Variation in phosphorus acquisition efficiency among maize varieties as related to mycorrhizal functioning

Phosphorus (P) is a main limiting factor for agricultural production, but overusing P fertilizer has brought serious environmental damages in China. Improving P acquisition efficiency of agricultural crops is an urgent topic. It has been proven repeatedly that arbuscular mycorrhizal fungi (AMF) and genetic diversity within one crop plant can play important roles in P uptake by crops. The main objective of this thesis was to understand the role of the arbuscular mycorrhizal symbiosis in P acquisition efficiency of different maize varieties. The specific objectives were to test: 1) how P uptake by maize varieties responds to colonization by the native AMF community in the field; and 2) whether AMF hyphae take up P for plants from phytate which is the most abundant organic P form in soil; 3) whether mixing maize cultivars can improve maize productivity and whether AMF can play a role in this system; and 4) how AMF species (or community) legacy affects successional maize growth. In this thesis, I combined field experiments and greenhouse experiments and made use of maize genetic diversity and (native) AMF to improve P (including inorganic and organic P) acquisition. The effects of one single AMF species on maize growth and nutrient uptake have been well studied, but how maize varieties respond to the native AMF community has been insufficiently studied. In Chapter 2, I focused on how maize varieties responded to the native AMF community by using rotated cores in the field, to compare mycorrhizal responsiveness among 20 maize varieties and the difference of the AMF native community of four maize varieties (two old landraces and two modern hybrids). The results indicated that, 1) increased P fertilizer significantly reduced mycorrhizal responsiveness in the field; 2) a complicated relationship exists between mycorrhizal responsiveness in the field and pot experiment; 3) there was no significant difference between old and modern maize varieties in terms of mycorrhizal responsiveness and colonization; 4) there were only small differences in AMF community composition among the four maize varieties. By comparing mycorrhizal responsiveness of maize varieties between in the pot experiment and in the field experiment (with in-growth cores), I found mycorrhizal responsiveness of maize varieties in the pot experiment was significantly larger than that in the field experiment. Thus, mycorrhizal responsiveness of varieties within one cereal plant species tested classically in pots may not present their realistically mycorrhizal responsiveness in field. Phytate is the most abundant form of organic P in soil. To explore the potential of phytate utilization by plants is agriculturally and environmentally essential. Increased P nutrition of mycorrhizal plants derived from phytate has been reported, indicating that phytate can be a potential P source. However, earlier studies assessed phytate use by using acid phosphatase rather than phytase, and did not consider that phytate adsorption could lead to phosphate release. Thus, I investigated the effect of mycorrhizal hyphae-mediated phytase activity on P uptake by maize in Chapter 3. I conducted a rhizobox experiment to explore phytate use by mycorrhizal hyphae for two maize varieties. The results showed that: 1) phytate addition increased phytase and acid phosphatase activity, and resulted in increased P uptake and plant biomass; 2) the increase in P uptake and biomass were correlated with the increase of phytase activity but not with the increase of acid phosphatase activity; 3) lower phytate addition rate increased, but higher addition rates decreased hyphal length density. I conclude that P from phytate can be used by mycorrhizal plants, but that the phytate contribution to plant nutrition is likely limited. Phytase activity is a more relevant indicator to assess phytate use. In addition, there was a significant interaction between maize varieties and AMF species in taking up P from phytate, which implies there is a possibility to combine different maize varieties to increase total yield using phytate. Besides, I used an empirical relationship to assess phosphate release due to phytate addition. My calculation implies that phosphate desorption cannot be ignored when assessing phytate use, particularly when a large amount of phytate is applied as a P source. In multispecies natural ecosystems, AMF can play a key role in enhancing plant productivity. However, their role in enhancing crop productivity in mixed cropping systems is still poorly understood. In Chapter 4, I conducted both greenhouse and field experiments to investigate whether mixing maize varieties with different P acquisition strategies could lead to overyielding, and what roles AMF play in this system with two maize varieties. The results showed that mixing maize varieties resulted in overyielding, both in P uptake and shoot biomass, but only when plants were mycorrhizal. At the same time, I found higher hyphal length density and higher AMF diversity in mixtures compared to the monocultures in the field experiment, and higher colonization rate and higher hyphal length density in mixtures in the pot experiment. Thus, I propose that overyielding by mixing maize varieties might be due to increased mycorrhizal performance leading to more P uptake. I also used the partitioning formula to calculate the contribution through the selection effect and complementarity effect to overyielding. I found that the increase of the total yield and P uptake in mixtures was largely due to complementarity effect, implying that relative overyielding and enhanced P uptake were not due to enhanced competitive ability by the larger variety. The results of Chapter 4 suggest that mixing mycorrhizal maize varieties might be beneficial for enhancing productivity and P uptake efficiency. Plant - soil feedback experiments have shown that AMF can play a crucial role in determining the direction and magnitude of that feedback. Most studies investigated plant - soil feedback dynamics between different plant species. However, it is unknown to what extent one variety of an agricultural crop can affect the performance of another variety of that same crop through plant - soil feedback. In Chapter 5, I carried out a two-phase experiment in a greenhouse, including conditioning phase and test phase to determine plant - soil feedbacks in the absence and presence of AMF species or community, to test the effects of AMF on feedback dynamics. The results in Chapter 5 showed that: 1) in the conditioning phase, both maize varieties were differentially influenced by different AMF species compared to non-mycorrhizal control; 2) in the feedback phase, non-mycorrhizal maize exhibited negative feedback dynamics for biomass and P-uptake; 3) on the feedback phase, mycorrhizal maize generally exhibited positive feedback dynamics for biomass and P-uptake. The interaction coefficient was largest with the mixture of three different AMF species. The interaction coefficient for shoot and P uptake were significantly correlated with the coefficient for mycorrhizal colonization. These results imply that different maize varieties are affected differently by different AMF species, thereby influencing the productivity of the subsequent maize variety. The results also raise questions how AMF influence rhizosphere biota and how maize varieties may select more beneficial AMF. In Chapter 6, I integrate the results from previous chapters. I discuss possible relationships between (negative) plant - soil feedback effect (due to pathogen) and the mycorrhizal effect on overyielding and improved P uptake due to mixing maize varieties (compared to the monoculture). I also discuss the linkage between phosphorus acquisition efficiency and mycorrhizal responsiveness within one crop species, and the relationship between plant genetic diversity and plant - soil feedback effects, and try to come up with a conceptual model how mixing maize varieties in the presence of AMF could be beneficial.