通过盆栽试验、静态箱气体采样和气相色谱分析技术测定土壤呼吸，研究了施氮150 mg N·kg-1和300 mg N·kg-1水平下，玉米喇叭口期、开花期和成熟期光合产物运往地下的分配。结果表明，玉米运往地下的碳占净同化碳的25%～39%，且随生长进程而降低；而积累于根的碳占6%～21%，高氮处理低于低氮处理，且随生长进程而降低；根际呼吸消耗的碳和转化为土壤有机质的碳分别占11%～14%和4%～8%。运往地下的同化碳中，根系碳、根际呼吸碳和转化为土壤有机质的碳分别占27%～53%、31%～53%和14%～22%；根系碳占运往地下碳的比例为低氮处理>高氮处理、喇叭口期>开花期>成熟期，而根际呼吸碳所占比例则为低氮处理<高氮处理、喇叭口期<开花期<成熟期。由此可见，早期玉米的同化产物运往地下的比例较高，且主要用于根系生长；后期的同化碳主要积累在地上部分，运往地下的主要用于根际呼吸；氮素不足时，促进玉米净同化碳和运往地下的同化碳向根系分配，而减少根际呼吸对后者的消耗，提高了后者的利用效率。总之，玉米通过促进根系的相对生长和提高运往地下碳的利用效率来适应低氮环境。
Carbon transported to underground is an important source of soil organic matter and affects on the availability of soil nutrient and activity of soil microbe, but its allocation has been rarely studied in China. The soil pot experiment with two nitrogen rates (150 and 300 mg N·kg-1) at different maize (Zea mays L.) growth stages was conducted. Carbon dioxides released from soil respiration were sampled by the static chamber method and measured by gas chromatography. The results showed that the aboveground biomass, assimilated carbon, total biomass and net assimilated carbon were significantly higher under high nitrogen rate than under low nitrogen rate at flowering and ripening stages, which indicated that the soil in this experiment was short of nitrogen. Carbon transported to underground in maize accounted for 25%–39% of net assimilated carbon and decreased with the growing process of time. Root carbon accounted from 6% to 21% of the net assimilated carbon and was lower at a high nitrogen rate than at a lower nitrogen rate, and also decreased with the growth process of time. The mounts of carbon respired in the rhizosphere and converted into soil organic matter were 11%–14% and 6%–8% of the net assimilated carbon, but they did not change significantly with the growing process of time and nitrogen rates. The quantity of rhizospheric respiration carbon was significantly higher in high nitrogen treatment than in low nitrogen treatment at flowering and ripening stages. Root carbon, rhizospheric respiration carbon and soil organic carbon derived from assimilation were 27%–53%, 31%–53% and 14%–22% of the carbon transported to underground, respectively. The ratio of root carbon to the carbon transported to underground was higher in lower nitrogen treatment and at the stages of trumpeting>flowering>ripening, but the ratio of rhizospheric respiration carbon to carbon transported to underground was lower at lower nitrogen treatment and at trumpeting<flowering<ripening stages. The increment of soil organic carbon was higher at ripening stage than at trumpeting and flowering stages, and in high nitrogen treatment than in lower nitrogen treatment, suggesting that maize growth and N application accelerated the function of soil as a carbon pool. It was concluded that assimilate produced by maize was mainly transported underground during early growth stage and mainly accumulated in aboveground parts during the late growth period. The carbon transported to underground was used for root growth during the early growth stage and for rhizoshperic respiration during late growth period. The increase of nitrogen rate increased the quantity of rhizospheric respiration carbon during the middle and late maize growth periods but did not change the contribution of rhizospheric carbon to net assimilated carbon. The shortage of nitrogen increased the allocation from net assimilated carbon and the carbon transported to underground to roots, but decreased the rhizospheric respiration from carbon transported to underground. Therefore, the use efficiency of assimilated carbon was high under the lower nitrogen treatment. In summary, maize adapted the stress of low nitrogen by increasing the root growth relatively and enhancing the use efficiency of carbon transported to underground.
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