis by PLFA showed that soil microbial community structure was not effected by land use type and cotton cultivated years.(2) Indigenous AM fungi improvd growth and nutrition uptake of host plants and salt resistance in saline field. In the present study, in-growth core system constructed with PVC cores and nylon mesh was employed to study indigenous AM fungal function for plant growth and salt resistance in saline field. The results showed that vibration did not affect the growth of the non-mycorrhizal plant-sugar beet, but significantly affected the growth of the mycorrhizal plants-cotton and maize. These data indicated that such core systems could provide a reliable method to quantify the functions of the AM fungal community in situ. Mycorrhizal colonization of cotton and maize significantly declined in the vibrating treatments compared to the static treatments. Biomass, P and N uptake of both cotton and maize were significantly higher in the presence of AM fungi (static treatments) than those vibrating treatments.. And the osmotic compounds were also promoted in the presence of AM fungal community. Salt resistance of fungi-plant symbiosis was correlated with soil salinity and crops species. Thses results indicate that indigenous AM fungi improve crops growth, salt resistance and nutrient uptake.(3) A two-year trial in2011and2013indicated that double-film mulch increased soil temperature in root layers and enchanced cotton seedlings emergence. Cotton seeds were emerged one day earlier with double-film mulch than those of with single-film mulch. And the emergence rates of early sowing and conventional sowing in the treatments of double-film mulch were increased by14%and40%, respectively. However, soil salinity and cotton yield were no significant differences under the conditions of double-film mulch.(4) Localized supply of P and ammonium fertilizer combined with ammonium fertigation in bolls stage enhanced cotton growth and yield. In our study, increment of P fertilizer as base fertilizer improved cotton shoot biomass not for root in seedling stage. No significant positive effect on cotton growth and cotton yield was observed in the later growing stages. P fertilizer forms did not promote cotton growth at total growth stages with same amounts of fertilizer.. Localized supply of P plus ammonium enhanced cotton growth in seedling stage compared with broadcast supply of P or localized supply P plus urea. Rhizosphere pH was significantly decreased in localized supply of P plus ammonium treatments because of increased proton released from root after absorbing NH4+, which consequently led to significant increment of P uptake. More nutrients were transferred from root to leave and reproductive organ in buds and bolls stages. Combined with ammonium fertigation in bolls stage, cotton yield significantly increased compared with broadcast supply of P or localized supply P plus urea.It is concluded that, firstly, soil salt concentration, nutrient content and soil microbial biomass were significantly effected by land use types and land use years in the processes of saline land improvement; secondly, indigenous AM fungi could improve crops growth, salt resistance and nutrient uptake; thirdly, early sowing combined with double-film mulch technique in early stage could promote soil temperature in root layer, resulting in increased emergence rate under the conditions of drip irrigation with mulch; fourthly, localized supply of P and ammonium fertilizer combined with ammonium fertigation in bolls stage enhanced cotton growth and yield. These results provided evidences for improving P bioavailability in saline field with optimized cultivating measures, fertilizer application and soil microbes.