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The North China agro–pastoral zone is a large, ecologically fragile zone in the arid and semi-arid regions. Quantitative remote sensing inversion of soil organic carbon (SOC) in this region can facilitate understanding of the current status of degraded land restoration and provide data support for carbon cycling research in the region. Deep learning (DNN) for SOC inversion has been W.a hot topic over the past decade, but there have been few studies at the regional scale in the arid and semi-arid zones. In this study, a DNN model with five hidden layers and five skip connections was established using 644 spatially distributed SOC samples and Landsat 8 OLI imagery. The model was compared with the random forest algorithm in terms of generalization ability. The main conclusions were as follows: 1. The DNN algorithm can establish a high-precision SOC inversion model (R2 = 0.52, RMSE = 0.7), with 90% of errors concentrated in the range of −2.5 to 2.5 kg·C/m2; 2. the Boruta variable-screening algorithm can effectively improve the model accuracy of the random forest algorithm, but due to the DNN's better ability to mine hidden information in the data, the improvement effect on the DNN model accuracy is limited; 3. the SOC samples in arid and semi-arid areas are highly positively skewed, with a significant impact on the modeling accuracy of DNN, and conversion is required to obtain a model with better generalization ability; and 4. in arid and semi-arid regions, SOC has a weak correlation with vegetation indices but a stronger correlation with temperature, elevation, and aridity. This study established a reliable deep learning model for SOC density in a large arid and semi-arid region, providing a reference and framework for the establishment of SOC inversion models in other regions. [ABSTRACT FROM AUTHOR]

Purpose: As the main component of terrestrial carbon pool, soil organic carbon (SOC) is vital to soil fertility and biogeochemical cycle. Quantifying the spatial distribution of regional SOC stocks (SOCS) provides critical support for climate change and food security decisions. Our aim was to explore the optimal interpolation method to improve the accuracy of spatial prediction of SOCS in temperate grasslands. Materials and methods: To support such research, we performed soil sampling to depths of 0 to 20 and 20 to 30 cm throughout the Hulun Buir grassland of Inner Mongolia. We compared prediction of the spatial patterns of SOCS using regression kriging (RK) and ordinary kriging (OK). We used topographic factors, climate variables, satellite data (the normalized-difference vegetation index (NDVI)), and soil texture as predictors in the RK method. Results and discussion: SOCS was significantly positively correlated with precipitation, NDVI, topographic variables, and clay content, but negatively correlated with temperature and sand content. NDVI explained more than 40% of the SOCS spatial variation and was the dominant factor. Geostatistical analysis showed strong and moderate spatial dependence of SOCS in the 0–20- and 20–30-cm soil layers, respectively. The RK and OK soil pools to a depth of 30 cm were 607.28 and 559.46 Tg, respectively. Conclusion: Compared with OK, the RK method improved the SOCS prediction accuracy by 20.4, 30.1, and 23.9% for soil depths of 0–20, 20–30, and 0–30 cm, respectively. Our findings suggest that OK may be acceptable where the environmental conditions are homogeneous, but that RK performs better in heterogeneous areas. [ABSTRACT FROM AUTHOR]

Purpose: Traditional soil organic carbon density (SOCD) measurement is time-consuming and costly, from field sampling to laboratory analysis. However, visible to near-infrared reflectance (vis-NIR) spectroscopy can rapidly estimate SOCD and thereby permit rapid, inexpensive measurements that support environmental management. Our aim was to explore the relationship between SOCD and vis-NIR spectra, with the goal of establishing a spectral SOCD estimation method. Materials and methods: In this study, we sampled soils throughout northern China's agro-pastoral ecotone and performed SOCD and spectral measurements. After pre-processing the data, we transformed the spectral reflectance into seven forms: the reciprocal logarithm, first-order differential, second-order differential, logarithmic first-order differential, logarithmic second-order differential, reciprocal first-order differential (RFOD), and reciprocal second-order differential. We then explored the relationship between SOCD and these indexes. We also analyzed spectral SOCD estimation models based on stepwise multiple linear regression and partial least-squares regression. Results and discussion: We found that the spectral reflectance decreased with increasing SOCD, and the most sensitive bands for SOCD were between 745 and 840 nm. RFOD greatly improved estimation accuracy. The performance of a stepwise multiple linear regression estimation model based on RFOD provided the best fit (R2 = 0.77) and the lowest root-mean-square error (0.75 kg C m−2), and the best ratio of percent deviation (2.11). Conclusion: Our results suggest a high potential for fast and reliable estimation of SOCD using spectral techniques and provides a theoretical basis and technical support for rapid monitoring of SOCD in northern China's agro-pastoral ecotone. [ABSTRACT FROM AUTHOR]

Abstract Knowledge of the spatial pattern of soil organic carbon (SOC) and its influencing factors is important for understanding the carbon cycle. We investigated the SOC density (SOCD) to a depth of 30 cm from ground surface at 644 sites in an agro-pastoral ecotone of northern China that covered 654,564 km2. SOC storage to a depth of 30 cm was 2360 Tg, and SOCD averaged 3.78 kg·m−2 for the entire study area. SOCD to a depth of 20 cm and from 20 to 30 cm showed strong and moderate spatial dependence, with a nugget to sill ratio of 0.7% and 43.5%, respectively. Overall, SOCD increased from southwest to northeast, it was more strongly correlated with latitude than with longitude. Forest and sandy land had the highest and lowest SOCD , at 6.28 and 1.76 kg·m−2, respectively. The SOCD (kg·m−2) of grassland decreased with decreasing vegetation cover: high (4.42) > medium (3.62) > low (2.50). For the common soils in the study area, the highest mean SOCD was 9.31 kg·m−2 in Grey Forest Soils, versus 1.75 kg·m−2 in Brown Pedocals. In the study area, temperature appears to be the dominant factor that influences SOC, and SOCD decreased with increasing temperature. However, topographic factors (elevation, slope, and aspect) had a weaker effect on SOC. Our results provide a valuable baseline for future research on the long-term evolution of SOCD. Highlights • Temperature is the dominant environmental factor influencing SOC in northern China's agro-pastoral ecotone. • The region had a relatively low SOC density of 3.78 kg·m−2 to a depth of 30 cm. • The total SOC storage was 2360.02 Tg in the study area. [ABSTRACT FROM AUTHOR]

The spatial distribution of soil organic carbon (SOC) and its storage in the topmost 100 cm of the soil were investigated in the Horqin Grassland of northeastern China. Soil samples were collected at 1465 sites, covering 12.03 × 10 4  km 2 . The region had a mean SOC density of 6.84 kg C m −3 , which is lower than China's mean (9.60 kg C m −3 ) and the world's mean (10.40 kg C m −3 ). The mean SOC density was much higher in the northern part of the study area (8.85 kg C m −3 ) than in the southern part (4.84 kg C m −3 ). The total SOC storage in the Horqin Grassland was 862.74 Tg. SOC storage decreased with increasing soil sampling depth. The SOC stored in the top 10, 20, 40, and 60 cm accounted for 17.7, 31.7, 53.8, and 71.3%, respectively, of the total amount in the top 100 cm. The region's extensive desertification appears to be one of the most important factors that led to the relatively low SOC content and the difference between the northern and southern parts of the Horqin Grassland. Our results provide an important baseline for evaluating past losses of SOC due to desertification, and for projecting the potential increase in SOC from the restoration of desertified land and how SOC will respond to climate change. [ABSTRACT FROM AUTHOR]

The spatial pattern of soil organic carbon (SOC) and total nitrogen (TN) densities plays a profound important role in estimating carbon and nitrogen budgets. Naiman Banner located in northern China was chosen as research site, a total of 332 soil samples were taken in a depth of 100 cm from the low hilly land in the southern part, sandy land in the middle part and an alluvial plain in the northern part of the county. The results showed that SOC and TN density initially decreased and then increased from the north to the south, The highest densities, were generally in the south, with the lowest generally in the middle part. The SOC and TN densities in cropland were significantly greater than those in woodland and grassland in the alluvial plains and for Naiman as a whole. The woodland SOC and TN density were higher than those of grassland in the low hilly land, and higher densities of SOC and TN in grassland than woodland in the sandy land and low hilly land. There were significant differences in SOC and TN densities among the five soil types of Cambisols, Arenosols, Gleysols, Argosols, and Kastanozems. In addition, SOC and TN contents generally decreased with increasing soil depth, but increased below a depth of 40 cm in the Cambisols and became roughly constant at this depth in the Kastanozems. There is considerable potential to sequester carbon and nitrogen in the soil via the conversion of degraded sandy land into woodland and grassland in alluvial plain, and more grassland should be established in sandy land and low hilly land. [ABSTRACT FROM AUTHOR]

Root decomposition is a critical feedback from the plant to the soil, especially in sandy land where strong winds remove aboveground litter. As a pioneer shrub in semi-mobile dunes of the Horqin sandy land, Artemisia halodendron has multiple effects on nutrient capture and the microenvironment. However, its root decomposition has not been studied in terms of its influence on soil organic carbon (SOC) and nitrogen (N). In this study, we buried fine (≤2 mm) and coarse roots in litterbags at a depth of 15 cm below semi-mobile dunes. We measured the masses remaining and the C and N contents at intervals during 434 days of decomposition. The soils below the litterbags were then divided into layers and sampled to measure the SOC and N contents. After rapid initial decomposition, both coarse and fine roots decomposed slowly. After 53 days, 36.2 % of coarse roots and 39.8 % of fine roots had decomposed. In contrast, only 18.4 % of coarse roots and 30.5 % of fine roots decomposed in the following 381 days. Fine roots decomposed significantly faster, and their decomposition rate after the initial rapid decay was strongly related to climate ( R = 0.716, P < 0.05). Root decomposition increased SOC and N contents below the litterbags, with larger effects for fine roots. The SOC content was more variable between soil layers than the N content. Thus, decomposition of A. halodendron roots cannot be ignored when studying SOC and N feedbacks from plants to the soil, particularly for fine roots. [ABSTRACT FROM AUTHOR]

Abstract: Overgrazing is a primary agent to cause and aggravate desertification in the Horqin Sandy Grassland of northern China that has reduced the capacity of carbon (C) and nitrogen (N) storage in the region. Grazing exclosure practice is recommended to control the desertification by vegetation restoration. How the restoration improves the C and N storage needs to be answered to the policy makers for their future land use planning and decisions. This storage in a plant–soil system down to 100cm inside grazing exclosure after practice terms of 7, 12, and 25 years was measured as compared to active sand dunes referred as exclosure practice term of 0. Organic C storage in the system increased by 341, 822, and 1786gm−2, respectively, after 7, 12, and 25 years of grazing exclosure, whereas the corresponding N storage increased by 30, 65, and 139gm−2. Soil is the major reservoir in the system to store C and N. Temporally along the exclosure terms of 0, 7, 12, and 25 years, the soil C storage comprised 97, 88, 77, and 84% of total plant–soil system C storage, respectively, whereas the corresponding soil total N storage comprised 99, 97, 94, and 94% of total plant–soil system N storage. Our results provide land use policy makers with rationale how to manage the grazing practice to restore the vegetation for C and N sequestration. [Copyright &y& Elsevier]

Abstract: Horqin Sandy Land is a seriously desertified and an ecologically fragile region of China. Soil degradation and desertification in this region are the result of several decades of overgrazing, non-manure cropping with short fallow, and arbitrary land use and management. We investigated whether the afforestation of active sand dunes with Mongolian pine (Pinus sylvestris var. mongolica Litv.) enhances the carbon (C) and nitrogen (N) storages and the overall soil quality. We compared soil physico-chemical properties, and C and N storages in the total and light fraction soil organic matter of active sand dunes, and of 25- and 35-year-old plantations. Soil water holding capacity, fine particle content and nutrients were significantly higher in plantations than in active sand dunes, with greater improvements occurring in the top 5cm than in the 5–15cm layer of the soil profile. Soil C and total N storages were 6.1 and 3.7 folds in 25-year-old, and 10.2 and 5.3 folds in 35-year-old plantations compared to active sand dunes, respectively. Carbon and N storages in the light fraction were 20.9 and 15.8 folds in 25-year-old, and 36.1 and 25.0 folds in 35-year-old plantations, respectively, relative to active sand dunes. The light fraction played an important role in soil C sequestration and its amount is an indicator of the effects of afforestation on C storage in sandy soil. The upper 15cm of the soil profile in the 35-year-old plantations of Mongolian pine has the capacity to sequester significant amount of C in the region, potentially offsetting part of the carbon that has been lost due to desertification over the past century in the Horqin Sandy Land. Our results conclude that afforestation with Mongolian pine has had positive impacts on soil quality and has increased the capacity for soil C sequestration in semi-arid degraded areas. [Copyright &y& Elsevier]

• We studied SOC changes in northern China since the 1980s based on survey data. • The SOC density decreased from 4.48 to 3.60 kg C m−2. • The SOC stock decreased from 2793.17 to 2248.34 Tg. • Grassland degradation was the main cause of decreased SOC. • Afforestation and grassland restoration were the main causes of increased SOC. Soil carbon sequestration and potential sequestration are key issues in global carbon research, as the size of the soil organic carbon (SOC) stocks and their changes over time are important for understanding their role in climate change. In northern China's agro-pastoral ecotone, we compared SOC data derived from 238 sampling sites during China's National Soil Inventory in the 1980s with the results of field surveys at 644 sites in 2018 to calculate the changes in SOC stocks between the 1980s and 2018. We found that the mean SOC stock to a depth of 30 cm decreased significantly, from 4.48 kg C m−2 in the 1980s to 3.60 kg C m−2 in 2018, and the total SOC stock decreased from 2793.17 Tg to 2248.34 Tg. This amounted to an average decrease of 27.74 g C m−2 yr−1 over the 30-year study period. Furthermore, we used a land use and cover change transition matrix to quantify the amount of SOC change caused by these transitions; the SOC reduction could be mainly ascribed to overgrazing, grassland reclamation for agriculture, and grassland desertification. Large-scale ecological restoration projects have increased SOC reserves, but not enough to compensate for the loss of SOC caused by factors such as desertification and ecosystem degradation. We also performed regression analysis for the relationship between the change in the SOC stock and the rates of temperature and precipitation change. We found that increasing temperature non-significantly decreased SOC, whereas precipitation was significantly (P < 0.05) related to SOC change, but the R 2 was very low (0.01). Our analysis suggests that humans have affected SOC in northern China's agro-pastoral ecotone more strongly than climatic factors. [ABSTRACT FROM AUTHOR]