Effects of microplastic input and straw addition on nitrogen leaching in fluvo-aquic soil and yellow-brown soil

This study aims to explore the effects of microplastic input and straw addition on soil nitrogen leaching in farmland. Fluvo-aquic soil and yellow-brown soil were taken as the research objects. Eight treatments were set for each soil, including control (CK), low microplastics (PE1), medium microplastics (PE2), high microplastics (PE3), straw (S), straw + low microplastics (S+PE1), straw + medium microplastics (S+PE2), and straw + high microplastics (S+PE3). A systematic investigation was made to clarify the effects of microplastic input on soil nitrogen leaching with/without the straw. The results showed that there was no significant difference in the TN leaching loss among PE1, PE2 and PE3 treatments in the fluvo-aquic soil with microplastics only, compared with the CK. In yellow-brown soil, PE1 treatment significantly increased the TN leaching loss. There was no significant difference in the TN leaching loss between PE2 and PE3 treatments. The addition of straw in the fluvo-aquic soil significantly reduced the leaching amount of NO₃^--N, NH₄⁺-N and TN, which decreased by 31.15%, 13.45% and 15.26%, respectively, compared with the CK. The addition of straw in the yellow-brown soil significantly increased the leaching amount of TN, which increased by 22.56%. Compared with no straw treatment, the cumulative leaching loss of NO₃^--N, NH₄⁺-N and TN under different concentrations of microplastic input in fluvo-aquic soil showed a decreasing trend, whereas, the low amount of microplastic input in the yellow-brown soil reduced the TN leaching loss, and the high amount of microplastic input increased the TN leaching loss. Partial least squares path model (PLS-PM) analysis showed that the addition of straw in the fluvo-aquic soil mainly affected nitrogen leaching via the pH of the leaching solution and NO₃^--N leaching loss, and the amount of added microplastics had no significant effect on nitrogen leaching. In the yellow-brown soil, the nitrogen leaching was affected by the leaching loss of NO₃^--N and NH₄⁺-N in the leaching solution. The microplastics mainly affected the nitrogen leaching for the leaching loss of NH₄⁺-N in the leaching solution. The research results can provide a sound basis for the control of the risk of microplastic pollution in farmland soil and the reduction of soil nitrogen leaching.