Investigating the spatio-temporal variability of soil organic carbon stocks in different ecosystems of China

Dynamics of Carbon and Soil Enzyme Activities under Arabica Coffee Intercropped with Brachiaria decumbens in the Brazilian Cerrado

The change in land use in the Brazilian Cerrado modifies the dynamics of soil organic matter (SOM) and, consequently, carbon (C) stocks and their fractions and soil enzyme activities. This study evaluated the effect of brachiaria (Brachiaria decumbens Stapf.) intercropped with Arabica coffee (Coffea arabica L.) on the stock and fractions of soil carbon and enzyme activities. The experiment was arranged in a completely randomized block design with three replications and treatments in a factorial design. The first factor consisted of coffee with or without intercropped brachiaria, the second of Arabica coffee cultivars ('I.P.R.103' and 'I.P.R.99') and the third factor of the point of soil sampling (under the canopy (UC) and in inter-rows (I)). Soil was sampled in layers of 0-10, 10-20, 20-30, 30-40, 40-60 and 60-80 cm. Soil from the 0-10 cm layer was also used to analyze enzymatic activity. Significant effects of coffee intercropped with brachiaria were confirmed for particulate organic carbon (POC), with highest contents in the 0-10 and 20-30 cm layers (9.62 and 6.48 g kg-1, respectively), and for soil enzymes (280.83 and 180.3 μg p-nitrophenol g-1 for arylsulfatase and β-glucosidase, respectively).

Spatial distribution and main drivers of soil selenium in Taihu Lake Basin, Southeast China

Selenium (Se) is an essential micronutrient that is both hazardous and beneficial to living organisms. However, few studies have examined soil Se distribution and its driving mechanisms on a large basin scale. Thus, multivariate statistics, geostatistics, boosted regression trees, and structural equation models were used to investigate the spatial distribution, driving factors, and multivariate interactions of soil Se based on 1753 topsoil samples (0-20 cm) from the Taihu Lake Basin. The results indicated that the soil Se concentration ranged from 0.12 to 57.26 mg kg-1, with a mean value of 0.90 mg kg-1. Overall, the spatial pattern of soil Se gradually decreased from south to north with approximately 1.06% of the soil contaminated with Se. Moisture index (MI), soil moisture (SM), and ≥ 0 ℃ accumulative temperature (AAT0) were the main determinants of soil Se accumulation. Additionally, the substantial effect of SM∩AAT0 on soil Se concentrations demonstrated that climate-soil interactions largely governed the spatial pattern of soil Se. The Se-enriched and Se-contaminated soils occurred mainly in regions with high precipitation, MI, SM, AAT0, and soil organic matter. This study provides a theoretical basis and practical guidance for the remediation of soil Se contamination and the sustainable development of Se-enriched agriculture.

Soil carbon sequestration potential of cultivated lands and its controlling factors in China

Understanding soil organic carbon (SOC) stocks and carbon sequestration potential in cultivated lands can have significant benefit for mitigating climate change and emission reduction. However, there is currently a lack of spatially explicit information on this topic in China, and our understanding of the factors that influence both saturated SOC level (SOCS) and soil organic carbon density (SOCD) remains limited. This study predicted SOCS and SOCD of cultivated lands across mainland China based on point SOC measurements, and mapped its spatial distribution using environmental variables as predictors. Based on the differentiation between SOCS and SOCD, the soil organic carbon sequestration potentials (SOCP) of cultivated land were calculated. Boosted regression trees (BRT), random forest (RF), and support vector machine (SVM) were evaluated as prediction models, and the RF model presented the best performance in predicting SOCS and SOCD based on 10-fold cross-validation. A total of 991 topsoil (0-20 cm) SOC measurements and 12 environmental variables explaining topography, climate, organism, soil properties, and human activity were used as predictors in the model. Both SOCS and SOCD suggested higher SOC levels in northeast China and lower levels in central China. The cultivated lands in China had the potential to sequester about 2.13 ± 0.96 kg m-2 (3.25 Pg) SOC in the top 20 cm soil depth. Northeastern China had the largest SOCP followed by Northern China, and Southwestern China had the lowest SOCP. The primary environmental variables that affected the spatial variation of SOCS were mean annual temperature, followed by clay content and normalized difference vegetation index (NDVI). The assessment and mapping of SOCP in China's cultivated lands holds significance importance as it can provide valuable insights to policymakers and researchers about SOCP, and aid in formulating climate change mitigation strategies.

Effects of land use and land cover change on soil organic carbon storage in the Hexi regions, Northwest China

Soil organic carbon (SOC) storage in arid inland regions is significantly affected by land use and land cover change (LUCC) associated with climate change and agricultural activities. A systematic evaluation to the LUCC effects on SOC storage could enable us to better manage soil carbon pools in arid inland regions. Here, we evaluated the effects of LUCC on SOC storage in the Hexi Regions based on high-resolution SOC and LUCC maps derived from Landsat imagery and digital soil mapping using machine learning algorithm and environmental covariates. The results showed that SOC generally increased from northwest to southeast over the Hexi Regions with an average stock of 7.15 kg C m^-2 at a soil depth of 100 cm and a total storage of 2783.05 Tg C. The SOC stock and storage in the Qilian Mountains (mountains) was about 3.90 and 4.55 times higher than that in the Hexi Corridor (plains), respectively. It was estimated that LUCC over the past four decades caused a net increase of 23.41 and 18.19 Tg C in total SOC storage for the Qilian Mountains and Hexi Corridor, respectively. Specifically, the development in grasslands quality as well as the land-use category conversion from the bare land to grassland mainly contributed to the increase in SOC storage of the Qilian Mountains, where the LUCC was mainly driven by climate change. By contrast, the SOC storage change in the Hexi Corridor was mainly associated with the conversion from sandy land and low-cover grassland to cropland as well as sandy land to grassland, being mainly affected by intense cropland expansion and desertification control. Our results highlighted the importance of climate change and cropland expansion in enhancing SOC storage of the Qilian Mountains and Hexi corridor, respectively.

Estimation of variability in soil water content in a forested critical-zone experimental catchment in Eastern China

Knowledge of soil water content (SWC) dynamics within soil profiles is crucial for the effective management of water and soil resources. This study aims to clarify the temporal variability and stability of SWC in a forested critical-zone experimental catchment, and further to improve the understanding of the temporal and spatial distribution of soil water in a typical hilly catchment in eastern China. The selected Nandadish (NDD) catchment covering 0.79 ha was instrumented with 34 SWC monitoring sites using Frequency Domain Reflectometry. The consecutive high-resolution monitoring data of soil water at different depths of the sites were collected from January 2017 to December 2019. The results showed that the SWC of the shallow layer (0-30 cm) had the strongest variability over time during the three hydrologic years. The interannual variability of SWC showed the opposite regularity with that of the seasonal variability. Specifically, the spatial variability of SWC in the dry years was greater than that in wet years; whilst the temporal stability of SWC in dry seasons was greater than that in rainy seasons. Precipitation and temperature were the two dominant factors influencing the temporal variation of SWC. Precipitation controlled the interannual variation of SWC, while temperature controlled the seasonal variation of SWC. Additionally, soil water had high temporal stability throughout the observation period in NDD catchment, and the most representative point was located at a relatively flat and central place, which can be used to simulate the variability of SWC under different rainfall conditions in the study area. The temporal stability of SWC patterns was controlled by topography, geographic location, throughfall, and the groundwater level in the study area, which was characterized by sloping terrain and forested cover. This research provides scientific bases for the optimum design of ground sampling, and the temporal and spatial prediction for soil moisture in a typical eastern hilly area with forest land uses.

Soil Organic Carbon Mineralization and Its Temperature Sensitivity under Different Substrate Levels in the Mollisols of Northeast China

Soil organic carbon (SOC) mineralization plays an important role in global climate change. Temperature affects SOC mineralization, and its effect can be limited by the substrate available. However, knowledge of the effects of temperature and substrate quality on SOC mineralization in the Mollisols of Northeast China is still lacking. In this study, based on a spatial transplant experiment, we conducted a 73-day incubation to examine the effects of temperature on SOC mineralization and its temperature sensitivity under different carbon levels. We found that the SOC content, incubation temperature and their interaction had significant effects on SOC mineralization. A higher SOC content and higher incubation temperature resulted in higher SOC mineralization. The temperature sensitivity of SOC mineralization was affected by the substrate quality. The temperature sensitivity of SOC mineralization, showed a downward trend during the incubation period, and the range of variation in the Q₁₀ declined with the increment in the SOC content. The study suggested that there was a higher SOC mineralization in high levels of substrate carbon when the temperature increased. Further, SOC mineralization under higher SOC contents was more sensitive to temperature changes. Our study provides vital information for SOC turnover and the CO₂ sequestration capacity under global warming in the Mollisols of Northeast China and other black soil regions of the world.

Terrestrial carbon sinks in China and around the world and their contribution to carbon neutrality

Enhancing the terrestrial ecosystem carbon sink (referred to as terrestrial C sink) is an important way to slow down the continuous increase in atmospheric carbon dioxide (CO₂) concentration and to achieve carbon neutrality target. To better understand the characteristics of terrestrial C sinks and their contribution to carbon neutrality, this review summarizes major progress in terrestrial C budget researches during the past decades, clarifies spatial patterns and drivers of terrestrial C sources and sinks in China and around the world, and examines the role of terrestrial C sinks in achieving carbon neutrality target. According to recent studies, the global terrestrial C sink has been increasing from a source of (-0.2±0.9) Pg C yr^-1 (1 Pg=10¹⁵ g) in the 1960s to a sink of (1.9±1.1) Pg C yr^-1 in the 2010s. By synthesizing the published data, we estimate terrestrial C sink of 0.20-0.25 Pg C yr^-1 in China during the past decades, and predict it to be 0.15-0.52 Pg C yr^-1 by 2060. The terrestrial C sinks are mainly located in the mid- and high latitudes of the Northern Hemisphere, while tropical regions act as a weak C sink or source. The C balance differs much among ecosystem types: forest is the major C sink; shrubland, wetland and farmland soil act as C sinks; and whether the grassland functions as C sink or source remains unclear. Desert might be a C sink, but the magnitude and the associated mechanisms are still controversial. Elevated atmospheric CO₂ concentration, nitrogen deposition, climate change, and land cover change are the main drivers of terrestrial C sinks, while other factors such as fires and aerosols would also affect ecosystem C balance. The driving factors of terrestrial C sink differ among regions. Elevated CO₂ concentration and climate change are major drivers of the C sinks in North America and Europe, while afforestation and ecological restoration are additionally important forcing factors of terrestrial C sinks in China. For future studies, we recommend the necessity for intensive and long term ecosystem C monitoring over broad geographic scale to improve terrestrial biosphere models for accurately evaluating terrestrial C budget and its dynamics under various climate change and policy scenarios.

Effect of Cropland Abandonment on Soil Carbon Stock in an Agroforestry System in Southwestern Spain

The low profitability of agricultural products in a globalized market context is causing the abandonment of less profitable agroforestry systems in Spain. This fact is implicated in a change in land use, increasing the forest area, which could alter the carbon stock in the soil. Thus, the objective of this study was to determine if the abandonment of rural areas and the change in land use has an impact on the soil organic carbon stock in agroforestry systems in southwestern Spain. Through historical aerial photographs and current satellite images, sites were identified where samples of abandoned agricultural soils in the 1950s were collected. They were compared with soil samples from adjacent locations whose agricultural activities continue to this day. After more than 60 years, the abandonment of agricultural activity is associated with a 54% increase in C concentration and 34.8% in soil organic carbon in the upper 30 cm of soil profiles. Therefore, the abandonment of agricultural land has influenced the carbon stock of this territory, becoming a carbon sink.

Soil carbon supplementation: Improvement of root-surrounding soil bacterial communities, sugar and starch content in tobacco (N. tabacum)

Soil carbon supplementation is known to stimulate plant growth by improving soil fertility and plant nutrient uptake. However, the underlying process and chemical mechanism that could explain the interrelationship between soil carbon supplementation, soil micro-ecology, and the growth and quality of plant remain unclear. In this study, we investigated the influence and mechanism of soil carbon supplementation on the bacterial community, chemical cycling, mineral nutrition absorption, growth and properties of tobacco leaves. The soil carbon supplementation increased amino acid, carbohydrates, chemical energy metabolism, and bacterial richness in the soil. This led to increased content of sugar (23.75%), starch (13.25%), and chlorophyll (10.56%) in tobacco leaves. Linear discriminant analysis revealed 49 key phylotypes and significant increment of some of the Plant Growth-Promoting Rhizobacteria (PGPR) genera (Bacillus, Novosphingobium, Pseudomonas, Sphingomonas) in the rhizosphere, which can influence the tobacco growth. Partial Least Squares Path Modeling (PLS-PM) showed that soil carbon supplementation positively affected the sugar and starch contents in tobacco leaves by possibly altering the photosynthesis pathway towards increasing the aroma of the leaves, thus contributing to enhanced tobacco flavor. These findings are useful for understanding the influence of soil carbon supplementation on bacterial community for improving the yields and quality of tobacco in industrial plantation.

Silvicultural Interventions Drive the Changes in Soil Organic Carbon in Romanian Forests According to Two Model Simulations

We investigated the effects of forest management on the carbon (C) dynamics in Romanian forest soils, using two model simulations: CBM-CFS3 and Yasso15. Default parametrization of the models and harmonized litterfall simulated by CBM provided satisfactory results when compared to observed data from National Forest Inventory (NFI). We explored a stratification approach to investigate the improvement of soil C prediction. For stratification on forest types only, the NRMSE (i.e., normalized RMSE of simulated vs. NFI) was approximately 26%, for both models; the NRMSE values reduced to 13% when stratification was done based on climate only. Assuming the continuation of the current forest management practices for a period of 50 years, both models simulated a very small C sink during simulation period (0.05 MgC ha−1 yr−1). Yet, a change towards extensive forest management practices would yield a constant, minor accumulation of soil C, while more intensive practices would yield a constant, minor loss of soil C. For the maximum wood supply scenario (entire volume increment is removed by silvicultural interventions during the simulated period) Yasso15 resulted in larger emissions (−0.3 MgC ha−1 yr−1) than CBM (−0.1 MgC ha−1 yr−1). Under ‘no interventions’ scenario, both models simulated a stable accumulation of C which was, nevertheless, larger in Yasso15 (0.35 MgC ha−1 yr−1) compared to CBM-CSF (0.18 MgC ha−1 yr−1). The simulation of C stock change showed a strong “start-up” effect during the first decade of the simulation, for both models, explained by the difference in litterfall applied to each scenario compared to the spinoff scenario. Stratification at regional scale based on climate and forest types, represented a reasonable spatial stratification, that improved the prediction of soil C stock and stock change.