A meta-analysis reveals increases in soil organic carbon following the restoration and recovery of croplands in Southwest China.

In China, the Grain for Green Program (GGP) is an ambitious project to convert croplands into natural vegetation, but exactly how changes in vegetation translate into changes in soil organic carbon remains less clear. Here we conducted a meta-analysis using 734 observations to explore the effects of land recovery on soil organic carbon and nutrients in four provinces in Southwest China. Following GGP, the soil organic carbon content (SOCc) and soil organic carbon stock (SOCs) increased by 33.73% and 22.39%, respectively, compared with the surrounding croplands. Similarly, soil nitrogen increased, while phosphorus decreased. Outcomes were heterogeneous, but depended on variations in soil and environmental characteristics. Both the regional land use and cover change indicated by the landscape type transfer matrix and net primary production from 2000 to 2020 further confirmed that the GGP promoted the forest area and regional mean net primary production. Our findings suggest that the GGP could enhance soil and vegetation carbon sequestration in Southwest China and help to develop a carbon-neutral strategy.

Global Responses of Soil Carbon Dynamics to Microplastic Exposure: A Data Synthesis of Laboratory Studies.

Microplastics (MPs) contamination presents a significant global environmental challenge, with its potential to influence soil carbon (C) dynamics being a crucial aspect for understanding soil C changes and global C cycling. This meta-analysis synthesizes data from 110 peer-reviewed publications to elucidate the directional, magnitude, and driving effects of MPs exposure on soil C dynamics globally. We evaluated the impacts of MPs characteristics (including type, biodegradability, size, and concentration), soil properties (initial pH and soil organic C [SOC]), and experimental conditions (such as duration and plant presence) on various soil C components. Key findings included the significant promotion of SOC, dissolved organic C, microbial biomass C, and root biomass following MPs addition to soils, while the net photosynthetic rate was reduced. No significant effects were observed on soil respiration and shoot biomass. The study highlights that the MPs concentration, along with other MPs properties and soil attributes, critically influences soil C responses. Our results demonstrate that both the nature of MPs and the soil environment interact to shape the effects on soil C cycling, providing comprehensive insights and guiding strategies for mitigating the environmental impact of MPs.

Effects of microplastics exposure on soil inorganic nitrogen: A comprehensive synthesis.

Microplastics, a growing environmental concern, impact soil inorganic nitrogen (N) transformation, specifically affecting water-extractable nitrate N (NO3--N) and ammonium N (NH4+-N). However, inconsistencies among relevant findings necessitate a systematic analysis. Accordingly, the present meta-analysis addresses these discrepancies by evaluating the effects of microplastics on soil inorganic N and identifying key influencing factors. Our meta-analysis of 216 paired observations from 47 studies demonstrates microplastics exposure causes an overall significant reduction of 7.89% in soil NO3--N concentration, but has no significant impact on NH4+-N concentration. Subgroup analysis further revealed effects of microplastics on soil inorganic N were modulated by microplastics characteristics, experimental conditions (exposure time, experimental temperature, plant effects), and soil properties (soil texture, initial soil pH, initial soil organic carbon, soil total N concentration). We found that microplastics exposure above 27 â„ƒ enhances soil NO3--N concentration, a finding linked to specific soil properties and conditions, underscoring the impacts of global warming. Importantly, the microplastics polymer type was the most influential predictor of effects on soil NO3--N concentration, while soil NH4+-N concentration was primarily affected by soil texture and microplastics type. These findings illuminate the complex effects of microplastics on soil inorganic N, informing soil management amid increasing microplastics pollution.

Biochar addition increased soil bacterial diversity and richness: Large-scale evidence of field experiments.

Biochar plays a crucial role in enhancing soil ecological functions and productivity, and mitigating environmental pollution. Despite the available studies conducted through high-throughput sequencing to understand the effects of biochar on soil bacterial diversity and richness, a comprehensive understanding remains elusive. Our global meta-analysis addresses this knowledge gap, incorporating 473 pairs of observations from 84 studies to assess soil bacterial diversity and richness response to biochar addition. We found that biochar application increased bacterial Shannon and Chao1 indices by 0.99 % and 6.45 % respectively. However, these positive effects were context-dependent, especially concerning bacterial diversity. Through aggregated boosted trees analysis, we determined that soil characteristics (including soil organic carbon and nitrogen contents, soil pH, and soil texture) had a more significant influence than biochar properties, experimental conditions, or climatic factors on soil bacterial diversity and richness post biochar addition. In particular, the soil carbon to nitrogen ratio stood out as the leading factor influencing the bacterial Shannon and Chao1 indices following biochar addition. Our findings offer new insights into biochar's influence on soil bacterial activity, taking into account biochar-mediated spatiotemporal variation. This information is pivotal for optimizing biochar characteristics and application to improve soil biological health.

Manure properties, soil conditions and managerial factors regulate greenhouse vegetable yield with organic fertilizer application across China.

To better understand the responses of vegetable yields in a greenhouse system to organic fertilizer through a quantitative evaluation based on peer-reviewed journal articles and in consideration of environmental managerial factors. We conducted a meta-analysis of 453 paired observations from 68 peer-reviewed journal articles to assess the response of vegetable yields in greenhouse vegetable systems in China to organic fertilization. Compared with the control (no organic fertilizer), organic fertilization significantly increased the yields of vegetables by 44.11% on average. The response of vegetable yields to organic fertilizer tended to increase with the increasing experimental duration. Organic fertilizer application had the greatest potential for leafy vegetables (+76.44%), in loamy soils (+53.94%), at moderate organic fertilizer carbon input levels (+54.13%), and in soils with moderate initial soil total nitrogen levels (+50.89%). Aggregated boosted tree analysis indicated that organic fertilizer carbon inputs, vegetable type and experimental duration were the predominant factors that manipulated the response of vegetable yields to organic fertilizer application. The rational application of farmyard manure would be a promising strategy for increasing vegetable yields in greenhouse vegetable systems in China. Factoring in vegetable type, carbon and nitrogen inputs of organic fertilizer, and soil texture would benefit vegetable yields with the application of organic fertilizer.

Ecosystem restoration and belowground multifunctionality: A network view.

Ecological restoration is essential to reverse land degradation worldwide. Most studies have assessed the restoration of ecosystem functions individually, as opposed to a holistic view. Here we developed a network-based ecosystem multifunctionality (EMF) framework to identify key functions in evaluating EMF restoration. Through synthesizing 293 restoration studies (2900 observations) following cropland abandonment, we found that individual soil functions played different roles in determining the restoration of belowground EMF. Soil carbon, total nitrogen, and phosphatase were key functions to predict the recovery of belowground EMF. On average, abandoned cropland recovered ~19% of EMF during 18 years. The restoration of EMF became larger with longer recovery time and higher humidity index, but lower with increasing soil depth and initial soil carbon. Overall, this study presents a network-based EMF framework, effectively helping to evaluate the success of ecosystem restoration and identify the key functions.

Mixed plantations enhance more soil organic carbon stocks than monocultures across China: Implication for optimizing afforestation/reforestation strategies.

Forests play an essential role in mitigating climate change by sequestering carbon dioxide from the atmosphere. The establishment of mixed plantations is a promising way to store carbon (C) in soil compared with monocultures. However, monoculture forests largely dominate the rapid increase in forest areas in China. To optimize afforestation strategies and maximize the subsequent potential of C sequestration, we conducted a meta-analysis with 427 observations across 176 study sites in China. The goal was to quantify changes in the stocks of soil organic carbon (SOC) in mixed plantations compared with monocultures and to identify the predominant drivers for the stocks of SOC, including geological location, climatic factors, land use history, edaphic properties, plantation age, the inclusion of nitrogen-fixing trees, mixing proportion, and mixed plant types. The results showed that mixed plantations significantly increased the SOC stocks by 12% compared with monocultures, and the mixing proportion should not exceed 55% to produce higher SOC stocks in mixed plantations compared with monoculture. Additionally, mixed plantations in barren land are the most likely to increase the SOC stocks with limited water or low temperatures for growth. Additional measures instead of mixed plantations should be explored to increase SOC stocks in north, central, and northwest China. The data from this study demonstrated the spatiotemporal variability on the storage of SOC driven by mixed trees and has valuable implications for the establishment and management of afforestation.

Factors shaping soil organic carbon stocks in grass covered orchards across China: A meta-analysis.

Orchard grass coverage has been widely adopted to increase fruit yield by improving soil fertility. However, the impact of the environment on the changes in soil organic carbon (SOC) stocks consecutive to orchard grass coverage remain poorly quantified at a large scale. The present study aimed to examine the responses of SOC stocks to grass coverage at a soil depth of 0-30 cm in orchards compared to clean tillage. A total of 342 observations across China from 139 peer-reviewed publications were subjected to meta-analysis. Aggregated boosted tree analysis was performed, evaluating the determinants of SOC stocks, such as plant traits (e.g., fruit tree type, grass type, orchard age, and grass age), edaphic variables (e.g., initial SOC and nitrogen concentration, soil pH, and soil clay content), climatic factors (e.g., mean annual precipitation (MAP) and mean annual temperature (MAT)), and management practices (e.g., grass source, grass growing mode, fertilization, grass mowing, placement of mowed residues, and irrigation). On average, orchard grass coverage significantly enhanced SOC stocks by 21.47% (percentage change) compared to clean tillage. Biotic and abiotic factors influenced this increase in SOC stocks following grass coverage in orchards to different extents. Grass age and soil clay content were the main determinants driving the variation in the SOC stocks following grass coverage in orchards. Thus, we propose an efficient way to optimize C sequestration in grass covered orchards, regarding plant traits, climatic factors, edaphic variables, and management practices. Longer than 12 months of surface grass coverage with cultivated grass species in mature deciduous fruit orchards (≥5 years) efficiently increased SOC stocks. This is particularly the case for acidic (pH < 6.5) soils with low C content (SOM < 15 g kg-1) in areas with suitable rainfall and temperature conditions (MAP ≥ 400 mm, MAT ≥ 10 °C). Collectively, this meta-analysis identified orchard grass coverage as a promising practice for significantly increasing SOC stocks at 0-30 cm across large geospatial locations in China.

Effects of dominant plant growth on the nutrient composition and bacterial community structure of manganese residues.

The rhizospheres of three dominant plant species (Miscanthus floridulus, Buddleja lindleyana, and Erigeron annuus) growing in manganese residue disposal sites in eastern Guizhou Province, China, were analyzed to study the effects of plant growth on the nutrient levels and bacterial community structure of two types of manganese residues. The results showed that the growth of the three species improved the nutritional composition of manganese residues; the available nitrogen (AN) contents of the manganese mine residue significantly increased by 29.56-60.78% while the available phosphorus (AP) contents of the electrolytic manganese residue significantly increased by 30.24-44.41% compared to those in unvegetated manganese residue. The diversity of the bacterial community in the manganese mine residue increased significantly due to plant growth. Proteobacteria, Acidobacteria, and Bacteroidetes were the dominant phyla in both manganese residues. Sphingomonas and GP6 were the dominant bacterial genera. The relative abundance of the Firmicutes phylum was significantly higher in the manganese mine residue than in the control and that of the Thiobacillus genus was lower, which indicated improvements in the microenvironment. Correlation analysis showed that OM and AN were the main nutrient factors affecting the bacterial community structure in the manganese mine residue.Novelty statement At present, research on the phytoremediation of manganese residue disposal sites focuses mostly on the investigation of different plant types and their heavy metal accumulation and transformation characteristics. However, comparative studies of the differences in growth matrix characteristics between plant growth areas and exposed areas are lacking. In addition, dominant plant species are regionally distributed. The previous studies were mostly concentrated in Chongqing, Guangxi, and Hunan in China. The eastern region of Guizhou Province is located in the "Manganese Triangle" area of China, where the manganese resources account for about 50% of the national total. There is no report on the phytoremediation of manganese residue disposal sites in this region. Therefore, the rhizospheres of three dominant plant species (Miscanthus floridulus, Buddleja lindleyana, and Erigeron annuus) growing in manganese residue disposal sites in eastern Guizhou Province, China, were analyzed to study the effects of plant growth on the nutrient levels and bacterial community structure of two types of manganese residues (manganese mine residue and electrolytic manganese residue). This study could provide useful theoretical information to benefit the ecological restoration of manganese residue disposal sites.

Biochar amendment effects on the activities of soil carbon, nitrogen, and phosphorus hydrolytic enzymes: a meta-analysis.

The aim of this meta-analysis was to synthesize the effects of biochar amendment on soil enzyme activities (SEAs) related to carbon (C), nitrogen (N), and phosphorus (P) cycling. Based on 401 paired comparisons from 43 published studies, the SEAs and main influential factors were analyzed in response to biochar characteristics, soil properties, and experiment conditions. Results showed that biochar additions to soils overall increased the N- and P-cycling SEAs by 14 and 11%, respectively. The enhancement of the N- and P-cycling SEAs was mainly attributable to the microbial stimulation by biochar properties (i.e., nutrient content and porosity) and soil nutrients (e.g., soil organic C and total N). The enhancement was the most significant under the conditions with biochars produced at low temperatures and using feedstock materials with high nutrient content, and biochar applications in acidic or neutral soils, coarse or fine soils, and farmland soils. Biochar additions to soils overall reduced the C-cycling SEAs by 6.3%. The C-cycling SEAs were greatly suppressed under the conditions with low and very high biochar loads, biochars produced at high temperatures and with feedstock materials of herb and lignocellulose, and biochar applications in alkaline, fine, and forest soils. The results were mainly related to the adsorption and inhibition effects of biochars and soil properties (e.g., liming effect, high biochar porosity and aromatic C content) on fungi and the enzymes. Biochar feedstock, C/N and load, and soil total N were the main influential factors on the SEAs. The results from this study demonstrate that biochar amendment is beneficial to improving soil N and P cycling and C sequestration.

Heavy grazing reduces grassland soil greenhouse gas fluxes: A global meta-analysis.

Grazing degrades worldwide grasslands and possibly suppresses soil greenhouse gas (GHG: CO2, CH4 and N2O) fluxes. However, the global patterns of these three gas fluxes in response to grazing and the general mechanisms remain poorly understood. Here, we performed a meta-analysis of 63 independent grazing studies that measured soil GHG fluxes across global grasslands. Our results revealed that light and moderate grazing had no significant effect on soil CH4 uptake, N2O and CO2 emission, but heavy grazing consistently reduced them. The magnitudes of their responses to grazing were regulated by grazing duration and precipitation. In comparison with CO2 emission, soil CH4 uptake and N2O emission were reduced much more under heavier grazing, longer grazing duration or less precipitation. The decrease in soil CO2 emission was possibly caused by grazing-induced reduction in root biomass and soil moisture, while the decline in soil CH4 uptake and N2O emission was due to decreased soil moisture and substrate availability. Overall, this study provides the first large-scale evaluation on three main soil GHG fluxes in response to grazing, highlighting grazing inhibition of GHG emission but at the cost of plant productivity and soil fertility. We call for future efforts to identify an appropriate grazing intensity that is optimal to balance these complicated impacts.

Global changes alter plant multi-element stoichiometric coupling.

Plant stoichiometric coupling among all elements is fundamental to maintaining growth-related ecosystem functions. However, our understanding of nutrient balance in response to global changes remains greatly limited to plant carbon : nitrogen : phosphorus (C : N : P) coupling. Here we evaluated nine element stoichiometric variations with one meta-analysis of 112 global change experiments conducted across global terrestrial ecosystems and one synthesis over 1900 species observations along natural environment gradients across China. We found that experimentally increased soil N and P respectively enhanced plant N : potassium (K), N : calcium (Ca) and N : magnesium (Mg), and P : K, P : Ca and P : Mg, and natural increases in soil N and P resulted in qualitatively similar responses. The ratios of N and P to base cations decreased both under experimental warming and with naturally increasing temperature. With decreasing precipitation, these ratios increased in experiments but decreased under natural environments. Based on these results, we propose a new stoichiometric framework in which all plant element contents and their coupling are not only affected by soil nutrient availability, but also by plant nutrient demand to maintain diverse functions under climate change. This study offers new insights into understanding plant stoichiometric variations across a full set of mineral elements under global changes.

Responses of soil microbial community structure changes and activities to biochar addition: A meta-analysis.

The objective of this study was to investigate responses of soil microbial community structure changes and activities to biochar addition under different biochar characteristics, soil properties, and experiment conditions. A meta-analysis was conducted based on 265 datasets from 49 published studies. Results showed that biochar addition significantly increased the ratios of soil fungi to bacteria (F/B) and the ratios of Gram-positive bacteria to Gram-negative bacteria (G+/G-), and microbial biomass and activities. The enhancement of F/B ratios was most significant with addition of biochars produced at low temperatures to soils with lower pH and nutrients in a long-term condition, which improved ecosystem stability of agricultural soils. The F/B ratios were mainly affected by biochar nutrients, soil nutrients, and soil pH values. Biochar nutrients and structural properties (i.e., surface area and porosity) also played the important role in enhancing G+/G-, total microbial biomass, and activities of bacteria, fungi, and actinomycetes. The G+/G- ratios increased the most with addition of biochars produced with medium temperatures and residue accompanied with fertilizers in dry land (dried farmland) soils. High biochar load greatly improved the total phospholipid fatty acids, and activities of bacteria, fungi, and actinomycetes in fine/coarse, paddy soils, and soils with low nutrients, in turn increased the soil nutrient cycling. In addition, the structural properties of biochars were the most influencing factor to increase total microbial biomass and actinomycete activity. Overall, the enhancement of microbial activities and community structure shifts under biochar addition should promote soil nutrients cycling and carbon sequestration, and improve crop yields.

A global meta-analysis of soil phosphorus dynamics after afforestation.

Afforestation significantly affects soil chemistry and biota, but its effects on the potentially growth-limiting nutrient phosphorus (P) had not to our knowledge been analyzed globally. We conducted a comprehensive meta-analysis of 220 independent sampling sites from 108 articles to evaluate global patterns and controls of soil P change following afforestation. Overall, total P concentration decreased by 11% and total P stock by 12% in the top 20 cm of mineral soil following afforestation, with no change in available P. Time since afforestation had no consistent effect on total P, while available P tended to increase with time. Prior land cover was the most influential factor for soil P change after afforestation, with available P increasing on native vegetation but decreasing on cropland. Afforestation increased available P by 22% without decreasing total P on formerly 'degraded' land, but depleted total P by 15% at nondegraded sites. Climate also influenced soil P response to afforestation, with larger P loss in the tropics. Afforestation did not appear to directly induce P limitation, as available P only decreased on cropland. However, substantial declines in total P may drive tropical plantations toward greater P limitation as the capacity to replenish available P decreases.

The environment, not space, dominantly structures the landscape patterns of the richness and composition of the tropical understory vegetation.

The mechanisms driving the spatial patterns of species richness and composition are essential to the understanding of biodiversity. Numerous studies separately identify the contributions of the environment (niche process) and space (neutral process) to the species richness or composition at different scales, but few studies have investigated the contributions of both types of processes in the two types of data at the landscape scale. In this study, we partitioned the spatial variations in all, exotic and native understory plant species richness and composition constrained by environmental variables and space in 134 plots that were spread across 10 counties in Hainan Island in southern China. The 134 plots included 70 rubber (Hevea brasiliensis) plantation plots, 50 eucalyptus (Eucalyptus urophylla) plantation plots, and 14 secondary forest plots. RDA based variation partitioning was run to assess the contribution of environment and space to species richness and composition. The results showed that the environmental variables alone explained a large proportion of the variations in both the species richness and composition of all, native, and exotic species. The RDA results indicated that overstory composition (forest type here) plays a leading role in determining species richness and composition patterns. The alpha and beta diversities of the secondary forest plots were markedly higher than that of the two plantations. In conclusion, niche differentiation processes are the principal mechanisms that shape the alpha and beta diversities of understory plant species in Hainan Island.