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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]

Understanding the temperature sensitivity of soil respiration ( Q 10 ) and its controlling factors plays an important role in accurately estimating soil respiration and carbon cycling in agro-ecosystems. This manuscript presents a case study on how the Q 10 value for soil respiration changes with soil temperature and moisture in the rotation phases. In a three-year crop rotation system (wheat/wheat/millet/pea) in a semi-arid region of China, the soil respiration rate, temperature and moisture were measured under different crop phases from July 2010 to June 2013. The soil respiration rate was significantly lower in the winter wheat phase (1.63 μmol m −2 s −1 ) than the millet phase (2.40 μmol m −2 s −1 ) and pea phase (2.21 μmol m −2 s −1 ). However, the Q 10 value was significantly higher in the wheat phase (2.76) than in the millet phase (1.85) and pea phase (1.47). The relationship between the Q 10 values and soil temperature followed an exponential decay function in the rotation system, and the Q 10 value was stable (1.8) with no obvious variation when the temperature exceeded 15 °C. The Q 10 value tended to increase with soil moisture until reaching a threshold of 14.7% soil moisture and then declined. Our results indicate that temperature-respiration empirical models should be parameterized according to crop type in the rotation phases, especially when estimating soil respiration in cold-resistant crops under global warming. [ABSTRACT FROM AUTHOR]

A better understanding of the response of soil respiration to land-use conversion has important practical implications for ecological restoration in degraded regions. In this study, in situ soil respiration was monitored in a typical land-use sequence on a ridge slope in Wangdonggou watershed of the Loess Plateau, China, during a three-year period from 2011 to 2013. The land-use conversion sequences included cropland (control), apple orchard, grassland, and woodland. The results clearly showed that soil respiration and temperature sensitivity ( Q 10 ) varied significantly with land-use conversion. Soil respiration was decreased by 10% after conversion of cropland to orchard, and increased by 7–46% after conversion of cropland to grassland and woodland. Q 10 was increased by 19% after conversion of cropland to woodland, and decreased by 9–26% after conversion of cropland to grassland and orchard. Soil respiration increased linearly with soil organic carbon (SOC) storage and fine root biomass (<2 mm). The results indicated that root biomass and SOC storage were the major factors influencing Q 10 after conversion of cropland to non-natural ecosystem, and substrate quality or root system adaptability may be the real reason for the difference in Q 10 after conversion of cropland to natural grassland ecosystem. Although soil temperature and moisture significantly influenced soil respiration among the four typical land-use types, their difference derived from land-use conversions could not well explain the difference in soil respiration among land-use conversions. In conclusion, the increases in SOC storage and fine root biomass were the major factors influencing soil respiration among land-use conversions. Thus, conversion of cropland to natural grassland seemed to be the most effective integrated small watershed management to increase soil carbon storage and decrease CO 2 concentration in the loess regions of China. [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]

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 CO 2 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 (Q 10). Nitrogen fertilization significantly increased mean annual cumulative soil respiration (R cum) by 25%–44%. R cum 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. R cum 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 Q 10 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 Q 10 values under N-fertilized plots also resulted from the lower aromaticity of dissolved carbon (SUVA 254) (7.40 vs. 10.53 L mg C−1 m−1). In addition, the altered R cum and Q 10 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 Q 10 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. • Nitrogen fertilizer application increased soil respiration rate (SR) but decreased Q 10. • Variation in SR and Q 10 was resulted from altered photosynthetic rate, C/N, SUVA 254 , and bacteria community. • Nitrogen fertilizer application increased carbon emission efficiency. [ABSTRACT FROM AUTHOR]

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  < 0.05) while temperature sensitivity decreased by 13.2% ( P  < 0.05) down the slope. Both bacterial richness (OTU) and diversity (Shannon diversity index) positively correlated with soil respiration. The copiotrophic groups (Acidobacteria and Zygomycota) negatively and oligotrophic groups (Proteobacteria and Basidiomycota) positively correlated with temperature sensitivity of soil respiration. Our findings revealed divergent responses of soil bacterial and fungal communities along the slope and highlighted the importance of microbial information in predicting the spatial variability of soil respiration in hillslope ecosystems. [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]

Land use plays an essential role in regional carbon cycling, potentially influencing the exchange rates of CO 2 flux between soil and the atmosphere in terrestrial ecosystems. Temperature sensitivity of soil respiration ( Q 10 ), 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. 1.80 μmol m − 2 s − 1 ), despite that the average Q 10 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. 39.09 nmol h − 1 g − 1 ) and 22% greater (21.39 vs. 17.50 nmol h − 1 g − 1 ) in the apple orchard than that in the cropland. Compared to the cropland, the lower Q 10 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 Q 10 values. Our findings call for further studies to include the varying effects of land use types into consideration when applying Q 10 values to predict the potential CO 2 efflux feedbacks between terrestrial ecosystems and future climate scenarios. [ABSTRACT FROM AUTHOR]

Nitrogen (N) and phosphorus (P) fertilization has the potential to alter soil respiration temperature sensitivity ( Q 10 ) by changing soil biochemical and crop physiological process. A four-year field experiment was conducted to determine how Q 10 responded to these biochemical and physiological changes in rain-fed agro-ecosystems on the semi-arid Loess Plateau. Soil respiration, as well as biotic and abiotic factors were measured in winter wheat ( Triticum aestivum L.), with three fertilization treatments: (no fertilization (CK), 160 kg N hm − 1 (N), and 160 kg N ha − 1 with 39 kg P ha − 1 (N + P). Mean annual soil respiration rate (calculated by averaging the four years) in the N treatment and N + P treatment was 18% and 48% higher than that in the CK treatment, respectively; and it was increased by 26% (14%–48%) in the N + P treatment as compared with that in the N treatment. The decrease of Q 10 in the N and N + P treatments against the CK treatment was not stable for each year, ranging from 0.01 to 0.28. The maximum decrease of Q 10 in the N and N + P treatments was 10% and 15% in 2014–2015, while in other years the decrease of Q 10 was numerical but not significant. Soil microbial biomass carbon (SMBC) was increased by 10% and 50%, dissolved organic carbon (DOC) was increased by 6% and 21%, and photosynthesis rate was increased ranging from 6% to 33% with N and N + P fertilization. The relative abundance of Acidobacteria, Actinobacteria and Chloroflexi were significantly higher by 32.9%–54.1% in N addition soils (N and N + P) compared to CK treatment, whereas additional P application into soils increased the relative abundance of the family Micrococcaceae, Nocardioidaceae and Chitinophagaceae. Soil respiration was positively related to SMBC, DOC and photosynthesis rate ( p < 0.05). However, variation in Q 10 may be related to the increase of soil mineral N content and variation of the relative abundance of soil microbial community in our study. Nitrogen and additional phosphorus fertilization regimes affect soil respiration and temperature sensitivity differently. [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]

Abstract: Elevated CO2 respiration rates have been observed in soils treated with complex mixtures versus single low molecular weight (LMW) organic substrates, and it has been postulated that a more diverse range of soil microorganisms responds to increasingly complex mixtures of LMW organic substrates. To test this hypothesis, 13C-labelled substrates (glycine, an amino acid mixture and an extract of water soluble compounds from plant roots) were applied at 15 μg C g−1 soil to an arable top soil. The soils were incubated and destructively sampled after 8, 24, 48, 120 and 240 h, and the 13C content of biomarker PLFA for Gram negative bacteria, Gram positive bacteria, Actinobacteria and fungi was determined. There was no significant increase in the concentration of the biomarker PLFA, apart from Actinobacteria at the end of the incubation (120 and 240 h). However there were significant changes in total PLFA concentration due to increases in the 16:0 and 18:0 PLFA, which cannot be assigned to specific functional groups of microorganisms. 13C incorporation into the biomarker PLFA of all microbial groups was significant at every time point, but more 13C was determined in the biomarker PLFA of all microbial groups after the application of the amino acid mixture compared to glycine. Calculations of the proportion of the incorporated of 13C in the different biomarker PLFA suggested the routing of substrate 13C between the microbial groups over time. This was related to the broad functional ecology (‘r’ or ‘K’ strategy) of the different microbial groups. In conclusion, we observed that the response of all microbial groups (detected as 13C incorporation) was increased by the addition of more complex mixtures of LMW organic substrates, but that different microbial groups responded differently over time. [Copyright &y& Elsevier]