Effects Of Limited Water And Fertilizer In Soil And Defoliation On Forage Growth And Mycorrhiza Formation

Grasslands are widely distributed globally and it is estimated that they make up one-fifth of the Earth’s land surface. Sustainable development of grassland is altered by soil moisture, fertilization and grazing. The low forage yield of alfalfa (Medicago sativa L.) in the Loess Plateau of China resulting from water-and nutrient deficiency in these regions restricts the development of animal husbandry. As alfalfa is the main leguminous grass in artificial grasslands, the introduction of graminaceous grass to the Loess Plateau is beneficial to develop the artificial grassland. In Western Australia, there are two types of grazing, cell-grazing and set-grazing. Set-grazing can result in degradation of grasslands if stocking levels are high, which limits sustainable development of animal husbandry. Based on the situation of alfalfa stands suffering from limited water and nutrients and a single leguminous grass species in artificial grassland in the Loess Plateau of China, this thesis includes a fertilization experiment of alfalfa stands in field conditions, the study of root morphology and physiology of alfalfa(Medicago sativa L.) subjected to limited water-and phosphorus (P), research on optimizing the establishment of switchgrass (Panicum virgatum L.) and response of root morphology and mycorrhiza formation with different grazing intensity for simulated ryegrass(Lolium rigidum) pasture.1. To clarify yield and yield components, soil water consumption in deep layers, and soil extracellular phosphatase affected by nitrogen(N) and phosphorus (P) fertility level, Field experiments were conducted in 2011 and 2012 during establishment and high yield phases (referred to as young and old alfalfa stands respectively) of alfalfa planted in 2010 and 2005 at the Dryland Agro-ecology Research Station (36°02’N,104°25’E), Lanzhou University. Alfalfa is important for crop-forage systems and animal husbandry in the semiarid Loess Plateau of China. Treatments were a factorial combination with two N rates (0 and 70 kg N ha-1) and three P rates (0, 17 and 34 kg P ha-1) arranged in a randomized complete block design with three replicates. Forage was recorded at harvest twice annually, and yield components were recorded at the first harvest. The forage yield of two alfalfa stands increased with N and P fertilization each year, which was positively correlated with shoot mass at the first harvest. Increased water depletion occurred with N and P fertilization in deeper soil layers in both alfalfa stands. N fertilization promoted soil alkaline phosphatase activity in the young alfalfa stand, while P fertilization inhibited acid- and alkaline phosphatase activity in the old alfalfa stand. Therefore, N and P fertilization was effective in increasing forage yield and WUE for the two alfalfa stands in Loess Plateau, without affecting the plant density.2. The aim of this study was to identify the morphological and physiological responses of roots to P-and moisture-limited conditions in alkaline soil of Loess Plateau. A pot experiment compared the growth and root characteristics of an introduced (Arkaxiya) and a locally-selected (Longzhong) alfalfa cultivar grown in alkaline soil, with an initial available soil P of 6.9 p.g P g-1 dry soil and a pH of 8.3, with four applied-P rates (0,4.2,8.4 and 16.8 μg P g-1 dry soil) an three soi moisture treatments [maintained at 75-90%,45-55% and 30-35% of field capacity (FC)]. At high soil P and high soil water content (SWC), high total root length contributed to high P uptake, high P-use efficiency, and greater plant growth in Arkaxiya compared to Longzhong. However at low SWC, Longzhong had a higher specific root length (thinner roots) and higher total root length than Arkaxiya. At low SWC greater biomass was allocated to roots, at low P-supply the specific root length increased, while the combination of low soil P and low SWC induced greater release of rhizosphere carboxylates. Overall, three was a positive correlation between P uptake and total root positively correlated with shoot mass at the first harvest. Increased water depletion occurred with N and P fertilization in deeper soil layers in both alfalfa stands. N fertilization promoted soil alkaline phosphatase activity in the young alfalfa stand, while P fertilization inhibited acid- and alkaline phosphatase activity in the old alfalfa stand. Therefore, N and P fertilization was effective in increasing forage yield and WUE for the two alfalfa stands in Loess Plateau, without affecting the plant density.2. The aim of this study was to identify the morphological and physiological responses of roots to P-and moisture-limited conditions in alkaline soil of Loess Plateau. A pot experiment compared the growth and root characteristics of an introduced (Arkaxiya) and a locally-selected (Longzhong) alfalfa cultivar grown in alkaline soil, with an initial available soil P of 6.9 p.g P g-1 dry soil and a pH of 8.3, with four applied-P rates (0,4.2,8.4 and 16.8 μg P g-1 dry soil) an three soi moisture treatments [maintained at 75-90%,45-55% and 30-35% of field capacity (FC)]. At high soil P and high soil water content (SWC), high total root length contributed to high P uptake, high P-use efficiency, and greater plant growth in Arkaxiya compared to Longzhong. However at low SWC, Longzhong had a higher specific root length (thinner roots) and higher total root length than Arkaxiya. At low SWC greater biomass was allocated to roots, at low P-supply the specific root length increased, while the combination of low soil P and low SWC induced greater release of rhizosphere carboxylates. Overall, three was a positive correlation between P uptake and total rootlength (r2=0.845, P< 0.001) and surface area (r2=0.851, P< 0.001). The results suggest that the introduced cultivar has traits that will benefit production in all but the driest and low-phosphate alkaline soils. This investigation is beneficial for cultivation and breeding of alfalfa in P-and moisture-limited conditions in alkaline soil.3. To develop biodiversity in the artificial grasslands of Loess Plateau, a field experiment was conducted to optimize the agricultural practice of switchgrass seedling emergence. Switchgrass (Panicum virgatum L.) is an important feedstock for bioenergy production, but establishment is difficult in cold arid areas. This study determined the conditions required to improve its germination and emergence in the field. Under controlled conditions, the effects of temperature, water status and light on germination were studied by exposure to 12-h alternating temperatures of 15/5,20/5,20/10,25/10,35/20,40/20,45/20℃ (day/night), and water potentials of 0,-0.2,-0.4,-0.6,-0.8,-1.2 MPa in culture medium. Maximum percentage germination occurred at 35/20℃, 0 MPa water potential and in continuous dark. In the field when planted in early spring (April), no emergence occurred when there was no surface cover or covered with straw mulch (SM), whereas using plastic film mulch (FM), emergence reached 383 seedlings per m2 with pre-sowing irrigation, but only 273 without irrigation. This was associated with higher soil temperatures and better soil moisture. However, in July seeding, SM lead to better water status and cooler soil conditions, and thus better stand establishment. In contrast, FM treatment had poor establishment due primarily to the high temperatures. This study indicates that emergence of switchgrass in an arid environment was (i) enhanced by pre-sowing irrigation and shallow seeding, (ii) made possible in spring by the use of FM, and (iii) improved with SM for summer seeding.4. This study is to clarify the differentiation among grazing practices that could induce degradation of pasture grassland in Western Australia. Defoliation of photosynthetic shoot tissue by grazing can alter the structure and function of grassland ecosystems. Grazing modifies the morphology of root systems and this influences colonisation of roots by arbuscular mycorrhizal (AM) fungi through the changes of carbon allocation to root and AM. There have been contradictory reports of impacts of defoliation on mycorrhiza formation but a meta-analysis by Barto and Rillig concluded that defoliation had little effect on mycorrhizas in annual crops and perennial forbs. The method of assessment of mycorrhizas may contribute to this uncertainty. We investigated mycorrhiza formation in simulated pasture swards of ryegrass (Lolium rigidum cv. Wimmera) grown for 6 months in a glasshouse in response to three defoliation regimes to clarify relationships between defoliation and mycorrhiza formation. The sward was grown in soil from an annual pasture in south-western Australia with a naturally occurring community of AM fungi. Plants were sampled 5 times during this period. Our objective was to quantify how defoliation frequency influenced mycorrhiza formation in relation to root traits (length, mass and specific root length), nutrient uptake and rhizosphere carbon. It was hypothesized that mycorrhizas assessed as % root length colonised would be little affected by defoliation, and that root biomass and root length would be reduced by defoliation. Therefore, mycorrhizas assessed as root length colonised was expected to be reduced by defoliation. Defoliation was expected to increase rhizosphere carbon. It was found that root biomass decreased with defoliation up to 87 days after sowing after which it remained stable. In contrast, root biomass continued to increase in non-defoliated plants. Consequently, the proportion of roots colonized by AM fungi increased with defoliation but root length colonised by mycorrhizal fungi was decreased by defoliation. Soluble carbon and microbial biomass carbon in rhizosphere soil were both reduced by defoliation. Root length colonized by AM fungi was more closely related to soluble carbon concentration and soil nutrient status of rhizosphere soil than was the proportion of roots colonized.It is identified that the study of root morphology and physiology of alfalfa subjected to limited water- and phosphorus and research on optimizing the establishment of switchgrass (Panicum virgatum L.) are beneficial for cultivation and management of grassland in semi-arid areas. Additionally, the high frequency defoliation reduced soil soluble carbon and root length colonized by mycorrhiza which could contribute to degradation of grassland.