Abstract:
Fertilizer inputs, such as phosphorus (P), to agricultural land are required to ensure adequate levels of crop production to meet the food and fiber demands of a growing population. Natural background P concentrations in soils are generally insufficient to sustain current levels of agricultural production. Consequently, agricultural soils are routinely supplemented with P fertilizers, often at rates exceeding immediate crop demand. (Habibiandehkordi et al., 2019; Roberts et al., 2020). Pronounced amounts of P have accumulated in cultural landscapes, due to prolonged fertilizer application over past decades (Bennett et al., 2001; Lannergård et al., 2020; Sharpley et al., 2013). A pronounced build-up of P stocks has occurred in many agricultural catchments, commonly known as legacy P, rendering agriculture the major source of diffuse P inputs into surface waters (Sandström et al., 2021; Sharpley et al., 1994). From agricultural soils, dissolved P is mobilized through surface runoff and erosion and subsequently transported via the transfer continuum (land-freshwater) through buffer strips until it eventually reaches adjacent drainage ditches (Efobo, 2023; Haygarth et al., 2005).
The loss of P from agricultural catchments increases the risk of eutrophication of water bodies and raise questions regarding the efficiency of current management practices. However, effective management strategies depend on a mechanistic understanding of P dynamics across the transfer continuum. The investigation of P dynamics in vegetated buffer strips (VBS, Section B) and drainage ditch sediments (DDS, Section C) clarifies how soil properties, hydrological variability, and prior P exposure control P retention, transformation, and mobility. While vegetated buffer strips and drainage ditch sediments are widely used to mitigate P transport from agricultural land to surface waters, their capacity to retain or remobilize P under fluctuating hydrological and redox conditions remains insufficiently understood. In particular, short- to medium-term transformations among labile, mineral-associated, and organic P pools in non-calcareous systems are poorly constrained. This knowledge gap is critical given that climate change is expected to intensify drying–rewetting cycles and extreme rainfall events, potentially enhancing the mobilization and re-release of legacy P from agricultural landscapes.
The aim of this dissertation was to understand the fate of P in VBS (Section B) and DDS (Section C) under dynamic environmental conditions (i.e., drying-rewetting cycles) by examining P pool transformations through an incubation experiment. Inorganic P (Pi) was added to fresh soil and sediment samples, followed by a 16- to 24-week incubation under different environmental conditions. A five-step sequential extraction scheme was implemented to follow temporal variations in the operationally defined P pools (Section B and Section C). In addition, for DDS (Section C) I applied an innovative isotopic labeling based on the δ18OPi analysis of different Pi pools. Moreover, hydrological regimes differing in the duration and frequency of drying and rewetting were applied to investigate their influence on biotic and abiotic processes affecting P retention at different time scales (Bai et al., 2017; Sugiyama et al., 2013).
The results showed that in VBS (Section B), additional Pi largely retained in dissolved and labile, surface-adsorbed pools (porewater-Pi and NaHCO₃-extractable Pi), with minimal transfer to stable Al- and Fe-bound pools such as NaOH-extractable Pi or CDB-extractable Pi. This pattern suggests sorption site saturation and highlights a risk of Pi mobilization, particularly under fluctuating moisture conditions that promote redox changes, which can trigger Fe(III)-(oxyhdr)oxide dissolution and alter Pi adsorption. Microbial and enzymatic activity also influenced P transformations, as evidenced by shifts in organic P (Po) pools and the mineralization of Po into more bioavailable forms. In contrast, forest soils without prior agricultural P input retained added Pi more effectively in stable NaOH-extractable Pi pools, demonstrating the importance of soil history and saturation in controlling P dynamics.
In DDS (Section C), temporally resolved 18OPi-labeling revealed rapid transfer of added Pi into labile NaHCO₃-extractable Pi and porewater pools immediately after application. During the early incubation phase, Pi was progressively redistributed into more strongly adsorbed NaOH-extractable Pi pools, while microbial activity and enzymatic processes transformed a portion of Pi into Po, particularly in association with Fe(III)-(oxyhydr)oxide surfaces. This microbial cycling was reflected in the changing isotopic composition of Pi, indicating intracellular and extracellular enzymatic turnover. Over time, the intermediate phase showed a pronounced microbial conversion of strongly bound Pi into Po, likely facilitated by biofilm formation, whereas the late phase was characterized by microbial dieback and subsequent decay of Po, resulting in the re-release of Pi and its surface adsorption. These results indicate that long-term Pi retention in non-calcareous drainage ditch sediments is primarily governed by surface adsorption rather than the formation of stable mineral P pools, with the CDB-extractable Po pool playing an unexpectedly significant role as an intermediate P reservoir.
Collectively, these studies underscore the interplay of biotic and abiotic factors in shaping P transformations and retention in agricultural land. Hydrological variability, microbial activity, and soil properties strongly influence whether Pi remains in labile, surface-adsorbed, or more stable forms. Importantly, the findings highlight the need for temporally resolved monitoring to capture the dynamic behavior of P pools and identify intermediate storage mechanisms such as CDB-extractable Po. For management strategies aiming to reduce P discharge from agricultural catchments, consideration of soil type, legacy P, hydrological variability, and microbial dynamics is essential, particularly under changing climatic conditions. These insights are crucial for improving the design and effectiveness of buffer zones, drainage ditches, and other landscape-scale measures intended to mitigate P transport to downstream aquatic ecosystems.
Agriculture