Increased planting density of Chinese milk vetch (Astragalus sinicus) weakens phosphorus uptake advantage by rapeseed (Brassica napus) in a mixed cropping system

Root morphological and physiological traits and arbuscular mycorrhizal fungi shape phosphorus-acquisition strategies of 12 vegetable species

Trait plasticity and integration mediate vegetable adaptive strategies. However, it is unclear how patterns of vegetables in root traits influence vegetable adaptation to different phosphorus (P) levels. Nine root traits and six shoot traits were investigated in 12 vegetable species cultivated in a greenhouse with low and high P supplies to identify distinct adaptive mechanisms in relation to P acquisition (40 and 200 P mg kg^-1 as KH₂PO₄). At the low P level, a series of negative correlations among root morphology, exudates and mycorrhizal colonization, and different types of root functional properties (root morphology, exudates and mycorrhizal colonization) respond differently to soil P levels among vegetable species. non-mycorrhizal plants showed relatively stable root traits as compared to solanaceae plants that showed more altered root morphologies and structural traits. At the low P level, the correlation between root traits of vegetable crops was enhanced. It was also found in vegetables that low P supply enhances the correlation of morphological structure while high P supply enhances the root exudation and the correlation between mycorrhizal colonization and root traits. Root exudation combined with root morphology and mycorrhizal symbiosis to observe P acquisition strategies in different root functions. Vegetables respond highly under different P conditions by enhancing the correlation of root traits. Low P supply could significantly improve the direct and indirect ways of mycorrhizal vegetable crops' root traits axis on shoot biomass, and enhance the direct way of non-mycorrhizal vegetable crops' root traits axis and reduce the indirect way of root exudates.

Complementary effects of phosphorus supply and planting density on maize growth and phosphorus use efficiency

Phosphorus (P) supply and planting density regulate plant growth by altering root morphological traits and soil P dynamics. However, the compensatory effects of P supply and planting density on maize (Zea mays L.) growth and P use efficiency remain unknown. In this study, we conducted pot experiments of approximately 60 days to determine the effect of P supply, i.e., no P (CK), single superphosphate (SSP), and monoammonium phosphate (MAP), and different planting densities (low: two plants per pot; and high: four plants per pot) on maize growth. A similar shoot biomass accumulation was observed at high planting density under CK treatment (91.5 g plot^-1) and low planting density under SSP treatment (94.3 g plot^-1), with similar trends in P uptake, root morphological traits, and arbuscular mycorrhizal colonization. There was no significant difference in shoot biomass between high planting density under SSP (107.3 g plot^-1) and low planting density under MAP (105.2 g plot^-1); the corresponding P uptake, root growth, and P fraction in the soil showed the same trend. These results suggest that improved P supply could compensate for the limitations of low planting density by regulating the interaction between root morphological traits and soil P dynamics. Furthermore, under the same P supply, the limitations of low planting density could be compensated for by substituting MAP for SSP. Our results indicate that maize growth and P use efficiency could be improved by harnessing the compensatory effects of P supply and planting density to alter root plasticity and soil P dynamics.

Mesorhizobium jarvisii is a dominant and widespread species symbiotically efficient on Astragalus sinicus L. in the Southwest of China

In order to identify rhizobia of Astragalus sinicus L. and estimate their geographic distribution in the Southwest China, native rhizobia nodulating A. sinicus were isolated and their genetic diversity were studied at 13 sites cultivated in four Chinese provinces. A total of 451 rhizobial isolates were trapped with A. sinicus plants from soils and classified into 8 different genotypes defined by PCR-based restriction fragment length polymorphism (RFLP) of 16S-23S rRNA intergenic spacer (IGS). Twenty-one representative strains were further identified into three defined Mesorhizobium species by phylogenetic analyses of 16S rRNA genes and housekeeping genes (glnII and atpD). M. jarvisii was dominant accounting for 76.3% of the total isolates, 22.8% of the isolates were identified as M. huakuii and five strains belonged to M. qingshengii. All representatives were assigned to the symbiovar astragali by sharing high nodC sequence similarities of more than 99%. Furthermore, the biogeography distribution of these rhizobial genotypes and species was mainly affected by contents of available phosphorus, available potassium, total salts and pH in soils. The most remarkable point was the identification of M. jarvisii as a widespread and predominant species of A. sinicus in southwest of China. These results revealed a novel geographic pattern of rhizobia associated with A. sinicus in China.