A conceptual framework and an empirical test of complementarity and facilitation with respect to phosphorus uptake by plant species mixtures

Plant species have different traits for mobilizing sparingly soluble phosphorus (P) resources,which could potentially lead to overyielding in P uptake by plant species mixtures compared to monocultures due to higher P uptake as a result of resource (P) partitioning and facilitation.However,there is circumstantial evidence at best for overyielding as a result of these mechanisms.Overyielding (the outcome) is easily confused with underlying mechanisms because of unclear definitions.We aimed to define a conceptual framework to separate outcome from underlying mechanisms and test it for facilitation and complementarity with respect to P acquisition by three plant species combinations grown on four soils.Our conceptual framework describes both mechanisms of complementarity and facilitation and outcomes (overyielding of mixtures or no overyielding) depending on the competitive ability of the species to uptake the mobilized P.Millet/chickpea mixtures were grown in pots on two calcareous soils mixed with calcium-bound P (CaP) and phytate P (PhyP).Cabbage/faba bean mixtures were grown on both acid and neutral soils mixed with P-coated iron (hydr)oxide (FeP) and PhyP.Wheat/maize mixtures were grown on all four soils.Rhizosphere carboxylate concentration and acid phosphatase activity (mechanisms) as well as plant P uptake and biomass (outcome) were determined for monocultures rhizosphere and species mixtures.Facilitation of P uptake occurred in millet/chickpea mixtures on one calcareous soil.We found no indications for P acquisition from different P sources,neither in millet/chickpea,nor in cabbage/faba bean mixtures.Cabbage and faba bean on the neutral soil differed in rhizosphere acid phosphatase activity and carboxylate concentration,but showed no overyielding.Wheat and maize,with similar root exudates,showed overyielding (the observed P uptake being 22%higher than the expected P uptake) on one calcareous soil.We concluded that although differences in plant physiological traits (root exudates) provide necessary conditions for complementarity and facilitation with respect to P uptake from different P sources,they do not necessarily result in increased P uptake by species mixtures,because of the relative competitive ability of the mixed species.

A trade-off between space exploration and mobilization of organic phosphorus through associated microbiomes enables niche differentiation of arbuscular mycorrhizal fungi on the same root

Ecology seeks to explain species coexistence,but experimental tests of mechanisms for coexistence are difficult to conduct.We synthesized an arbuscular mycorrhizal(AM)fungal community with three fungal species that differed in their capacity of foraging for orthophosphate(P)due to differences in soil exploration.We tested whether AM fungal species-specific hyphosphere bacterial assemblages recruited by hyphal exudates enabled differentiation among the fungi in the capacity of mobilizing soil organic P(P_(o)).We found that the less efficient space explorer,Gigaspora margarita,obtained less ^(13)C from the plant,whereas it had higher efficiencies in P_(o)mobilization and alkaline phosphatase(Al Pase)production per unit C than the two efficient space explorers,Rhizophagusintraradices and Funneliformis mosseae.Each AM fungus was associated with a distinct alp gene harboring bacterial assemblage,and the alp gene abundance and P_(o)preference of the microbiome associated with the less efficient space explorer were higher than those of the two other species.We conclude that the traits of AM fungal associated bacterial consortia cause niche differentiation.The trade-off between foraging ability and the ability to recruit effective P_(o)mobilizing microbiomes is a mechanism that allows co-existence of AM fungal species in a single plant root and surrounding soil habitat.

Positive intercropping effects on biomass production are species-specific and involve rhizosphere enzyme activities: Evidence from a field study

Less attention has been given to soil enzymes that contribute to beneficial rhizosphere interactions in intercropping systems.Therefore,we performed a field experiment by growing faba bean,lupine,and maize in mono and mixed cultures in a moderately fertile soil.We measured shoot biomass and the kinetic parameters(maximal velocity(V max)and Michaelis-constant(K m))of three key enzymes in the rhizosphere:Leucine-aminopeptidase(LAP),β-1,4-N-acetylglucosaminidase(NAG),and phosphomonoesterase(PHO).Faba bean benefitted in mixed cultures by greater shoot biomass production with both maize and lupine compared to its expected biomass in monoculture.Next,LAP and NAG kinetic parameters were less responsive to mono and mixed cultures across the crop species.In contrast,both the V max and K m values of PHO increased in the faba bean rhizosphere when grown in mixed cultures with maize and lupine.A positive relative interaction index for shoot P and N uptake for faba bean showed its net facilitative interactions in the mixed cultures.Overall,these results suggest that over-productivity in intercropping is crop-specific and the positive intercropping effects could be modulated by P availability.We argue that the enzyme activities involved in nutrient cycling should be incorporated in further research.