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Land-use Control on Abiotic and Biotic Mechanisms of P Mobilization

dc.contributor.advisorKuzyakov, Yakov Prof. Dr.
dc.contributor.authorMaranguit, Deejay Sabile
dc.titleLand-use Control on Abiotic and Biotic Mechanisms of P Mobilizationde
dc.contributor.refereeDippold, Michaela Prof. Dr.
dc.description.abstractengPhosphorus (P) is the most limiting nutrient for plant growth and productivity in many regions worldwide especially in the tropics. Aside intrinsic low P availability controlled by physicochemical and biological reactions, erosion and yield harvest are also very crucial in P depletion. These processes are massively intensified through anthropogenic activities, such as land-use change, the predominant global change of this century due to increasing population and food demand. Land-use change in consequence, affects P mobilization directly or indirectly through major modification of soil properties and functions. Hence, profound knowledge on abiotic and biotic factors affecting various P pools is necessary to understand the P dynamics and mobilization and to obtain a more effective soil management practices towards P conservation. Most studies were focused only on assessing the effects of land-use change on available P, but the other P pools such as Fe-bound P and microbial biomass P which are very important as reserve P pools especially in P-depleted soil were rarely considered. Therefore, this thesis aims at assessing the impacts of land-use on abiotic and biotic processes controlling forms, distribution and availability of P in soil. The P sequential fractionation approach following Hedley method (1982) was used to assess the various P pools. The Hedley fractionation method estimates the P forms that have potential contribution to available P over a growing season. The extent of the method on extracting P from various pools and the mechanisms behind P dynamics was validated in an incubation experiment using 33P tracer isotope. The incorporation of 33P-labeled KH2PO4 was traced in available P, microbial biomass P and Fe-bound P pools in an acidic P-depleted soil (Cambisol) depending on availability of carbon and nitrogen provided via applying glucose and ammonium sulfate, respectively. The Hedley fractionation was very efficient and accurate in extracting various P forms. The P immobilization via microbial uptake and fixation by the Fe and Al oxides was almost instantaneous. Applying glucose boosted microbial growth and so demand for P, resulting in increased 33P recovery and P content in microbial biomass. The microbial biomass P, as the most important labile P reservoir prohibits P fixation and increases the availability of P to plants during biomass turnover. In contrast, the high 33P recovery in Fe-bound P pool showed the dominance of P adsorption by Fe and Al oxides on P fixation and so less availability for plants. The potential contribution of earthworms (another biotic factor) on P availability was also investigated. By coupling 14C imaging and direct zymography for the first time, we visualized and localized the effects of earthworms on distribution of litter and C compounds as well as enzymes activity throughout soil profile. Earthworms bury above ground litter, produce casts and mucus that enhance the activity of beneficial soil microorganisms, colonizing earthworms’ biopores and so affect the P mobilization. Indeed, increase in microbial biomass P in the biopores and the activity of phosphatase enzymes which is responsible in hydrolyzing recalcitrant forms of organic P to become available for plants, were recorded. In the second part of this thesis, we found out that the change of forests to: (a) intensively-managed oil palm and rubber plantations in the tropics and; (b) organic and conventional farming in sub-tropics alters the distribution of P pools through controlling abiotic and biotic reactions in soil. Organic and inorganic fertilizers application increases easily-available inorganic P. However, by decrease of easily-available organic P, moderately-available and non-available P intensifies. This means that fertilization maintains soil fertility only for a short time and fertilization is not sustainable in the long run due to the depletion of P reserves. The mechanisms of depletion in this easily-available P pool through land-use change are: 1) soil erosion; 2) microbial mineralization of soil organic matter (SOM) and 3) P export via yield products. The intensified reduction in SOM contents induced by land-use change is the major influencing factor on P mobilization. Decreasing SOM furthermore, promotes soil compaction and reduces soil water holding capacity that leads to flooding. In the third part of this thesis, we demonstrated that anaerobic conditions which may take place following flooding accompanying decreasing SOM contribute to P mobilization and so the potential uptake of P by plant roots. The extent of microbial-mediated reduction process leading to dissolution of ferric oxides is apparently determined by the SOM content. SOM is the source of carbon and energy which enables microorganisms to efficiently reduce Fe3+. Therefore, soils under forest and agroforest, with relatively high SOM content, resulted in a faster and higher P release than the plantation soils. Furthermore, increasing bulk density and in consequence flooding in soils under rubber and particularly under oil palm plantations led to lengthier anaerobic conditions and so more Fe3+ reduction and P release. In conclusion, land-use change leads to major modification of soil properties and functions that affect abiotic and biotic mechanisms controlling the dominant type of P pool and their distribution in a soil, and determine the dynamics of P pools transformation and P availability for plants. Among all the affecting factors, the mechanisms controlling P mobilization and availability are more closely linked to SOM content. Thus, ecologically-based managements to reduce SOM content loss are necessary to have the highest P availability for plants and so higher
dc.contributor.coRefereeCarminati, Andrea Prof. Dr.
dc.contributor.thirdRefereePolle, Andrea Prof. Dr.
dc.contributor.thirdRefereeJoergensen, Rainer Prof. Dr.
dc.contributor.thirdRefereeSauer, Daniela Prof. Dr.
dc.contributor.thirdRefereeFlessa, Heinz Prof. Dr.
dc.subject.engPhosphorus availabilityde
dc.subject.engPhosphorus formsde
dc.subject.engSoil organic matterde
dc.subject.engAbiotic mechanismsde
dc.subject.engBiotic mechanismsde
dc.subject.eng33P isotope labellingde
dc.subject.engHedley P fractionationde
dc.subject.engHighly weathered soilde
dc.subject.engMicrobial biomass Pde
dc.subject.engRainforest deforestationde
dc.subject.engSoil floodingde
dc.subject.engIron reductionde
dc.subject.engAnaerobic conditionsde
dc.affiliation.instituteBiologische Fakultät für Biologie und Psychologiede
dc.subject.gokfullBiologie (PPN619462639)de

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