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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.

Purpose: Rivers transfer eroded sediment with soil organic carbon (SOC) from terrestrial land to ocean basins, playing a key role in the global carbon cycle. During sediment delivery, the coarse particles can be preferentially settled and the fine particles may be transported to downstream depositional sinks. However, the size-specific quantification of soil microbial communities and SOC composition and their roles in SOC stability have not been fully elucidated. Materials and methods: In this study, two soils with similar textures but different aggregate structures were fractionated using a settling tube apparatus into three size classes according to their settling velocities. The individual soil particle-size aggregates and unfractionated soil were analyzed to determine (1) the diversity and composition of the soil microbial communities via 16S rRNA and ITS1, and (2) the SOC composition using mid-infrared diffuse reflectance spectroscopy. Results: The results showed that as the soil aggregate size decreased, the ratio of alkyl groups to carboxyl groups (C−H/COO, C−H/C=O) and soil CO2 emission per gram of SOC significantly decreased; however, the richness and diversity of the soil bacterial and fungal communities increased. Meanwhile, the aggregates of smaller size had lower relative abundances of Proteobacteria and Basidiomycota but higher relative abundances of Actinobacteria and Ascomycota. Rhizobiales was an indicator taxon for bigger soil aggregate communities, while Deinococcales, Capnodiales, and Venturiales were indicator taxa for communities with smaller soil aggregates. SOC composition showed a stronger direct effect on SOC stability in bigger soil aggregates (path coefficient, 0.68). However, SOC composition exerted a greater indirect impact on SOC stability by regulating soil bacterial and fungal community composition in smaller size aggregates (path coefficient, 0.65). Conclusions: Overall, our study provides a fundamental understanding of the chemical and microbial control of SOC stability at the aggregate level and highlights the potential impacts of aggregate-scale SOC decomposition on the fate of eroded SOC. [ABSTRACT FROM AUTHOR]

• 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]

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]

Understanding the response mechanisms of litter respiration to soil moisture in water-limited semi-arid regions is of vital importance to better understanding the interplay between ecological processes and the local carbon cycle. In situ soil respiration was monitored during 2010–2012 under various conditions (normal litter, no litter, and double litter treatments) in a 30-year-old artificial black locust plantation (Robinia pseudoacacia L.) on the Loess Plateau. Litter respiration with normal and double litter treatments exhibited similar seasonal variation, with the maximum value obtained in summer (0.57 and 1.51 μmol m−2s−1 under normal and double litter conditions, respectively) and the minimum in spring (0.27 and 0.69 μmol m−2s−1 under normal and double litter conditions, respectively). On average, annual cumulative litter respiration was 115 and 300 g C m−2 y−1 under normal and double litter conditions, respectively. Using a soil temperature of 17°C as the critical point, the relationship between litter respiration and soil moisture was found to follow quadratic functions well, whereas the determination coefficient was much greater at high soil temperature than at low soil temperature (33–35% vs. 22–24%). Litter respiration was significantly higher in 2010 and 2012 than in 2011 under both normal litter (132–165 g C m−2 y−1 vs. 48 g C m−2 y−1) and double litter (389–418 g C m−2 y−1 vs. 93 g C m−2 y−1) conditions. Such significant interannual variations were largely ascribed to the differences in summer rainfall. Our study demonstrates that, apart from soil temperature, moisture also has significant influence on litter respiration in semi-arid regions. [ABSTRACT FROM AUTHOR]

Projected increases in the frequency and magnitude of extreme precipitation profoundly impact terrestrial ecosystems. However, the response of soil respiration to changing precipitation patterns has not been systematically investigated, especially in arid and semi-arid areas. Precipitation amounts of 300 and 600 mm (T300 and T600) were set to simulate normal and extremely wet summers, respectively, and were individually conducted by wetting soils in a series of sub-events—10 or 100 mm (P10, P100)—over equal time intervals. Maize straw (1300 kg ha−1) and N fertilization (200 kg N ha−1) were applied in parallel to extend our simulation to natural cropland conditions. Soil respiration pulses were observed after T300-P100, and inhibited effects occasionally occurred 1 day after T600-P100, primarily because frequent and harsh storms in T600-P100 induced saturation stress on soil respiration. Cumulative soil respiration in T600 was greater than that in T300, owing to prolonged suitable water conditions for soil respiration. Similarly, for the same total precipitation amount, P100 produced 40–44% more soil respiration than P10. Applying N and maize straw did not change response trends but produced different magnitudes of response to changing precipitation patterns. Maize straw significantly stimulated soil respiration, and this effect became more evident in P100 with improved water conditions, producing a 122–185% increase in cumulative soil respiration. Nitrogen slightly inhibited soil respiration; this effect was enhanced when maize straw was also added but was insignificant. Cumulative soil respiration was significantly correlated with β- d -glucosidase and soil microbial biomass carbon content. Such soil respiration responses highlight the need to accurately account for soil respiration contributions when projecting global carbon cycling in future climate scenarios. • Extreme precipitation events lead to soil respiration pulses, occasionally inhibiting and enhancing soil respiration rates. • Extreme precipitation patterns greatly enhanced the cumulative soil respiration. • Straw addition significantly amplified the response of soil respiration to extreme precipitation patterns. • N showed minor effects on soil respiration under extreme precipitation patterns. [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]

Temperature sensitivity of soil respiration ( Q 10 ) 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 Q 10 responds to chemical fertilization under different levels of initial soil fertility. On the Chinese Loess Plateau, changes in soil respiration rates and Q 10 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  < 0.05). The Q 10 after NP fertilization was significantly decreased by 6.9% in low fertility soil, but was unchanged in the high fertility soil. The Q 10 variation might be attributed to the different response of microbial respiration Q 10 in the two soils. The decreased Q 10 with NP fertilization in the low fertility soil was possibly due to N-induced increase of substrate quality for soil microbes and increased activities of both cellobiohydrolase and polyphenol oxidase. In the high fertility soil, the unchanged Q 10 with NP fertilization may be the integrated result of less affected substrate quality and neutral response of polyphenol oxidase activity. Overall, our results suggested that the effects of NP fertilization on soil respiration and its temperature sensitivity varied with soil initial fertility levels, and therefore must be properly accounted for when estimating potential effects of local agricultural management to regional agroecosystems under future climate conditions. [ABSTRACT FROM AUTHOR]

Temperature sensitivity of soil respiration ( Q 10 ) is an important mechanism for the possible feedback between global carbon cycle and climate system. Knowledge of how crop types and nitrogen (N) fertilization affect Q 10 is critical for estimating soil respiration and carbon cycling in agro-ecosystem. A two-year field experiment was conducted with cold-resistant (winter wheat; Triticum aestivum L.) and thermophilic (spring maize; Zea mays L.) crops at two N fertilization levels (no fertilization (CK) and 160 kg N hm −1 ) from October 2013 to September 2015 in semi-arid Loess Plateau. Annual mean soil respiration and Q 10 in maize were 20% (1.85 vs. 1.54 μmol m −2 s −1 ) and 36% (2.49 vs . 1.83) higher than that in wheat. Nitrogen fertilization resulted in a 35% increase in annual mean soil respiration (1.95 vs. 1.44 μmol m −2 s −1 ) and a 11% decrease in Q 10 (2.05 vs . 2.28) compared with the CK treatment. Soil respiration was positively related to root biomass, whereas no significant relationship was found between root biomass and Q 10 . Therefore, it can be concluded that soil respiration and temperature sensitivity of soil respiration are significantly influenced by crop types and N fertilization regimes, which should be considered in calculating carbon budget in agro-ecosystem using carbon models. [ABSTRACT FROM AUTHOR]

A better understanding of the factors affecting interannual variability in litter respiration is critical to precisely understand local carbon cycling, especially under the changing climate. In this study, litter respiration was obtained by subtracting in situ soil respiration in a control (LCK) treatment by that in a litter removal (LR) treatment for the period 2009–2013 in a 30-year-old black locust plantation ( Robinia pseudoacacia L.) on a ridge slope in a small watershed of Loess Plateau, China. Annual cumulative litter respiration ranged from 48 ± 15 to 165 ± 36 g C m −2 y −1 , with mean value of 113 ± 45 g C m −2 y −1 and coefficient of variation (CV) of 40%; annual contribution rate of litter respiration to total soil respiration (hereafter refer to as litter contribution rate) also exhibited a similar interannual variability (ranged from 8 ± 3% to 20 ± 7%; mean = 15 ± 5%; CV = 31%). Additionally, annual mean soil moisture was highest in 2010 (53.2 ± 8.4% WFPS) and lowest in 2013 (31.4 ± 9.5% WFPS), with mean value of 41.6 ± 7.9% WFPS and CV of 19%; annual litter production rates also exhibited a similar interannual variability (ranged from 379 ± 34 to 565 ± 69 g m −2 y −1 ; mean = 477 ± 71 g m −2 y −1 ; CV = 15%). Annual mean soil moisture was mainly affected by the frequency and distribution of precipitation, and annual litter production rates varied with summer precipitation. Annual cumulative litter respiration and litter contribution rate increased linearly with both annual litter production rates and mean soil moisture content. The contribution of soil water to litter respiration was larger than that of litter production rates. Therefore, litter production rates and soil moisture resulted from precipitation needs to be taken into account for precisely predicting litter respiration in the dryland ecosystems, especially under the changing climate. [ABSTRACT FROM AUTHOR]