Since 2015, the determination of microplastics (MPs; < 5 mm) in soil has gained increasing attention. However, usual analytical protocols still render a comparison of results challenging. This structured review integrates an in-depth screening of scientific articles (n = 106; from 1980 to 2022) that focused on determining MPs in soils. The different studies were divided into groups regarding (i) MPs input pathways, (ii) study site information, (iii) sampling design, (iv) sample preparation, and (v) analytical methods for MPs detection. Most of the studies (63%) focused on a defined study site influenced by a known point source, whereas 37% measured MPs background contents related to delocalized MPs emission sources. Even though soil was the target compartment, only 26% classified the soil type, mostly using the World Reference Base (WRB) as a classification system. Additional information on soil properties was supplied within only 20% of the studies. However, this information is mandatory for evaluation of MPs recovery tests and analytical results. In nearly all studies, the mass of the single samples equaled the final mass of the composite sample, with a mean of 1.32 kg ± 1.07 kg. However, other procedures that involve a larger sample mass seem promising but are still seldom applied. Our structured review revealed that a standard operation procedure with harmonized methods is urgently needed with a coherent and comprehensive workflow, including field sampling and sample preparation. Such a procedure would ensure the reproducibility and representativeness of analytical results, which are mandatory for evaluating and restricting MPs pollution in soils in the future. Graphical Abstract

Soil temperature (ST) is an important property of soils and driver of below ground biogeochemical processes. Global change is responsible that besides variable meteorological conditions, climate-driven shifts in ST are observed throughout the world. In this study, we examined long-term records in ST by a trend decomposition procedure from eleven stations in western Germany starting from earliest in 1951 until 2018. Concomitantly to ST data from multiple depths (5, 10, 20, 50, and 100 cm), various meteorological variables were measured and included in the multivariate statistical analysis to explain spatiotemporal trends in soil warming. A significant positive increase in temperature was more pronounced for ST (1.76 ± 0.59 °C) compared with air temperature (AT; 1.35 ± 0.35 °C) among all study sites. Air temperature was the best explanatory variable to explain trends in soil warming by an average 0.29 ± 0.21 °C per decade and the trend peaked during the period from 1991–2000. Especially, the summer months (June to August) contributed most to the soil warming effect, whereby the increase in maximum ST (ST_max) was nearby fivefold with 4.89 °C compared with an increase of minimum ST (ST_min) of 1.02 °C. This widening between ST_max and ST_min fostered enhanced diurnal ST fluctuations at ten out of eleven stations. Subsoil warming up to + 2.3 °C in 100-cm depth is critical in many ways for ecosystem behavior, e.g., by enhanced mineral weathering or organic carbon decomposition rates. Thus, spatiotemporal patterns of soil warming need to be evaluated by trend decomposition procedures under a changing climate. Graphical abstract

Platinum (Pt)‐tipped electrodes are frequently employed to measure the soil redox potential ( E _H ). Thereby, the timely transition from reducing towards oxidising soil conditions is one of the most important biogeochemical changes that can occur in soil. This condition is mainly linked to the air‐filled pore volume ( ε ) and pore geometries. However, even when the Pt electrodes are located in close vicinity to each other, E _H readings behave non‐uniformly, presumably due to the millimetre scaled heterogeneity of pore spaces controlling oxygen (O ₂ ) availability and transport. In this study, we examined the ε distribution and pore connectivity in the close vicinity of a Pt electrode during an artificial evaporation experiment using an undisturbed soil sample (Ah‐horizon, Calcaric Gleysol). We combined physio‐chemical methods with non‐destructive X‐ray computed microtomography (μCT) and 3D‐image analysis. μCT scans were conducted at three‐time points, that is, reducing conditions with E _H < −100 mV (CT‐1), the transition from reducing towards oxidising conditions with an E _H increase > 5 mV h ⁻¹ (CT‐2), and oxidising conditions with E _H > 300 mV (CT‐3). We observed that the shift from reducing towards oxidising conditions took place at an air‐filled porosity ( ε _CT ) of ~0.03 cm ³ cm ⁻³ , which matches very with gravimetrically calculated data obtained by tensiometry of ε ~0.05 cm ³ cm ⁻³ . Besides the relation of E _H and ε , image analysis revealed that a connected ε _CT ( ε _{CT_conn} ) of ~0.02 cm ³ cm ⁻³ is needed to enable enhanced O ₂ diffusion from the soil surface towards the Pt surface and facilitate a straightforward E _H response. We conclude that ε _{CT_conn} is a critical parameter to assess aeration processes in temporarily water‐saturated soils to characterise a switch in redox conditions. Highlights: Usually, soil redox dynamics are related to the air‐filled porosity ( ε _CT ) but here its connected portion ( ε _{CT_conn} ) was found more relevant. 3D X‐ray computed microtomography imaging close to a redox electrode enabled us to understand the soil aeration process. Connected ε _CT ( ε _{CT_conn} ) of ~0.02 cm ³ cm ⁻³ facilitated oxidising soil conditions. ε _{CT_conn} is a critical parameter to assess the aeration process in temporarily water‐saturated soils.