Soil C:N:P stoichiometry in rhizosphere and non-rhizosphere of Pinus sylvestris var. mongolica forests

The aim of this study was to reveal the stoichiometric characteristics of carbon, nitrogen and phosphorus in rhizosphere and non-rhizosphere soils of Pinus sylvestris var. mongolica in the Hulunbuir desert. We investigated the contents and stoichiometry of organic carbon, total nitrogen, and total phosphorus contents of rhizosphere and non-rhizosphere soils across different stand ages (28, 37 and 46 a) of P. sylvestris var. mongolica plantations, with P. sylvestris var. mongolica natural forest as the control. We analyzed the correlation between soils properties and soil stoichiometry. The results showed that rhizosphere effect significantly affected soil N:P, and stand age significantly affected soil organic carbon content in P. sylvestris var. mongolica plantation. Soil organic carbon content in plantation was significantly lower than that in natural forest. Soil organic carbon and total nitrogen contents of plantations in both rhizosphere and non-rhizosphere soils firstly decreased and then increased with increasing stand age, while total phosphorus firstly increased and then decreased in rhizosphere soils, and firstly decreased and then increased in non-rhizosphere soils. There was significant positive correlations between C:N and C:P in rhizosphere soils but not in non-rhizosphere soils, suggesting that higher synergistic rhizosphere soil N and P limitation. The mean N:P values of rhizosphere and non-rhizosphere soils were 4.98 and 8.40, respectively, indicating that the growth of P. sylvestris var. mongolica was restricted by soil N and the rhizosphere soils were more N-restricted. The C:N:P stoichiometry of rhizosphere and non-rhizosphere soils were significantly influenced by soil properties, with available phosphorus being the most important driver. The growth of P. sylvestris var. mongolica was limited by N in the Hulunbuir desert, and root system played an obvious role in enriching and maintaining soil nutrients. It was recommended that soil nitrogen should be supplemented appropriately during the growth stage of P. sylvestris var. mongolica plantation, and phosphorus should be supplemented appropriately according to the synergistic nature of nitrogen and phosphorus limitation.

Rhizosphere effects of moso bamboo and dominant tree species of secondary broadleaved forest on soil organic carbon mineralization

The rhizosphere effect of plants affects soil organic carbon (SOC) mineralization. It is still unclear for the mechanism by which the rhizosphere effect of dominant plants in secondary broadleaved forest habitats invaded by moso bamboo affects SOC mineralization. Taking broadleaved tree species (Quercus glauca and Cunninghamia lanceolata) and moso bamboo, dominating respectively in uninvaded secondary broadleaved forest and bamboo forest formed after the invasion as test materials, we investigated rhizosphere effect of plants on the SOC mineralization in laboratory incubation experiments. The results showed that carbon mineralization rates of Phyllostachys edulis (PE), Quercus glauca (QG) and Cunninghamia lanceolata (CL) rhizosphere soils were 20%, 26%, and 21% higher than bulk soils, respectively. Carbon mineralization of bulk soils of QG and CL was 22% and 26% higher, while that of rhizosphere soils was 14% and 11% higher than PE, respectively. The contents of water-soluble organic carbon and organic carbon in rhizosphere soils of the three species were significantly higher than those of bulk soil, and the abundance of rhizosphere soil bacteria was higher than that of non-rhizosphere. The contents of microbial biomass carbon, water-soluble organic carbon, and total nitrogen were important factors influencing carbon mineralization in rhizosphere, while water-soluble organic carbon and microbial metabolic quotient were important factors influencing carbon mineralization in non-rhizosphere. On the whole, the rhizosphere effect increased total SOC mineralization, driving by changes in microbial biomass carbon, water-soluble organic carbon, and total nitrogen content. The results could provide a theoretical basis for plant-soil interaction on soil carbon cycling in bamboo invasion habitats.

New Insights into the Accumulation, Transport, and Distribution Mechanisms of Hexafluoropropylene Oxide Homologues, Important Alternatives to Perfluorooctanoic Acid, in Lettuce (Lactuca sativa L.)

Hexafluoropropylene oxide (HFPO) homologues, which are important alternatives to perfluorooctanoic acid, have been frequently identified in crops. Although exposure to HFPO homologues via crops may pose non-negligible threats to humans, their impact on crops is still unknown. In this study, the accumulation, transport, and distribution mechanisms of three HFPO homologues in lettuce were investigated at the plant, tissue, and cell levels. More specifically, HFPO trimer acid and HFPO tetramer acid were primarily fixed in roots and hardly transported to shoots (TF, 0.06-0.63). Conversely, HFPO dimer acid (HFPO-DA) tended to accumulate in lettuce shoots 2-264 times more than the other two homologues, thus resulting in higher estimated daily intake values. Furthermore, the dissolved organic matter derived from root exudate enhanced HFPO-DA uptake by increasing its desorption fractions in the rhizosphere. The transmembrane uptake of HFPO homologues was controlled by means of a transporter-mediated active process involving anion channels, with the uptake of HFPO-DA being additionally facilitated by aquaporins. The higher accumulation of HFPO-DA in shoots was attributed to the larger proportions of HFPO-DA in the soluble fraction (55-74%) and its higher abundance in both vascular tissues and xylem sap. Our findings expand the understanding of the fate of HFPO homologues in soil-crop systems and reveal the underlying mechanisms of the potential exposure risk to HFPO-DA.

Species of fast bulk-soil nutrient cycling have lower rhizosphere effects: A nutrient spectrum of rhizosphere effects

Tree roots not only acquire readily-usable soil nutrients but also affect microbial decomposition and manipulate nutrient availability in their surrounding soils, that is, rhizosphere effects (REs). Thus, REs challenge the basic understanding of how plants adapt to the environment and co-exist with other species. Yet, how REs vary among species in response to species-specific bulk soil nutrient cycling is not well-known. Here, we studied how plant-controlled microbial decomposition activities in rhizosphere soils respond to those in their corresponding bulk soils and whether these relations depend on species-specific nutrient cycling in the bulk soils. We targeted 55 woody species of different clades and mycorrhizal types in three contrasting biomes, namely a temperate forest, a subtropical forest, and a tropical forest. We found that microbial decomposition activities in rhizosphere soils responded linearly to those in their corresponding bulk soils at the species level. Thereafter, we found that REs (parameters in rhizosphere soils minus those in corresponding bulk soils) of microbial decomposition activities had negative linear correlations with microbial decomposition activities in corresponding bulk soils. A multiple factor analysis revealed that soil organic carbon, total nitrogen, and soil water content favored bulk soil decomposition activities in all three biomes, showing that the magnitude of REs varied along a fast-slow nutrient cycling spectrum in bulk soils. The species of fast nutrient cycling in their bulk soils tended to have smaller or even negative REs. Therefore, woody plants commonly utilize both positive and negative REs as a nutrient-acquisition strategy. Based on the trade-offs between REs and other nutrient-acquisition strategies, we proposed a push and pull conceptual model which can bring plant nutrient-acquisition cost and plant carbon economics spectrum together in the future. This model will facilitate not only the carbon and nutrient cycling but also the mechanisms of species co-existence in forest ecosystems.

Effects of nitrogen addition on rhizosphere soil properties in a salinized grassland

We explored the impacts of nitrogen (N) inputs and the rhizosphere effect on the properties of rhizosphere and bulk soils in a salinized grassland in Northern Shanxi under N addition rates of 0, 1, 2, 4, 8, 16, 24 and 32 g N·m^-2·a^-1. The results showed that N addition significantly decreased soil pH, but significantly increased Ca²⁺, NO₃^--N and inorganic nitrogen contents in rhizosphere and bulk soil. With the increases of N addition rates, the contents of Ca²⁺, NO₃^--N, inorganic nitrogen in rhizosphere and bulk soils and total nitrogen in rhizosphere soil increased gradually, whereas the contents of Na⁺, K⁺, Mg²⁺, NH₄⁺-N and amino acid in rhizosphere soil, and total nitrogen in bulk soil first increased and then decreased. Results of the principal component analysis showed that the responses of soil properties to low (≤8 g·m^-2·a^-1) and high nitrogen addition rates (>8 g·m^-2·a^-1) were significantly different. Compared with bulk soil, soil pH, the contents of organic acids and amino acids in rhizosphere soil were significantly lower by 0.71 units, 44.3% and 9.8%, respectively, while the contents of K⁺, Ca²⁺, Mg²⁺, NH₄⁺-N, inorganic nitrogen, total carbon and total nitrogen in rhizosphere soil were significantly higher by 51.0%, 47.6%, 20.8%, 215.5%, 139.3%, 31.7% and 65.3%, respectively. These results indicated that rhizosphere effect on soil properties was stronger than that of nitrogen addition.

Impacts of the rhizosphere effect and plant species on organic carbon mineralization rates and pathways, and bacterial community composition in a tidal marsh

Despite the growing recognition regarding the carbon cycle in the rhizosphere of upland ecosystems, little is known regarding the rhizosphere effect on soil organic carbon (SOC) mineralization in tidal marsh soils. In the current study, in situ rhizobox experiments (including rhizosphere and inner and outer bulk soil) were conducted in an estuarine tidal marsh. Our results showed that a higher abundance of total bacteria, Geobacter, dsrA and mcrA and lower α-diversity were observed in the rhizosphere relative to the bulk soil. Rhizosphere effects shifted the partition of terminal metabolic pathways from sulfate reduction in the bulk soil to the co-dominance of microbial Fe(III) and sulfate reduction in the rhizosphere. Although the rhizosphere effect promoted the rates of three terminal metabolic pathways, it showed greater preference towards microbial Fe(III) reduction in the tidal marsh soils. Plant species had little impact on the partitioning of terminal metabolic pathways, but did affect the potential of total SOC mineralization together with the abundance and diversity of total bacteria. Both the rhizosphere effect and plant species influenced the bacterial community composition in the tidal marsh soils; however, plant species had a less pronounced impact on the bacterial community compared with that of the rhizosphere effect.

[Responses of polyphenoloxidase and catalase activities of rhizosphere and bulk soils to warming during the growing season in an alpine scrub ecosystem.]

To understand the effects of climate warming on rhizosphere ecological processes in the alpine scrub ecosystem, the responses of polyphenoloxidase and catalase activities in the rhizosphere and bulk soils to experimental warming (1.3 ℃) were examined during the growing season in a Sibiraea angustata scrub ecosystem on the eastern Qinghai-Tibetan Plateau, China. The results showed that the activities of polyphenoloxidase in rhizosphere and bulk soils in the middle growing season were significantly higher than those in the early or late growing season. The activities of catalase in the bulk soil increased gradually during the growing season, while they showed no seasonal changes in the rhizosphere soil. In the bulk soil, warming significantly increased the activity of polyphenoloxidase by 17.5% in the late growing season and increased that of catalase by 2.2% in the middle growing season, whereas it did not affect soil enzyme activities in early or late growing seasons. In the rhizosphere soil, warming only significantly increased the activities of polyphenoloxidase and catalase by 6.5% and 1.3% in the early growing season. The rhizosphere effect of soil polyphenoloxidase activity was positive throughout the growing season, while there was no obvious rhizosphere effect for soil catalase activity. Furthermore, warming significantly decreased the rhizosphere effect of soil polyphenoloxidase activity by 15.2% during the late growing season. These results indicated that the activities of polyphenoloxidase and catalase activities differed between rhizosphere and bulk soils, with consequences on the rhizosphere soil ecological processes under climate warming in the alpine scrub ecosystem on the eastern Qinghai-Tibetan Plateau.

[Effects of warming on microbial biomass carbon and nitrogen in the rhizosphere and bulk soil in an alpine scrub ecosystem]

To understand the effects of climate warming on the rhizosphere ecological process in the alpine scrub ecosystem, the responses of microbial biomass carbon and nitrogen in the rhizosphere and bulk soil to experimental warming were examined in a Sibiraea angustata scrubland on the eas-tern Qinghai-Tibetan Plateau, China. The results showed that the concentrations of microbial biomass carbon and nitrogen in the rhizosphere and bulk soil in the early growing season were significantly higher than those in the middle and late growing seasons. Experimental warming did not significantly affect the concentrations of microbial biomass carbon and nitrogen of the rhizosphere soil in the most growing seasons. In the bulk soil, however, the effects of experimental warming on the microbial biomass carbon and nitrogen differed among the growing season. Experimental warming significantly decreased microbial biomass carbon but increased microbial biomass nitrogen in the early growing season. In the middle growing season, warming significantly increased both microbial biomass carbon and nitrogen. In the late growing season, there was no significant effect. The rhizosphere effects of soil microbial biomass carbon and nitrogen also differed with the growing season. The rhizosphere effects of microbial biomass carbon and nitrogen were negative in the early growing season but positive in the middle growing season. In the late growing season, there were negative rhizosphere effects of soil microbial biomass carbon and positive rhizosphere effects of soil microbial biomass nitrogen. Furthermore, experimental warming significantly increased the rhizosphere effects of soil microbial biomass carbon and nitrogen in the early growing season, but decreased those in the middle and late growing seasons. These results uncovered the changing mechanism of the biologi-cal process in the rhizosphere and bulk soil in the alpine scrub ecosystems under the background of climate warming.

Irrigation management and phosphorus addition alter the abundance of carbon dioxide-fixing autotrophs in phosphorus-limited paddy soil

In this study, we assessed the interactive effects of phosphorus (P) application and irrigation methods on the abundances of marker genes (cbbL, cbbM, accA and aclB) of CO2-fixing autotrophs. We conducted rice-microcosm experiments using a P-limited paddy soil, with and without the addition of P fertiliser (P-treated-pot (P) versus control pot (CK)), and using two irrigation methods, namely alternate wetting and drying (AWD) and continuous flooding (CF). The abundances of bacterial 16S rRNA, archaeal 16S rRNA, cbbL, cbbM, accA and aclB genes in the rhizosphere soil (RS) and bulk soil (BS) were quantified. The application of P significantly altered the soil properties and stimulated the abundances of Bacteria, Archaea and CO2-fixation genes under CF treatment, but negatively influenced the abundances of Bacteria and marker genes of CO2-fixing autotrophs in BS soils under AWD treatment. The response of CO2-fixing autotrophs to P fertiliser depended on the irrigation management method. The redundancy analysis revealed that 54% of the variation in the functional marker gene abundances could be explained by the irrigation method, P fertiliser and the Olsen-P content; however, the rhizosphere effect did not have any significant influence. P fertiliser application under CF was more beneficial in improving the abundance of CO2-fixing autotrophs compared to the AWD treatment; thus, it is an ideal irrigation management method to increase soil carbon fixation.

[Effects of short-term elevated CO2 concentration and drought stress on the rhizosphere effects of soil carbon, nitrogen and microbes of Bothriochloa ischaemum.]

A water control pot experiment was conducted in climate controlled chambers to study soil carbon, nitrogen and microbial community structure and their rhizosphere effects in the rhizosphere and non rhizosphere soil of Bothriochloa ischaemum at elevated CO2 concentrations (800 μmol·mol^-1) under three water regimes, i.e., well watered (75%-80% of field capacity, FC), moderate drought stress (55%-60% of FC), and severe drought stress (35%-40% of FC). The results showed that elevated CO2 concentration and drought stress did not have significant impacts on the content of soil organic carbon, total nitrogen or dissolved organic carbon (DOC) in the rhizosphere and bulk soils or their rhizosphere effects. Elevated CO2 concentration significantly decreased dissolved organic nitrogen (DON) content in the rhizosphere soil under moderate drought stress, increased DOC/DON, and significantly increased the negative rhizosphere effect of DON and positive rhizosphere effect of DOC/DON. Drought stress and elevated CO2 concentration did not have significant impacts on the rhizosphere effect of total and bacterial phospholipid fatty acids (PLFA). Drought stress under elevated CO2 concentration significantly increased the G⁺/G^- PLFA in the rhizosphere soil and decreased the G⁺/G^- PLFA in the bulk soil, so its rhizosphere effect significantly increased, indicating that the soil microbial community changed from chemoautotroph microbes to heterotrophic microbes.

ex-Steud

Rhizobacterial communities associated with Phragmites australis (Cav.) Trin. ex Steud. in a hypersaline pond close to Wuliangsuhai Lake (Inner Mongolia - China) were investigated and compared with the microbial communities in bulk sediments of the same pond. Microbiological analyses have been done by automated ribosomal intergenic spacer analysis (ARISA) and partial 16S rRNA gene 454 pyrosequencing. Although community richness was higher in the rhizosphere samples than in bulk sediments, the salinity seemed to be the major factor shaping the structure of the microbial communities. Halanaerobiales was the most abundant taxon found in all the different samples and Desulfosalsimonas was observed to be present more in the rhizosphere rather than in bulk sediment.