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Plant–plant competitive and facilitative interactions might strongly affect the biodiversity effect and field productivity of intercropping system under the condition of limited resources such as available phosphorus (P). To test this hypothesis, the maize–grass pea intercropping system with five P application gradients (from low- to high-P) was experimentally investigated and the global meta-analysis validation was simultaneously performed. The main objectives of this study included as: 1) to examine the characteristics of plant–plant interaction variations along soil P addition gradients; 2) to identify the shift from interspecific complementarity to selection effect regarding the productivity and nutrient uptake and its driving mechanism; 3) to verify the universality of above phenomenon via global meta-analysis and explore the general pattern of P-dependent plant–plant interactions in the intercropping systems. Regardless of growing seasons, the total net effect of intercropping system was always >0, showing net facilitative effects of plant–plant interactions on field productivity and nutrient uptake efficiency, relative to monoculture systems. On average, the yield, biomass, N and P uptake were elevated by 3.0–13.7%, 3.2–12.8%, 7.5–20.1% and 5.6–18.9% in intercropping respectively, relative to those of monoculture. On the other hand, the contribution of complementarity effect to the total net effect was up to 78.7%, much more than that of selection effect (21.3%). The relative interaction index tended to decline remarkably with increasing soil P availability (from 0.097–0.126 to 0.027–0.029), suggesting that the facilitation (rather than the competition) dominated in intercropping under the low-P condition. Furthermore, the intercropped grass pea decreased the harvest index and allocated more biomass to vegetative growth as a result of the enhanced interspecific competition under the P-sufficient condition. Finally, the meta-analysis demonstrated that intercropping system had greater relative benefits regarding field productivity and N/P acquisition and utilization than monoculture system. In the meanwhile, the intensity of interspecific facilitative effect was enhanced under the P-deficient condition. Field observations presented the evidences regarding the P-dependent plant-plant interactions and its effects on field productivity in the maize-grass pea intercropping system. The meta-analysis confirmed that high P application decreased the interspecific facilitative effects in terms of field productivity and nutrient utilization in the intercropping systems. But the low-P condition led to the opposite trend, i.e. facilitative effect dominated in the intercropping for greater productivity efficiency. Graphical abstract elucidates the plant-plant interaction pattern in maize-grass pea intercropping system according to field observation and meta-analysis [Display omitted] • Effect of interspecific competition & facilitation on field productivity along soil P gradients in intercropping system is unclear • To examine how the plant–plant interactions affected crop productivity and nutrient uptake via field observations & meta-analyses • Intercropping led to greater net effects on productivity and N/P uptake efficiency via facilitative interactions than monoculture • Complementarity effect was dominant to ensure productivity benefit in low-P soil, yet competitive effect dominated in high-P soil • P-dependent plant-plant interaction efficiently mediated relative field productivity and resource benefits in intercropping system. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) play a vital role in mediating rhizosphere interactions and plant growth, especially in resource-constrained soils. However, few studies have documented the effects of AMF on plant–plant interaction patterns and resource use efficiency along stress gradients. In this study, a growth environment-controlled experiment was carried out to evaluate the effects of AMF inoculation on maize-grass pea intercropping along three phosphorus (P) and two water gradients. The data indicated that soil water and P availability determined the plant interspecific interaction pattern, and that interaction was strongly mediated by AMF inoculation. Grass pea and maize were mutually facilitated under low P and water conditions and transitioned to a new pattern, i.e., that maize was facilitated but grass pea was neutral role or facilitator with increasing resource availability. The facilitative effect was particularly significant under P- and water-deficient conditions. Moreover, the transition of the plant–plant interaction pattern was mechanically driven by improving rhizospheric phosphatase activities by 5.08–20.02 %, rhizospheric acidification and enhanced microbial activities under resource deficit conditions. Critically, AMF inoculation was observed to significantly improve rhizospheric soil P availability and microbial biomass (p < 0.05); however, it enhanced the complementary utilization of P and water and weakened interspecific competition under high-resource conditions. Therefore, AMF inoculation positively transformed the plant–plant interaction pattern along resource gradients for higher P, nitrogen and water use efficiency. This study validates the context-dependent interspecific interaction and the role of AMF in optimizing resource use in a nutrient-impoverished intercropping system. Illustrating the effect of AMF inoculation on plant growth and plant–plant interaction in maize-grass pea intercropping system. [Display omitted] • Plant diversity increased rhizosphere microbial activity and phosphatase secretion. • AMF inoculation enhanced facilitation intensity with increasing stress conditions. • AMF lowered interspecific competition intensity for higher resource use efficiency. [ABSTRACT FROM AUTHOR]

Reducing the yield loss of inferior crop species is crucial for improving the production of intercropping systems, particularly in drought-prone regions. Yet, the physiological mechanism of yield loss caused by interspecific competition in response to soil water availability is unclear. A two-year (2018–2019) field experiment was conducted to explore the impacts of interspecific competition on yield formation of the inferior crop in the maize-faba bean intercropping system with and without plastic film mulching in a semiarid rainfed region. The plant-water relation, and yield formation in association with nitrogen (N) remobilization were determined and compared. Results indicated that the land equivalent ratio of the intercropping system was greater than 1, regardless of whether treated with plastic mulching. However, the net effect of the intercropping system was significantly greater than 0 only under plastic mulching. Intercropping with faba beans prolonged the anthesis-silking interval of maize by 93.7% (p < 0.05) and the bald tip length by 56.2% (p < 0.05). This trend resulted in insufficient pollination of tassels and an increased kernel abortion rate in maize (p < 0.05). Moreover, the amount of dry matter and N transfer to grain in vegetative organs decreased by 34.0% and 23.2% (p < 0.05), respectively. Physiologically, this brought about yield loss in the intercropped maize. Water competition from faba bean significantly reduced the soil water availability in the intercropped maize strips, regardless of whether treated with plastic mulching. However, plastic mulching alleviated the competition for water between faba bean and maize, and thus kernel abortion in maize was significantly reduced. Our findings reveal the physiological mechanism of yield loss in intercropped inferior species, by connecting kernel formation and remobilization function, in the semi-arid and rainfed intercropping systems. Specific water or nutrient management strategies might be needed for inferior crops to further improve the total productivity of cereal-legume intercropping system in drought-prone environments. [Display omitted] • The yield loss mechanism of inferior crops in legume-based intercropping system is unclear. • Growth loss of maize in cogrowth period cannot be compensated in late seed filling period. • Water competition lowered remobilization function and increased kernel abortion of maize. • Plastic mulching (PM) weakened plant competition and conducive to maize final yield formation. • PM is a promising option to improve the productivity of intercropping system in rainfed regions. [ABSTRACT FROM AUTHOR]

The intercropping-led diversified tillage is crucial for sustainable development in dryland agriculture. However, the plant-plant interaction transition and its effects on soil physiochemical characteristics along soil phosphorus (P) levels are not clear. To address this issue, maize-grass pea intercropping system was used to investigate the variations in plant-plant facilitation along soil P gradients (low- to high-P) and their effects on rhizosphere soil N and P availability and microbial biomass. The data showed that total net effect (NE) was always positive, indicating facilitative effects of intercropping on total community productivity. Specifically, the relative yield (RY) for both species was greater than 0.5 in the P-deficient soils, representing mutually facilitated effects. Yet, under the P-sufficient condition, maize and grass pea acted as facilitated species and facilitator respectively. Rhizosphere phosphatase activity was significantly enhanced in the P-deficient soils, accordingly fostering P mineralization for higher P availability and N utilization. The contents of microbial biomass carbon, nitrogen and phosphorus (i.e., MBC, MBN and MBP) were evidently elevated in intercropping systems than those of monoculture. Also, global meta-analysis verified above phenomenon, showing that high P addition decreased facilitation intensity with type shift, regarding plant productivity, N and P utilization and vice versa. We confirmed a relatively full picture of P-driven facilitation intensity and type shift along stress gradients by field observation and meta-analysis. The findings provided a new insight of biodiversity in soil physiochemical traits and also revealed a critical mechanism affecting resource utilization in the P-limited soils. [Display omitted] • Plant facilitation was significantly enhanced in low soil phosphorus conditions. • Meta-analysis confirmed P availability altering interspecific facilitation model. • The universality of stress gradient hypothesis was validated in intercropping. [ABSTRACT FROM AUTHOR]

Intercropping, the simultaneous production of two or more crops on the same field, offers opportunities for sustainable agricultural intensification by providing higher yield per unit of land than monoculture. However, intercropping is largely ignored when evaluating regional carbon (C) cycle and ecosystem service function. We hypothesized that interspecific interactions in intercropping systems might induce changes in rhizosphere soil properties and accordingly affect carbon emission. To test the hypothesis, three intercropping systems including maize-soybean (2 M:4 S, 2-row maize: 4-row soybean), wheat-soybean (4 W:4 S), and maize-wheat (2 M:4 W) were established to explore the effects of interspecific interactions on land equivalent ratio (LER), C emission and soil properties in a typical semiarid rainfed site from 2019 to 2020. The results showed that interspecific facilitation (LER>1 and RII>0) was observed when soybean was intercropped with maize and wheat. On average, soil C emission was substantially increased by 10.6%, compared with their respective monocultures. Particularly, the intercropping with soybean not only alleviated N deficiency in the rhizosphere of maize or wheat, but also improved water use efficiency (WUE) of maize or wheat via complementary effect. Moreover, facilitating process evidently promoted the input of dissolved organic carbon (DOC) and easily oxidized organic carbon (EOOC) in rhizosphere soils (P < 0.05). This resulted in greater microbial activities, N mineralization rate, and inorganic N supply in the maize and wheat strips. However, the belowground competition from the intercropped wheat significantly lowered soil moisture and inorganic N content in maize strips. Also, soil labile C input (DOC and EOOC) and microbial activities were declined, while carbon emission was evidently lowered by 7.4%. Interestingly, the trade-off between grain yield production and carbon emission showed that interspecific facilitation promoted the yield per unit of carbon emission, indicating higher carbon emission efficiency. Yet, interspecific competition led to an opposite trend. The findings enhanced the understandings on carbon emission efficiency and its potential management strategy regarding crop diversity and green production in semiarid agroecosystems. [Display omitted] • Soybean intercropped with cereal promoted crop yield, showing plant facilitation. • Plant facilitation was resulted from water complementary use and N enrichment. • Increased labile carbon input and microbial activities enhanced carbon emission. • Yet, wheat-maize mixture showed an opposite trend due to interspecific competition. • Soybean-led rhizosphere interaction promoted carbon production efficiency & yield. [ABSTRACT FROM AUTHOR]

The sole and integrated effects of N rates and cropping patterns on grain yield advantage (a-c), and carbon emission efficiency advantage (d-f). Notes: CEE, the carbon emission efficiency; N1-150, N2-225, and N3-300 represented three N levels of 150, 225 and 300 kg ha−1, respectively. [Display omitted] • Soybean intercropped with wheat or maize promoted LER, showing plant facilitation. • Plant facilitation increased LER and reduced nitrogen application by increasing NUE. • Facilitation promoted carbon emission (CE) with C&N degradation enzyme activity. • Wheat-maize intercropping showed opposite trend due to interspecific competition. • Plant facilitation increased carbon emission efficiency (yield/CE) but lowered N application. It is critical to reduce carbon (C) emission and nitrogen (N) input in agroecosystems under a changing climate. If crop diversification is introduced, interspecific plant–plant interactions as an effective pathway may achieve this goal. However, the related process and its mechanism are poorly understood. A two-year field study was conducted to explore the effects of intercropping systems including soybean-maize, soybean- wheat and maize-wheat on the land equivalent ratio (LER), nitrogen use efficiency (NUE), seasonal carbon emission, and soil properties in a typical semiarid environment. Three N rates (N1: 150 kg ha−1, N2: 225 kg ha−1, N3: 300 kg ha−1) were applied. The result indicated that the intercropping with soybean significantly increased system productivity with LER > 1, showing a typical plant–plant facilitation. However, the LER of maize–wheat intercropping was significantly lower than 1, representing interspecific competition. With the increasing N rate, the productivity of monoculture wheat or maize was evidently promoted. Particularly, the productivity under N2 and N3 remained at a similar level due to interspecific facilitation. This trend was mechanically driven by the improved N uptake (NLER > 1) and NUE under the presence of interspecific facilitation. Critically, interspecific facilitation was observed to promote carbon emissions (CE) by 4.0%-6.3%, since root input, microbial activities and the C&N decomposition enzyme activities were significantly enhanced. To say, interspecific facilitation evidently enhanced carbon emission efficiency (Yield/CE) whereas interspecific competition turned to lower it. To sum up, plant facilitation improved crop productivity and carbon emission efficiency by reducing N input. Our findings provided a new insight into the exploration of green solution in terms of reducing emissions and increasing efficiency, as well as lowering N fertilizer application in the natural and agricultural ecosystems. [ABSTRACT FROM AUTHOR]

The relative intensity of facilitative effect among coexisting plant species is frequently affected by soil water and nutrient availability, such as nitrogen (N), phosphorus (P), potassium (K), and organic fertilizer. However, to date, the general pattern of nutrient gradient effects on plant–plant interactions remains inconsistent or even opposite in the intercropping systems. To address this issue, we performed a global meta-analysis and three experiments to investigate the production efficiency of intercropping along fertilizer input gradient (low vs. high) by assessing the land equivalent ratio (LER). Trial 1 included five P gradient in the maize–grass pea intercropping system. Trial 2 consisted of two N gradient in the maize–wheat, maize–soybean and wheat–soybean intercropping systems. Trial 3 contained two water and three P gradients in the same intercropping systems as Trial 1. The meta-analysis result indicated that the average LER for yield (LER Y), biomass (LER B), N uptake (LER N), and P uptake (LER P) were 1.49, 1.25, 1.35, and 1.66, respectively. It implied that intercropping would save 49 %, 25 %, 35 % and 66 % lands to achieve the same yield, biomass, N and P uptake amount as monoculture, respectively. Also, high fertilizer inputs significantly decreased the relative productivity and nutrient uptake. Average effect sizes of high inputs on LER Y , LER B , LER N , and LER P were up to − 0.0505 (−0.0550 to −0.0461), − 0.0267 (−0.0303 to −0.0230), − 0.0321 (−0.0395 to −0.0247) and − 0.0237 (−0.0370 to −0.0103), respectively. These negative values demonstrated that the LER markedly declined under the high-nutrient conditions. On the other hand, the results of Trial 1 confirmed that the LER Y , LER B , LER N , and LER P tended to decrease significantly with increasing P addition. In Trial 2 , the LER Y of intercropping systems tended to significantly decrease with increasing N application. In Trial 3 , the LER Y , LER B , LER N , and LER P also displayed a descending trend with increasing P addition and soil water availability. Collectively, the three trials confirmed the reliability and universality of the meta-analysis conclusions. This study provided a broad support for the hypothesis of the nutrient-dependent relative benefits in intercropping systems. [Display omitted] • Average land equivalent ratio was greater than 1 in meta-analysis database. • High nutrient input decreased the land equivalent ratio relative to low input. • Field and pot-culture trials confirmed the reliability of meta-analysis results. • Relative productivity benefits were species specific and context dependent. • Results confirmed the universality of stress gradient hypothesis in intercropping. [ABSTRACT FROM AUTHOR]

We hypothesized that soil phosphorus (P) utilization might be tightly associated with interspecific interactions. To clarify this issue, a maize-grass pea intercropping system was investigated in a rainfed field with P addition (P0 and P +), water (rain-fed and plastic film mulching) and isolation (root barrier and root intermingling) treatments. The data indicated that total net effect was higher than 0 under all the conditions, displaying facilitative effects of biodiversity on agroecosystem productivity. The dynamics of total land equivalent ratio demonstrated that the intensity of plant-plant facilitation became greater under P-deficient condition. Maize was the facilitated species consistently, while grass pea changed from facilitated species under low soil P and moisture to facilitator under high soil P and moisture. The driving mechanism was associated with soil acidification and microbial community promotion effect under P-deficient condition. Intercropped rhizosphere soil phosphatase activity was increased by 4.9–17.2%, and Olsen-P availability was evidently enhanced relative to monoculture. Interestingly, drought stress significantly enhanced this trend. Critically, root isolation confirmed that plant-plant facilitation was briefly driven by interspecific rhizosphere interactions. Conclusively, this study confirmed the context-dependent facilitation shift in agroecosystem and provided a novel insight into the importance of rhizosphere interaction on soil P availability and utilization. [Display omitted] • Plant-plant facilitation was observed in the maize-grass pea intercropping system. • The intensity of facilitation decreased from low to high P and water gradients. • P deficiency promotes rhizosphere phosphatase activity and acidification process. • Drought and P deficiency led to the optimal soil microbial biomass and activities. • Intercropped grass pea fostered soil P mineralization and resource use efficiency. [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]