Agricultural plastic pollution reduces soil function even under best management practices

Soil plastic contamination is considered a threat to environmental health and food security. Plastic films which are widely used as soil mulches are the largest single source of agricultural plastic pollution. Growing evidence indicates that high concentrations of plastic negatively affect critical soil functions. However, the relationships between agricultural plastic accumulation and its biogeochemical consequences in regions with relatively low levels of soil plastic pollution remain poorly characterized. We sampled farms across the California Central Coast (a region of global agricultural importance with extensive plastic mulch-based production) to assess the degree and biogeochemical consequences of plastic pollution in fields subject to ‘best practice’ plastic mulching application and removal practices over multiple years. All farms exhibited surface soil plastic contamination, macroplastic positively correlated with microplastic contamination levels, and macroplastic accumulation was negatively correlated with soil moisture, microbial activity, available phosphate, and soil carbon pool size. These effects occurred at less than 10% of the contamination levels reported to degrade field soils, but were relatively subtle, with no detectable relationship to microplastic concentration. Identifying declines in soil quality with low levels of macroplastic fragment accumulation suggests that we must improve best management plasticulture practices to limit the threat to soil health and agricultural productivity of unabated plastic accumulation.

Methods
Macroplastics and surface soil samples have been collected from 12 fields belongs to 5 different farms. Macro and micro plastics contamination was measured in form of number and mass concentration per ha, surface area per ha, mass per surface area etc., only number concentration (coung per kg soil) was estimated for microplastics from collected soil samples.

Different Biogeochemical properties such as gravimetric moisture (soil_moist), total inorganic nitrogen (TiN), Olsen-P, particulate organic matter (POM), minera associated organic matter (MAOM), soil respiration and microbial biomass was also estimated.

The relationship between macroplastic and microplastic accumulation in soil was tested using a linear mixed effect model, with ‘field’ nested within ‘farm’ treated as a random effect. The influence of macroplastic (count per ha, surface area per ha, mass per ha, mass per surface area) and microplastic (count kg-1 soil) on soil properties was also tested with ‘field’ nested within ‘farm’ treated as a random effect. Because clay content strongly influences soil biogeochemical properties and can increase the retention of microplastics in the soil even after typical mulch removal practices , average clay content at the field level was included as a covariate. Response variables were log transformed when assumptions of normality were not met and the transformation improved the residuals (gravimetric soil moisture, P content, POM and MAOM). Response variables that had values which fell below detection (e.g., ‘0’) were left untransformed (observed for TIN, soil respiration, microbial biomass, and microplastic count kg-1 soil). Predictor variables (macroplastic count per ha, and macroplastic mass/surface area) were rescaled by log transformation to improve model fit. The “ggeffect” function was used to predict the response variable and associated 95% confidence interval based on the “lmer” model to fit the obtained data points. Data analysis was completed using R (version 4.3.1) and RStudio (version 2023.09.0-463) using the packages ‘lme4’ and ‘lmerTest’. All plots were made using the ggplot2 package . Field and farm identity is anonymized for data presentation.