Effects of microplastic contamination on soil nitrogen and its bioavailability in soybean-maize rotation system

Microplastic contamination may affect nitrogen stock and bioavailability in the soil-plant system, to differing extents depending on plastic material properties. A field experiment was conducted to assess the effects of low-density polyethylene and poly (butylene adipate-co-terephthalate) debris (LDPE-D and PBAT-D, 500–2000 µm), and low-density polyethylene particles (LDPE-P, 500–1000 μm) on soil total nitrogen (TN), nitrate nitrogen (NO - 3 -N) and ammonium nitrogen (NH + 4 -N) dynamics, as well as nitrogen-associated enzyme activities, from the soybean planting, fallow, to maize harvesting stages of the crop rotation cycle. Results showed that PBAT-D, LDPE-D and LDPE-P significantly inhibited increases in TN during the soybean growth period, but promoted decreases during the fallow period ( p < 0.05). PBAT-D and LDPE-D also inhibited increases in NH + 4 -N significantly during the soybean growth period, but had no significant effect on NO - 3 -N. PBAT-D, LDPE-D and LDPE-P significantly promoted increases in NO - 3 -N during the fallow period, and stimulated the conversion of NH + 4 -N to NO - 3 -N during the maize growth period. In addition, microplastics significantly inhibited leucine-aminopeptidase and alanine aminotransferase enzyme activities in soybean and maize rhizosphere soils, but only stimulated N-acetyl-β-glucosaminidase enzyme activities in soybean rhizosphere soil. These findings indicate that microplastic contamination may inhibit soil N increases associated with biological N fixation by legume crops. Furthermore, the enhancement of N converting from NH + 4 -N to NO - 3 -N may lead to increased risks of leaching loss under microplastic contamination. These experimental outcomes suggest that microplastic contamination may negatively impact N sustainability and bioavailability in agricultural soils under soybean-maize rotation. • Microplastics (MPs) affected soil N and soybean-maize rotation sustainability. • MPs inhibited N stock (soybean stage) but enhanced N transformation (fallow stage). • MPs facilitated the conversion of ammonium to nitrate (maize stage). • MPs disrupted nitrogen-enzyme activities in maize rhizosphere soil.