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Effects of PLA microplastic particle size and addition level on soil phosphorus cycling and Chinese cabbage growth in a Luvisol from Southwest China: A controlled pot experiment
Summary
Researchers conducted a pot experiment in phosphorus-limited Luvisol soil and found that PLA microplastics at 1% w/w — especially 10 µm particles — cut Chinese cabbage shoot weight by roughly 50% by disrupting the rhizosphere microbiome and shifting phosphorus into less bioavailable organic fractions, with smaller particles driving stronger immobilization.
Biodegradable microplastics such as polylactic acid (PLA) are increasingly used in agriculture as "eco-friendly" alternatives, but they can persist in soil and may disrupt nutrient cycling in phosphorus-limited subtropical systems. We carried out a pot experiment using P-deficient Luvisol soil. We measured how PLA microplastic particle size (10 μm vs 500 μm) and addition level (0.1% vs 1.0% w/w) affect soil P cycling and Chinese cabbage (Brassica rapa var. pekinensis) growth. We measured plant performance, P uptake, antioxidant enzymes, soil P fractions, rhizosphere microbial community composition, and P-cycling gene abundances. We then used partial least squares path modeling (PLS-PM) to link these factors. Cabbage growth responses depended on both particle size and concentration: at 1.0% (w/w), PLA-particularly the 10 μm particles-significantly reduced cabbage growth (shoot fresh weight decreased by ∼50%) and P uptake, whereas 0.1% (w/w) PLA had little effect. Under 1.0% PLA, SOD and POD activities increased, so oxidative stress increased. PLA also reduced labile P pools (Olsen-P and Resin-P decreased by 28%-38%). At the same time, P shifted from more available forms to less available organic fractions, and this immobilization was stronger with smaller particles. These changes happened together with rhizosphere disruption, including lower microbial diversity, more Proteobacteria, and higher abundances of several P-scavenging genes. PLS-PM showed that PLA limited plant growth mainly in an indirect way, because it changed the rhizosphere microbiome and lowered P availability. Overall, PLA microplastics can still disturb soil P dynamics and reduce crop growth in P-limited soils, so fragmentation and accumulation in agroecosystems should be managed.