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Background and aims Although silicon (Si) is known to improve plant growth under low phosphorus (P) conditions, the in planta mechanisms responsible for this effect are still unknown. Here, we investigated the role of Si on P uptake along with the expression of Pi transporters in wheat (Triticum aestivum L.) grown in low P acid soil in comparison with P fertilization and liming. Methods A combined approach was performed including analyses of rhizosphere soil, tissue P content, the expression of the root Pi transporter genes (TaPHT1.1 and TaPHT1.2), and the root exudation of citrate and malate. Results Supply of Si in a form of Na2SiO3 increased shoot P concentration to an adequate level in the range of P-fertilized plants. Silicon ameliorated low soil pH and high Al3+ comparable to the effect of liming. The in planta effect of Si on up-regulating the expression of TaPHT1.1 and TaPHT1.2 was several fold higher and consequently P uptake doubled compared to both P fertilization and liming. In addition, Si directly stimulated root Pi acquisition by prominently increasing both malate and citrate exudation rate. Conclusions Application of Si increased root exudation of organic acids that mobilize Pi in the rhizosphere and up-regulated Pi transporters in wheat roots.

The effects of calcium-magnesium phosphate, rock phosphate, lime, fly ash, and animal manure as liming agents on the microbial community composition, enzyme activities involved in C, N, P, and S cycling and rice yields of acid sulfate soils were studied in a three-year field trial. Significant increases in soil pH caused by five ameliorants, particularly lime and fly ash, were observed after 3 years. Both soil exchangeable Al³⁺and H⁺were significantly (P < 0.05) and negatively correlated with soil pH. Increased pH led to 61–102 % increase in rice yield after 2 and 3 years but not after 1 year. Soil phospholipid fatty acid (PLFA) profiles and enzyme activities were significantly changed after 3 years of application of the soil amendments. Enzyme activities increased along gradients of soil pH, indicating that the influences of inorganic or organic ameliorants on soil enzyme activities were mainly due to the effect on soil pH value. PLFA analysis showed that this pH effect played a more important role in shaping microbial community composition than specific effects of organic and inorganic amendments. All rice yield-associated enzymes and PLFA biomarkers (e.g., gram-negative bacteria and actinomycetes) were regulated by soil pH after 3 years. These results revealed that pH-induced changes in soil enzyme activity and microbial composition might be an important mechanism in alleviating acid stress in soil cropped to rice by various ameliorants.

Aims Characterizing the roles of abundant and rare soil microorganism is vital for clarifying microbial functions and help optimize agricultural management strategies, but their responses to various amendments in acid soils and correlations with crop biomass remain poorly understood. Methods A 5-year field experiment was conducted to evaluate the influence of amendments, including canola straw (SC), animal manure (OM), and alkaline slag (AS), on abundant and rare bacteria or fungi in acidic bulk and rhizosphere soils in aspect of microbial diversity, compositions, potential functions, and their relationship with canola biomass. Results Amendments and canola planting significantly affected both bacterial and fungal community structures. Applying AS had pronounced effect on bacterial communities and sub-communities, and the addition of OM showed the strong influence on fungal communities and sub-communities. The sub-community structures of rare bacterial and rare fungal significantly correlated to canola biomass; this correlation was stronger in the rhizosphere soil than that in the bulk soil. The α-diversity of rare rhizosphere bacteria, but not rare bulk bacteria and rare fungi, was significantly positively related to canola biomass. Rare bacteria accounted for the high proportion of nodes and most keystone species in microbial networks, and the number of edges belonging to rare bacteria positively correlated to canola biomass. The predicted functions of rare bacteria also make large contributions to crop biomass. Conclusions These findings highlight the important roles that rare microbes play in ameliorating acid soils and suggest that rare bacteria should be more closely associated with crop growth than rare fungi.

A microcosm study was established using three arable soils differing in both physicochemical properties and AOB or AOA dominance of ammonia oxidization. Nitrification and N2O emissions were quantified under 10 Pa C2H2, 1-octyne, or 3,4-dimethylpyrazole phosphate (DMPP), and the community composition and abundance of AOA and AOB were assessed. Amendment of soil with DMPP had a comparable effect to 1-octyne on AOB activities, composition, and N2O emissions across soils. By blocking AOB activity, both DMPP and 1-octyne supported higher rates of AOA growth in both the acid and alkaline soils. Approximately, 50% of ammonia-oxidizing activity in the acid soil was attributed to AOB activity by phylotypes affiliated with Nitrosomonas communis. When AOB were blocked by DMPP or 1-octyne across all soils, recovery of AOA communities was associated with the increased abundance of Nitrososphaera viennesis.

Background and AimsSuitable N source supply is critical to improve plant growth and N uptake, but the importance of nitrate (NO3−) for rice (Oryza sativa L.) and microbiota is often neglected in acidic paddy soils where ammonium (NH4+) is dominant. This study aimed to explore the differential effects of NH4+ and NO3− on rice growth, fertilizer nitrogen recovery efficiency (FNRE), and rhizosphere bacterial community in acid soil.MethodsTwo rice varieties, Kasalath (Al-sensitive indica) and Koshihikari (Al-tolerant japonica), were exposed to different N sources with or without lime in an acid soil.ResultsLiming and NO3− application solely improved the growth and FNRE of the Al-sensitive rice, namely, by increasing soil pH and alleviating Al toxicity. Compared with liming and rice variety, N source had a more pronounced influence on rhizobacterial community composition. Of the two sources, NO3− had a stronger effect on the rhizobacterial community than NH4+. Remarkably, rice plants fed with NH4+ specifically recruited Desulfosporosinus and Desulfitobacterium associated with ferric NH4+ oxidation in the rhizosphere, whereas those exposed to NO3− recruited Alicyclobacillus with NO3−-reducing iron oxidation ability. Three keystone taxa were identified in a rhizobacterial co-occurrence network analysis: Alicyclobacillus, which was positively associated with rice growth and FNRE, and Acidobacteriales and WPS-2, both with negative associations.ConclusionCompared with NH4+, NO3− enhances the growth and FNRE of Al-sensitive rice and exerts dominant effects on the rhizobacterial community, which indicates the importance of NO3− for rice and has instructive implications for N management in acid soil.

Aims We investigated the link between tree community composition and soil microbial community biomass and structure in central-eastern Spain. Methods The effects of the forest stand composition on the soil organic matter dynamics and on the structure and activity of the soil microbial community have been determined using phospholipid fatty acid profiles and soil enzymatic activities. Results The soil and litter N and C contents were higher in Pinus nigra Arn. ssp. salzmannii and Quercus ilex mixed forest stands (SBHO) and in long-term unmanaged Pinus nigra Arn. ssp. salzmannii forest stands (SBPC) than in pure Pinus nigra Arn. ssp. salzmannii forest stands (SBPA) and Pinus nigra Arn. ssp. salzmannii and Juniperus thurifera mixed forest stands (SBSJ). The bacterial biomass was significantly higher in SBSJ and SBPA than in SBPC and SBHO. The results show an uncoupling of the soil microbial biomass and its activity. pH is related to microbial biomass and its community structure under a Mediterranean humid climate. Conclusions The tree species seem to affect the biomass of the soil microbial community and its structure. The pH, but not the C/N ratio, is a factor influencing the microbial dynamics, biomass, and community structure.

Aims We assessed the effects of native and exotic tree leaf litter on soil properties in two contrasting scenarios. The native Quercus robur and Pinus pinaster tree species coexist with the aliens Eucalyptus globulus and Acacia dealbata in acid soils of NW Spain. The native trees Fraxinus angustifolia and Ulmus minor coexist with the aliens Ailanthus altissima, Robinia pseudoacacia and Ulmus pumila in eutrophic basic riparian soils in Central Spain. Methods Four plastic trays per species were filled with homogenized top-soil of the site and covered with leaf litter. Before and after 9 months of incubation, litter mass, soil pH, organic matter, mineral and total N were measured. Available mineral N (NO3−-N and NH4+-N) was assessed every 2 months. Results Soil biological activity was higher in the basic than in the acid soil. Litter of the exotic trees tended to decompose less than litter of native species, probably due to the presence of secondary metabolites in the former. Soil pH, mineral and total N responded differently to different litter types, irrespective of their exotic or native origin (acid soil), or was similar across litter treatments (basic riparian soil). The similar response of the basic soil to the addition of different litter types may be due to the low contrast of litter quality between the species. E. globulus litter inhibitied soil microbial activity much more than the rest of the studied litter types, leading to a drastic impoverishment of N in soils. Conclusion Litter of exotic N-fixing trees (A. dealbata and R. pseudoacacia) did not increase soil N pools because of the inhibition of microbial activity by secondary compounds. Therefore, secondary metabolites of the litter played a major role explaining exotic litter impact on soil properties.

Aims Aluminum (Al) toxicity in acid soil significantly reduces plant growth, agricultural productivity and ecosystem health. The Al-tolerant barley cultivars were reported to mainly rely on the Al-activated efflux of citrate from root apices, but the key mechanisms for Al tolerance may differ for wild relatives of barley adapted to acid soil. Methods Here, we investigated plant Al tolerance from evolutionary physiological, molecular, and ecological perspectives. Results Phylogenetic analysis of Al tolerance-associated gene families showed that most of these genes were conserved from streptophyte algae to angiosperms, indicating land plants have evolved gradually in adaption to Al-rich acid soil during plant terrestrialization. Vacuolar phosphate transporter SPX-major facility superfamily (SPX-MFS) and inorganic phosphate transporter 1 family (PHT1s) of streptophyte algae showed high genetic similarity to land plants. PHT1s exhibited a significant expand during the evolution from streptophyte algae to liverworts and then eudicots. Al-tolerant Tibetan wild barley accession, XZ29 showed high levels of P-containing glycolytic intermediates including Glu-6-P, Fru-6-P, 3-PGA, 2-PGA and PEP under Al stress. Some primary metabolites were evolutionarily conserved in liverwort, gymnosperm and three tested angiosperms. Furthermore, we found that Al-induced Pi efflux from root elongation zone to chelate rhizosphere Al 3+ , and immobilization of Al with P at the inner epidermal layer of root mature zone to reduce Al accumulation in the cortical layer in barley. Conclusions These results indicated that Tibetan wild barley has evolved unique P transport and metabolism for the adaptation to harsh conditions in eastern and southeastern Tibet where acid soils contain high P.

Aims We aimed to determine the influence of the distribution of different broadleaved tree species on soil chemical properties in a mature deciduous forest in Central Germany. Methods Triangles of three neighboring trees (tree clusters) that consisted of either one or two species of European beech (Fagus sylvatica L.), European ash (Fraxinus excelsior L.) or lime (Tilia cordata Mill. or Tilia platyphyllos Scop.) were selected and analyzed for their litterfall chemistry and chemical properties of the forest floor and mineral soil (0–10 cm and 10–20 cm). Results Base saturation, pH-value and the stock of exchangeable Mg2+ (0–10 cm) were highest under ash and lowest under beech. The proportion of exchangeable Al3+ was smallest under ash and highest under beech. The stock of exchangeable Mg2+ and Ca2+ correlated positively with the annual input of the respective nutrient from leaf litterfall. Ash leaf litterfall contained highest amounts of Mg and Ca. Beech leaf litterfall showed the highest C:N ratio and lignin: N ratio. Soil pH, stocks of organic C, total N and exchangeable Mg2+ and Ca2+ correlated positively with increasing proportions of ash leaf litter to total leaf litterfall. Conclusions Our results indicate that the abundance of ash in beech dominated forests on loess over limestone had a positive effect on soil chemical properties and reduced soil acidification. The intermixture and distribution of ash in beech-dominated stands resulted in an increase of the horizontal and vertical diversity of the soil habitat.

Purpose The key factors influencing pH buffering capacity of acid soils from tropical and subtropical regions, and effects of soil evolution and incorporation of biochars on pH buffering capacity were investigated to develop suitable methods to increase pH buffering capacity of acid soils. Materials and methods A total of 24 acid soils collected from southern China were used. The pH buffering capacity was determined using acid–base titration. The values of pH buffering capacity were obtained from the slope of titration curves of acid or alkali additions plotted against pH in the pH range 4.0–7.0. Two biochars were prepared from straws of peanut and canola using a low temperature pyrolysis method. After incubation of three acid soils, pH buffering capacity was then determined. Results and discussion pH buffering capacity had a range of 9.1–32.1 mmol kg –1 pH –1 for 18 acid soils from tropical and subtropical regions of China. The pH buffering capacity was highly correlated ( R 2  = 0.707) with soil cation exchange capacity (CEC) measured with ammonium acetate method at pH 7.0 and decreased with soil evolution due to the decreased CEC. Incorporation of biochars at rates equivalent to 72 and 120 t ha −1 increased soil pH buffering capacity due to the CEC contained in the biochars. Incorporation of peanut straw char which itself contained more CEC and alkalinity induced more increase in soil CEC, and thus greater increase in pH buffering capacity compared with canola straw char. At 5% of peanut straw char added, soil CEC increased by 80.2%, 51.3%, and 82.8% for Ultisol from Liuzhou, Oxisol from Chengmai and Ultisol from Kunlun, respectively, and by 19.8%, 19.6%, and 32.8% with 5% of canola straw char added, respectively; and correspondingly for these soils, the pH buffering capacity increased by 73.6%, 92.0%, and 123.2% with peanut straw char added; and by 31.3%, 25.6%, and 52.3% with canola straw char added, respectively. Protonation/deprotonation of oxygen-containing functional groups of biochars was the main mechanism for the increase of pH buffering capacity of acid soils with the incorporation of biochars. Conclusions CEC was a key factor determining pH buffering capacity of acid soils from tropical and subtropical regions of China. Decreased CEC and content of 2:1-type clay minerals during evolution of tropical soils led to decreased pH buffering capacity. Incorporation of biochars generated from crop straws did not only ameliorate soil acidity, but also increased soil pH buffering capacity.