Qiqi Sun

Temperature sensitivity of soil respiration: Synthetic effects of nitrogen and phosphorus fertilization on Chinese Loess Plateau.

Nitrogen (N) and phosphorus (P) fertilization has the potential to alter soil respiration temperature sensitivity (Q10) by changing soil biochemical and crop physiological process. A four-year field experiment was conducted to determine how Q10 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), 160kgNhm-1 (N), and 160kgNha-1 with 39kgPha-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 Q10 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 Q10 in the N and N+P treatments was 10% and 15% in 2014-2015, while in other years the decrease of Q10 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 Q10 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.

Phosphorus Accumulation and Sorption in Calcareous Soil under Long-Term Fertilization

Application of phosphorus (P) fertilizers to P-deficient soils can also result in P accumulation. In this study, soil P status and P uptake by apple trees were investigated in 5-, 10-, and 15-year-old orchards in the semi-arid Loess Plateau, China, and subset soils with different soil P statuses (14–90 Olsen-P mg kg−1) were selected to evaluate the characteristic P adsorption. Due to the low P-use efficiency (4–6%), total soil P increased from 540 mg kg−1 to 904 mg kg−1, Olsen-P ranged from 3.4 mg kg−1 to 30.7 mg kg−1, and CaCl2-P increased from less than 0.1 mg kg−1 to 0.66 mg kg−1 under continuous P fertilization. The P sorption isotherms for each apple orchard were found to fit the Langmuir isotherm model (R 2 = 0.91–0.98). K (binding energy) and Q m (P sorption maximum) decreased, whereas DPS (degree of phosphorus sorption) increased with increasing P concentration. CaCl2-P increased significantly with the increase of Olsen-P, especially above the change point of 46.1 mg kg−1. Application of surplus P could result in P enrichment in P-deficient soil which has high P fixation capacity, thus posing a significant environmental risk.

Contrasting responses of soil respiration and temperature sensitivity to land use types: Cropland vs. apple orchard on the Chinese Loess Plateau

Land use plays an essential role in regional carbon cycling, potentially influencing the exchange rates of CO2 flux between soil and the atmosphere in terrestrial ecosystems. Temperature sensitivity of soil respiration (Q10), 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. 1.80μmolm-2s-1), despite that the average Q10 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. 39.09nmolh-1g-1) and 22% greater (21.39 vs. 17.50nmolh-1g-1) in the apple orchard than that in the cropland. Compared to the cropland, the lower Q10 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 Q10 values. Our findings call for further studies to include the varying effects of land use types into consideration when applying Q10 values to predict the potential CO2 efflux feedbacks between terrestrial ecosystems and future climate scenarios.

Spatial variations of soil respiration and temperature sensitivity along a steep slope of the semiarid Loess Plateau

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.