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The objectives of this study were (i) to determine the stock and distribution of soil organic carbon (SOC) and the quantity and quality of fine root; and (ii) the correlation between SOC stock and fine root quantity and quality in soils of artificial and natural vegetation in the Loess Plateau. Three vegetation types (grassland, shrubland and woodland) and two restoration approaches (artificially and naturally restored from cropland) were investigated in the Yangou watershed of the Loess Plateau. SOC stock, fine root biomass and root C/N ratio at the 0–20, 20–40, 40–60, 60–80, and 80–100 cm depths were determined. The mean SOC stock of natural vegetation at the 0–100 cm depth was significantly greater than that of artificial vegetation, with an increase of 100% for woodland, 15% for shrubland, and 23% for grassland. Natural vegetation restoration led to a significantly greater SOC stock up to a depth of 100 cm for woodland, 40 cm for shrubland, and 40 cm for grassland. The fine root biomass of natural vegetation at the 0–100 cm depth was also significantly greater than that of artificial vegetation, with an increase of 170% for woodland, 140% for shrubland, and 20% for grassland. Natural vegetation restoration led to a significantly greater fine root biomass up to a depth of 100 cm for woodland, 60 cm for shrubland, and only 20 cm for grassland. There was a significant linear correlation between SOC stock and fine root biomass. Thus, natural vegetation restoration could lead to a significantly greater SOC stock, fine root biomass, and fine root C/N ratio than the artificial vegetation restoration. Fine root was an important factor influencing the differences in the SOC stock between artificial and natural vegetation. [ABSTRACT FROM AUTHOR]

Abstract: Nitrogen (N) and precipitation are the main limiting factors in food production under rain-fed cropping systems in arid and semiarid regions. A long-term field experiment was conducted from 1984 to 2009 in the Changwu State Key Agro-Ecological Experimental Station of the Loess Plateau, China, to study interactions between precipitation and N fertilization. Winter wheat (Triticum aestivum L.) was grown with N fertilization at 0, 45, 90, 135, and 180kgNha−1. With 25-year N fertilization, soil organic carbon and total N were increased by 18% and 26%, respectively. Mean yields and precipitation use efficiency (PUE) levels with N fertilization were increased by onefold to twofold compared with unfertilized soils. Generally, during the period of 1984–2009, grain yields with 45–`180kgN ha−1 increased at the rate of 38–49kgha−1 year−1 (p <0.05), whereas PUE increased at the rate of 0.071–0.088kgmm−1 year−1. These increases over time are largely attributable to improvements in soil organic carbon and total N. Variations in yield were proportionally reduced by N fertilization, but heavily depended on the fluctuation of precipitation. Grain yields with 45–180kgN ha−1 decreased by approximately 10%–45% (p <0.05) in the dry years (drought index<−0.35) compared with the normal years, whereas significant increases were not detected in the wet years (drought index>0.35). Grain yields with 90–180kgN ha−1 were positively correlated (p <0.05) with fallow season precipitation, but insignificantly correlated with growing season precipitation. The optimum N fertilization rates in the dry, normal and wet years increased following an increasing sequence of 45, 135 and 180kgNha−1. These results have significant implications for optimizing N fertilization and maximizing yield and PUE in rain-fed cropping systems in arid and semiarid regions. [Copyright &y& Elsevier]

Abstract: Land use and topography strongly influence soil organic C (SOC) and N accumulation in eroded hilly regions. However, their combined effects and the underlying mechanism remain unclear. In this study, five land uses and three topographic positions across an eroded hilly watershed of the Loess Plateau were selected to investigate their effects on SOC and N accumulation. The restored grassland, shrubland and woodland (25 to 30years) increased SOC and total N by 32% to 119% in the slope and 17% to 81% in the gully, respectively compared with the cropland. These restored vegetation increased soil dissolved organic C (DOC) and microbial biomass C and N (MBC and MBN) by 1.1- to 3.0-fold in the slope and 30% to 108% in the gully, respectively. Similar increases were observed in soil aggregates, MBN/total N, soil C:N and microbial C:N ratios. These improvements were higher in the shrubland than in the other land uses. The SOC significantly decreased from the gully to the slope, and the magnitudes decreased in a sequence of cropland, grassland, shrubland and woodland. Soil DOC, total N, and MBN also decreased from the gully to the slope in the cropland, but remained unchanged in the grassland, shrubland, and woodland. Land use (P <0.05), topographic position (P <0.05), and their interaction (P <0.1) influenced SOC, total N, DOC, MBC, MBN, soil C:N and microbial C:N ratios. There were close relationships among soil aggregates, SOC and total N, and DOC, MBC and MBN. Therefore, land use, topographic position (erosion and deposition), and their interactions regulate SOC and N accumulation and their labile fractions in the eroded hilly region at a watershed scale. Our results suggested that converting cropland to shrubland is an initial strategy to restore degraded ecosystems and increase soil C sequestration in eroded hilly region of the Loess Plateau. [Copyright &y& Elsevier]

[Display omitted] • Lateral carbon emissions from gully and terrace sidewalls were 26 % of that from the flatland. • The actual soil exposure area on the sidewalls was 46 % larger than its ortho -projected area. • Responses of lateral CO 2 emissions to soil water and temperature differently from the flatland. • The sidewall CO 2 emissions were sensitive to bank failure and drying-rewetting cycles. • Upscaling based on ortho -projection would misrepresent the carbon flux in complex terrains. The "U" or "V" shaped gullies on the Chinese Loess Plateau are highly susceptible to landslides and bank failure. The lateral carbon emissions from the bare scarps (valley banks or gully sidewalls) in theory promise a non-negligible carbon source, but had long been ignored due to their inaccessibility and underrepresentation in orthographic projection. In this study, the all-year-round CO 2 fluxes from the flatland, gully sidewalls and terrace sidewalls were monitored in-situ in a gully-dominated catchment on the Chinese Loess Plateau. The instantaneous responses of CO 2 fluxes to bank failure and drying-rewetting cycles were further examined by manual detachment and rainfall simulation. We observed that: 1) The sidewalls emitted CO 2 in all seasons, from warm rainy summer to freezing dry winter. The average CO 2 emission rates from the sidewalls reached 26% of that from the flatland, even though the soil organic carbon, dissolved organic carbon and soil microbial biomass carbon content of the barren soil on the sidewalls was disproportionally lower as 57%, 44%, and 76% of that on the flatland. 2) The simulation of bank failure and drying-rewetting cycles could rejuvenate the sidewalls and thus enhance the CO 2 emission rates, respectively by 49%, 27%, and collectively by 82%. 3) Based on the digital terrain model developed from the imagery acquired by the unmanned aerial vehicle, the actual exposed area of the study site was 46% larger than its ortho-projected area. This not only challenges the plausibility of conventional orthographic projection to estimate the actual soil surface area in similar regions predominated with eroded scarps and bench terraces, but also highlights the non-negligible contributions from lateral CO 2 emissions to local carbon source strength. The vigorous responses of lateral CO 2 emissions to soil water, temperature and layer detachment further emphasize the vulnerability of bank failure, which may potentially introduce more exposed area and thus trigger more CO 2 fluxes under future climate conditions. Therefore, the lateral CO 2 emissions from steep slopes or eroded scarps should not be ignored but deserve systematic investigations in complex terrains similar to the Chinese Loess Plateau. [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]

Fungal communities play an important role in soil biogeochemical cycles and ecosystem function. The redistribution of water and soil causes significant differences in the soil physicochemical properties in eroding landscapes. However, changes in soil fungal diversity and key-stone taxa in soil-eroding catena remain poorly understood in these landscapes. In the current study, the diversity and key-stone taxa of the fungal community in topsoil samples were analyzed from three typical erosive topographic units (autonomous, transitional, and depositional zones). The Chao 1 index showed a decrease from the autonomous and transitional zones (1759.7 and 1752.6, respectively) to the depositional zone (1535.3), but there were no significant differences in the Shannon index. Transitional and depositional networks had fewer edges (318 and 387, respectively) and clustering coefficients (0.236 and 0.301, respectively) than the autonomous network (911 and 0.362, respectively). At the phylum level, the nutritional strategists of key-stone taxa tended to shift from r - to K-strategists along the soil-eroding catena, indicating that the majority of key-stone taxa were dominated by the phylum Ascomycota in the autonomous and transitional zones, whereas the phylum Basidiomycota was dominant in the depositional zone. The recalcitrant carbon composition (C C/C O, autonomous vs. transitional vs. depositional: 3.82 vs. 5.09 vs. 7.52) may have contributed to the decreased fungal richness along the soil-eroding catena. The primary controlling factors affecting key-stone taxa were soil organic carbon (SOC), total nitrogen (TN) and Olsen-P in the autonomous zone, SOC and C H/C O in the transitional zone, and C C/C O in the depositional zone. Exploring the changes in fungal diversity and key-stone taxa among different erosive topographic units can contribute to an in-depth understanding of the biogeochemical cycle in the eroding environment. • Fungal richness decreased along the soil-eroding catena. • High recalcitrant carbon content could induce low fungal richness in the depositional zone. • Fungal key-stone taxa tended to shift from r- to K-strategists along the soil-eroding catena. • Changes in soil properties could lead to spatial heterogeneity in fungal key-stone taxa. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Many ecosystem functions and processes depend on biodiversity, however, the effect of root diversity in agroecosystems on soil bacterial communities and processes remained largely unknown. Our objectives were to examine the importance of increased root diversity through crop rotation on soil bacterial community composition and its relationship with soil carbon (C) and nitrogen (N) accumulation, which play an important role in soil fertility. In a field experiment with 30-year crop rotation, where there was no difference in root biomass input in top soil, soil C and N accumulation rates, soil microbial activities and bacterial community composition were investigated. Soil C and N accumulation rates and microbial biomass content were generally increased after rotation, with a greater increase in legume-cereal rotation than in cereal-cereal rotation. Crop rotation also increased soil microbial activity (soil respiration, potential N mineralization), but did not affect soil bacterial diversity. The increased bacterial abundance and changes in bacterial community structure and abundances of dominant bacterial phyla in rotation soils were related to increases in soil C and N accumulation and microbial activity. Our results suggest that increased root diversity through rotation can influence soil bacterial community structure and increase soil fertility by enhancing C and N accumulation rates, and cause positive effects on soil organic C and fertility. The influence of legume-cereal rotation was greater than that of cereal-cereal rotation. [ABSTRACT FROM AUTHOR]

As the essential driving force of soil redistribution, runoff and sediment are not often considered when quantifying and integrating the effects of soil erosion and deposition on soil CO 2 emissions. Therefore, in this study, variations in soil CO 2 emissions from erosion-deposition plots were regularly monitored in China's Loess Plateau (2015–2019). The cumulative soil CO 2 emissions from the depositional zones with slope gradients of 5°, 10°, and 20° were increased by 0.4–16.7%, 20.1–32.6%, and 31.9–51.5%, respectively, than those of their respective eroding slopes. Relative to the 5° eroding slopes, the cumulative soil CO 2 emissions decreased by 2.8–13.5% and 11.3–15.6% on the steeper 10° and 20° eroding slopes, respectively. Conversely, cumulative soil CO 2 emissions increased by 1.0–7.9% and 6.9–13.3% in the depositional zones of 10° and 20°, respectively, as compared with that in the 5° depositional zones. Considering both the eroding slopes and depositional zones, the total amount of CO 2 emissions from the 10° and 20° erosion-deposition plots were 114 g CO 2 -C y−1 and 177 g CO 2 -C y−1 lower than those from the 5° erosion-deposition plots, respectively. This can be attributed to the combined effects of the amounts of runoff and sediment displacement on soil CO 2 emissions from eroding slopes and depositional zones. Our results indicate that, soil erosion and deposition together have great potential to mitigate soil organic carbon mineralization and preserve soil carbon pools in slope lands. Furthermore, soil organic carbon mineralization at the slope scale is sensitive to displaced runoff and sediment. Therefore, possible variations in soil-atmospheric carbon exchanges induced by erosion-displaced runoff and sediment should be properly accounted for when attributing carbon fluxes in regions dominated by sloping landscapes. • Soil CO 2 emissions on eroding slopes were lower than that in the depositional zones. • Increasing slope gradients exacerbated the reduction of soil CO 2 emissions on eroding slopes. • Soil CO 2 emissions in the depositional zones enhanced by increasing slopes gradients. • Total soil CO 2 emissions from the whole erosion-deposition plots decreased as slopes steepened. [ABSTRACT FROM AUTHOR]

• Ten years of irrigation with brackish water resulted in a less SOC-rich soil. • The soil microbial communities in the Brackish field were less connected and more stressed. • The correlations of soil microbial hub species with soluble salts were ion-specific. Irrigation with brackish water has been widely applied in many regions of the world as an efficient alternative to conserve freshwater resources. Although it has limited soluble salt content, brackish water can alter the physio-chemical properties and microbial communities of soil over time. In this study, a cotton field that utilized drip irrigation with brackish water for 10 years (Brackish field) was compared with a nearby field that utilized drip irrigation with fresh water (Fresh field). Our results show the following: 1) After 10 years, the Brackish field was saltier with a greater electrical conductivity and sodium adsorption ratio, as well as less soil organic carbon content when compared with the Fresh field. 2) The more polarized distribution of the most abundant operational taxonomic unit (OTU) was observed in the Brackish field, accompanied by significantly smaller beta diversity, a less clustered network and fewer co-occurrences (edges) within the soil microbial community compared with the less salty Fresh field, altogether demonstrating that long-term brackish irrigation was very likely to form a less diverse and more stressed environment for soil microbes to co-exist. 3) The mismatches between the edge ranking of hub species and the ranking of relative abundances highlight that the predominant abundances of salt tolerant species from genera Acinetobacter and Pseudomonas may help cotton to grow better in the saline soil but may not help to stabilize soil microbial communities. In future studies, sophisticated sampling protocols with high time-resolution are required to fully capture the possible soil microbial responses to event-sensitive ion redistributions across cotton rows as well during repeated irrigation cycles. [ABSTRACT FROM AUTHOR]