Differential Effects of Biodegradable Polymers and Polymer-Phosphorus Composites on Tomato Performance and Phosphorus Uptake

While biopolymers have the potential to enhance agrochemical delivery and mitigate environmental impacts such as runoff, previous plant studies have often been limited to examining single biopolymers in isolation. This approach has hindered effective comparisons of plant outcomes due to variations in plant type, growth duration, and soil characteristics. The current study addresses this gap by incorporating six separate milled biopolymers: pectin, starch, chitosan, polycaprolactone (PCL), polylactic acid (PLA), or polyhydroxybutyrate (PHB) into soil and directly comparing their impacts on tomato (Solanum lycopersicum) plants cultivated under identical environmental parameters. Plant outcomes were also studied when biopolymers were modified via the inclusion of two phosphorus (P) salts, forming two types of Polymer-P-containing salt composites with amorphous CaPO₄ (CaP) and CaHPO₄ (DCP). Our results revealed that chitosan-based treatments significantly improved tomato root and shoot biomass, with increases of 200–300% compared to the control plants. Chitosan-CaP and Chitosan-DCP also enhanced P uptake, though the effect was significantly more pronounced in the former, suggesting a synergy between chitosan and CaP. Neither Chitosan-P-containing salt treatment, however, mitigated P leaching from soil when compared to CaP or DCP applied in isolation. The two most hydrophilic biopolymers, pectin and starch, as well as their P-salt-containing counterparts, showed the most substantial reductions in biomass (∼80%) with respect to control plants, while similarly lowering P uptake and P retention in soil compared to CaP- and DCP-only plants. PCL- and PHB-based treatments also adversely influenced biomass and plant P, though these effects were not as drastic as those observed with pectin and starch. PLA-based soil amendments had no effect on any plant performance metric, though PLA-CaP, specifically, was the only treatment to appreciably mitigate P leaching (−63%). Based on these findings, subsequent tomato growth experiments were conducted over a longer 8-week period with CaP, DCP, Chitosan, Chitosan-CaP, and Chitosan-DCP. While all chitosan-treated plants showed similar enhancements in biomass, plants treated with Chitosan-CaP and Chitosan-DCP were the only ones to fruit, demonstrating the benefit of using chitosan in conjunction with a P source as compared to either treatment in isolation. These findings contribute to an expanding body of evidence that biopolymer carriers can offer a more sustainable approach to improving the precision of nutrient delivery, while also highlighting the pivotal role of biopolymer and nutrient type in the development of these carriers.