Optimizing grazing exclusion duration for carbon sequestration in grasslands: Incorporating temporal heterogeneity of aboveground biomass and soil organic carbon.

Grasslands account for approximately one-third of the global terrestrial carbon stocks. However, a limited understanding of the impact of grazing exclusion on carbon storage in grassland ecosystems hinders progress towards restoring overgrazed grasslands and promoting carbon sequestration. In this study, we conducted a comprehensive meta-analysis to investigate the effects of grazing exclusion on aboveground biomass (AGB) and soil organic carbon (SOC) in four grasslands: alpine grasslands (AP), tropical savannas (TS), temperate subhumid grasslands (TG), and a semi-desert steppe (SD). Our meta-analysis indicated that grazing exclusion significantly enhanced carbon sequestration in grassland ecosystems, and the benefits of carbon sequestration were most pronounced in the AP, followed by the TG, SD, and TS. Grazing exclusion duration (DUR) was a significant factor associated with the response of aboveground biomass (AGB) and soil organic carbon (SOC) to grazing exclusion. Moreover, the relationships between AGB and DUR were nonlinear, with existence thresholds of 18, 21, 12, 19, and 23 years in global grasslands (ALL), AP, TS, TG, and SD, respectively. However, the relationship between SOC and DUR was linear, with SOC continuing to increase as DUR increased (up to 40 years). The multi-objective optimization indicated that the optimal duration of grazing exclusion for grassland carbon sequestration was 18-20, 21-23, 12-14, 19-21, and 23-25 years for ALL, AP, TS, TG, and SD, respectively. Our study contributes to the enhancement of grazing management and offers better options for increasing carbon sequestration in grasslands.

Soil freeze-thaw cycles affect spring phenology by changing phenological sensitivity in the Northern Hemisphere.

The use of frozen soil-vegetation feedback for predictive models is undergoing enormous changes under rapid climate warming. However, the influence of soil freeze-thaw (SFT) cycles on vegetation phenology and the underlying mechanisms remain poorly understood. By synthesizing a variety of satellite-derived data from 2002 to 2021 in the Northern Hemisphere (NH), we demonstrated a widespread positive correlation between soil thawing and the start of the growing season (SOS). Our results also showed that the SFT cycles had a significant impact on vegetation phenology mainly by altering the phenological sensitivities to daytime and nighttime temperatures, solar radiation and precipitation. Moreover, the effects of SFT cycles on the sensitivity of the SOS were more pronounced than those on the sensitivity of the end of the growing season (EOS) and the length of growing season (LOS). Furthermore, due to the degradation of frozen soil, the changes in phenological sensitivity in the grassland and tundra biomes were significantly larger than those in the forest. These findings highlighted the importance of incorporating the SFT as an intermediate process into process-based phenological models.

Source partitioning using phosphate oxygen isotopes and multiple models in a large catchment.

Excessive phosphorus (P) loadings cause major pollution concerns in large catchments. Quantifying the point and nonpoint P sources of large catchments is essential for catchment P management. Although phosphate oxygen isotopes (δ18O(PO4)) can reveal P sources and P cycling in catchments, quantifying multiple P sources in a whole catchment should be a research focus. Therefore, this study aimed to quantitatively identify the proportions of multiple potential end members in a typical large catchment (the Yangtze River Catchment) by combining the phosphate oxygen isotopes, land use type, mixed end-element model, and a Bayesian model. The δ18O(PO4) values of river water varied spatially from 4.9‰ to18.3‰ in the wet season and 6.0‰ to 20.9‰ in the dry season. Minor seasonal differences but obvious spatial changes in δ18O(PO4) values could illustrate how human activity changed the functioning of the system. The results of isotopic mass balance and the Bayesian model confirmed that controlling agricultural P from fertilizers was the key to achieving P emission reduction goals by reducing P inputs. Additionally, the effective rural domestic sewage treatment, development of composting technology, and resource utilization of phosphogypsum waste could also contribute to catchment P control. P sources in catchment ecosystems can be assessed by coupling an isotope approach and multiple-models.

Control mechanisms of water chemistry based on long-term analyses of the Yangtze River.

Long-term series data can provide a glimpse of the influence of natural and anthropogenic factors on water chemistry. However, few studies have been conducted to analyze the driving forces of the chemistry of large rivers based on long-term data. This study aimed to analyze the variations and driving mechanisms of riverine chemistry from 1999 to 2019. We compiled published data on major ions in the Yangtze River, one of the three largest rivers in the world. The results showed that Na+ and Cl- concentrations decreased with increasing discharge. Significant differences in riverine chemistry were found between the upper and middle-lower reaches. Major ion concentrations in the upper reaches were mainly controlled by evaporites, especially Na+ and Cl- ions. In contrast, major ion concentrations in the middle-lower reaches were mainly affected by silicate and carbonate weathering. Furthermore, human activities were the drivers of some major ions, notably SO42- ions associated with coal emissions. The increased major ions and total dissolved solids in the Yangtze River in the last 20 years were ascribed to the continuous acidification of the river and the construction of the Three Gorges Dam. Attention should be given to the impact of anthropogenic activities on the water quality of the Yangtze River.

Rare earth elements in soil around coal mining and utilization: Contamination, characteristics, and effect of soil physicochemical properties.

REEs are emerging contaminants, and soils nearby coal and coal ash with high REEs composition are vulnerable to REEs contamination. Besides, coal industry can alter surrounding soil characteristics. However, there is information paucity about REEs contamination and geochemical behaviors along with soil characteristics around coal industrial areas, which are essential for understanding their toxicity and mobilization. The study was conducted in soils surrounding Kriel coal-fired power plant (KCM) and Greenside coal mining in Witbank (GSCM), South Africa. Multivariate statistical analysis, pollution and fractionation indices, and BCR sequential extraction were applied. The ∑REEs in the soils were compared to ∑REEs abundance in the upper earth's crust (UEC), and slightly higher ∑REEs was found in KCM but slightly lower in GSCM. Generally, LREEs are abundant. The soil REEs were normalized using the Post-Archean Australian Shale (PAAS) and Eu and Gd in KCM and Gd in GSCM were >1. Contamination assessment revealed that the soils are slightly to moderately contaminated by REEs. ∑REEs in KCM was significantly correlated with soil particle sizes of 2.00-50.00 µm, Al2O3, Fe2O3, and MnO, while with 2.00-3.00 µm and Al2O3 in GSCM. Fractionation characteristics showed a positive Ce anomaly, with positive linear regressions with Fe2O3 and MnO. In contrast, a negative Eu anomaly was found with positive linear regressions with Al, Ca, and Mg-oxides. Oxidizable fractioned REEs accounted for 32.33% of the ∑REEs in GSCM and 35.85% in KCM, and their high EF suggests enrichment that could be due to coal mining and utilization. Most soil physicochemical properties appear to be negatively correlated with the exchangeable REEs. Overall, the soils are contaminated by REEs and the REEs characteristics are considerably influenced by major elements oxide, U, and Th contents. Therefore, more attention should be paid to REEs contamination and impacts around coal mining and utilization.

Influence of arbuscular mycorrhizal fungi on bioaccumulation and bioavailability of As and Cd: A meta-analysis.

Increasing industrial activity has led to a growing risk of arsenic (As) and cadmium (Cd) accumulations and biomagnifications in plants and humans. Arbuscular mycorrhizal fungi (AMF) have been extensively studied as a soil amendment owing to their capability to reduce the accumulation of As and Cd in plant tissues. However, a quantitative and data-based consensus has yet to be reached on the effect of AMF on As and Cd bioaccumulation and bioavailability. Here, a meta-analysis was conducted to quantitatively evaluate the impact of AMF using 1430 individual observations from 194 articles. The results showed that AMF inoculation caused a decrease in shoot and root As and Cd accumulation compared to control, and the reduction rates were affected by experimental duration, P fertilizer, AMF species, plant family, plant lifecycle, and soil properties. Intermediate experimental duration (lasting 56-112 days) and no P fertilizer favored AMF to reduce the shoot As and root Cd accumulation. Compared to other plant families, the reduction in As and Cd accumulation in legumes was the greatest, following AMF inoculation. The soils with alkaline, high organic carbon (OC), and low available phosphorus (AP) appeared to be more favorable for AMF to reduce As accumulation in plant tissues, while soils with low AP were more conducive to reducing the Cd accumulation in plant tissues. In addition, AMF inoculation increased pH (1.92%), OC (6.27%), easily-extractable glomalin-related soil protein (EE-GRSP) (29.36%), and total glomalin-related soil protein (T-GRSP) (29.99%), and reduced bioavailable As (0.52%) and Cd (2.35%) in soils compared to control. Overall, the meta-analysis provides valuable guidelines for the optimal use of AMF in different plant-soil systems.

A review on moss nitrogen and isotope signatures evidence for atmospheric nitrogen deposition.

Moss nitrogen (N) concentration and isotopic composition (δ15N) values can reveal a better understanding of atmospheric N deposition patterns. Here, we summarize the moss N content and δ15N signatures using data compiled from 104 papers. Based on the dataset, we summarize the models for assessing the level and reduced (NHx): oxidised compounds (NOx) ratio of atmospheric N deposition. Results showed a historical increase in N concentration and 15N depletion of specimen mosses close to anthropogenic N sources from intensive animal production and agricultural activities (NHx emission) since the 1800s. However, an increase of moss N with a less negative 15N observed in the last three decades could be due to a substantial fossil fuel combustion contributed NOx emission. Spatially, N deposition in Europe decreased due to successful control actions, but Asia has become a hotspot for NHx emission from agriculture. The present results highlight the importance of moss N and δ15N values for estimating atmospheric N deposition patterns at spatio-temporal trends.

Effects of elevated CO2 on plant C-N-P stoichiometry in terrestrial ecosystems: A meta-analysis.

A substantial number of experiments have so far been carried out to study the response of the C-N-P stoichiometry of terrestrial plants to the rising CO2 level of the earth. However, there is a need of systematic evaluation for assessing the impact of the elevated CO2 on plant C-N-P stoichiometry. In the present investigation, a comprehensive meta-analysis involving 386 published reports and including 4481 observations has been carried out. The goal of the research was to determine the response of plants to their C-N-P stoichiometry due to elevated levels of global atmospheric CO2. The results showed that rising CO2 altered the concentration of C (+2.19%, P < 0.05), N (-9.73%, P < 0.001) and P (-3.23%, P < 0.001) and C:N (+13.29%, P < 0.001) and N:P ratios (-7.32%, P < 0.0001). Overall, a slightly increasing trend in the C:P ratio (P > 0.05) in the plant was observed. However, plant leaf, shoot and herbaceous type of plants showed more sensitivity to rising CO2. CO2 magnitude exhibited a positive effect (P < 0.05) on C:N ratio. Additionally, "CO2 acclimation" hypothesis as proposed by the authors of the current paper was also tested in the study. Results obtained, especially, show changes of C and N concentrations and C:P ratio to an obvious down-regulation for long-term CO2 fumigation. At spatial scales, a reduction of plant N concentration was found to be higher in the southern hemisphere. The CO2 enrichment methods affected the plant C-N-P stoichiometry. Compared to FACE (free-air CO2 enrichment), OTC (open top chamber) showed larger changes of C, N, P, and N:P. The results of the present study should, therefore, become helpful to offer a better understanding towards the response of the terrestrial plant C-N-P stoichiometry to an elevated global atmospheric CO2 in the future.