Middle concentration of microplastics decreasing soil moisture-temperature and the germination rate and early height of lettuce (Lactuca sativa var. ramosa Hort.) in Mollisols

Microplastics (MPs) have been widely found in soils, however, the mechanism of MPs influencing plant growth is still debated and possibly attributed to the soil environment changed by MPs. In this study, 0.0 %, 0.1 %, 0.5 %, 1.0 %, 2.0 %, and 5.0 % (w/w) content of low-density polyethylene MPs (LDPE-MPs) with the particle sizes of 75–2000 μm was used to test how MPs alter the germination and the early growth of lettuce (Lactuca sativa var. ramosa Hort.) in Mollisols under both natural condition and regular incubation condition. Soil temperature (ST), soil moisture (SM) and the ratio of cracks area to surface soil area (CA) and cracks length to surface soil area (CL) were monitored. As well, the dynamics of water and nutrient infiltration reported by our previous publication were combined to analyze the relationship between soil properties and crop growth influenced by MP concentration. The main results showed that: (1) compared with CK (0.0 %), the germination and plant height of lettuce were lowest in treatments with the middle concentration of MPs (0.5 % and 1 %, w/w), but was highest in treatments of high concentration of MPs (5.0 %, w/w) during the whole 14 days of incubation; (2) increasing MP concentration weakened the influence of SM on ST in Mollisols; (3) the average of SM and ST were highest at 5 % of MP concentration, while was lowest at 0.5 % and 1 % of MP concentration from the 2nd to the 9th day; (3) compared with CK and other treatments, the CA and CL were lowest in 1.0 % MP concentration, but were highest in 0.1 % and 5.0 % of MP concentration. This study provides insight that middle, rather than high and low levels of MP concentration, significantly decrease the SM and ST and increase nitrogen leaching which further leads to negative impacts on emergent and early growth of crops in soils with heavy texture (Mollisols).