Abstract:

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.

Key words: Nitrogen rates, Maize, Growing stages, Assimilates, Carbon transported to underground