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Erosion processes spatially redistribute soil particles and the associated carbon across landscapes. Their spatial redistribution pattern is governed by the transport distances of individual displaced soil particles, which is not only dependent on their settling velocity, but also affected by slope topography. However, the potential impacts of fine-scale variation of slope topography on the erosion-induced lateral and carbon fluxes are often over-generalized by coarse digital elevation models. In this study, two topo-sequences, convex and concave, over a long gentle slope in the northeast China were investigated. Surface soils were sampled at predetermined space intervals from upslope to downslope along the two toposequences, and then fractionated by the settling velocity of individual fractions into four classes: > 250, 63 – 250, 20 – 63 and

Nitrogen (N) fertilization has been repeatedly reported to strongly influence soil properties and crop growth. However, there is little information about the combined effects of soils and crops on soil CO2 fluxes under N fertilization. In this study, in situ soil respiration, soil physicochemical properties, microbial communities and crop properties were measured for eight years (2008 to 2016) on the Loess Plateau. Five rates of N fertilization were applied to different plots to compare the soil respiration rate and its temperature sensitivity (Q10). Nitrogen fertilization significantly increased mean annual cumulative soil respiration (Rcum) by 25%–44%. Rcum had a positive correlation with grain yield, and the carbon emission efficiency (grain yield produced per unit of carbon emission) under N-fertilized plots was 1.62–2.52 times that of unfertilized plots. Rcum also had a positive correlation with root biomass and the root N concentration but showed a negative correlation with the root C/N ratio. The Q10 values under N-fertilized plots decreased by half at a diurnal scale, but had a smaller reduction (i.e., 0.04–0.09) at an annual scale compared to unfertilized plots. The decreased Q10 values under N-fertilized plots also resulted from the lower aromaticity of dissolved carbon (SUVA254) (7.40 vs. 10.53 L mg C−1 m−1). In addition, the altered Rcum and Q10 values were affected by the varied bacteria community derived from N fertilization, which was related to Acidobacteria, Chloroflexi, Proteobacteria and Bacteroidetes. Therefore, N fertilizer applications regulate the combined effects of soil and crop parameters on soil respiration and the Q10 value. This study suggests that, due to the lower carbon emission efficiency and higher SOC concentration under N-fertilized plots, N fertilizer applications may be used to sustain crop yields and increasing SOC storage while minimizing carbon emission impacts to the environment on the Loess Plateau.

The natural environment provides material essentials for human survival and development. The characteristics, processes, regional differentiation and forcing mechanisms of the elements of the natural environment (e.g. geomorphology, climate, hydrology, soil, etc.) are the main objects of research in physical geography. China has a complex natural environment and huge regional differentiation and therefore it provides outstanding reserach opportunities in physical geography. This review summarizes the most important developments and the main contributions of research in the physical geography and human living environment in China during the past 70 years. The major topics addressed are the uplift of the Tibetan Plateau and the evolution of its cryosphere, the development of fluvial systems, the acidification of the vast arid region of the Asian interior, variations in the monsoon and westerly climate systems on multiple timescales, the development of lakes and wetlands, the watershed system model, soil erosion, past human-environment interactions, biogeography, and physical geographic zonality. After briefly introducing international research developments, we review the history of research in physical geography in China, focusing on the major achievements and major academic debates, and finally we summarize the status of current research and the future prospects. We propose that in the context of the national demand for the construction of an ecological civilization, we should make full use of the research findings of physical geography, and determine the patterns and mechanisms of natural environmental processes in order to continue to promote the continued contribution of physical geography to national development strategies, and to further contribute to the theory of physical geography from a global perspective.

Net soil CO2 emissions are not independent of topography but tend to decline with increasing slope gradients. Such decline has been attributed to increased runoff and greater soil loss on steep slopes, leaving the soil less habitable for microorganisms. However, the specific variations of slope gradients and thus the associated soil properties relevant for CO2 emissions, especially from terraced slopes, are often disguised by the coarse resolution of digital terrain models (DTMs) based on commonly available open-source data. Such misrepresentation of the relationship between topography and soil CO2 emissions carries the risk of a wrong assessment of soil-atmosphere interaction. By applying a slope dependent soil CO2 emission model developed from erosion plots to nearby sloping and partially terraced cropland using two DTMs of different spatial resolutions, this study tested the significance of these resolution-induced errors on CO2 emission estimates. The results show that the coarser-resolution Shuttle Radar Topography Mission (SRTM) underestimated CO2-C emission by 27% compared to the higher-resolution DTM derived from Unmanned Aerial Vehicles (UAV) imagery. Such difference can be mostly attributed to a better representation of the proportion of flat slopes in the high-resolution DTM. Although the observations from erosion plots cannot be directly extrapolated to a larger scale, the 27% underestimation using the coarser-resolution SRTM DTM emphasizes that it is essential to represent microreliefs and their impact on runoff and erosion-induced soil heterogeneity at an appropriate scale. The widespread impact of topography on erosion and deposition on cropland, and the associated slope-dependent heterogeneity of soil properties, may lead to even greater differences than those observed in this study. The greatly improved estimation on CO2 emissions by the UAV-derived DTM also demonstrates that UAVs have a great potential to fill the gap between conventional field investigations and commonly applied coarse-resolution remote sensing when assessing the impact of soil erosion on global soil-atmosphere interaction.

Chronic synthetic nitrogen (N) application can result in a significant accumulation of nitrate in the subsoil, which could alter subsoil N cycle and subsequently affect subsoil N levels. To understand how elemental interactions affect the cycle and storage of subsoil N, we examined the soils receiving no fertilizer control (CK), 30-year applications of synthetic fertilizer (CF), and CF plus organic manure (CF + OM). The N cycling microbial groups and activity were investigated through analyzing abundance of bacteria, nitrifiers and denitrifiers, potential nitrification (PNA) and denitrification (DEA) rates in the topsoil (0–20 cm) and subsoil depths (20–80 cm). Compared with the CK, the CF application increased subsoil nitrate but reduced or did not change subsoil microbial biomass N and total N. Corresponding to the increased nitrate, the abundances of denitrifiers increased in the CF subsoils. By contrast, the abundances of nitrifiers increased in the CF topsoil. Significant correlation between the abundances of nitrifiers and soil PNA was found in the topsoil, while significant correlation was also found in the subsoil between the abundances of nirS- and/or nirK-type denitrifiers and DEA. These results suggest that the depleted or less changed subsoil total N by CF application might be partly related to the enriched denitrifiers groups and the related potential activity. The contrasting responses of nitrifiers and denitrifiers in the CF subsoil indicate a decoupling of both processes. Our findings highlight that the leached nitrate by synthetic fertilizer addition not only occurs as an environmental risk causing groundwater contamination but may also alter the subsoil N cycle through the denitrifier groups.

Background Crust formation affects soil erosion by raindrop impacted flow through changing particle size and cohesion between particles on the soil surface, as well as surface microtopography. Therefore, changes in soil microtopography can, in theory, be employed as a proxy to reflect the complex and dynamic interactions between crust formation and erosion caused by raindrop-impacted flow. However, it is unclear whether minor variations of soil microtopography can actually be detected with tools mapping the crust surface, often leaving the interpretation of interrill runoff and erosion dynamics qualitative or even speculative. Methods In this study, we used a laser scanner to measure the changes of the microtopography of two soils placed under simulated rainfall in experimental flumes and crusting at different rates. The two soils were of the same texture, but under different land management, and thus organic matter content and aggregate stability. To limit the amount of scanning and data analysis in this exploratory study, two transects and four subplots on each experimental flume were scanned with a laser in one-millimeter interval before and after rainfall simulations. Results While both soils experienced a flattening, they displayed different temporal patterns of crust development and associated erosional responses. The laser scanning data also allowed to distinguish the different rates of developments of surface features for replicates with extreme erosional responses. The use of the laser data improved the understanding of crusting effects on soil erosional responses, illustrating that even limited laser scanning provides essential information for quantitatively exploring interrill erosion processes.

Soil erosion influences both lateral soil organic carbon (SOC) re-distribution and vertical soil CO2 emissions. While potential SOC mineralization during transport and the burial effects of SOC at depositional sites have been addressed in previous reports, erosion induced on-site CO2 emissions are still under-studied. In this study, two soils (Loess soil and Black soil) with similar texture but contrasting aggregate structure and SOC content were subject to a set of 60-min long simulated rainfall events. There were two different rainfall intensities (30 and 90 mm h−1) at three slope gradients (5°, 15° and 25°). Runoff and sediment from erosion plot were collected at 10-min intervals over 60 min. Soil CO2 emissions from eroding slopes, SOC and particle size distribution of the eroding soil were measured after the erosion events. The results show that the runoff rates from the two soils were comparable, but the sediment rates from the Loess soil roughly three times that from the Black soil. In general, the SOC erosion from the Loess soil was 1.8 times that from the Black soil, even though the SOC concentration in the original Black soil was 56% higher than the Loess soil. The cumulative soil CO2 emissions from the eroding slopes of the Loess soil ranged from 15.4 to 19.7 g C m−2, which was doubled on the Black soil (from 28.1 to 59.6 g C m−2). When the rainfall intensity raised from 30 mm h−1 to 90 mm h−1, the cumulative soil CO2 emissions from the Black soil decreased by 38.2%, but only declined by 10.0% on the Loess soil. When the slope gradient increased from 5° to 25°, the cumulative soil CO2 emissions decreased by 23.8% on the Black soil but by 12.6% on the Loess soil. Therefore, our observations suggest that the soil CO2 emissions on the Black soil was much more sensitive to the variations of rainfall intensity and slope gradients than the Loess soil. Greater SOC erosion should not be directly translated to less on-site soil CO2 emissions. The selective depletion/enrichment of SOC and the lability of individual components must be fully understood when accounting for slope-scale carbon balances.

Two-dimensional (2-D) numerical models have been widely used to predict soil water-salt transport under mulched drip irrigation. However, conventional 2-D models often neglect the spatial variation of soil water-salt transport along drip lines and over-simplifies it by averaging values. This calls for a more robust model to better reflect the actual spatial and temporal variation of soil water-salt distribution in the field, especially in arid regions using brackish water with additional salt. In this study, mulched drip irrigation with brackish water was applied to a cotton field with loam soil in an arid region of southern Xinjiang, northwest China. The changes of soil water potential and total dissolved solid (TDS) of soil water over two irrigation events were intensively measured on hourly base to establish and calibrate a 3-D model, and another five irrigation cycles were monitored on daily base to validate the 3-D model. The mean absolute relative errors between measured and calculated soil water potential and TDS of soil water were 13.7% and 10.7% during hourly-based model calibration, even though there might be bias inevitably introduced by pre-determined sampling intervals and volumes. The calculated values during model validation were reasonably in line with the temporal patterns of soil moisture and TDS before and after irrigation at different irrigation cycles. This 3-D model was then applied to predict the spatial and temporal variations of soil water-salt transport during and after irrigation. Our results show that (1) the 3-D model with additional consideration on point-source discharge from individual drippers effectively reflected the wet front interferences along the drip lines. (2) A semi-elliptic cylindrical wet bulb together with relatively low salinity was formed along the drip lines, which matched well with the cotton layout (one mulch, two drip lines and four rows). However, the uneven overlapping of wet front interferences among individual drippers required to optimize the design of dripper intervals and irrigation regime to provide desirable soil water-salt conditions for cotton growth in this loam soil. (3) During the 96 h after irrigation, the TDS of soil water was always greatest in the dense root zone, increasing from approximately 3.8 g L−1 to 7.0 g L−1 as a result of root water uptake. Such localized re-salinization patterns demand repeated leaching in continued irrigation cycles to ensure cotton growth. Our study suggests that a smaller interspace between drippers with short but more frequent irrigation cycles would be much more helpful to timely and spatial-precisely meet plant water demanding.

Spatial heterogeneity of soil microbes introduces great uncertainty to our understanding of microbe-mediated soil carbon cycling, yet was few studied on sloping lands. Along a steep-slope grassland (35°) on the Chinese Loess Plateau, soils of 0–10 cm were sampled in 2016 at three slope positions (upper, middle and bottom) to determine microbial community composition (by Illumina Hiseq sequencing) and function (enzymes involved in carbon cycling, the in situ soil respiration and temperature sensitivity). The bacterial alpha-diversity were greater at middle- and bottom- than at upper slope position, while fungal alpha-diversity varied little across slope positions. The bacterial phylum Proteobacteria was 9.7% and 19.4% lower but Acidobacteria was 36.5% and 41.3% greater at bottom- than at upper- and middle- slope positions. The fungal community transitioned from being Basidiomycota-dominant (relative abundance of 46.8%) at upper slope position to Zygomycota-dominant (relative abundance of 36.5%) at bottom slope position. The β- d -glucosidase activity generally declined down the slope while β- d -xylosidase and cellobiohydrolase activities hiked at middle slope position. All the enzyme activities were suppressed at bottom slope position. Soil respiration increased by 49.1% (P

Temperature sensitivity of soil respiration (Q10) is an important parameter when modeling the effects of global warming on terrestrial ecosystem carbon release. Widely applied chemical fertilizers can significantly affect soil productivity and carbon cycling in agroecosystems. However, little is known about how Q10 responds to chemical fertilization under different levels of initial soil fertility. On the Chinese Loess Plateau, changes in soil respiration rates and Q10 were investigated in soils of two fertility levels: low fertility (L) and high fertility (H). For each soil fertility level, there was one control plot and one chemical fertilized plot (+NP), which in total formed four treatments: L, L + NP, H and H + NP. All the treatments were replicated for three times on a continuous winter wheat cropping system. Respiration rates of surface soil in each treatment were in situ monitored from October 2010 through September 2015. Our results showed that after NP fertilization, soil respiration rates were increased by 46% in low fertility soil, yet only by 14% in high fertility soil (P

Land use plays an essential role in regional carbon cycling, potentially influencing the exchange rates of CO2 flux between soil and the atmosphere in terrestrial ecosystems. Temperature sensitivity of soil respiration (Q10), as an efficient parameter to reflect the possible feedback between the global carbon cycle and climate change, has been extensively studied. However, very few reports have assessed the difference in temperature sensitivity of soil respiration under different land use types. In this study, a three-year field experiment was conducted in cropland (winter wheat, Triticum aestivum L.) and apple orchard (Malus domestica Borkh) on the semi-arid Loess Plateau from 2011 to 2013. Soil respiration (measured using Li-Cor 8100), bacterial community structure (represented by 16S rRNA), soil enzyme activities, and soil physicochemical properties of surface soil were monitored. The average annual soil respiration rate in the apple orchard was 12% greater than that in the cropland (2.01 vs. 1.80μmolm-2s-1), despite that the average Q10 values in the apple orchard was 15% lower than that in the cropland (ranging from 1.63 to 1.41). As to the differences among predominant phyla, Proteobacteria was 26% higher in the apple orchard than that in the cropland, whereas Actinobacteria and Acidobacteria were 18% and 36% lower in the apple orchard. The β-glucosidase and cellobiohydrolase activity were 15% (44.92 vs. 39.09nmolh-1g-1) and 22% greater (21.39 vs. 17.50nmolh-1g-1) in the apple orchard than that in the cropland. Compared to the cropland, the lower Q10 values in the apple orchard resulted from the variations of bacterial community structure and β-glucosidase and cellobiohydrolase activity. In addition, the lower C: N ratios in the apple orchard (6.50 vs. 8.40) possibly also contributed to its lower Q10 values. Our findings call for further studies to include the varying effects of land use types into consideration when applying Q10 values to predict the potential CO2 efflux feedbacks between terrestrial ecosystems and future climate scenarios.

The influence of vegetation coverage and topography on soil organic carbon (SOC) stocks has been intensively studies. However, very few of the studies have recognized the potential combining effects of vegetation types and topographic positions onto SOC distribution, especially on the Chinese Loess Plateau where vegetation recovery has generated complex combination of fragmented topography and vegetation coverage. This study systematically sampled soil cores (259) from four vegetation types (woodland, grassland, cropland, and orchard) at three topographic positions (tableland, slope and valley bottom). Each soil core was divided into three layers: surface soil (0–20 cm), subsoil (20–60 cm) and deep soil (60–200 cm). Our results show that: (1) the SOC concentration declined over soil depths, regardless topographic positions or vegetation types. The absence of ancient cultivation layers at the valley bottoms further made the SOC stocks deep to 200 cm there much less than the tableland with thick loess soil layers (8.3 kg km –2 vs. 13.4 kg km –2 ). (2) The SOC concentration of cropland varied evidently with topographic positions, with the greatest on the tableland (8.0 g kg –1 ), and the least along the slope (5.3 g kg –1 ). However, grassland was rather stable across the three topographic positions. (3) In addition, the SOC concentrations of the three vegetation types were comparable on the tableland (6.1 g kg –1 ), while differed noticeably at the valley bottoms (5.0 g kg –1 ). Overall, our findings in this study call for the account for each combination of topographic position and vegetation type, so as to properly assess regional SOC stocks for sustainable land use.

Knowledge of CO2 emissions under different slope gradients and positions and its controlling factors is critical in accurately estimating CO2 emissions and carbon cycling on the slopes of eroded regions. In this study, three east-facing plots of 100 m2 (20 m × 5 m) with a slope gradient of 0.5° (S0.5), 1° (S1), and 3° (S3) were established in an eroded gully of the semi-arid Loess Plateau, China. The CO2 emission, temperature, moisture, runoff, sediment, fine root biomass and grain yield of these three plots were measured from October 2013 to September 2015 to investigate the relationship between slope gradients and soil CO2 emissions. The results showed that the mean annual cumulative CO2 emissions at S1 and S3 (731.0 ± 65.1 and 628.3 ± 74.8 g C m−2 year−1) were about 13.4% and 25.5% lower than that at S0.5 (843.7 ± 84.9 g C m−2 year−1). The CO2 emissions were higher at bottom slope than at upper slope, with an increase of 26.2% at S3, 22.9% at S1 and 14.5% at S0.5, respectively. The mean soil moisture ranged from 40.8% to 44.8% water-filled pore space (WFPS) among the slope gradients, and from 35.8% to 45.6% WFPS among the slope positions. There was a significant difference in mean fine root biomass among different slope gradients (S0.5 > S1 > S3, P

Soil redistribution by terrace construction, as one of the most evident anthropogenic imprints on hill-slopes, may influence soil organic carbon (SOC) dynamics through re-shaping topography and altering water and oxygen availability. However, the fundamental role and mechanisms by which terrace construction affects in situ soil CO

Application of biochar to soil has been suggested as a nonstructural best management practice to mitigate runoff potential and soil loss. However. empirical data substantiating these effects are still scarce. Rainfall was presently simulated under controlled conditions to study the effects of biochar on erosion and runoff potential with sandy loam soil sampled from a field experiment 2 years after biochar amendment (2 kg m(-2)). The field experiment comprised four untreated and four biochar-treated plots that were amended with biochar produced from birchwood. In the laboratory, aggregate stability and organic carbon (OC) in topsoil (0-0.2 m) were analyzed. Soil loss and runoff were measured during a 3.5-h rainfall simulation using a round flume setup. Organic C was also determined in the eroded sediment, soil crust, and bulk soil of each flume after simulations. While runoff was not affected by treatment biochar-treated soils showed significantly lower soil loss rates compared with untreated soils. reducing the cumulative soil loss by 41%. This was attributed to the aggregating effects of OC from the biochar application and delayed crust formation. The reduced erosion of OC from biochar-treated soils also resulted in lower OC enrichment ratios in the sediment and thus less OC export than from untreated soils. The results illustrated that biochar application already in the short term (2 years) can lead to an improvement of soil aggregation, thereby enhancing aggregate stability and leading to a reduction in soil loss. However, extrapolation of these positive results must be done with caution because the scale of the flume limited the erosive forces acting on the soil. Before an extrapolation of the results, an upscaling of erosion tests to field scale is therefore required.

Soil C-acquiring enzymes are good indicators for the biological mechanism of soil nutrients and organic matter cycles. However, they have been used less frequently to assess the ecological stability and soil C cycle in eroding landscapes due to a lack of knowledge of the responses of C-acquiring enzyme activities to soil erosion and deposition. In the present study, a 3-year field simulation experiment was conducted to examine the variations in the activities of C-acquiring enzymes (β-1,4-xylosidase (βX), β-1,4-glucosidase (βG) and β-D-cellobiohydrolase (CBH)) from erosion-deposition plots with different slope gradients (5°, 10° and 20°) on the Loess Plateau in China (2016–2018). The activities of βX, βG and CBH were higher in the depositional plots than in the erosional plots, and those differences were enlarged with increasing slope gradients. Compared to the 5°-erosional plot, the activities of βX, βG and CBH respectively declined by 3.2–4.5%, 14.3–37.5% and 12.7–29.1% in the 10°-and 20°-erosional plots. The βX, βG and CBH activities were 2.2–18.1%, 17.3–32.1% and 14.8–86.2% higher in the 10°- and 20°-depositional plots than in the 5°-depositional plot. Moreover, the total soil CO2 emissions from the whole erosion-deposition plots decreased as slopes steepened. The displaced runoff and sediment depleted soil moisture, SOC, clay and microbial biomass in the erosional plots but enhanced these resources in the depositional plots, which can account for the changes in C-acquiring enzyme activities. The spatial distribution of enzyme activities affected soil CO2 emissions in a positive linear function. The sensitive responses of the C-acquiring enzyme activities and the controlling effects of C-acquiring enzyme activities on soil CO2 emissions during erosion and deposition processes, should be properly considered in assessing the biological mechanism for nutrition cycling in regions predominated with fragmented eroding landscapes.

Soil prokaryotes composition and diversity are the key to uncovering the mechanisms that drive variations in soil biogeochemical processes. Soil erosion is a primary factor that affects the spatial distribution of the soil prokaryotic community, but how soil prokaryotes in soil-eroding catena respond to environmental factors related to topography remains largely unclear. In this study, topsoils were sampled from three typical erosion geomorphic units (autonomous, transitional and depositional zones) in 2018 to identify the soil prokaryotic community and interactions among the species on the Chinese Loess Plateau. The alpha-diversity was greater but the beta-diversity was lower in the autonomous and transitional zones than in the depositional zone. Gammaproteobacteria and Bacteroidetes were 73% and 68% lower in the autonomous and transitional zones than in the depositional zone. In addition, Deltaproteobacteria, Acidobacteria, Firmicutes, Actinobacteria, Chloroflexi, Gemmatimonadetes and Nitrospirae were significantly higher in the autonomous and transitional zones. A less clustered network and fewer co-occurrences within the prokaryotic community and functional groups of processes were found in the depositional zone than in the autonomous and transitional zones. The alpha-diversity index was significantly negatively correlated with clay particles, soil water content, soil organic carbon (SOC), and ratio of SOC and nitrogen (C/N) but positively correlated with total nitrogen (TN). The higher relative abundances of copiotrophic groups (including Gammaproteobacteria, Bacteroidetes, etc.) in the depositional zone was mainly due to the increased SOC caused by the deposition of SOC-rich clay. Reassembly of the soil physico-chemistry characteristics among the topographic units significantly altered the soil prokaryotic community along the soil-eroding catena.

Rocks in civil and mining engineering usually experience rather complex stress disturbance. Rock dynamic mechanical behaviors under constant stress amplitude condition have been widely studied. However, the rock fracture evolution characteristics subjected to multi-level cyclic loading conditions are not well understood. In this work, multi-level cyclic compressive loading experiments were performed on marble with interbed orientation of 0°, 30°, 60° and 90°. Anisotropic fracture evolution characteristics were revealed using dynamic stress strain descriptions and post-test CT scanning technique. Results show that rock fatigue deformation, strength, lifetime, dynamic elastic modulus and damping ratio are all impacted by rock structure. The interbed structure plays a dominant role in fracture evolution compared with natural fracture and pyrite band. Rock stiffness degrades and damping ratio increases with the increase of interbed orientation. In addition, a two-phase damage accumulation pattern was found for marble under multi-level cyclic loading condition. A damage evolution model was first established to model damage accumulation, the proposed model fits well to the experimental data. Moreover, post-test CT scanning reveals the internal crack coalescence pattern and failure mode is impacted by the interactions of interbeds, natural fracture and pyrite bands. The rock bridge structure in marble with 60° and 90° interbed orientation alters the crack propagation path. Although the crack pattern is relatively simple at rock bridge segment, much more energy is needed to drive crack propagation. The testing results are expected to improve the understanding of the influence of rock structure on fracture evolution when subjected to variable stress amplitude loading conditions.

Rill erosion is a small-scale but universally occurring phenomenon. Given its potential to concentrate into larger-scale erosion and its non-negligible contributions to soil loss, substantial research has been dedicated to understanding its processes. In this article, we conducted a holistic review of the major achievements in rill erosion research over the past few decades, mainly from the following perspectives: 1) Hydraulic parameters to describe rill development; 2) morphological indicators to represent rill morphology; 3) commonly used measuring methods for rill morphology and rill flow; and 4) advantages and limitations of rill erosion modelling. In each of the perspectives, we also identified the challenges faced by current rill erosion research. Concrete suggestions on the pressing needs to help advance rill erosion research in the future are further presented.

The net effect of soil erosion by water, as asink or source of atmospheric carbon dioxide (CO2), isdetermined by the spatial (re-)distribution and stability oferoded soil organic carbon (SOC), and the dynamicreplacement of eroded C by the production of newphotosynthate. The depositional position of eroded SOCis a function of the transport distances of soil fractionswhere the SOC is stored. In theory, the transport distancesof soil fractions are related to their settling velocitiesunder given flow conditions. Yet, very few field investigationshave been conducted to examine the actualmovement of eroded soil fractions along hillslopes,let alone the re-distribution pattern of SOC fractions.Eroding sandy soils and sediment were sampled after aseries of rainfall events along a slope on a freshly seededcropland in Jutland, Denmark. All the soil samples werefractionated into five settling classes using a settling tubeapparatus. The spatial distribution of soil settling classesshows a coarsening effect immediately below the erodingslope, followed by a fining trend at the slope tail. Thesefindings support the validity of the conceptual modelproposed by Starr et al. (Land Degrad Dev 11:83–91,2000) to predict SOC redistribution patterns alonghillslopes. The d13C values of soil fractions were morepositive at the footslope than on the slope shoulder or atthe slope tail, suggesting enhanced decomposition rate offresh SOC input at the footslope during or after erosioninducedtransport. Pronounced CO2 emission rates at theslope tail also suggest a higher potential for decompositionof the eroded SOC after deposition. Overall, ourresults illustrate that immediate deposition of fast settlingsoil fractions and the associated SOC at footslopes, andpotential CO2 emissions during or immediately aftertransport, must be appropriately accounted for in attemptsto quantify the role of soil erosion in terrestrial Csequestration. A SOC erodibility parameter based onactual settling velocity distribution of eroded fractions isneeded to better calibrate soil erosion models.

The validity of soil erosion data is often questioned because of the variation between replicates. This paper aims to evaluate the relevance of interreplicate variability to soil and soil organic carbon (SOC) erosion over prolonged rainfall. Two silty loams were subjected to simulated rainfall of 30 mm h−1 for 360 min. The entire rainfall event was repeated ten times to enable statistical analysis of the variability of the runoff and soil erosion rates. The results show that, as selective removal of depositional particles and crust formation progressively stabilized the soil surface, the interreplicate variability of runoff and soil erosion rates declined considerably over rainfall time. Yet, even after the maximum runoff and erosion rates were reached, the interreplicate variability still remained between 15 and 39 %, indicating the existence of significant inherent variability in soil erosion experiments. Great caution must be paid when applying soil and SOC erosion data after averaging from a small number of replicates. While not readily applicable to other soil types or rainfall conditions, the great interreplicate variability observed in this study suggests that a large number of replicates is highly recommended to ensure the validity of average values, especially when extrapolating them to assess soil and SOC erosion risk in the field.

During slope-scale erosion events, re-distribution of eroded soil and the associated soil organic carbon (SOC) is not always uniform, but very often affected by preferential transport and deposition. Under given flow conditions, the site of SOC deposition depends on the transport distances of sediment particles containing the SOC. Very often, soil and SOC erosion risk are assessed by applying mineral particle specific SOC distributions to erosion models. However, soil is not always eroded as individual mineral particles, but mostly in a form of aggregates. Aggregates are likely to increase settling velocities of individual mineral particles, which may considerably reduce the transport distance of sediment fractions and the associated SOC, skewing SOC redistribution and its subsequent fate. Yet, little is known about the potential effects of aggregation on the movement and fate of eroded SOC. To assess the effect of preferential deposition, a simulated rainfall was applied to two soils in this study, with the Movelier silty clay having greater SOC content and aggregate stability than the Mohlin silty loam. The eroded sediments of the two soils were fractionated by a settling tube apparatus according to their potential transport distances. The CO2 emissions of the fractionated and incubated sediments were then measured for 50 days enabling the assessment of the bioactivity of eroded SOC for weeks after deposition. Our results show that: 1) the re-deposition of eroded SOC into terrestrial systems increased by 64% if considering the actual aggregate specific rather than the mineral particle specific SOC distribution. 2) The CO2 emission rates differed across settling fractions, with the most pronounced rates in the finest fractions from the Mohlin silty loam sediment and in the medium-size fractions from the Movelier silty clay sediment. 3) Over 50-day incubation, the CO2 emissions from the Mohlin silty loam sediment was 114% greater than that from the non-eroded Mohlin soil, whereas CO2 emissions were roughly equivalent for the Movelier sediment and non-eroded Movelier silty clay. These data demonstrate that erosion and preferential deposition of SOC-enriched aggregates can enhance terrestrial SOC deposition. This can further result in greater CO2 efflux than commonly applied mineral particle size specific SOC distribution would suggest. The different performances of deposition and CO2 emissions between the two soils also suggest that these effects can vary with soil type. Our observations certainly illustrate that sediment aggregation requires further investigation to assess the redistribution and subsequent fate of eroded SOC appropriately.

Climate change is expected to affect both the amount of global crop production, and annual variability in food supply. Agriculture is a major source of greenhouse gas emissions, but also considered to mitigate climate change. Conservation tillage, as a climate-smart agricultural practice, is repeatedly reported to mitigate net greenhouse gas emissions by increasing soil organic carbon (SOC). However, with reduced tillage, less litter is moved from the surface deeper into the soil profile, so SOC increase is very likely constrained to topsoil layers. Further adaptation benefits, such as increasing crop yield and resilience to famine, have recently been questioned after averaging yields from field studies. However, such global averaging masks the geographic extent individual studies apply to. This paper attempts a holistic regional analysis on the benefits of conservation tillage, in particular its fundamental principle no-tillage (NT), on the Chinese Loess Plateau. Based on a review of almost 20 years of conservation tillage plot experiments, the potential of NT to increase SOC stocks and to adapt to lower but more variable rainfall in the future has been assessed. The results show that the difference of total SOC stocks between NT and CT decreased with soil depth, confirming that the SOC benefits of NT are concentrated to the immediate topsoil still subject to direct seeding. The topsoil achieved maximum SOC stocks after about 10 years of NT. Crop yields from NT increased by up to 20% for years with average and below average precipitation, demonstrating the advantages of NT in stabilizing crop yields in dry years. However, the results in previous reports are not weighted by the actual spatial extent of drylands and humid regions after counting individual plot studies. As a consequence of such global and unweighted averaging, the benefits from NT to increase SOC stocks are likely to misrepresent the actual impact. Therefore, given the size of the Loess Plateau and its relevance for food security in China, our analysis illustrates the need to assess the benefits of a tillage and residue management system for each combination of eco-region and farming practice, weighted by their area and the affected population, rather than just using a global average for policy development on sustainable productivity.

The quantification of point source (PS) pollution and non-point source (NPS) pollution in different regions is needed so that targeted management strategies for water environment managers can be prepared. The aim of this study was to establish a method for quantifying PS and NPS pollution by measuring ion concentrations. The Danjiangkou Reservoir and its main tributaries were selected as the study area. The spatiotemporal variation in the concentration of nutrients and ions was analyzed. The results showed that the main ions did not display any distinct spatiotemporal variations on the reservoir, but spatiotemporal variations were evident in the tributaries: Mg2+, Na+, SO42− and Cl− were lowest in the wet season, K+ was highest in the dry-to-wet transition season, and Si was highest in the wet season. We selected Cl−/TN, SO42−/TN, and Na+/TN as PS pollution tracking indicators and K+/TN, Ca2+/TN, and Si/TN as NPS pollution tracking indicators. The mean contribution rate of NPS pollution calculated using Cl−/TN, Na+/TN, and Si/TN, and by the isotope approach, had a significant linear relationship (p

Soil redistribution is an important movement process of surface material that drastically affects the dynamic cycling of essential elements and soil productivity in the terrestrial ecosystem. Although a number of studies have focused on the soil organic carbon (SOC) pool under soil redistribution, few studies have comprehensively assessed the effects of soil redistribution on the ecosystem carbon pool. A five-year experiment of simulated topsoil redistribution (removal and addition, R-A) was conducted to explore the effects of soil redistribution on soil CO2 emissions, the SOC pool and the plant C pool in the semi-arid Loess Plateau of China. The soil properties, soil CO2 emissions, and crop parameters were measured after the topsoil removal (R20) treatment and topsoil addition (A20) treatment and in the undisturbed control (CK). Our results showed that 1) compared with the undisturbed control, the mean soil CO2 emissions of the R-A treatment decreased by 8% while the mean SOC pool increased by 3.0%; 2) the plant C pool was significantly reduced by 53.8% in 2015 and 5.8% in 2019 compared with the undisturbed control; 3) the ecosystem C pool (the sum of the SOC pool and plant C pool) of the R-A treatment was reduced by 10.3%, 3.7% and 1.1% compared with the ecosystem C pool of the undisturbed CK for 2015, 2016 and 2017, respectively, but increased by 0.6% and 1.9% from 2018 to 2019. During the first three experimental years, the decrease in the ecosystem C pool of the R-A treatment was caused by the reduced plant C pool in R20 exceeding the elevated plant C pool in A20. Over time, the soil bulk density (BD) decreased in R20 and the SOC, microbial biomass carbon (SMBC) and total nitrogen (TN) increased, which jointly improved the plant C pool in R20 and the ecosystem C pool by 2019. The gradual recovering pattern of the ecosystem C pool in the later years highlights the key role of plants in restoring soil properties and stabilizing ecosystem C cycling. Overall, our findings highlight that any attempt to assess the net impacts of soil redistribution, whether as a sink or source for atmospheric CO2, should comprehensively consider the changes in the SOC pool and plant C pool.

Slope induced runoff and erosion processes redistribute water, soil and nutrients, thus potentially influence soil CO2 emissions and perturb the carbon balances on sloping land. However, most of the previous studies on soil CO2 emissions were conducted on flat land. In regions with complex terrains and fragmented fields with terraces, such as the Chinese Loess Plateau, it is particularly important to understand slope dependent soil CO2 emissions. This requires investigations to unravel the relationship between soil erosional responses and CO2 emissions on different slopes. In this study, erosion plots, set at angles of 0.5°, 5°, 10°, and 20°, were refilled with loess soils from local farmland on the Chinese Loess Plateau. Changes of soil properties and CO2 emission rates were measured at three positions (upper, middle, lower) on each plot from October 2014 to September 2015. Our results show that greater runoff from the steeper slopes evidently depleted soil water and nutrients, directly responsible for the less soil CO2 emissions. Furthermore, the average yearly soil CO2 emissions decreased exponentially with slope gradients, from approximately 832.7 g m−2 yr−1 on the 0.5° slope, to 380.9 g m−2 yr−1 when the slope gradient was 20°. This not only proves the dependency of soil CO2 emission to slope gradients, but also provides the fundamental dataset to develop an empirical model between slope gradients and soil CO2 emissions. A coefficient β has been parameterized for the first time in this study to represent the sensitivity of soil CO2 emissions to slope gradients. The model of slope-dependent CO2 emissions identified in this study tentatively demonstrate non-negligible implications for global carbon fluxes from sloped and terraced field to the atmosphere. Further investigations on slope-scale carbon balances should test and integrate topography specific CO2 emissions.