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Aims: Plant-plant interactions are determined by resource availability. However, its responses to differences in soil phosphorus (P) and water and the rhizospheric driving mechanism have not been systematically revealed. Methods: A pot-culture experiment was conducted in monoculture and in mixture with maize and grass pea. Resource stress treatments were created by organic and inorganic P applications (phytate and KH2PO4) as well as drought and well-watered treatments to address the above issue. Results: Intercropping increased system yield and biomass by 3.5% and 4.5% on average in soils with sufficient P and water, and by 12.0% and 11.7% under stressful conditions respectively. With increasing water and P availability, the yield-based interaction types transited from mutually facilitated to maize facilitated with grass pea as facilitator. Comparatively, the biomass-based facilitation shifting from maize facilitated and grass pea neutral to both being facilitated with increasing stress. This shift was probably mediated by complementary use of soil water in the two species, and rhizospheric acidification in grass pea. Also, rhizospheric interactions promoted phosphatase secretion by 5.3–22.2%, and mobilized organic P to buffer P limitation under P-deficient soils. Regardless of crop species, reproductive and vegetative biomass fell into a typical allometric pattern (α > 1). The rhizospheric interactions drove the differentiated biomass allocation patterns and the species-specific interaction transition. Conclusion: High P and water environments intensified the asymmetric interspecific competition. The trait-dependent facilitation shift was mechanically driven by rhizospheric interaction in intercropping systems. The findings update the understanding on stress gradient hypothesis in a modified model. [ABSTRACT FROM AUTHOR]

Note: P (phosphorus), W (water), + (positive or increase), – (negative or decrease). [Display omitted] • Plant facilitation was found in the intercropping system of maize and grass pea. • Facilitation shifted from +/+, +/0 to +/- from low to high water and P gradients. • In low P, rhizosphere soil acidification of grass pea fostered P mineralization. • Soil microbial biomass P was improved for higher productivity in +/+ facilitation. • Soil water and phosphorus availability altered interspecific facilitation model. Plant-plant facilitation is widely studied to increase productivity and resource utilization in cereal-legume intercropping system. However, physiological and ecological mechanisms driving interspecific interaction shift along the environmental gradients is largely unknown. To clarify this issue, we first tested plant-plant facilitation along with four phosphorus (P) gradients in maize-grass pea intercropping system. Results illustrated a progressive transition of seed yield-based facilitation from mutually facilitated (+/+) to maize facilitated but grass pea as facilitator (+/-) along with low to high P gradients. Secondly, above trend was evidently enhanced when combining with drought stress gradients, in which severe drought amplified facilitative effects, whereas the magnitude of facilitation was relatively weak under well-watered condition. Interestingly, biomass-based facilitation transition did not synchronize with seed-based one, in which occurred in a broader threshold range of water and P gradients. Specifically, total yield, biomass, N and P uptake increased by 0.5%, 4.1%, 1.8% and 2.9% under the sufficient P and water availability, whereas these indicators increased by 25.3%, 18.5%, 20.5% and 21.4% in P and water deficient soils. And the total net effect was positive under all the environmental conditions. Rhizosphere interaction plays a crucial role in facilitation judgment, and the driving mechanism was associated with soil acidification and microbial community promotion under P-deficient condition. Under low soil moisture and available P, soil acidification and lower rhizosphere soil pH of intercropped maize were observed. Rhizosphere phosphatase secretion were significantly activated in P-deficient soils and accelerated the mineralization of soil organophosphorus, and the microbial biomass P was improved for stronger facilitation. Taken together, our findings confirmed the P and water driven facilitation shift along with stress gradients and highlighted the roles of rhizosphere interaction in affecting species diversity advantage. In conclusion, our work provided a relatively full picture for plant facilitation evaluation and more accurate management regarding intercropping productivity. [ABSTRACT FROM AUTHOR]