Köster, Moritz Johannes (2021). Climate−dependent phosphorus forms and their utilization by plants and microorganisms. (Thesis). Universität Bern, Bern

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Abstract

Phosphorus (P) in soil is an important nutrient for biological activity and often limits plant and microbial growth in natural ecosystems. Due to its chemical nature and conditions in soil, its mobility in soil is limited by strong sorption to metal(hydr)oxides and precipitation with calcium (Ca), iron (Fe), and aluminum (Al) ions. P speciation and fixation in soil changes during pedogenesis from mainly Ca-bound P in early developmental stages to Fe-, and Al-bound P and organic P in more weathered soils. The rate of mineral weathering and soil development strongly depend on climatic conditions. Therefore, ecosystems under varying climate, but with soils that originate from similar parent material, are expected to show different states of weathering and P speciation in soil. Plants and microorganisms possess adaptations to mobilize P from various sources. It is believed that P cycling mechanisms and processes change with the P species that are available in soil. The aim of this work was to determine climate-dependent P speciation and biotic P cycling strategies along an aridity gradient in the Chilean Coastal Cordillera. The study sites comprised a sequence of ecosystems from an arid shrubland in the north to a Mediterranean woodland as intermediate site and a humid-temperate forest in the south. The studies of this work focused on the question whether P nutrition in a given ecosystem depends mainly on the acquisition of P from mineral sources, or if recycling of organic P is the main P source. Moreover, the focus was on the impact of root-derived labile C on P cycling under varying climatic conditions. The contribution of low-molecular-weight organic acids (LMWOAs) to mineral weathering (weathering agents) but also to organic P recycling was scrutinized. This work consists of four studies that investigated P speciation in soil and the mechanisms driving P cycling within the plant-soil-microbe system. P speciation in the rhizosphere and bulk soil was determined by X-ray absorption near edge structure (XANES), a powerful tool to determine the speciation of inorganic P in soil. Moreover, P was extracted as operationally defined P pools as water extractable, NaHCO3 extractable, ammonium-oxalate extractable (NH4-oxalate), and dithionite-citrate-bicarbonate (DCB) extractable P. The sorption capacity of soils for P was determined by 33P application. The adsorption of the tracer to the soil solid phase was measured and the sorption capacity of soils was calculated. Microbial uptake of sorbed inorganic P was quantified under steady state conditions and under the addition of glucose (rhizosphere conditions). A method to extract low-molecular-weight organic acids was adapted to measure the 13C content in these weathering agents. LMWOA were extracted after 13C enriched CO2 application (pulse labeling). This allowed not only to relate P species to the exudation of organic acids in general, but also to quantify plant and microbial C allocation to these weathering agents. Primary Ca-bound P contents were highest in the arid shrubland ecosystem and declined towards the Mediterranean woodland and humid-temperate forest. Secondary precipitated Fe- and Al-P followed the same trend, while inorganic P sorbed to Fe- and Al-(hydr)oxides and organic P increased with decreasing aridity. Accumulation of organic P in root proximity was detected in the Mediterranean woodland and humid-temperate forest, reflecting high biological activity at these sites. P speciation was correlated with LMWOA contents, phosphatase kinetics and microbial biomass carbon. It was demonstrated, that under arid climate and in the subsoil of the Mediterranean woodland, LMWOA contributed to mineral weathering. They fundamentally changed their role in the Mediterranean woodland’s topsoil and humid-temperate forest soil, where LMWOA liberated organic P, likely for enzymatic degradation. It was inferred that P cycling under arid conditions is driven by the acquisition of mineral inorganic P but towards decreasing aridity, the importance of organic P recycling increases. The P sorption capacity of soils was highest under a humid climate and highly weathered soils and was explained by high contents of ferrihydrite, ammonium-oxalate leachable Al and clay. Microbial uptake of sorbed inorganic P from the soil’s solid phase was highest in the A-horizon of the Mediterranean woodland soil and was low in the humid-temperate forest. In the latter, phosphatase activity was high, indicating organic P recycling and a low relevance of sorbed inorganic P to meet the P requirements of the biota. Inorganic P sorbed to the soil solid phase was most intensively utilized in the Mediterranean woodland soil and microbial uptake of sorbed inorganic P increased when glucose was added. This demonstrates that P limitation in the Mediterranean woodland soil can be reduced when readily available C is exuded in the rhizosphere. In contrast, microbiota in the other ecosystems along the ecosequence were either not primarily P limited or relied on different P sources. Moreover, in the humid-temperate forest a plethora of C compounds are available for microbial utilization, reducing the importance of labile root exuded C. In accordance with the XANES results, operationally defined P pools showed an increase of plant available P and a decrease of precipitated P with decreasing aridity. While the content of LMWOA in soil was higher in the humid-temperate forest than the Mediterranean woodland and arid shrubland, 13C allocation to LMWOA was of equal magnitude in the humid-temperate forest and Mediterranean woodland. High enzyme activities in the humid-temperate forest imply the utilization of organic P. A variety of P mobilizing processes can be assumed as weathering agents were only partly derived from recently exuded root C but also from unlabeled C sources. For the arid shrubland and Mediterranean woodland it was shown that the content of fungi-derived oxalic acid is positively correlated with well crystallized inorganic P (DCB leachable P), XV pointing to fungi as important organisms to mobilize hardly available P sources in these ecosystems. 13C allocation patterns in the arid shrubland are similar to those in the Mediterranean woodland, even though contents are on a lower level. This is interpreted as adaption to drought in both off these ecosystems and an indication that aridity is driving belowground C allocation. In summary it was demonstrated that biological weathering is an important process of P acquisition under arid and Mediterranean climate, but not under humid-temperate climate, where P recycling becomes more important. At the same time, LMWOA are important P cycling agents in soil; under arid conditions they support mineral weathering, while under humid climate they facilitate organic P recycling. C limitation of P acquisition is high under arid and Mediterranean climate and relieved under humid climate. With respect to climate dependent P cycling in soil it is concluded that each site has its specific P speciation, P availability, and stoichiometric nutrient constraints. It was, therefore, not possible to identify a systematic trend of these parameters along the ecosequence. Abiotic changes (i.e., climate change) likely result in nonlinear changes in P speciation, P availability, and stoichiometric nutrient limitations, impeding our ability to predict changes in P cycling as a result of changing abiotic conditions at a site.

Item Type:	Thesis
Dissertation Type:	Cumulative
Date of Defense:	11 October 2021
Subjects:	900 History > 910 Geography & travel
Institute / Center:	08 Faculty of Science > Institute of Geography
Depositing User:	Hammer Igor
Date Deposited:	01 Dec 2021 15:28
Last Modified:	01 Dec 2021 15:34
URI:	https://boristheses.unibe.ch/id/eprint/3197

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