Variation of Soil Organic Carbon Density with Plantation Age and Initial Vegetation Types in the Liupan Mountains Areas of Northwest China

Carbon sequestration of plantations formed by three kinds of forestation (natural forest to plantation (NP), grassland to plantation (GP), and cropland to plantation (CP)) greatly depends on the change of soil organic carbon density (SOCD) compared with its initial SOCD before forestation. However, this dependence was rarely studied, especially in semi-humid/arid regions with strong site variation. This limits the precise assessment and management of SOCD. Therefore, the SOCD variations of 0–100 cm soil layers in these three kinds of plantations were studied in the semi-humid/arid Liupan Mountains in northwestern China. The NP with high initial SOCD showed firstly a decrease and then an increase of SOCD up to 293.2 t·ha−1 at 40 years. The CP and GP with low and relatively high initial SOCD showed negligible and slight SOCD decrease after forestation, but then an increase up to 154.5 and 266.5 t·ha−1 at 40 years. After detecting the main factors influencing SOCD for each forestation mode, statistic relationships were fitted for predicting SOCD variation. This study indicates that besides forest age and biomass growth, the effects of initial vegetation, site-dependent initial SOCD, and SOCD capacity, also precipitation and air temperature in some cases, should be considered for more precise assessment and management of SOCD of plantations.

Effects of fine roots on the distribution of soil water and soil organic carbon in a shelterbelt modified agricultural system

<p>Shelterbelts, also termed windbreaks, play a significant role in reducing soil erosion, sand drift, and protecting crops, livestock and farmstead. Cropland shelterbelts are known to improve the microclimate and provide protection against sand-related damages to crops. However, the protection of shelterbelts to crops may be complicated by fine roots near the edge of the shelterbelt, which directly affects the absorption of soil water and the accumulation of soil organic carbon. In this study, we investigated the effects of shelterbelt fine roots on farmland soil water content (SWC) and soil organic carbon density in an agricultural system located in northern China. The distribution characteristics of fine-root biomass density, soil water content and soil organic carbon density were measured at the 0-200 cm soil depth in a farmland shelterbelt system at distances of 0.3H (H is the windbreak height), 0.5H, and 0.7H from the shelterbelt with three replicates. Soil samples were randomly collected in the center of the cropland. The results showed that fine roots of shelterbelts concentrated at the depth of the 20-60 cm soil layer, whereas the fine roots of the crop summer squash (Cucurbita pepo L.) were mainly found within the top 20 cm of the soil profile. With the presence of shelterbelt fine roots, strong SWC deficiency that occurred up to100 cm in the soil profile was observed. The SWC was deficient at all sampling points, the overall shallow layer deficient was larger than the deep layer. In contrast, the soil organic carbon was cumulative, and the overall deep accumulation was greater than the shallow layer. The effects of fine roots on SWC and soil organic carbon density variations in different soil layers were inconsistent. The information accrued in this study can be used to evaluate the effect of farmland shelterbelt on soil water and soil carbon in the Hetao irrigation area of Inner Mongolia, northern China.</p>

Modeling and Predictive Mapping of Soil Organic Carbon Density in a Small-Scale Area Using Geographically Weighted Regression Kriging Approach

Different natural environmental variables affect the spatial distribution of soil organic carbon (SOC), which has strong spatial heterogeneity and non-stationarity. Additionally, the soil organic carbon density (SOCD) has strong spatial varying relationships with the environmental factors, and the residuals should keep independent. This is one hard and challenging study in digital soil mapping. This study was designed to explore the different impacts of natural environmental factors and construct spatial prediction models of SOC in the junction region (with an area of 2130.37 km2) between Enshi City and Yidu City, Hubei Province, China. Multiple spatial interpolation models, such as stepwise linear regression (STR), geographically weighted regression (GWR), regression kriging (RK), and geographically weighted regression kriging (GWRK), were built using different natural environmental variables (e.g., terrain, environmental, and human factors) as auxiliary variables. The goodness of fit (R2), root mean square error, and improving accuracy were used to evaluate the predicted results of the spatial interpolation models. Results from Pearson correlation coefficient analysis and STR showed that SOCD was strongly correlated with elevation, topographic position index (TPI), topographic wetness index (TWI), slope, and normalized difference vegetation index (NDVI). GWRK had the highest simulation accuracy, followed by RK, whereas STR was the weakest. A theoretical scientific basis is, therefore, provided for exploring the relationship between SOCD and the corresponding environmental variables as well as for modeling and estimating the regional soil carbon pool.

Simulated Soil Organic Carbon Density Changes from 1980 to 2016 in Shandong Province Dry Farmlands Using the CENTURY Model

The changes in cultivated soil organic carbon (SOC) have significant effects on soil fertility and atmospheric carbon dioxide (CO2) concentration. Shandong Province is an important agricultural and grain production area in China. Dry farmland accounts for 74.15% of the province’s area, so studies on dynamic SOC changes would be helpful to understand its contribution to the Chinese national carbon (C) inventory. Using the spatial overlay analysis of the soil layer (1:10,000,000) and the land use layer (1:10,000,000), 2329 dry farmland soil polygons were obtained to drive the CENTURY model to simulate SOC dynamics in Shandong Province from the period 1980 to 2016. The results showed that the CENTURY model can be used to simulate the dry farmland SOC in Shandong Province. From the period 1980 to 2016, the soil organic carbon storage (SOCS) and soil organic carbon density (SOCD) showed an initial increase and then decreased, especially after reaching a maximum in 2009. In 2016, the SOCS was 290.58 × 106 t, an increase of 26.99 × 106 t compared with 1980. SOCD in the dry farmland increased from 23.69 t C ha−1 in 1980 to 25.94 t C ha−1 in 2016. The dry farmland of Shandong Province was a C sink from 1980 to 2016. Among the four soil orders, inceptisols SOCD dominated, and accounted for 47.81% of the dry farmland, followed by >entisols > vertisols > alfisols. Entisols SOCD growth rate was the highest (0.23 t C ha−1year−1). Compared to 1980, SOCD in 2016 showed an increasing trend in the northeast, northwest and southeast regions, while it followed a downward trend in the southwest.