Stable gullies provide a suitable habitat for functional insects and reduce the threat of pests on crops in farmland of Northeast China.

Gullies with lower altitudes compared to the surrounding environment are widely distributed in farmland of the watershed and their numbers are still expanding. However, it is still unclear how these gullies regulate the functional insects in farmland. In this study, land use types combined with the herbaceous plant, herbicide application, soil moisture, topography and climatic factors during crop growth were considered to understand how gullies influence the dynamics of functional insects in farmland from a watershed (240 ha) of Northeast China. The primary findings demonstrate that the richness and abundance of functional insects are generally greatest in gullies, particularly in stable gullies, and decrease in the following order: forest belts, grasslands, and farmlands within the watershed. Notably, the ratios of beneficial insects to pests (BI/Pest) in terms of richness and abundance were lower in gullies before July but reversed after July, in comparison to farmland. Stable gullies exhibited higher BI/Pest abundance and diversity ratios than developing gullies. The richness and abundance of functional insects were higher in the middle sections of gullies compared to their heads and tails. Furthermore, the ratios of BI/Pest were generally lower in farmlands than in any gully position. Functional insect dynamics were mainly determined by season, followed by plant abundance and biomass in the gullies, and rarely by soil moisture in the both watershed and single gullies scales. Generally, the richness and abundance of functional insects in farmland were mainly influenced by gullies, especially influenced by the gully middle position. Insect composition in farmland influenced by stable gullies was stronger than by developing gullies, and stable gullies were more beneficial in reducing the threat of pests to crops in the farmland of the watershed.

Middle concentration of microplastics decreasing soil moisture-temperature and the germination rate and early height of lettuce (Lactuca sativa var. ramosa Hort.) in Mollisols.

Microplastics (MPs) have been widely found in soils, however, the mechanism of MPs influencing plant growth is still debated and possibly attributed to the soil environment changed by MPs. In this study, 0.0 %, 0.1 %, 0.5 %, 1.0 %, 2.0 %, and 5.0 % (w/w) content of low-density polyethylene MPs (LDPE-MPs) with the particle sizes of 75-2000 µm was used to test how MPs alter the germination and the early growth of lettuce (Lactuca sativa var. ramosa Hort.) in Mollisols under both natural condition and regular incubation condition. Soil temperature (ST), soil moisture (SM) and the ratio of cracks area to surface soil area (CA) and cracks length to surface soil area (CL) were monitored. As well, the dynamics of water and nutrient infiltration reported by our previous publication were combined to analyze the relationship between soil properties and crop growth influenced by MP concentration. The main results showed that: (1) compared with CK (0.0 %), the germination and plant height of lettuce were lowest in treatments with the middle concentration of MPs (0.5 % and 1 %, w/w), but was highest in treatments of high concentration of MPs (5.0 %, w/w) during the whole 14 days of incubation; (2) increasing MP concentration weakened the influence of SM on ST in Mollisols; (3) the average of SM and ST were highest at 5 % of MP concentration, while was lowest at 0.5 % and 1 % of MP concentration from the 2nd to the 9th day; (3) compared with CK and other treatments, the CA and CL were lowest in 1.0 % MP concentration, but were highest in 0.1 % and 5.0 % of MP concentration. This study provides insight that middle, rather than high and low levels of MP concentration, significantly decrease the SM and ST and increase nitrogen leaching which further leads to negative impacts on emergent and early growth of crops in soils with heavy texture (Mollisols).

Nitrogen and phosphorus change the early natural vegetation restoration in degraded Phaeozems of gullies.

The influence of nutrients during natural vegetation restoration (NVR) in complicated landscapes and hydrologic conditions has often been debated. This study aimed to clarify how nitrogen (N) and phosphorus (P) runoff influences plant biomass and biodiversity during early restoration stages in gullies. In this study, the influence of runoff containing N, P, and N + P on the biomass and diversity of ten predominant herbaceous species was simulated in two degraded Phaeozems of gullies by under controlled conditions for two years. Increasing N in runoff increased the biomass in both low-degradation Phaeozems (LDP) and high-degradation Phaeozems (HDP), and N input could increase the competitive ability of No-Gramineae (NG) and constrain G biomass in the second year. N and P increased the biomass by increasing the species abundance and individual mass but not the diversity. N input typically decreased biodiversity, while P input influenced the dynamics of biodiversity was nonmonotonic increased or decreased. Compared with sole N input, additional P accelerated the competition of NG, restrained G mass, and decreased the total biomass in LDP, while increasing the total biomass in HDP in the first year. However, additional P input did not change the N effects on biodiversity in the first year, while high P input improved the herbaceous diversity in the second year of gullies. Generally, N in runoff was the key factor influencing NVR, especially for biomass in early NVR stages. The P dose and the ratio of N:P in the runoff were the main determinants of P mediation on the N effect on NVR.

Microplastics change the leaching of nitrogen and potassium in Mollisols.

Nowadays, the dynamics of nutrients leaching from the soils and their driving mechanism have been focused on, however, it is still unclear how microplastics (MPs) influence the nutrients' leaching in soils. In this study, five concentrations (w/w, 0.0 %, 0.5 %, 1 %, 2 %, 3 %) and three sizes of MPs of polyethylene (PE) (0.15-0.36 mm, 0.36-0.60 mm and 0.60-1.00 mm) influencing the leaching of NO3--N and water-soluble potassium (WSK) was simulated by a column method in Mollisols, and both the pre-fertilization and post-fertilization were considered. The results showed that, before KNO3 addition, there was a negative power function relationship between the NO3--N concentration and the leaching solution volume/leaching time. The amount and concentration of NO3--N leaching was higher in the early leaching stage. Compared with the CK, PE0.5% significantly reduced the leaching amount of WSK, while increased the leaching amount of NO3--N but not significantly. The leaching amount of WSK decreased with the increasing size of PEMP when the PEMP concentration was the same, while NO3--N was opposite. PE0.60-1.00 increased the leaching amount of NO3--N, while reduced the leaching amount of WSK. After KNO3 addition, compared with CK, PE1% significantly reduced the leaching amount of NO3--N, and PE1% had the lowest leaching amount of WSK. However, when the PEMP concentration in the soil reached a certain threshold (w/w, >1 %), the leaching amount of NO3--N and WSK increased gradually with PEMP increasing. PE0.60-1.00 reduced the leaching amount of NO3--N and WSK most obviously. In general, low concentrations (w/w, <1 %) and large sizes (0.60-1.00 mm) of PEMP promoted NO3--N leaching and inhibited the WSK leaching from the soil before the addition of KNO3, however, they both inhibited the leaching of NO3--N and WSK from the soil after addition of KNO3.

Microplastic migration and distribution in the terrestrial and aquatic environments: A threat to biotic safety.

Plastics production has been increasing over years, while their recycling rate is lower, resulting in huge amounts of microplastics (MP) accumulating in the environment. Although the environmental behaviors of MPs have been focused on in recent years, the migration, distribution and adverse effects of MPs in terrestrial and aquatic environments are still not systematically understood. In this review, based on the newest publications from the core database of the Web of Science, both the migration and distribution of MPs were summarized, as well as MPs transfer in biota and their biological effects were also focused on. Generally, the complicated and numerous pathways of MPs migration lead to their distribution throughout or nearly all environments on a global scale. However, the migration mechanisms of MPs with various sizes, shapes, and colors by physicochemical and biological processes, and the prediction models of MP migration and distribution, are deficient, despite these properties being highly related to MPs migration and bio-safety. Although MPs have already invaded microorganisms, plants, animals, and even human beings, the biological effects still need more study, so far as their sizes and shapes and also their composition and adsorption are concerned. Moreover, based on the highlights and deficiencies of current studies, further studies have also been proposed. This review aims to help people re-evaluate the uncertain behaviors of MPs in various environments, and could be helpful to fully understand their biological effects in different environmental conditions.

Heterogeneity of plastic residue was determined by both mulch film and external plastic pollutants in the farmland of Northeast China.

Plastic pollution in farmland ecosystems has been widely concerned. However, the heterogeneity and driving mechanisms of plastic residue (PR) remain unclear in the farmland surrounded by complex pollution points. In this study, the abundance, mass, and accumulation areas of PR of mulch film (MF) and non-MF (NMF) were investigated in a large area of the vegetable field covered by plastic mulching in a long-term in Northeast China. Geostatistics combined with classical statistics were used to clarify how pollution source and migration factors change the PR heterogeneity in the farmlands. Results indicated that the MF type was only polyethylene (PE) (79.1 % of total PR), while NMF accounted for 20.9 % of total PR. As well, NMF-polypropylene (PP) and NMF-PE accounted for 45.3 % and 39.7 % of total NMF respectively, followed by polystyrene accounting for 7.5 %. In the 0-20 cm soil layer, the spatial autocorrelation of mass and accumulation areas of MF were significantly (p < 0.05) positive, and their spatial pattern tended to cluster. The accumulation areas of MF was predoniment in northwest and southeast near the roadside in all soil layers, while the accumulation areas of NMF was higher near the landfill in the 0-20 cm soil layer. Landfill and residential areas were critical sources of PR for the farmland. Southwest wind and southeast wind were the main driving force of PR migration and their annual migration rates were 23.7 and 19.8 m·year-1. The functional groups on the surface of plastics were different after degradation (including different types and different utilization methods), and PR was oxidized could release or adsorb toxic substances from the soils. Generally, in order to reduce PR pollution, in addition to source control and recycling, farmland tillage should be avoided in the weather when the wind speed is strong, especially near the PR pollution source.

Field management changes the distribution of mesoplastic and macroplastic in Mollisols of Northeast China.

Mesoplastic (MaP) and macroplastic (MeP) coming from plastic mulching tend to cause negative effects on biota in ecosystems. However, it is still not clear how field management influences the distribution of MeP/MaP in soils. In this study, MeP/MaP was investigated in 0-20 and 20-30 cm soil layers of three vegetable fields (3.4-6.5 ha) after 13 years plastic-mulching in Mollisols of Northeast China under different management methods (MM) of fertilization and tillage frequency. The tillage frequency was MM2 > MM1 > MM3, while the fertilization was MM1 > MM2 > MM3. The results showed that polyethylene (PE), polypropylene (PP), polystyrene, polyvinyl chloride, polyethylene terephthalate (PET), polyamide, melamine-formaldehyde resin and polyether urethane were found in soil, and PE (>83.76%, from plastic mulching) was the predominant type of MeP/MaP. MeP abundance was significantly (p < 0.05) higher in MM1 and MM2 than that in MM3 in the 0-20 cm soil layer. MM1 and MM2 had the highest abundance of MeP/MaP of size <4 cm2 and 4-16 cm2, while MM3 had the highest abundance at the size >16 cm2. The broken index of MeP/MaP was significantly (p < 0.05) lower in MM2 compared with MM1 and MM3 in the 20-30 cm soil layer. Both tillage frequency and fertilization accelerate the breaking of plastics, especially since the influence was stronger from fertilization. Compared with original plastics, the PE, PP and PET's carbonyl index was significantly (p < 0.05) higher in the three MMs. Generally, fertilization and frequent tillage can reduce the physical effects of large-sized plastic debris on crop growth and increases the negative effects of small-sized plastic and new pollutants formed on biota in the agroecosystems. MeP/MaP recycling should be strengthened, and the irrigation and rotation of farmland should be carried out when the wind speed is weak to avoid plastic invasion.

Crops Change the Morphology, Abundance, and Mass of Microplastics in Mollisols of Northeast China.

Degradation of microplastics (MPs) by both physicochemical and biological processes in the natural environment is determined by the enzymes inside the soil, and which was severely influenced by crop growth and straw amendment (SA). However, it is still unclear how crop growth and SA influence degradation of MPs in soils. In this study, both catalase and sucrase were measured, and the stereomicroscope combined with microscopic infrared spectroscopy and scanning electron microscope (SEM) was used to detect the morphology and quantity of low-density polyethylene microplastic (LDPE-MP) and low-density polypropylene microplastic (LDPP-MP), after crop growth (maize and soybean, with and without SA, 1 and 2% MP) in an outdoor pot experiment, in the Mollisols. The results showed that the growth of the crops changed the morphology, functional groups (e.g., methylene, carbonyl), total mass, and abundance ratio of MPs of different sizes. These were possibly caused by enzymes that were significantly influenced by crop types, abundance, and types of MPs in the soils. Maize growth decreased the mass of LDPE-MP and LDPP-MP by 28.7 and 32.7%, respectively, and 2% (w/w) of LDPP-MP addition in soil decreased mass of 9%, which was higher than that in 1% (w/w) LDPP-MP addition in soil. Soybean growth with SA decreased the mass of LDPE-MP and LDPP-MP by 36.6 and 20.7%, respectively, than the control treatment (CK). Compared with CK, both crop growth and SA changed the abundance of MPs of different sizes and decreased the mean size of MPs. The LDPE-MP could be more easily degraded by enzymes in the soils compared to LDPP-MP when the MP size was smaller with surface roughness. Generally, both maize and soybean growth can accelerate MP change in soils, and MP change process was mainly determined by SA, MP types, and the dose effect of MP.

Key factors determining soil organic carbon changes after freeze-thaw cycles in a watershed located in northeast China.

The transformation and migration process of soil organic carbon (SOC) could be changed during freeze-thaw cycles (FTCs) and may further affect the SOC distribution in the watershed. In this study, both field investigation and lab incubation combined with geostatistics were used to clarify how environmental factors influence the SOC heterogeneity in Mollisol after FTCs, from a watershed in northeast China. The results showed that after FTCs, SOC decreased in 68.5% of the total watershed area at 0-20 cm soil depth, and the mean value decreased by 7.4%. Spatial autocorrelation (Moran's І) decreased in the 0-5 cm and 10-20 cm soil depths after FTCs (P < 0.01), but did not change in 5-10 cm and 0-20 cm soil depths. SOC increased at the top slope positions, the watershed outlet, and the upper slope position of the intersection area between farmland and forestland. The SOC decrease was 25 times and 14 times greater at the 0-5 cm and 0-20 cm soil depths respectively in the forestland than in the farmland. The SOC decrease was significantly higher on the 6-8° slopes than on the 0-2° slopes in 0-20 cm soil depth. SOC increased in most areas of cross-slope tillage (CST), but decreased in most areas of downslope tillage. The increase of SOC (positive change) decreased with increasing soil depth under soybeans field, while the decrease of SOC (negative change) increased with increasing soil depth under corn. Topographical factors alone, and topographical factors combined with land use types all influenced the SOC change in this watershed. High levels of ferrous minerals tended to reduce the rate of SOC after FTCs. SOC change is positively correlated to soil bulk density, FTC frequency and soil moisture after FTCs at the soil depth of 0-30 cm. Equations based on soil properties before FTCs, topographical factors, sediment transport index, runoff intensity index, and tillage method can be used for coarsely predicting SOC distribution after FTCs (45% < R2 < 78%, P < 0.01). Generally, SOC dynamics were mainly determined by topography, land use, and to tillage methods that possibly attributes to soil and water loss during FTCs. Both erosion caused by snowmelt runoff and vertical migration of SOC could be the key factors that changed the SOC spatial pattern. CST was beneficial to conserve SOC during FTCs, while forestland could reduce SOC loss by reducing snowmelt erosion.

Non-biodegradable microplastics in soils: A brief review and challenge.

Non-biodegradable microplastics (MPs) pollution long-termly existed in soils, and was only concerned in recent years. In order to better understand MP behavior in soils, the sources, migration, distribution, biological effects, degradation and analytical methodology of non-biodegradable MPs in soils were quantificationally summarized from 170 publications based on Web of Science in 1950-2020. From the publications, we found these studies were mainly carried out in the Asia (60.0%) and Europe (23.3%), and most were on agricultural soils (68.5%). Polyethylene-MP (78.8% of the studies), Polypropylene-MP (78.8%), and Polystyrene-MP (45.5%) were the MPs most frequently found in the soils, with a MP size of 20-5000 µm being most common. Of the soil samples 64.3% contained MP 1000-4000 items kg-1, and the colour frequency ranking is blue (66.7%) > white (61.1%) ≈ red ≈ black. MPs changed the soil microenvironment and microorganism activity, and caused the negative effects on both soil animals (100%) and plants (57.9%). MP degradation was influenced by the photooxidation reactions, microorganism activities, enzymatic effects, environmental conditions, and by the composition, size and morphology of the MPs. An optional analytical method was suggested in this study. At the end of paper, the urgent and important research work in the future was prospected.

Effects of freeze-thaw cycles on the spatial distribution of soil total nitrogen using a geographically weighted regression kriging method.

Freeze-thaw cycles (FTCs) change the soil physicochemical properties and biogeochemical cycles and possibly also change the spatial heterogeneity of soil total nitrogen (TN) in the watershed. In this study, 912 soil samples were collected at 0-5 cm, 5-10 cm and 10-20 cm soil depths in the autumn and the spring of next year after FTCs of 2016-2017 and 2017-2018 in a Mollisol watershed (1.86 km2) of northeast China. The field investigations combined with classical statistics and geographically weighted regression kriging (GWRK) were used to explore the spatiotemporal distribution of TN before and after FTCs. Terrain information (e.g., slope aspect) and land management (e.g., tillage method) was main covariates were used for GWRK. The results showed the following. (1) TN decreased by 3.7-5.7% after FTCs at 0-20 cm soil depths at the watershed scale, decreasing more than 60% of the total watershed area. (2) The spatial pattern of TN did not change in the field with slope aspects and tillage methods after FTCs, but it changed with slope steepness and land uses. (3) TN was mainly influenced by snowmelt erosion during FTCs. TN increased in parts of the top slope, at land use intersection, in gully banks and at the watershed outlet. (4) Simulation accuracy of GWRK was higher than ordinary kriging (OK) for predicted TN at 0-20 cm soil depths before and after FTCs. (5) Spatial distribution of soil TN after FTCs can be predicted (R2 = 0.521, p < 0.0001) and validated (R2 = 0.494, p < 0.0001) using the data before FTCs based on GWRK. Generally, to reduce N loss and increase farmland fertility after FTCs, conservational techniques, e.g., tillage and straw amendment, could be used, especially in the middle slope positions. Moreover, fertilization should be appropriately reduced in parts of the watershed after FTCs, especially on the top slope, land use intersection and watershed outlet.

Quantitative studies of gully slope erosion and soil physiochemical properties during freeze-thaw cycling in a Mollisol region.

Gully erosion has been widely studied during the rainy season due to soil loss that seriously reduces arable area and decreases soil quality. However, very few publications have focused on gully slope erosion (GSE) during freeze thaw cycle (FTC). In this study, GSE on both active and stable gullies in Mollisol fields was investigated by 3D-photogrammetry. Soil bulk density (BD), soil moisture (SM), soil temperature (ST), daily maximum difference in soil temperature (MDT), saturated water (SW), field capacity (FC), soil organic carbon (SOC), soil total nitrogen (TN), water-stable soil aggregate (WA), vegetation cover rate (VC), root dry weight (RW), root length (RL), slope length (SL) and slope steepness (SS) were compared before- and after FTCs. The main results are as follows: (1) combined with both front and profile views, 3D photogrammetry can be used to monitor GSE; (2) GSE mainly occurred at the early stage of FTCs (approximately 80%) and was mainly determined by snowmelt of both the gully slope and farmland and was driven by the solar radiation in activity gully; (3) the high ST in surface soil layers (0-5 cm) of active gullies accelerated the GSE; (4) GSE on the active gully slope was 7.3-9.8 times greater than that on the stable gully slopes; (5) the plough pan as the important layer can effectively reduce GSE at upper slope positions in an active gully; (6) low values of VC, BD, SOC, RW, RL and macro-WA and high values of SL, SW and MDT in the middle of the gully slope typically accelerate the GSE; (7) the index SS*SL/VC can be used to predict GSE on Mollisol gully slopes. Generally, GSE was greatest after FTCs compare to the soil loss tolerance in the Mollisol region, especially in the middle slope position of the active gully, and should urgently be controlled.