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• The α-diversity of rare bacteria was greater in the depositional zones than in the eroding slopes. • The α-diversity of abundant bacteria between the eroding slopes and depositional zones showed no difference. • The community compositions of abundant and rare bacteria differed between the eroding slopes and depositional zones. • Abundant and rare species jointly maintained the stability of co-occurrence network in eroded ecosystem. • The α-diversity of rare bacteria showed closer linkage with SCCF than abundant bacteria. Elucidating the interaction between microorganisms and soil carbon cycling functionality (SCCF) in the eroded ecosystems, can improve our understanding of the mechanisms by which soil erosion and deposition affect soil organic carbon pool. Abundant and rare sub-communities are the integral components in driving SCCF. However, changes in abundant and rare sub-communities, as well as their contributions to SCCF, remain unclear in the eroded ecosystems. In this study, changes in abundant and rare sub-communities and their correlations with SCCF were comparatively explored with respect to soil erosion and deposition in the Loess Plateau. The α-diversity of rare sub-communities in the depositional zones was 4% higher than that in the eroding slopes, but there was no significant difference in α-diversity of abundant sub-communities between the eroding slopes and depositional zones. The compositions of abundant and rare sub-communities differed between the eroding slopes and depositional zones. Regarding the co-occurrence network of the eroded ecosystems, abundant sub-communities compared with rare sub-communities presented a higher degree and betweenness centrality; however, rare sub-communities possessed more frequent interactions with other species than abundant sub-communities, especially positive interactions. The α-diversity of abundant and rare sub-communities in ecological clusters showed positive links with SCCF. The link between α-diversity of abundant sub-communities and SCCF was weaker than that with rare sub-communities, which was related to the significant disparity in the abundance of functional genes involved in SCCF and the extent of functional redundancy between abundant and rare sub-communities in the eroded ecosystems. The functional difference of abundant and rare sub-communities in improving SCCF under soil erosion and deposition, is important for accurately assessing the effects of microbial communities on soil organic carbon pool in regions predominated with fragmented eroded landscapes. [ABSTRACT FROM AUTHOR]

• Upstream has the highest prokaryotic alpha-diversity index. • Upstream had a larger clustered network in the < 63 μm fractions. • Varied sediment fractions during soil erosion altered the prokaryotic community. The transfer of particle-bound prokaryotes and substrates from slopes to rivers during soil erosion is a crucial dynamic process that greatly influences terrestrial biological and geochemical cycles. Particle size strongly affects the transport distance of suspended sediments and likely induces prokaryote and substrate variations along the course of the river. Previous studies have mainly focused on the spatial redistribution of soil prokaryotes, whereas the variation in prokaryotic diversity and community composition among different particle size-fractions in suspended sediments remain unclear. In this study, suspended sediments were collected at four plots (upstream, middle stream, downstream, and dam) along a river that runs through a valley. Sampled sediments were size-fractionated into > 63 μm and < 63 μm particles (referred to as small and large particle fractions hereafter), to identify the corresponding prokaryotic community composition, interactions, and functions. The richness index (Chao1 and observed species) and the diversity index (Shannon) of sediment-associated prokaryotic communities significantly decreased from upstream towards dam plots, except in the small particle fraction in downstream and dam plots. The highest β-diversity index value was found for the large particle fractions, while the geospatial contribution to β-diversity varied. The concentration of Proteobacteria in downstream (41.39 %) and dam (34.86 %) plots was lower than that in the upstream (47.65 %) and middle stream (45.90 %) plots. In contrast, Cyanobacteria (4.89 % and 6.75 %) and Verrucomicrobiota (5.98 % and 11.00 %) concentrations were greater in the downstream and dam plots, respectively, than those in the upstream (0.93 % and 2.43 %) and middle stream (3.37 % and 3.11 %) plots, respectively. Additionally, the concentrations of Verrucomicrobiota and Bacteroidota were significantly higher in the small particle fractions (2.26–8.12 % and 4.33–7.72 %) than those in the large particle fractions (2.03–5.94 % and 3.70–7.11 %). Compared to the other plots, upstream plots had a larger clustered network and a greater number of co-occurrences within the prokaryotic community in the bulk sediment and in the small particle fraction, whereas more complex interactions among prokaryotic communities were found in the large particle fraction from the dam plots. Soil organic carbon (SOC), total nitrogen (TN), and Olsen phosphorus were the three most influential factors, explaining 67.41 % of the variation in prokaryotic community. Proteobacteria and Myxococcota correlated negatively with SOC, TN, and Olsen P but positively with C = O. Conversely, Cyanobacteria and Verrucomicrobiota correlated positively with SOC, TN, and Olsen P but negatively with C = O. Functional groups linked to the biogeochemical cycling of carbon (methanol oxidation and methylotrophy) were found in high concentrations, whereas those linked to nitrogen (nitrogen and nitrate respiration) were found in low concentrations in upstream and middle stream plots. Reassembly of the sediment physicochemical characteristics among geospatial plots along the transport transect significantly altered the prokaryotic community composition and metabolic functional groups. [ABSTRACT FROM AUTHOR]

Abstract Soil erosion influences both lateral soil organic carbon (SOC) re-distribution and vertical soil CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 emissions decreased by 23.8% on the Black soil but by 12.6% on the Loess soil. Therefore, our observations suggest that the soil CO 2 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 CO 2 emissions. The selective depletion/enrichment of SOC and the lability of individual components must be fully understood when accounting for slope-scale carbon balances. Highlights • Loess soil has lower SOC content but greater SOC erosion than Black soil. • On-site soil CO 2 emissions on Black soil decreased with erosion intensity. • On-site soil CO 2 emissions on Loess soil barely changed with erosion intensity. • Greater SOC erosion does not mean less on-site soil CO 2 emissions. • On-site CO 2 emissions from Black soil may not be regulated by erosional removal but by inherent SOC. [ABSTRACT FROM AUTHOR]

• Enzyme activity decreased by erosion leading to an increase in the depositional plot. • Slope enhanced the difference in enzyme activity between the erosional and depositional plot. • Enzyme activity mitigated total soil CO 2 emission in the erosion-deposition plots. 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 CO 2 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 CO 2 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 CO 2 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. [ABSTRACT FROM AUTHOR]

• Influence of soil properties on rill erosion is not adequately considered. • Study of timely changes in rill morphology and flow require advanced techniques. • Rill network requires more investigations to help prevent erosion at large scales. • Controlled experiments are case-specific and should be systematically integrated. • Rill erosion models are often simplified and require more knowledge on processes. 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. [ABSTRACT FROM AUTHOR]

• Soil CO 2 emissions decreased exponentially with slope gradients. • Erosion-distributed water and nutrient responsible for slope dependent CO 2 emissions. • First study to parameterize the dependency of soil CO 2 emissions to slope gradients. Slope induced runoff and erosion processes redistribute water, soil and nutrients, thus potentially influence soil CO 2 emissions and perturb the carbon balances on sloping land. However, most of the previous studies on soil CO 2 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 CO 2 emissions. This requires investigations to unravel the relationship between soil erosional responses and CO 2 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 CO 2 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 CO 2 emissions. Furthermore, the average yearly soil CO 2 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 CO 2 emission to slope gradients, but also provides the fundamental dataset to develop an empirical model between slope gradients and soil CO 2 emissions. A coefficient β has been parameterized for the first time in this study to represent the sensitivity of soil CO 2 emissions to slope gradients. The model of slope-dependent CO 2 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 CO 2 emissions. [ABSTRACT FROM AUTHOR]