The microbial performance regarding the mobilization of soil P has been an object of investigation for decades. Thus, different types of microbial enzymes, which enable the specific breakdown of various organic-P compounds have been studied on functional level, in terms of the substrate specificity, the reaction mechanism or the reaction kinetics. Moreover, in recent years, there has been an increasing supply of sequencing data of targeted enzymes from a broad diversity of microorganisms. This in turn helped to uncover the gene expression pattern of major P cycle associated enzymes. The discovery of the bacterial mps phenotype, together with further microbial traits that enhance the P supply of plants, manifested the prominent role of soil microorganisms in plant growth promotion and suggested their prospective role as biofertilizers. Still, one major issue yet remained unacknowledged: The majority of the previously described data was gained from laboratory and microcosm experiments, where merely few, selected and well-characterized isolates were applied.
Thus, it remains unclear whether the entire set of beneficial microbial traits is actually present in real, unfactitious ecosystems. Moreover it is questionable, if the hitherto described PGPB indeed represent the key players of the microbial soil P turnover, or if further yet unknown taxa are of major importance. Another unknown variable in this respect is the impact of the soil P stock. Depending on the size and the proportion of organic- and inorganic-P, the individual processes of the P turnover might be differentially pronounced accordingly.
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Therefore, the following hypotheses were tested in this study:
i) The microbial potential for the solubilization of inorganic-P is dominating in close to nature forest soils with high amounts of total- and mineral-P in the topsoil. As opposed to this, the microbial P turnover in P-depleted forest soils is more relying on recycling processes of soil organic-P instead.
ii) Moreover, a stronger microbial potential for efficient P transporters is expected in P-depleted forest soils, due to the tighter recycling processes of soil P to prevent nutrient losses from the ecosystem.
iii) The majority of soil microorganisms is simultaneously occurring at distinct forest sites and forms a stable core microbiome, despite significant differences regarding the soil nutrient stocks.
iv) The microbial potential for the mineralization of soil organic-P strongly depends on the soil horizon and is highly variable on temporal and spatial scale.
To address these research questions, several experiments were performed in soils that were derived from a P geosequence of five German close to nature forest ecosystems with comparable management practices and similar main tree species. Apart from that, the forest sites were spatially separated and highly diverse regarding the soil type, the nutrient stocks and the environmental conditions. The main objectives of the underlying study comprised:
• The entire microbial P turnover in forest soils was reconstructed and the respective microbial key processes and players were assigned. Therefore, shotgun sequencing (SGS) of total genomic DNA was performed, which was derived from the mineral topsoil of the two most diverse forest sites of the P geosequence, regarding the contents of total- and mineral-P (M1).
• Novel oligonucleotide primer systems were developed in silico, which target marker genes that code for the major steps of the microbial P mobilization and uptake in soils (phoD, phoN, phnX, appA, gcd, pitA and pstS), considering the results obtained by metagenomics (M1). The in-depth diversity analysis of the associated microbial communities was conducted by amplicon sequencing of total genomic DNA derived from the organic layer of the P-richest forest site (M3).
• The applicability of the introduced oligonucleotide primers for quantitative analysis of environmental samples was proven for the phoN gene. To investigate the rate of seasonal fluctuation and highlight the impact of the distinct soil horizons on the microbial potential for
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the mineralization of soil organic-P compounds, soil samples from the organic layer and the mineral topsoil of two different forest soils were analyzed as a function of the season (M5).
• The impact of distinct soil nutrient stocks on the diversity and the composition of microbial communities in different forest soils was investigated. As previous data was primarily derived from manipulation experiments, where the P content of agricultural sites was artificially increased by fertilization or manuring, it remained unclear, whether the response pattern of soil microbial communities to P fertilization is transferable to ecosystems with naturally diverging P stocks. Therefore, total genomic DNA from the mineral topsoil of the five forest sites was applied to bacterial fingerprinting of the 16S rRNA gene and subsequently correlated to various soil physicochemical parameters in terms of the total P, N, C contents and pH. The occurrence and the size of a common core microbiome in the soils of the P geosequence was investigated by the calculation of a VENN diagram based on fingerprinting results (M2).
• In frame of an interlaboratory test, six variations of the two basic principles for the measurement of microbial biomass P were compared. Microbial biomass P was determined in the mineral topsoil of the five forest sites, using gaseous or liquid fumigation based methods, and subsequently compared regarding the absolute values and the relative gradient along the soils of the P geosequence (M4).
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