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The spatial heterogeneity of soil respiration and its temperature sensitivity pose a great challenge to accurately estimate the carbon flux in global carbon cycling, which has primarily been researched in flatlands versus hillslope ecosystems. On an eroded slope (35°) of the semiarid Loess Plateau, soil respiration, soil moisture and soil temperature were measured in situ at upper and lower slope positions in triplicate from 2014 until 2016, and the soil biochemical and microbial properties were determined. The results showed that soil respiration was significantly greater (by 44.2%) at the lower slope position (2.6 μmol m–2 s–1) than at the upper slope position, as were soil moisture, carbon, nitrogen fractions and root biomass. However, the temperature sensitivity was 13.2% greater at the upper slope position than at the lower slope position (P < 0.05). The soil fungal community changed from being Basidiomycota-dominant at the upper slope position to being Zygomycota-dominant at the lower slope position, corresponding with increased β-D-glucosidase activity at the upper slope position than at the lower slope position. We concluded that soil respiration was enhanced by the greater soil moisture, root biomass, carbon and nitrogen contents at the lower slope position than at the upper slope position. Moreover, the increased soil respiration and decreased temperature sensitivity at the lower slope position were partially due to copiotrophs replacing oligotrophs. Such spatial variations along slopes must be properly accounted for when estimating the carbon budget and feedback of future climate change on hillslope ecosystems. [ABSTRACT FROM AUTHOR]

Nitrogen (N) fertilization has a considerable effect on food production and carbon cycling in agro-ecosystems. However, the impacts of N fertilization rates on the temperature sensitivity of soil respiration (Q10) were controversial. Five N rates (N0, N45, N90, N135, and N180) were applied to a continuous winter wheat (Triticum aestivum L.) crop on the semi-arid Loess Plateau, and the in situ soil respiration was monitored during five consecutive years from 2008 to 2013. During the growing season, the mean soil respiration rates increased with increasing N fertilization rates, peaking at 1.53 μmol m−2s−1 in the N135 treatment. A similar dynamic pattern was observed during the non-growing season, yet on average with 7.3% greater soil respiration rates than the growing season. In general for all the N fertilization treatments, the mean Q10 value during the non-growing season was significantly greater than that during the growing season. As N fertilization rates increased, the Q10 values did not change significantly in the growing season but significantly decreased in the non-growing season. Overall, N fertilization markedly influenced soil respirations and Q10 values, in particular posing distinct effects on the Q10 values between the growing and non-growing seasons. [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]

• 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]