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Deep soil organic carbon (SOC) plays an important role in carbon cycling. Precisely predicting deep SOC at the regional scale is crucial for the accurate assessment of carbon sequestration potential in soils but has been challenging for a century. Herein, we developed a depth distribution function-based empirical approach to predict SOC in deep soils at the regional scale. We validated this approach with a dataset from four regions of the world and examined the application of this approach in China's Loess Plateau. We found that among the reported depth distribution functions describing vertical patterns of SOC, the negative exponential function performed best in fitting SOC along the soil profile in various regions. Moreover, the parameters (i.e., Ce and k) of the negative exponential function were linearly correlated to surface SOC (0–20 cm) and the changing rates of SOC within the topsoil (0–40 cm). Based on the above relationships, the empirical equations for predicting the negative exponential parameters are established. The validation results from site-specific and regional dataset showed that combining the negative exponential function and such empirical equations can precisely predict SOC concentration in soils down to 500 cm depth. Our study provides a simple, rapid and accurate method for predicting deep soil SOC at the regional scale, which could simplify the assessment of deep soil SOC in various regions. [ABSTRACT FROM AUTHOR]

Highlights • Converting grassland to shrubland increased soil N availability and mineralization. • The positive effect on soil mineral N was higher in lower slope than upper slope. • The positive effect on soil mineral N was similar across soil textures. • The effect on net N mineralization varied with slope position and soil texture. Abstract Grassland afforestation, which is a major form of land-use change, has expanded considerably in the last few decades. However, the effect of conversion from grassland to shrubland on soil nitrogen (N) cycling and availability with regard to soil textures and slope positions is unclear. This lack of clarity has hindered our understanding of how grassland afforestation influences soil N availability and transformation on a larger spatial scale. In this study, we compared the net N mineralization rates and mineral N concentrations in soils from native grassland and under legume shrub (Caragana korshinskii Kom.) with contrasting soil textures (sandy-loam vs. loamy-sand) and slope positions (upper vs. lower) in the northern region of China's Loess Plateau. The objective of this study is to understand how soil texture and slope position regulate the response of soil N cycling to grassland afforestation. The results showed that the concentrations of soil nitrate and total mineral N and the rates of net nitrification and mineralization were higher in soils under shrub (0.64 ± 0.10 g N m−2, 1.01 ± 0.11 g N m−2, 23.19 ± 5.22 mg N m−2 d-1 and 22.07 ± 5.44 mg N m−2 d-1) than in grassland (0.22 ± 0.03 g N m−2, 0.56 ± 0.04 g N m−2, 1.48 ± 1.51 mg N m−2 d-1 and 1.36 ± 1.96 mg N m−2 d-1). However, soil ammonium concentration and net ammonification rate were not affected by grassland afforestation. The effect of afforestation on nitrate and total mineral N concentrations was greater in the lower slope than in the upper slope but was similar between the sandy-loam and loamy-sand soils. The effect of this land-use change on net nitrification and mineralization rates varied with soil texture and slope position, with greater effects in the upper slope than in the lower slope in more clayey soils, but with the opposite influencing pattern in more sandy soils. These results suggested that planting C. korshinskii in grassland ecosystems increased soil N mineralization and availability. More importantly, soil texture and slope position should be considered when assessing the effects of grassland afforestation in complex landscape conditions. [ABSTRACT FROM AUTHOR]

• Soil in eroding zone had higher mineral N and net mineralization rates than in depositional zone. • Vegetation removal increased soil mineral N but not affected soil net N mineralization rates. • The effects of vegetation removal on mineral N pools depended on landscape position, site, and season. • The responses of net N mineralization rates to vegetation removal were consistent across landscape positions and sites, but varied with season. The soil nitrogen (N) availability is a key limiting element of terrestrial ecosystem productivity and functioning, especially in fragile ecosystems (e.g., eroding environments). However, to date, little is known about how vegetation and erosion individually or jointly affect soil N mineralization under field conditions across a range of sites varying in soil and climate properties on a long-term observational scale. In this study, we elucidated how soil net N mineralization responds to vegetation removal (plots with vs. without vegetation) and landscape position (eroding vs. depositional zones) during a two-year manipulation experiment using an in situ incubation method on the China's Loess Plateau. The results indicated that the nitrate and total mineral N concentrations and net nitrification and mineralization rates were higher in the eroding zone (2.74 ± 0.12 mg kg−1, 4.47 ± 0.13 mg kg−1, 0.046 ± 0.005 mg kg−1 d-1 and 0.046 ± 0.006 mg kg−1 d-1) than those in the depositional zone (2.01 ± 0.10 mg kg−1, 3.80 ± 0.12 mg kg−1, 0.030 ± 0.005 mg kg−1 d-1 and 0.032 ± 0.006 mg kg−1 d-1) on the China's Loess Plateau. Vegetation removal resulted in an increase in the nitrate (2.88 ± 0.13 vs. 1.87 ± 0.09 mg kg−1) and total mineral N concentrations (4.56 ± 0.14 vs. 3.71 ± 0.11 mg kg−1), but did not affect the soil net N mineralization rates on the China's Loess Plateau. The effects of vegetation removal on soil mineral N pools depended on the site, season and landscape position, with stronger effects in the eroding zone than those in the depositional zone. However, the responses of the net N mineralization rates to vegetation removal were consistent across the various landscape positions and sites, but varied with the season. Variance partitioning revealed that soil properties rather than climate and vegetation conditions were the most dominant contributors to the variations of soil N mineralization in both the eroding and depositional zones, explaining 37.6–64.6% of the variation. These results emphasized the substantial spatial (landscape position and site) and temporal (seasonal) dependence of the effects of vegetation removal on soil N mineralization at large scale. [ABSTRACT FROM AUTHOR]

Soil carbon (C) and nitrogen (N) combining with physical and chemical properties play important roles in terrestrial C and nutrient cycling; however, how C and N in deep soils respond to land-use change is less understood, hindering the precise evaluation of soil C dynamics and budgets. Here, we present results regarding soil C, N and related soil properties at 0–500 cm depth as affected by afforestation at two sites (Fufeng and Yongshou) with contrasting soil inorganic C (SIC) levels in China's Loess Plateau. The concentrations of SIC ranged from 12.94 to 20.17 g kg−1 in Fufeng and from 0.18 to 0.36 g kg−1 in Yongshou. The objectives of this study were to examine how soil C and N in deep soils are altered following afforestation and how these effects vary with soil depth and site. We found that the establishment of woodland on cropland increased soil porosity and saturated hydraulic conductivity but decreased the bulk density in the topsoils (0–20 cm). The conversion of cropland to woodland resulted in an increase of soil electric conductivity in the topsoils and a decrease in the deep soils in Fufeng but had an opposite influence with soil depth in Yongshou. This land-use change increased organic C and total N concentrations along the 0–500 cm soils at both sites but had divergent effects on the concentrations of SIC and soil total C (STC) between the two sites. Overall, the concentrations of SIC and STC increased in Yongshou but decreased in Fufeng after woodland establishment. The results from this study indicated the determination of the initial SIC concentration on the response of C to afforestation in semiarid soils, and such determination should be considered in biogeochemical models. • Afforestation decreased pH and increased SOC and TN regardless of sites. • Effects of afforestation on soil C were dependent on initial SIC level. • Afforestation increased SIC and STC in low SIC site and reduced them in high SIC site. [ABSTRACT FROM AUTHOR]

Soil nitrogen (N) mineralization is an important ecological process influencing N availability and ecosystem functionality, and is sensitive to vegetation presence and soil erosion. However, how vegetation presence affects N mineralization and its response to erosion has not been directly examined, hindering our ability to predict N cycling in eroding environments. Herein, we compared the effect of vegetation presence on N mineralization and mineral N dynamics in two growing seasons between erosion vs. deposition sites in three sub-watersheds in the transition zone between water erosion and wind erosion of North China. We aim to examine how soil N cycling responds to erosion and vegetation presence at temporal and spatial scales. The mineral N concentration in the soil solution and mineralization rates between May 2017 and October 2018 in plots with and without vegetation were measured to assess the effects of vegetation presence. We showed that N mineralization rates did not significantly vary among the three sub-watersheds or between erosion and deposition sites. The concentrations of mineral N in the soil solution were higher in relatively larger sub-watersheds for the erosion sites and in smaller sub-watersheds for the deposition sites. Vegetation decreased soil NO 3 --N and total mineral N and increased NH 4 +-N concentrations in the early and middle periods of the growing season. The effects of vegetation presence on net N mineralization rates varied with size of sub-watersheds, with increased rates of net nitrification and mineralization at both deposition and erosion sites in relatively larger sub-watershed, but reduced rates of net nitrification and mineralization in smaller sub-watersheds. Moreover, the presence of vegetation shifted the seasonal pattern of N mineralization. Therefore, vegetation presence had important ecological regulation on soil N cycling at spatial and temporal scales in the transition zone between water erosion and wind erosion. • Soil mineral N varied with sub-watershed scales and landscape positions. • Vegetation decreased nitrate and total mineral N but increased ammonium. • The effects of vegetation on N mineralization varied with sub-watershed scales. • Vegetation presence shifted the seasonal pattern of N mineralization. [ABSTRACT FROM AUTHOR]

• Afforestation increased significantly soil OC and N mineralization. • The increase of OC and N mineralization were larger in sites with high clay content. • The temperature sensitivity decreased in sites with relative high clay content. • Soil texture regulated OC and N mineralization and their temperature sensitivity. Afforestation on abandoned cropland has great potential to increase soil organic carbon (OC) and nitrogen (N). Identifying the effect of afforestation on mineralization of OC and N as well as their temperature sensitivity at large spatial and temporal scales is essential to understand soil OC and N dynamics. In this study, we measured mineralization of soil OC and N at standard temperatures (15 and 25 °C) by laboratory incubation, and calculated their temperature sensitivity (Q 10). Soil samples were collected from cropland (0 years, selected as a control) and adjacent woodlands with different afforestation age (10, 20 and 30 years) at five sites from north to south on China's Loess Plateau. The objectives were to understand how long-term afforestation affects OC and N mineralization and their Q 10 and whether such effects depend on afforestation age and soil texture. The results showed that afforestation increased OC and N mineralization, but this effect was not affected by afforestation age (10–30 years). The Q 10 of OC and N mineralization increased in the first 10 years of afforestation but decreased in the late stage. Furthermore, the increases of OC and N mineralization by afforestation were higher at sites with fine-textured soils than those at sites with coarse-textured soils. However, the Q 10 increased at sites with a relatively high sand content but decreased or remained unchanged at sites with a relatively high clay content. The changes in OC and N mineralization and their Q 10 were related to soil OC and N contents, pH and clay content. Overall, we concluded that long-term afforestation would contribute to the stability of soil OC and N in afforested ecosystems. Soil texture is an important factor controlling OC and N mineralization and their Q 10 , and should be considered when predicting soil OC and N dynamics in response to temperature change in different landscapes. [ABSTRACT FROM AUTHOR]