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Aims: Cropland revegetation is an effective measure to curb soil erosion on eroding hillslopes and increase farmers' income at depositional check dams. However, how soil microbial metabolic limitation responds to cropland revegetation in erosion and deposition landscapes remains poorly understood, which has substantial implications for carbon (C) retention and nutrient cycling in the eroding environment. Methods: We sampled 0–2 m soils in cropland and revegetated forest and grassland at upslopes and check dams in the hilly-gully region of the Loess Plateau, China. The activities of soil C-, nitrogen (N)-, and phosphorus (P)-acquiring enzymes were analyzed. The improved enzyme vector model (V-T model) based on balance points was used to quantify microbial metabolic limitations based on ecoenzymatic stoichiometry. Results: Microorganisms suffered from no energy (C) limitation, but the relative microbial C limitation was greater in revegetated forest than in cropland. At upslopes, the revegetated forest was primarily limited by P, while the revegetated grassland and cropland were limited by N, which was indicated by the VTN/P limitation values of 4.35, -2.74, and − 1.87, respectively. Microbial P limitation was greater in deep soils of revegetated forest due to the assimilation of P by the root system. At check dams, land-use change had no significant influence on microbial N/P limitations owing to abundant soil C and nutrients and a wet environment. Conclusion: Concludingly, cropland revegetation had a weaker influence on microbial metabolic limitation at the lower-lying topography, which compensates for the current understanding of resource restrictions on microorganisms at slopes or flat areas. [ABSTRACT FROM AUTHOR]

Soil erosion is a major form of land degradation, especially in agroecosystems, which has been effectively controlled by vegetation restoration. However, the interactive role of erosion and cropland revegetation on soil enzyme activities and microbial nutrient limitations is less understood. To address this issue, we examined carbon (C), nitrogen (N), and phosphorus (P) in bulk soils and microbial biomass, enzyme activities, and microbial nutrient limitations in the 0–200 cm soils in eroded and deposited landscapes occupied by cropland, revegetated forest, and grassland. The results showed that the activities of C-, N-, and P-acquiring enzymes were larger in the deposited landscape than in the eroded landscape for 0–20 cm soils in forest and grassland but not in cropland. Microbial metabolism was co-limited by N and P, and the threshold element ratio (TERL) indicated that P was the most limiting factor. Microbial N limitation was lower in the deposited than the eroded zone, especially in surface soils in revegetated forest and grassland. The TERL value was larger at the deposited than at the eroded zone, and a greater difference was found in the surface soils of forest and grassland. Microbial nutrient limitations were mostly explained by C/P and N/P. Conclusively, the deposited areas were characterized by ameliorated enzyme activities, decreased microbial N limitation but relatively strengthened microbial P limitation compared to the eroded area, and such variations existed in the revegetated forest and grassland but not in the cropland, which thus contributes to a better understanding of C and nutrient cycling for agroecosystems and revegetation ecosystems in eroded environments. [ABSTRACT FROM AUTHOR]

• Eroding soils and sediments were collected at 0–2 m depth across the Loess Plateau. • Sediments had higher clay than eroding soils at sites with higher erosion modulus. • Sediments had higher BD, lower Ks, FC, porosity than soils at sites with higher MAP. • Reaggregation of deposited sediments occurred, which were enriched in clay and SOC. • Spatial heterogeneity in deposition zone was not lower than that in erosion zone. Soil erosion, the most widespread form of soil degradation worldwide, has been acknowledged to cause onsite soil physical deterioration. However, the off-site impact of erosion on the soil physical and hydraulic properties of deposited sediments remains unclear, especially at catchment or larger scales. Here, we measured soil physical and hydraulic properties in erosion topography (hillslopes) and deposition topography (check dams) at 0–2 m depth in five eroding catchments across China's Loess Plateau. Results indicated that the deposition zone exhibited a higher clay content (+13%) at the southern sites with a finer texture and a higher erosion modulus, while the same zone exhibited a higher sand content (+25%) at the northern sites with a coarser texture and a higher extreme rainfall erosivity. The deposition zone exhibited a higher bulk density (+9%) and a lower saturated hydraulic conductivity (-13%), field capacity (−9%), and porosity (-5%) than those of the erosion zone, which was strengthened by a higher mean annual precipitation. Soil macroaggregate proportion and mean weight diameter of aggregates were significantly lower in the deposition zone (14.36% and 0.69 mm, respectively) than those in the erosion zone (15.71% and 0.75 mm, respectively), and the differences were larger at sites with higher rainfall erosivity levels and in surface 20 cm soils. However, sediment reaggregation occurred due to the accumulation of binding agents such as clay and organic carbon at sites with lower rainfall erosivity levels. The spatial heterogeneities of sediment physical and hydraulic properties were greater than or similar to those of soils in the erosion zone. Results of our study highlight the dependence of variations in soil physical and hydraulic properties driven by erosion and deposition on erosion factors, which contributes to a better understanding of the hydrology cycle and carbon stability in erosion and deposition topographies. [ABSTRACT FROM AUTHOR]

• WDOM parameters in erosion and deposition zone were tested across Loess Plateau. • Sediment below 20 cm had higher WEOC, aromaticity, hydrophobicity, humic components. • Eroded soils above 20 m had higher WEOC and exogenous features with plant inputs. • WEOM variations were higher in deep soils and in sites with higher erosion modulus. • WEOM variations were mostly driven by vegetation in top and by ΔSOC in deep soils. Water-extractable organic matter (WEOM) is the most active and mobile fraction of soil organic matter and responds sensitively to soil erosion. However, previous studies on the chemical nature of WEOM in the erosion and deposition landscapes were controversial at site-level studies and mainly focused on surface soils, limiting our assessment of WEOM's environmental fate at larger and deeper scales. Here, we collected eroded soils and deposited sediments from a depth of 0–200 cm in five eroding catchments with an erosion intensity gradient across the Loess Plateau. We examined the concentration, composition, and potential sources of WEOM using UV–visible absorption and fluorescence excitation-emission matrix-parallel factor analysis. Soil WEOM quantity and quality varied significantly in the erosion and deposition landscapes, with greater variations in soils below 20 cm, where WEOM in the deposition zone was characterized by higher concentration, aromaticity, hydrophobicity, and UVA and UVC humic-like components derived from terrestrial and soil organic matter. WEOM variations in deep soils increased with increasing erosion modulus among sites, and were mainly driven by changes in soil organic carbon concentration, aggregate stability, and carbon to nitrogen ratio. However, the pattern of WEOM parameters between erosion and deposition zones at 0–20 cm was opposite to that in soils below 20 cm. Vegetation inputs compensated the loss of organic matter in sites with suitable climatic status and weak erosion, and led to higher concentrations and exogenous features of WEOM in the surface erosion area. Concludingly, the WEOM concentration, components, and sources differed significantly between erosion and deposition zones, and these differences, as well as their response to erosion intensity, revealed an opposing pattern in surface and deep soils. [ABSTRACT FROM AUTHOR]

Check dams are small engineering structures. They are used worldwide for soil and water conservation, gully rehabilitation, hydrological regulation, and food supply. Eroded materials trapped and buried in check dams constitute an important carbon (C) sink. However, as a typical deposition site, systematic information about C cycling processes in check dams is relatively unknown, despite its crucial role in estimating the fate of buried C and elucidating the ongoing C sink/source debate on soil erosion. This study reviewed the stock, source, enrichment, distribution, stability, and underlying mechanisms of sediment C in check dams. Current studies have found a large amount of C stored in check dams (approximately 1–30 Mg C km−2 yr−1), and have demonstrated that C retention ability relied more on factors governing sediment production and trapping at larger scales than on check dam trapping efficiency at smaller scales. Stable isotopes, radiocarbon, geochemical properties, biomarkers, and spectroscopy methods have proven to be practical in identifying the sources of sediment C but can only be used in certain situations because of their intrinsic limitations. Both enrichment and impoverishment of sediment C compared with source soils have been reported, and have depended on soil erosion type, land-use type, sediment connectivity, and the loss of C during transportation. Sediment C generally increases from the back of the check dam to the front along the flow pathway and fluctuates vertically, depending on the clay and sand layers of the deposition couplets. Information about C mineralization and sequestration is relatively limited, and the existing studies have shown reduced C mineralization and promoted C-fixing potential in check dam sediments. Given the specific benefits of deposition couplets in reflecting information about burial period, erosion intensity, and historical land use, further studies should integrate C stability with soil erosion and deposition information for the deposit profiles. The responses of C cycling to climate change and human disturbances, such as warming, dam-break, tillage, and fertilization for check dam sediments require further research. • Check dams store significant C depending on sediment input and trapping efficiency. • Both enrichment and impoverishment of sediment C occur compared with source soils. • Sediment C raises with flow path horizontally and fluctuates with couplets vertically. • Stable C fractions dominated in the sediments and changed little with depth. • Effects of warming, tillage, fertilization, and dam-break on C stability are needed. [ABSTRACT FROM AUTHOR]

Soil erosion is continually overlooked in the hilly red soil region of southern China, and there is a substantial lack of knowledge about the effects of soil and water conservation measures (SWCMs) on erosion prevention and soil carbon and nitrogen accumulation for the eroded sloping arable land in this region. Herein, based on a long-term experiment in the red soil hilly region in China, we investigated the effects of 17 years of SWCMs on soil organic carbon (SOC), total nitrogen (TN), microbial biomass carbon and nitrogen (MBC and MBN, respectively) as well as soil erodibility (K factor) at soil depths of 0–100 cm in the top, middle and bottom parts of the slope. The SWCMs included straw mulching (SM), downslope ridge cultivation (DC), downslope ridge cultivation + contour hedgerow planting (DCH), and contour ridge cultivation tillage (CC). A bare land treatment without any crops was used as a control (CK). We found that SWCMs increased SOC, TN, MBC, and MBN by 8–57%, 1–14%, 25–68%, and 17–35%, respectively, with greater increases in these variables in the CC and SM treatments and smaller increase in the DC treatment. Additionally, the effects of the SWCMs were greater at the top and middle of the slope than at the bottom of the slope, and the effects varied with soil depth. Moreover, the SWCMs decreased soil erodibility, with a lower K factor in the SWCMs treatments (0.018–0.049 t ha h ha−1 MJ−1 mm−1) than in the CK (0.042–0.068 t ha h ha−1 MJ−1 mm−1) and a greater decrease in the CC treatment. The effects of the SWCMs on soil erodibility mainly occurred through the regulation of SOC according to structural equation modeling. Overall, this study suggested that straw mulching and contour ridge cultivation tillage were the most effective strategies for restoring soil carbon and nitrogen and preventing soil erosion. • The SWCMs improved soil carbon and nitrogen levels but reduced erodibility. • The effects of the SWCMs on soil erodibility mainly through the regulation of SOC. • Straw mulching and contour ridge cultivation tillage prevent soil erosion conspicuously. [ABSTRACT FROM AUTHOR]

• Up-slope runoff and sediment increased most nutrient contents on the wide slope. • Up-slope runoff and sediment decreased most nutrient contents on the narrow slope. • Up-slope runoff and sediment reduced random spatial heterogeneity of soil nutrients. Upslope runoff and sediment critically influence soil erosion, while their impact on the subsequent downslope nutrient distribution has not been examined, consequently hindering the exact assessment of the effects of erosion on soil biogeochemical cycles. Herein we selected two groups of wide and narrow slopes on the middle Loess Plateau, with each slope group including slopes with and without upslope runoff and sediment. For each slope, we collected soil samples from topsoil along the slope at different distances from the upper slope to the lower slope, and collected samples from depths of 0–60 cm at the upper, middle and lower positions of the slope. The contents of soil organic matter and nutrients were analysed. The purpose of this study was to disentangle the effects of upslope runoff and sediment on spatial distribution of soil nutrients on slopes of different widths. We showed that compared with slopes without upslope runoff and sediment, upslope runoff and sediment increased the content of most nutrients in topsoil on the wide slope, but had a contrasting effect on the narrow slope. Upslope runoff and sediment reduced the random spatial heterogeneity of soil nutrients on both the wide and narrow slopes, with greater effects observed on the narrow slope. The influence of upslope runoff and sediment on the profile distribution of soil nutrients varied with slope position and slope width, and was nutrient type-dependent. These results indicated that slope topographical conditions interact with upslope runoff and sediment in influencing vertical and horizontal patterns of soil nutrients on slopes. [ABSTRACT FROM AUTHOR]

• Higher erosion intensity and smaller size of check dam resulted in greater variation in soil particles. • Soil OC and N in check dams decreased with the distance from the dam body to the gully head. • Silt content was more important than clay content in influencing the distribution of OC and N in check dam soils. Check dams are effective soil and water conservation structures worldwide that play an important role in preserving sediments containing carbon (C) and nitrogen (N). However, in terms of the depositional characteristics of C and N in check dams, research on the relative effects of erosion intensity and depositional area is limited, hindering our understanding of the fates of eroded C and N. This study compared the horizontal and vertical distributions of soil particles, organic C (OC), and total N (TN) in check dams of various sizes and erosion intensities in the Loess Plateau. The results showed greater variations in the horizontal and vertical patterns of soil particles of smaller check dams and highly serious erosion (erosion modulus = 15040 t km-2 a-1) than those that of larger check dams and strong erosion regions (erosion modulus = 5151 t km-2 a-1). Soil OC and TN contents decreased from the dam body to the gully head, especially in the highly serious erosion region. Notably, the effect of smaller check dam on the vertical distributions of soil OC and TN was more intense than that of the larger check dam in the strong erosion region. In contrast, a minor effect of check dam size was observed in the highly serious erosion region. Moreover, a closer relationship between the distribution of OC and TN with silt content rather than with clay content in check dams was observed, and this relationship was enhanced by an increase in erosion intensity. Overall, the synergistic effects of erosion intensity and check dam size affected the spatial patterns of OC and TN in sediments on the Loess Plateau. Careful consideration of these two factors is beneficial for accurately assessing the C and N fates in depositional areas. [ABSTRACT FROM AUTHOR]

Cultivation after the converting natural ecosystems to agricultural land is often accompanied by accelerated soil erosion. To better understand how land-use conversion and agricultural erosion separately affect aggregate-associated nitrogen (N) and organic carbon (OC), we examined the responses of aggregate-associated N and OC to both the converting forest to agricultural land and erosion in the agricultural region of Northeast China. Non-erosion sites in forest and agricultural land converted from forestland 60 years ago were selected to address the effects of land-use change. The sites with various erosion intensities in an eroding area of sloping cropland (i.e., non-erosion, light erosion, moderate erosion and heavy erosion) were selected to compare the effects of the erosion intensity. The measured soil metrics included the soil bulk density, water-stable aggregates, and aggregate-associated OC and N along 0–70 cm soil profiles. Converting forest to agricultural land did not significantly influence the OC and N concentrations in each aggregate, while the land-use change significantly reduced the OC and N stocks in macroaggregates but increased them in microaggregates and silt+clay fraction. However, erosion significantly decreased the OC and N concentrations and stocks in each aggregate. The changes in the OC and N stocks in aggregates were mainly due to the depletion of soil aggregates in responding to the conversion of forest to agricultural land but due to decreases in OC and N concentrations in aggregates in responding to soil erosion. Our results indicated that the effects of erosion on aggregate-associated OC and N were greater compared with the effects of converting forest to agricultural land in this Mollisol region. • Land-use change didn't significantly affect OC and N concentrations in aggregates. • Erosion decreased OC and N concentrations in aggregates. • OC and N stocks in aggregates were determined by their concentrations during erosion. • OC and N stocks in aggregates were dominated by aggregates mass following land-use change. [ABSTRACT FROM AUTHOR]

• Simulated erosion decreased mineralization of OC and N in soils • Decrease in OC and N mineralization was mainly occurred in small aggregate sizes • Changes in OC and N availability determined decrease in OC and N mineralization • The OC and N mineralization increased with OC and N contents but texture independent Soil erosion has important influence on soil processes and particularly soil nutrient dynamics, which in turn affects the terrestrial C and N cycling and greenhouse gases emission. However, the responses of carbon (C) and nitrogen (N) cycling in soils and soil fractions to erosion remain largely unclear, hindering the precise prediction of the fate of C and N in the terrestrial ecosystem. Herein, we compared the mineralization of organic carbon (OC) and N at two different temperatures (15 and 25 °C) in bulk soils and aggregate fractions of 19 soils before and after a simulated rainstorm. The temperature sensitivity (Q 10) of OC and N mineralization was calculated. Soil samples were collected from 9 sites across China, with soil types of Alfisols, Inceptisols, Mollisols, Ultisols, and land uses of croplands, grasslands, and woodlands. The soil aggregate fractions were >2, 2-1, 1-0.25 and <0.25 mm, respectively. We hypothesized that erosion will decrease OC and N mineralization due to the loss of labile organic matter from soils and that the effects of erosion would decrease with clay content and vary with different aggregate fractions. In support of these hypotheses, the cumulative mineralized OC and N (C m and N m) in the soils and aggregate fractions were decreased of 78-85% and 37-52% after erosion, and the temperature sensitivities of OC and N mineralization were decreased of 80-84% and 4-43%, respectively. The effects of erosion varied with site where the soil was collected. The decreases in C m and N m mainly occurred in small aggregate sizes (1-0.25 and <0.25 mm fractions). The C m and N m in bulk soils and aggregates increased significantly with the contents of OC, N, labile OC and mineral N but were texture independent. These results highlighted significantly reduction of OC and N mineralization at eroding sites, which should be carefully considered when assessing emissions of greenhouse gases in eroding environments. [ABSTRACT FROM AUTHOR]