Abstract
Microplastics may affect soil ecosystem functioning in critical ways, with previously documented effects including changes in soil structure and water dynamics; this suggests that microbial populations and the processes they mediate could also be affected. Given the importance for global carbon and nitrogen cycle and greenhouse warming potential, we here experimentally examined potential effects of plastic microfiber additions on CO2 and N2O greenhouse gas fluxes. We carried out a fully factorial laboratory experiment with the factors presence of microplastic fibers (0.4% w/w) and addition of urea fertilizer (100 mg N kg− 1) using one target soil. The conditions in an intensively N-fertilized arable soil were simulated by adding biogas digestate at the beginning of the incubation to all samples. We continuously monitored CO2 and N2O emissions from soil before and after urea application using a custom-built flow-through steady-state system, and we assessed soil properties, including soil structure. Microplastics affected soil properties, notably increasing soil aggregate water-stability and pneumatic conductivity, and caused changes in the dynamics and overall level of emission of both gases, but in opposite directions: overall fluxes of CO2 were increased by microplastic presence, whereas N2O emission were decreased, a pattern that was intensified following urea addition. This divergent response is explained by effects of microplastic on soil structure, with the increased air permeability likely improving O2 supply: this will have stimulated CO2 production, since mineralization benefits from better aeration. Increased O2 would at the same time have inhibited denitrification, a process contributing to N2O emissions, thus likely explaining the decrease in the latter. Our results clearly suggest that microplastic consequences for greenhouse gas emissions should become an integral part of future impact assessments, and that to understand such responses, soil structure should be assessed.
Highlights
As a result of human activities, the load of reactive nitrogen compounds (NH3/NH4, NO3−, NOx, N2O) on the earth has more than doubled in recent decades [1, 2]
At the time of the highest emissions, the variants with microplastics showed slightly lower fluxes for N2O and slightly higher fluxes for CO2 than the control variants without microplastics. This was accompanied by a differentiated effect of microplastics on the flux dynamics
The addition of urea at the beginning of the second incubation phase suddenly caused a strong short-term stimulation of the CO2 release and a longer-term stimulation of the N2O release, whereby the flux rates, especially for N2O, were significantly higher than in the first incubation phase (Fig. 2). Both gases reacted to the presence of microplastics with the same reaction pattern as in the first incubation phase with regard to the level and dynamics of the emissions
Summary
As a result of human activities, the load of reactive nitrogen compounds (NH3/NH4, NO3−, NOx, N2O) on the earth has more than doubled in recent decades [1, 2] This was accompanied by a doubling of the intensity of the global nitrogen cycle. A main driver of this development are intensified agricultural practices entailing increased (2021) 1:3 and nitrogen budgets of soils are closely linked. This is especially true for the mineralization of soil organic matter as a source of CO2 release from soils [10]. Despite its potential importance, compared to other factors of global change, we have so far only scratched the surface in terms of assessing microplastic impacts on soil properties and processes in general [15,16,17,18]
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