Increasing concentration of pure micro- and macro-LDPE and PP plastic negatively affect crop biomass, nutrient cycling, and microbial biomass.

Over the last 50 years, the intense use of agricultural plastic in the form of mulch films has led to an accumulation of plastic in soil, creating a legacy of plastic in agricultural fields. Plastic often contains additives, however it is still largely unknown how these compounds affect soil properties, potentially influencing or masking effects of the plastic itself. Therefore, the aim of this study was to investigate the effects of pure plastics of varying sizes and concentrations, to improve our understanding of plastic-only interactions within soil-plant mesocosms. Maize (Zea mays L.) was grown over eight weeks following the addition of micro and macro low-density polyethylene and polypropylene at increasing concentrations (equivalent to 1, 10, 25, and 50 years mulch film use) and the effects of plastic on key soil and plant properties were measured. We found the effect of both macro and microplastic on soil and plant health is negligible in the short-term (1 to <10 years). However, ≥ 10 years of plastic application for both plastic types and sizes resulted in a clear negative effect on plant growth and microbial biomass. This study provides vital insight into the effect of both macro and microplastics on soil and plant properties.

LDPE and biodegradable PLA-PBAT plastics differentially affect plant-soil nitrogen partitioning and dynamics in a Hordeum vulgare mesocosm.

Micro and macroplastics are emerging contaminants in agricultural settings, yet their impact on nitrogen (N) cycling and partitioning in plant-soil-microbial systems is poorly understood. In this mesocosm-scale study, spring barley (Hordeum vulgare L.) was exposed to macro or microplastic produced from low density polyethylene (LDPE) or biodegradable plastic at concentrations equivalent to 1, 10 and 20 years of plastic mulch film use. Partitioning of 15N-labelled fertiliser into plant biomass, soil and leachate yielded a partial mass balance. Soil N partitioning was probed via compound-specific 15N-stable isotope analyses of soil microbial protein. Concentration-dependent decreases in plant 15N uptake occurred with increased leached nitrogen for LDPE microplastic. Assimilation into soil microbial protein was higher for biodegradable plastics, which we associate with early-stage biodegradable plastic degradation. Partitioning of 15N into inorganic soil N pools was affected by LDPE size, with lower assimilation into the microbial protein pool. While microplastics and macroplastics altered soil N cycling, the limited impacts on plant health indicated the threshold for negative effects was not reached at agriculturally relevant concentrations. This study highlights the difference between conventional and biodegradable plastics, and emphasises that the interplay of micro and macroplastics on soil N cycling must be considered in future studies.

Do plants use root-derived proteases to promote the uptake of soil organic nitrogen?

AIMS: The capacity of plant roots to directly acquire organic nitrogen (N) in the form of oligopeptides and amino acids from soil is well established. However, plants have poor access to protein, the central reservoir of soil organic N. Our question is: do plants actively secrete proteases to enhance the breakdown of soil protein or are they functionally reliant on soil microorganisms to undertake this role? METHODS: Growing maize and wheat under sterile hydroponic conditions with and without inorganic N, we measured protease activity on the root surface (root-bound proteases) or exogenously in the solution (free proteases). We compared root protease activities to the rhizosphere microbial community to estimate the ecological significance of root-derived proteases. RESULTS: We found little evidence for the secretion of free proteases, with almost all protease activity associated with the root surface. Root protease activity was not stimulated under N deficiency. Our findings suggest that cereal roots contribute one-fifth of rhizosphere protease activity. CONCLUSIONS: Our results indicate that plant N uptake is only functionally significant when soil protein is in direct contact with root surfaces. The lack of protease upregulation under N deficiency suggests that root protease activity is unrelated to enhanced soil N capture.

Methodological bias associated with soluble protein recovery from soil.

Proteins play a crucial role in many soil processes, however, standardised methods to extract soluble protein from soil are lacking. The aim of this study was to compare the ability of different extractants to quantify the recovery of soluble proteins from three soil types (Cambisol, Ferralsol and Histosol) with contrasting clay and organic matter contents. Known amounts of plant-derived 14C-labelled soluble proteins were incubated with soil and then extracted with solutions of contrasting pH, concentration and polarity. Protein recovery proved highly solvent and soil dependent (Histosol > Cambisol > Ferralsol) and no single extractant was capable of complete protein recovery. In comparison to deionised water (10-60% of the total protein recovered), maximal recovery was observed with NaOH (0.1 M; 61-80%) and Na-pyrophosphate (0.05 M, pH 7.0; 45-75% recovery). We conclude that the dependence of protein recovery on both extractant and soil type prevents direct comparison of studies using different recovery methods, particularly if no extraction controls are used. We present recommendations for a standard protein extraction protocol.