Soil nitrogen (N) availability and pH are two determinants affecting plant growth, both of which are influenced by long‐term N deposition. However, the physiological mechanism of plants response to the changes in soil N availability and pH are not fully understood. To investigate the response of Picea asperata to both factors, seedlings of P. asperata were exposed to 50 or 1000 µM NH4NO3 with pH 5 or pH 7. In the current study, P. asperata, regardless of N availability and pH in growth medium, exhibited invariably a
NH4 + preference. Lower root biomass, root : shoot mass ratio, total root length and area, and root vitality were detected in high N condition compared to those in low N supply, corresponding well to lower net influxes of
NH4 + and
NO3 - at the root surface in both pH treatments. These results indicate that P. asperata may employ an active‐forge strategy to exploit nutrient resources for growth under low N availability, probably by increased below‐ground carbon allocation and net influxes of
NH4 + and
NO3 - . Although low pH, to some extent may generate more malondialdehyde, P. asperata would enhance pH tolerance by increased detoxification, i.e., antioxidant enzymes (peroxidase), free proline and soluble protein as well as improved carbohydrate status (i.e., soluble sugar and starch).
In the present study, we investigated the absorption and transfer of photosynthetic carbon among the different alfalfa parts. Alfalfa was planted under controlled conditions and pulse‐labeled with carbon–13 dioxide (13CO2; gas) during its branching stage. Then, the amounts of 13CO2 on the alfalfa leaves, stems and roots were measured, and soil samples were taken after 6, 12, 18, 24 and 30 days. The carbon‐13 (13C) distribution ratios of roots, stems and leaves increased over the sampling time and reached the maximum value at day 18; after that, the values decreased but still higher than day 6 and day 12. The 13C distribution ratio in the roots increased gradually to 2.77% at day 30. Over the extended labeling time, 13C loss caused by root and microbial respiration increased. There was no clear trend visible in the 0–10 cm soil layer, but in the 10–20 cm soil layer, the 13C distribution ratio increased gradually from 2.46% at day 6 to 3.05% at day 30. The 13C content in the 0–10 cm soil layer at days 6, 12, 18, 24 and 30 after labeling ranged from 0.90 ppm to 1.53 ppm and in the 10–20 cm layer increased from 0.83 ppm (6 days) to 1.12 ppm (30 days). The 13CO2 released by microbial respiration increased gradually from 2.02 mg at day 6 to 23.30 mg at day 30. The 13CO2 content released by alfalfa root respiration accounted for the majority of total respired 13C content and increased from 24.06 mg (6 days) to 54.15 mg (18 days) and remained constant hereafter. This study suggests that carbon was rapidly cycled between the different parts of alfalfa and soil by means of photosynthesis and respiration. The results obtained in this study will be of importance for building the mechanistic model of photosynthetic carbon flow in alfalfa‐soil system.
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