Dynamics of Phosphorus Availability and Uptake in Hydroponics and Soils
by Aamir Manzoor
Date of Examination:2024-10-21
Date of issue:2024-12-09
Advisor:Evgenia Dr Blagodatskaya
Referee:Prof. Dr. Michaela Dippold
Referee:Prof. Dr. Klaus Dittert
Persistent Address:
http://dx.doi.org/10.53846/goediss-10935
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Abstract
English
The availability of phosphorus (P) in terrestrial ecosystems is primarily determined by plant genotypes and soil physiochemical characteristics that influence the chemical forms of P in soil and its biotransformation by soil microorganisms. The mineralization of dissolved soil organic-P by extracellular phosphatases regulates the biogeochemical cycling of P and is influenced by the root exudation of metabolic products and the presence of adsorptive minerals (i.e., AI- and Fe-oxides) in the soil. It is critical to understand the balance between the direct uptake of inorganic P (Pi) by plant roots through P fertilizers and/or organic waste products like sewage sludge and biogas digestate, or recycled Pi acquired from soil organic-P through phosphatase activity for effective P management in agroecosystems. However, the mechanisms by which phosphatase activity is regulated and how it affects P dynamics and root uptake of Pi in the rhizosphere remain poorly understood. This thesis explores the availability of Pi through phosphatase activity, the effects of soil physicochemical factors on organic-P mineralization, and the uptake mechanisms of Pi by maize roots in hydroponics. Our initial study involved a detailed review and analysis of soil and plant factors to examine soil P dynamics and bioavailability via phosphatase activity. The demand for P by plants is critical in the uptake of Pi by roots and the mineralization of soil organic-P through phosphatase activity. Root exudation of metabolic products, i.e., mucilage and low-molecular weight organic acids (LMWOAs) influences the processes of Pi acquisition and organic-P mineralization in the soil. Mucilage enhances soil hydraulic conductivity and pore connectivity, leading to increased phosphatase activity. On the other hand, LMWOAs promote soil P mineralization through acidification, chelation, and exchange reactions. These exuded metabolites play a vital role in modifying the rhizosphere physiochemical conditions to facilitate Pi uptake by plant roots and the restoration of Pi and organic-P in the soil. Our review revealed a biphasic effect of root metabolic products on extracellular phosphatases, resulting in notable changes to their catalytic mechanism. As per our proposed two-phase conceptual framework, soil Pi is acquired by plants and microorganisms in a coupled manner characterized by the release of their metabolic products. When root exudation is limited or inactive, plants recycle P by adsorbing it on the soil matrix, consequently reducing the rhizosphere phosphatase activity. Our second study analyzed the uptake of Pi by maize roots through two mechanisms: active uptake via P transporters at the root surface and passive diffusion driven by Pi concentration difference between root cytosolic P and external nutrient solution. Using Michaelis-Menten kinetics, we established (i) a threshold limit between active uptake and passive diffusion of Pi ions into maize roots in the hydroponics, (ii) determined potential Pi release through phosphatase activity in the soil surrounding the tips of seminal maize roots, and (iii) compared the kinetic parameters of maximum Pi uptake (Imax) and release rate (Vmax) within the localized hotspot area of 1 mm distance from the maize root surface. Pi uptake at concentrations below 161.4 µmol L –1 adhered to Michaelis-Menten kinetics with an Imax of 11.44 µmol L –1 h –1 and a KmP value of 60 µmol L –1 . Following the initial 8 h of a Pi depletion experiment, Pi uptake was transitioned to passive diffusion by maize roots, resulting in a linear increase up to the maximum Pi concentration of 322 µmol L –1 . The Vmax was determined to be 0.79 µmol g –1 h –1 with a KmS of 49.4 µmol L –1 . When normalized per cm of root length, the Imax of 0.060 (µmol h –1 ) closely corresponded to the Vmax of 0.066 (µmol h –1 ) in the rhizosphere at a distance of 1.5 mm from the maize root surface. Our study demonstrated that the rise in Pi ion concentration in the rhizosphere resulting from its release from soil organic-P and root uptake can lead to an increase in phosphatase activity, which is directly linked to the coordinated processes of active and diffusive Pi uptake. However, further research is needed to understand the spatial and temporal shifts between active uptake and passive diffusion of Pi ions in plant roots during different plant developmental stages. Our third study focused on the effects of four different types of sewage sludge on soil microbial biomass carbon (MBC) and nitrogen (MBN), extractable organic C (EOC) and N (EN), nutrient uptake (P, Fe, K, and Ca) by maize plants, and soil phosphatase activity. The sewage sludge sources investigated were sanitary wastewater (SS), SS treated through combustion at 800°C (SSA), humified EKO-Plant sludge produced naturally through digestion with reed cultures, and one biogas (BG) digestate derived from anaerobic digestion. The results demonstrated a positive correlation between P concentration in maize roots and soil phosphatase activity, which was consistently higher in all sludge applications compared to the control. Phosphatase activity was significantly higher in SS and EKO-plant applications due to enhanced soil moisture content and/or soil EN level. EKO-plant sludge also led to increased soil MBN and MBC contents compared to other sludge types. It is strongly recommended to consider using SS and EKO-plant sludge sources as they are beneficial for increasing enzyme activity and nutrient availability in the soil. BG digestate exhibited lower EOC and EN contents compared to other sludge types, and significantly increased maize shoot and root biomass. SS and SSA sludges resulted in higher concentrations of P, Fe, K, and Ca in maize shoot dry biomass. The utilization of both sludge and BG digestate resulted in a significant enhancement in root translocation efficiency, as demonstrated by a decrease in mineral concentration in the root biomass following the incorporation of sludge into the soil. Our fourth study examined the mineralization of organic P in soil samples taken from litter, humified organic, and mineral topsoil layers of two beech forest ecosystems with contrasting P stocks. Additionally, leached soil solutions were collected using zerotension lysimeters to assess the impact of phosphatase leaching on the ratio of soil organic-P mineralization to Pi availability in organic soil layers. The Michaelis-Menten parameters (Vmax, Km) were found to be highly sensitive to the soil physiochemical properties at both forest sites, indicating a negative correlation between phosphatase activity and Pi status in the soil. Rapid mineralization of soil organic-P (76% within 24 h of sampling) was observed at the P-deficient site, suggesting an underestimation of 15% of the soil Pi availability (0.03 mg L –1 ). The results from the leached soil solutions suggested that soil organic-P could be mineralized at deeper layers due to potential phosphatase leaching which was significantly higher at the P-rich forest site compared to the P-poor forest site. The determination of phosphatase activities in leached soil solutions offers a valuable strategy in conjunction with the traditional soil extraction method for analyzing various forms of P and their mobility in forest soils. This thesis contributes to a more comprehensive understanding of soil P dynamics and P nutrition of plants within a broader ecological framework. The proposed two-phase conceptual framework offers both theoretical and process-based insights into soil P dynamics, which may aid in the comprehension of Pi acquisition (substrate turnover) and P restoration (phosphatase adsorption by soil) in various terrestrial ecosystems. Plant roots secrete phosphatases to fulfill their P needs, especially when dissolved Pi is limited in the soil. The release of Pi in nutrient solutions via phosphatase activity or desorption from mineral soil surfaces plays a critical role in Pi uptake by plant roots. Depletion of Pi from nutrient solutions may result from leaching to subsoil horizons, potentially underestimating the role of soil organic-P in Pi availability and mobility in the soil. Total P remains unaffected by enzyme-driven hydrolysis of soil organic-P, making it the most reliable variable for assessing Pi availability in the root zone. Further research is needed to establish a standardized measurement protocol for comparing leaching of different forms of P across various soil horizons to enhance our understanding of soil P dynamics and how plant´s P nutrition may be influenced by different soil environmental conditions. The use of sewage sludge products and BG digestate was found beneficial in increasing mineral uptake, soil phosphatase activity, and maize root and shoot biomass in the current study. However, caution should be exercised in their direct application due to the potential addition of toxic heavy metals and the possible drawbacks (e.g., high costs/energy demand) of SSA treatment before wide-scale implementation in agriculture.
Keywords: Phosphatase activity; Phosphorus bioavailability; Hydroponics; Sewage sludge; Michaelis-Menten kinetics; Phosphorus uptake by plants
