Effects of Polypropylene Microplastics and Aging Time on Nitrogen and Phosphorus Dynamics and Enzyme Activities in Soil

To investigate the effects of polypropylene microplastics (PP MPs) on soil nitrogen and phosphorus dynamics as well as enzyme activities, an incubation experiment was conducted using different PP MPs mass fractions (CK, 5%, and 10%) and aging times (7, 15, 25, 35, and 45 d). The results showed that, compared with the control treatment, PP MPs addition significantly decreased the contents of available phosphorus (AP) and the activities of soil nitrate reductase (S-NR), catalase (S-CAT), neutral phosphatase (S-NP), and urease (S-UE), whereas the NO- 2-N content was significantly increased. Aging time was significantly negatively correlated with AP, S-NR, S-CAT and S-NP, but significantly positively correlated with NO- 2-N content. Conversely, PP MPs mass fraction showed significant positive correlations with NO- 3-N. Furthermore, the results indicated that PP MPs indirectly affected soil enzyme activities by altering soil nitrogen and phosphorus fractions, particularly NO- 2-N. Overall, the effects of PP MPs on soil nitrogen and phosphorus contents and enzyme activities were mainly associated with aging time.