[Prediction of Spatial Distribution of Heavy Metals in Cultivated Soil Based on Multi-source Auxiliary Variables and Random Forest Model].

Spatial prediction of the concentrations of soil heavy metals (HMs) in cultivated land is critical for monitoring cultivated land contamination and ensuring sustainable eco-agriculture. In this study, 32 environmental variables from terrain, climate, soil attributes, remote-sensing information, vegetation indices, and anthropogenic activities were used as auxiliary variables, and random forest (RF), regression Kriging (RK), ordinary Kriging (OK), and multiple linear regression (MLR) models were proposed to predict the concentrations of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn in cultivated soils. In comparison to those of RK, OK, and MLR, the RF model had the best prediction performance for As, Cd, Cr, Hg, Pb, and Zn, whereas the OK and RK models had highest prediction performance for Cu and Ni, respectively, showing that R2 was the highest, and mean absolute error (MAE) and root mean square error (RMSE) were the lowest. The prediction performance of the spatial distribution of soil HMs under different prediction methods was basically consistent. The high value areas of eight HMs concentrations were all distributed in the southern plain area. However, the RF model depicted the details of spatial prediction more prominently. Moreover, the importance ranking of influencing factors derived from the RF model indicated that the spatial variation in concentrations of the eight HMs in Lanxi City were mainly affected by the combined effects of Se, TN, pH, elevation, annual average temperature, annual average rainfall, distance from rivers, and distance from factories. Given the above, random forest models could be used as an effective method for the spatial prediction of soil heavy metals, providing scientific reference for regional soil pollution investigation, assessment, and management.

[Effects of Supplementation of Different Amendments on Soil Heavy Metals and Enzyme Activities in Coastal Saline Land].

The supplementation of soil amendments may not only improve the soil physical and chemical properties but also lead to the accumulation of heavy metals in soil. This experiment included six treatments:control (CK), organic manure (OM), polyacrylamide+organic manure (PAM+OM), straw mulching+organic manure (SM+OM), buried straw+organic manure (BS+OM), and bio-organic manure+organic manure (BM+OM) to explore the effects of different soil amendments on heavy metals and soil enzyme activities in coastal saline land and the relationship between them. The results revealed that compared with that in the CK treatment, the contents of soil Cr, Cu, Ni, and Pb exhibited an upward trend after the supplementation of soil amendments, among which the SM+OM and PAM+OM treatments had the most significant effects on the contents of soil Cr and Cu, respectively, whereas the BM+OM treatment had the most significant effects on the contents of soil Ni and Pb. Compared with those in the CK treatment, the activities of soil invertase and urease increased significantly following supplementation of soil amendments, and the BM+OM treatment had the best effect. The alkaline phosphatase activity exhibited a slightly upward trend after the supplementation of soil amendments, whereas the catalase activity did not change significantly. The redundancy analysis revealed that the first two axes cumulatively accounted for 70.3% of the variability in enzyme activities, and the importance of single soil heavy metals on soil enzyme activity was as follows:Ni>Cu>Cr>Pb.

[Effect of Spartina alterniflora Invasion on Soil C:N:P Stoichiometry in Coastal Wetland of Hangzhou Bay].

The invasion of Spartina alterniflora poses a great threat to coastal wetland ecosystems. In this study, the stoichiometric characteristics of soil carbon, nitrogen, and phosphorus under a Spartina alterniflora invasion were explored using ANOVA in a coastal wetland in Hangzhou Bay, and the driving coupling relationship between soil environmental factors and soil C:N:P stoichiometric characteristics of the coastal wetland were further explored based on the redundancy analysis (RDA), boosted regression tree (BRT), and partial least squares-structural equation (PLS-SEM) model. The results showed that:① after the invasion of Spartina alterniflora, soil N:P and total nitrogen (TN) in the wetland increased significantly, and with the increase in invasion time, TN and N:P decreased significantly, whereas soil organic carbon (SOC), C:N, and C:P increased significantly. ② The RDA model revealed that the main factors affecting the stoichiometric characteristics of topsoil C:N:P were SOC>electrical conductivity (EC)>TN in winter and SOC>bulk density (BD)>TN in summer. ③ The BRT model showed that under the invasion of Spartina alterniflora, TN was the key factor affecting soil C:N and N:P, and SOC was the key factor affecting C:P. ④ The PLS-SEM model showed that clay and water content directly affected SOC, thus affecting C:N and C:P; the clay and EC directly affected total phosphorus (TP), thus affecting N:P and C:P; and the EC directly affected TN, thus affecting C:N and N:P. In conclusion, the invasion of Spartina alterniflora had a significant impact on soil C:N:P stoichiometric characteristics in the study area. Soil physical properties and nutrient content directly or indirectly affected soil C:N:P stoichiometric characteristics to varying degrees.

[Spatial Heterogeneity of PM2.5 Concentration in Response to Land Use/Cover Conversion in the Yangtze River Delta Region].

The sustainable management direction of PM2.5 concentrations in the Yangtze River Delta region remains unclear due to regional spatial effects. This study combined the random forest model, spatial econometric model, and multi-scale geographically weighted regression model (MGWR) to explore the multi-scale spatial response of PM2.5 concentration to land use/cover conversion. The results show that:① PM2.5 concentrations in the Yangtze River Delta region from 2000 to 2018 showed four types of spatial-temporal patterns of spatially continuous aggregation, with strong regional synchronous changes. ② The relative influence of land conversion on PM2.5 concentrations showed a complex performance, and the source-sink effect of cultivated land and forest land was obvious. Neighborhood analysis indicated that the effect of surrounding aggregated land use conversion was generally more significant than that of single cells on PM2.5 concentration change, and the spatial effect was obvious. ③ PM2.5 concentration changes were mostly significantly negatively correlated with forest land and grassland conversion types and significantly positively correlated with conversion types between cropland, construction land, and water bodies. The importance ranking of the random forest model and correlation coefficient intensity indicated that the conversion between cropland-cropland (29.65%; 0.650), forest land-forest land (26.98%; 0.726), construction land-cropland (22.57%; 0.519), cropland-forestland (17.84%; 0.602), and cropland-construction land (16.34%; 0.424) contributed more to the variation in PM2.5 concentration. The spatial Durbin model revealed a significant spatial dependence of the change in PM2.5 concentration and a strong spatial spillover effect. ④ The MGWR model revealed the scale effects and non-stationary characteristics of the spatial relationships between different land use conversions acting on PM2.5 concentration change, and its spatial relationship showed strong differences in transfer types. However, the multi-models revealed that different land conversions drove the PM2.5 concentration change in different ways, so it is necessary to formulate targeted joint management strategies in a categorical and hierarchical manner.

[Impacts of Spartina alterniflora invasion on coastal wetland ecosystem: Advances and prospects].

Coastal wetland, affected by the interaction of land and marine ecosystems, is a typical fragile and sensitive zone. Spartina alterniflora is the most successful invasive species in global coastal zone, with important impact on coastal wetland ecosystems. We systematically summarized available literature, and reviewed the effects of S. alterniflora invasion on biogeochemical cycling (soil carbon, nitrogen, phosphorus cycling, and soil heavy metal migration) and biological community (microorganisms, plants, and animals) in coastal wetlands. Then, three perspectives were proposed for the future research: 1) strengthening the mechanism of the impact of S. alterniflora invasion on ecosystem health of coastal wetlands; 2) focusing on the coupling mechanism of the interaction between S. alterniflora community and coastal wetland environment in the context of global change; 3) carrying out long-term positioning monitoring to clarify the responses of coastal wetlands at different stages of S. alterniflora invasion. This review could provide guidance for the ecological utilization and management of S. alterniflora.

[Assessment of Soil Quality in Coastal Tidal Flat Reclamation Areas Based on MDS-TOPSIS Model].

Investigating the change of soil quality during reclamation can provide scientific guidance for desalinization, fertilization, etc. Soil samples were collected from natural tidal flat areas (0 years) and coastal tidal flat reclamation areas with different reclamation years (5, 30, 38, and 61 years) to assess the changes in soil quality after 60 years of reclamation. Soil quality was evaluated using a soil quality index (SQI), which was calculated by the selected minimum data set (MDS) and TOPSIS method. The ratio of clay content (CLAY), soil salt content (SSC), soil organic carbon (SOC), total potassium (TK), and bulk density (BD) were selected as soil quality indicators involving soil physical and chemical properties based on the MDS. The results showed that soil particle size tended to refine gradually with the continuous decrease of sand content and increase of silt and clay content. Simultaneously, soil water content (SWC) and soil BD decreased gradually during the reclamation period. Additionally, SSC, pH and sodium adsorption ratio (SAR) decreased significantly with increase in reclamation years, whereas the nutrients and available nutrients were accumulated gradually, such as soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), available nitrogen (AN), and available phosphorus (AP). However, C/N, TK and available potassium (AK) showed a decrease with increasing reclamation years, and the cation exchange capacity (CEC) showed a trend of first decrease and then increase. Finally, SQI values varied from 24.06 in the bare flat soils to 63.08 in the soils after 63 years of reclamation, and showed that bare flat (20.04±11.48) < reclamation for 5 years (29.33±10.65) < reclamation for 30 years (51.52±8.76) ≈ reclamation for 38 years (49.98±10.75) < reclamation for 61 years (58.37±3.15), which has gone through approximately three stages of "initial stability-rapid improvement-relative stability." Soil CLAY content had the highest obstacle degree towards SQI among the selected MDS indicators, followed by SSC, SOM, TK, and SWC, and thereby the lower clay content and higher salt content were the main obstacle factors for promotion of soil quality in reclamation areas.

[Response of Soil Enzyme Activities and Their Relationships with Physicochemical Properties to Different Aged Coastal Reclamation Areas, Eastern China].

Land reclamation transforms a tidal flat from a marine ecosystem to terrestrial ecosystem, which significantly changes the soil ecosystem process, affecting the soil enzyme activities. Soil enzyme is involved in almost all soil biochemical processes. Clarifying the relationship between soil physicochemical properties and soil enzyme activities can reveal the changes of soil quality after reclamation and provide a theoretical basis for the scientific regulation of the soil ecosystem. In this study, a total of 72 soil samples were collected from a coastal reclamation area with different years of reclamation (0, 7, 32, 40, and 63 a) in Rudong County. The changes in soil physicochemical properties and enzyme activities within different reclamation years were analyzed by classical statistical analysis, and the relationship between soil enzyme activities and physicochemical properties was analyzed using a redundancy analysis. The results showed that longterm land reclamation had significant effects on soil physicochemical properties and enzyme activities. pH and electrical conductivity showed a decreasing trend after reclamation, while the content of soil organic carbon, total nitrogen, and total phosphorus showed a trend of "rapidly accumulate-gradually consume-accumulate again" with increasing years of reclamation. Compared with the bare flat area (0 a), amylase, dehydrogenase, and acid phosphatase activity initially increased and then decreased with increasing years of reclamation, while urease and alkaline phosphatase activities were characterized by an increase-decrease-increase trend. Meanwhile, the enzyme activities within different reclamation ages were basically higher in surface soil than deep layers. Redundancy analysis revealed that the soil physicochemical properties explained 69.8% of the variation in soil enzyme activities, and an obvious relationship existed between pH, electronic conductivity, soil organic carbon, total nitrogen, total phosphorus, and soil enzyme activities (P<0.01). The order of importance of soil physicochemical properties to soil enzyme activities was as follows:total nitrogen > soil organic carbon > pH > total phosphorus > electronic conductivity.