A systematic analysis and review of the impacts of afforestation on soil quality indicators as modified by climate zone, forest type and age

A systematic analysis and review of soil organic carbon stocks in urban greenspaces

Urban greenspaces typically refer to urban wetland, urban forest and urban turfgrass. They play a critical role in carbon sequestration by absorbing carbon from the atmosphere; however, their capacity to retain and store carbon in the form of soil organic carbon (SOC) varies significantly. This study provides a systematic analysis and review on the capacity of different urban greenspace types in retaining and storing SOC in 30 cm soil depth on a global scale. Data came from 78 publications on the subject of SOC stocks, covering different countries and climate zones. Overall, urban greenspace types exerted significant influences on the spatial pattern of SOC stocks, with the highest value of 18.86 ± 11.57 kg m-2 (mean ± standard deviation) in urban wetland, followed by urban forest (6.50 ± 3.65 kg m-2), while the lowest mean value of 4.24 ± 3.28 kg m-2 was recorded in urban turfgrass soil. Soil organic carbon stocks in each urban greenspace type were significantly affected by climate zones, management/environmental settings, and selected soil properties (i.e. soil bulk density, pH and clay content). Furthermore, our analysis showed a significantly negative correlation between SOC stocks and human footprint in urban wetland, but a significantly positive relationship in urban forest and urban turfgrass. A positive correlation between SOC stocks and human footprint indicates that increased human activity and development can enhance SOC stocks through effective management and green infrastructure. Conversely, a negative correlation suggests that improper management of human activities can degrade SOC stocks. This highlights the need for sustainable practices to maintain or enhance SOC accumulation in urban greenspaces.

The Long-Term Effects of Barren Land Afforestation on Plant Productivity, Soil Fertility, and Soil Moisture in China: A Meta-Analysis

As global ecological degradation intensifies, the long-term impacts of afforestation on productivity and soil fertility in barren lands have become critical in improving global ecological security and productivity. Through meta-analysis, this study integrates data from 109 barren land afforestation sites across China, aiming to comprehensively analyze the effects on plant productivity and soil fertility while identifying the key environmental drivers of these changes. We found that afforestation consistently enhances plant productivity across 60 years. However, soil fertility and moisture initially surged significantly after afforestation but gradually declined after the first decade, indicating the limited long-term benefits. Climatic factors, namely precipitation and humidity index, are crucial in enhancing plant productivity, while geographic factors, specifically lower elevations and gentler slopes, are associated with greater increases in soil fertility. Elevation and slope are two key factors that influence soil moisture after afforestation. These findings highlight the need for ongoing soil management and ecological maintenance in afforestation projects to sustain the soil fertility benefits. Our study provides a robust scientific foundation for afforestation strategies aimed at barren land restoration and offers valuable insights for policy formulation in barren land afforestation.

An overlooked soil carbon pool in vegetated coastal ecosystems: National-scale assessment of soil organic carbon stocks in coastal shelter forests of China

Protection and restoration of vegetated coastal ecosystems provide opportunities to mitigate climate change. Coastal shelter forests as one of vegetated coastal ecosystems play vital role on sandy coasts protection, but less attention is paid on their soil organic carbon (OC) sequestration potential. Here, we provide the first national-scale assessment of the soil OC stocks, fractions, sources and accumulation rates from 48 sites of shelter forests and 74 sites of sandy beaches across 22° of latitude in China. We find that, compared with sandy beaches, shelter forest plantation achieves an average soil desalination rate of 92.0 % and reduces the soil pH by 1.3 units. The improved soil quality can facilitate OC sequestration leading to an increase of soil OC stock of 11.8 (0.60-64.2) MgC ha^-1 in shelter forests. Particulate OC (POC) is a dominant OC fraction in both sandy beaches and shelter forests, but most sites are >80 % in shelter forests. The low δ¹³C values and higher C:N ratios, which are more regulated by climate and tree species, together with high POC proportions suggest a substantial contribution of plant-derived OC. Bayesian mixing model indicates that 71.8 (33.5-91.6)% of the soil OC is derived from local plant biomass. We estimate that soil OC stocks in Chinese shelter forests are 20.5 (7.44-79.7) MgC ha^-1 and 4.53 ± 0.71 TgC in the top meter, with an accumulation rate of 45.0 (6.90 to 194.1) gC m^-2 year^-1 and 99.5 ± 44.9 GgC year^-1. According to coastal shelter forest afforestation plan, additional 1.72 ± 0.27 TgC with a rate of 37.9 ± 17.1 GgC year^-1 can be sequestrated in the future. Our findings suggest that construction of coastal shelter forests can be an effective solution to sequester more soil carbon in coastal ecosystems.

Changes in Organic Carbon Stock in Soil and Whole Tree Biomass in Afforested Areas in Latvia

This study investigates the soil organic carbon (SOC) and whole tree biomass carbon (C), soil bulk density (BD) as well as changes in these parameters in afforested areas in Latvia. The study covered 24 research sites in afforested areas-juvenile forest stands dominated by Scots pine, Norway spruce and Silver birch. The initial measurements were conducted in 2012 and repeated in 2021. The results show that afforestation mostly leads to a general decrease in soil BD and SOC stock in 0-40 cm soil layer and an increase in C stock in tree biomass across afforested areas with various tree species, soil types, and former land uses. The physical and chemical properties of the soil could explain the differences in changes in soil BD and SOC caused by afforestation, as well as the impact of past land use may have persisted. When comparing the changes in SOC stock with the increase in C stock in tree biomass due to afforestation, taking into account the decrease in soil BD and the resulting elevation of soil surface level, the afforested areas at juvenile development stage can be considered a net C sink.

Effect of different vegetation restoration on soil organic carbon dynamics and fractions in the Rainy Zone of Western China

Vegetation restoration on purple soil (Eutric Leptic Regosols) slopes aiming at reducing soil erosion in the Rainy Zone of Western China has significantly altered soil organic carbon (SOC) storage and distribution. A better understanding of the effects of different vegetation restoration types on SOC dynamics and fractions is critical in devising better policy to protect or enhance SOC stocks to improve soil quality and ecosystem function. In the present study, total, labile, and non-labile organic carbon (TOC, LC, and NLC), and carbon management index (CMI) of Cryptomeria fortunei (CF), mixed C. fortunei and Betula luminifera (MF), Neosinocalamus affinis (NA), and Camellia sinensis (CS) were compared with those of Zea mays field (ZM) on purple soil slopes in the Rainy Zone of Western China in order to develop more effective ways to implement vegetation restoration in the future. Different vegetation restoration types (CF, MF, NA and CS) increased TOC stock by 47.79%-118.31% and NLC stock by 56.61%-129.52% in the 0-50 cm soil layer compared with that of ZM. The direction and magnitude of changes in LC stock and CMI, however, depended strongly on the vegetation restoration type. Compared with ZM, CF had the largest increase of LC stock and CMI, whereas NA had the largest decrease of LC stock and CMI in the 0-50 cm soil layer. The LC:TOC ratio in four reforested species all declined significantly compared with that of ZM (p < 0.01), indicating decreased SOC activity after afforestation. The vegetation type and soil depth together explained more than 90% of the changes of TOC and its fractions in the plantations on purple soil slopes. Our study demonstrates that transforming the ZM into the CS is optimal to achieve the sustainable development goal, whereas transforming the ZM into the NA reduces the SOC activity and availability.

Horizontal and vertical variation of soil clay content and its controlling factors in China

Detailed information on the variation of soil clay content at different soil depths are important for water repellency, management of soil fertility, and crop growth. However, studies on the variation of soil clay content at different depths at large scales are limited. In this study, 1446 soil profiles covering most areas of China were extracted from the World Soil Information Service (WoSIS) database and classified into 0-20, 20-60, and 60-100 cm depths using the equal-area quadratic splines. Random forest (RF) and Shapley additive explanations (SHAP) were used to investigate the relationship between soil clay content and environmental factors (climate, topography, land use, and soil types). The results showed that soil clay content and its variability increased with soil depth. The RF model predicted soil clay content at the three soil depths. The mean absolute error (MAE), root mean square error (RMSE), coefficient of determination (R²), and Lin's concordance correlation coefficient (CCC) ranged from 6.95 % to 8.74 %, 8.90 % to 11.30 %, 42 % to 46 %, and 56 % to 59 %, respectively. Mean annual temperature (MAT), mean annual precipitation (MAP), solar radiation (solarR), and elevation were the key factors that controlled the variation of soil clay content at depths of 0-20, 20-60, and 60-100 cm. In addition, the main effect values of SHAP can identify environmental thresholds consistent with significant areas by capturing the relationship between soil clay content and MAT, MAP, solarR, and elevation. Significant differences were found in soil clay content on both sides of the threshold at MAT of 15 °C, MAP of 800 mm, solarR of 15,000 Jm^-2 yr^-1, and elevation of 1200 m. The results provide clues to the management of national soil security under global and regional climate change.

Stream water quality optimized prediction based on human activity intensity and landscape metrics with regional heterogeneity in Taihu Basin, China

The driving effects of landscape metrics on water quality have been acknowledged widely, however, the guiding significance of human activity intensity and landscape metrics based on reference conditions for water environment management remains to be explored. Thus, we used the self-organized map, long- and short-term memory (LSTM) algorithm, and geographic detectors to simulate the response of human activity intensity and landscape metrics to water quality in Taihu Lake Basin, China. Fitting results of LSTM displayed that the accuracy was acceptable, and scenario 2 (regional heterogeneity) was more efficient than scenario 1 (regional consistent) in the improvement of water quality. In the driving analysis for the reference conditions, clusters I and II (urban agglomeration areas) were mainly affected by the amount of production wastewater per unit of developed land and the amount of livelihood wastewater per unit of developed land, respectively. Their optimal values were 0.09 × 10³ t/km² (reduction of 35.71%) and 0.2 × 10³ t/km² (reduction of 4.76%). Cluster III (agricultural production areas) was mainly affected by interference intensity, and the optimal value was 2.17 (increased 38.22%), and cluster IV (ecological forest areas) was mainly affected by the fragmentation of cropland, and the optimal value was 1.14 (reduction of 1.72%). The research provides a reference for the prediction of water quality response and presents an ecological and economic sustainability way for watershed governance.

Understory Plant Abundance Is More Important than Species Richness in Explaining Soil Nutrient Variation Following Afforestation on the Eastern Loess Plateau, China

Afforestation plays an important role in controlling soil erosion and nutrient loss on the Loess Plateau in China. However, previous studies on soil nutrient changes have mainly focused on the effects of tree species, whereas little is known about how changes in shrubs and herbs caused by afforestation drive soil nutrient changes. In this study, we examined the variation characteristics of understory vegetation and soil nutrients for different vegetation types. The results showed that compared to abandoned farmland, plantations significantly increased soil organic carbon and total nitrogen but had no significant effect on total phosphorus. Robinia pseudoacacia L. forests were more effective than Pinus tabuliformis Carr. forests in increasing soil nutrient content. In addition, herbaceous vegetation in the R. pseudoacacia forest better explained the soil nutrient variation, and herb abundance was the best explanatory variable; however, shrub vegetation contributed more to soil nutrient variation in the P. tabuliformis forest, and shrub abundance contributed the most. Accordingly, we determined that understory plant abundance, rather than species richness, may be the most important factor driving soil nutrient changes. Specifically, herb abundance in the R. pseudoacacia forest may drive soil nutrient changes mainly by regulating herb biomass and litter biomass. By contrast, shrub abundance in the P. tabuliformis forest indirectly affected soil organic carbon mainly by altering shrub biomass. Furthermore, although the phylogenetic relationships had less effect on soil nutrients than species composition, they also made important contributions. Therefore, the phylogenetic relationships should also be considered in addition to species composition when assessing the impact of vegetation on soil properties in the future.

Changes in Soil Physico-Chemical and Microbiological Properties During Natural Succession: A Case Study in Lower Subtropical China

Vegetation succession can change the function and quality of the soil. Exploring the changes in soil properties during secondary forest restoration is of great significance to promote forest restoration and improve the ecological service function of subtropical ecosystems in South China. In this study, we chose three typical forests in subtropical China as restoration sequences, broadleaf-conifer mixed forest (EF), broad-leaved forest (MF), and old-growth forest (LF), to study the changes in soil physico-chemical and biological properties and the changes of soil comprehensive quality during the secondary succession of subtropical forest. The results showed that the soil physical structure was optimized with the progress of forest succession. The soil bulk density decreased gradually with the progress of forest restoration, which was significantly affected by soil organic carbon (p < 0.01). In LF, the soil moisture increased significantly (p < 0.05), and its value can reach 47.85 ± 1.93%, which is consistent with the change of soil porosity. With the recovery process, soil nutrients gradually accumulated. Except for total phosphorus (TP), there was obvious surface enrichment of soil nutrients. Soil organic carbon (15.43 ± 2.28 g/kg), total nitrogen (1.08 ± 0.12 g/kg), and total phosphorus (0.43 ± 0.03 g/kg) in LF were significantly higher than those in EF (p < 0.05). The soil available nutrients, that is, soil available phosphorus and available potassium decreased significantly in LF (p < 0.05). In LF, more canopy interception weakened the P limitation caused by atmospheric acid deposition, so that the soil C:P (37.68 ± 4.76) and N:P (2.49 ± 0.24) in LF were significantly lower than those in EF (p < 0.05). Affected by TP and moisture, microbial biomass C and microbial biomass N increased significantly in LF, and the mean values were 830.34 ± 30.34 mg/kg and 46.60 ± 2.27 mg/kg, respectively. Further analysis showed that total soil porosity (TSP) and TP (weighted value of 0.61) contributed the most to the final soil quality index (SQI). With the forest restoration, the SQI gradually increased, especially in LF the value of SQI was up to 0.84, which was significantly higher than that in EF and MF (p < 0.05). This result is of great significance to understanding the process of restoration of subtropical forests and improving the management scheme of subtropical secondary forests.

Variability in Soil Macronutrient Stocks across a Chronosequence of Masson Pine Plantations

Plantations play a vital role in the global nutrient cycle because they have large stocks of soil macronutrients. However, the impacts of plantations on soil macronutrient stocks combined with stand age and soil physicochemical properties have not been well quantified. We compared soil macronutrient stocks at soil depths of 0−20 and 20−40 cm across a 7-, 14-, 25-, and 30-year chronosequence of Masson pine (Pinus massoniana Lamb.) plantations. The results showed that the nitrogen (N), phosphorus (P), and potassium (K) stocks first increased and then decreased with stand age. The highest N and P stocks were observed in the 14-year-old plantation, and the 25-year-old plantation displayed the highest K stock. The C, N, and P stocks declined with increasing soil depth across all sites, whereas the reverse trend was found in the K stock. Carbon stocks were highest for all plantations, followed by the K, N, and P stocks. Plantation soils exhibited a higher C:P ratio and a lower P:K ratio at various soil depths. The dominant controlling factors for the soil macronutrient stocks varied significantly at different stand ages and soil depths according to statistical analysis. For the total soil system, the C stock was affected by the available nutrients, organic matter, and stoichiometry; the available nutrients and organic matter were the determinant factors of the N and P stocks. Aggregate stability could be the primary parameter affecting the K stock. Organic matter explained most of the variation in soil macronutrient stocks, followed by the P:K ratio and available K. Collectively, our results suggest that the response of soil macronutrient stocks to stand age and soil depth will be dependent on different soil physicochemical properties, and P and K may be important limiting factors in Masson pine plantation ecosystems.

Soil Organic Carbon Stocks in Afforested Agricultural Land in Lithuanian Hemiboreal Forest Zone

In the context of the specificity of soil organic carbon (SOC) storage in afforested land, nutrient-poor Arenosols and nutrient-rich Luvisols after afforestation with coniferous and deciduous tree species were studied in comparison to the same soils of croplands and grasslands. This study analysed the changes in SOC stock up to 30 years after afforestation of agricultural land in Lithuania, representing the cool temperate moist climate region of Europe. The SOC stocks were evaluated by applying the paired-site design. The mean mass and SOC stocks of the forest floor in afforested Arenosols increased more than in Luvisols. Almost twice as much forest floor mass was observed in coniferous than in deciduous stands 2–3 decades after afforestation. The mean bulk density of fine (<2 mm) soil in the 0–30 cm mineral topsoil layer of croplands was higher than in afforested sites and grasslands. The clear decreasing trend in mean bulk density due to forest stand age with the lowest values in the 21–30-year-old stands was found in afforested Luvisols. In contrast, the SOC concentrations in the 0–30 cm mineral topsoil layer, especially in Luvisols afforested with coniferous species, showed an increasing trend due to the influence of stand age. The mean SOC values in the 0–30 cm mineral topsoil layer of Arenosols and Luvisols during the 30 years after afforestation did not significantly differ from the adjacent croplands or grasslands. The mean SOC stock slightly increased with the forest stand age in Luvisols; however, the highest mean SOC stock was detected in the grasslands. In the Arenosols, there was higher SOC accumulation in the forest floor with increasing stand age than in the Luvisols, while the proportion of SOC stocks in mineral topsoil layers was similar and more comparable to grasslands. These findings suggest encouragement of afforestation of former agricultural land under the current climate and soil characteristics in the region, but the conversion of perennial grasslands to forest land should be done with caution.