Objective  Effects on plant metabolism of the widely practiced utilizing spent straws for fertilization in northeastern China corn fields were studied.

  Method  In a field experimentation, application of either the conventional chemical fertilization (CK) or 15% reduction of chemical fertilizer with an addition of humic acid (HA), spent straws (SR), or both HA and SR (SRH) was implemented on a black soil lot of Zea mays. Physiological responses including carbon and nitrogen metabolisms to the treatments of current-season corn plants were monitored.

  Result   The replacement of chemical fertilizer by SR enhanced the phosphoenolpyruvate carboxylase (PEPC) activity, that by HA decreased the 1,5-ribulose bisphosphate carboxylase (Rubisco) and PEPC activities, and that by SRH interacted on PEPC in the corn leaves. Although SR decreased, HA increased the sucrose phosphate synthase (SPS) activity. Both SR and HA (SRH) treatment raised the activities of sucrose synthase (SS), and combined use of HA and HA(SRH) increased the acid invertase (AI), and neutral invertase (NI) promoting the synthesis of reducing sugars that resulted in an increased total soluble sugar (TSS) and starch synthesis. SR had no significant effect, but HA had a positive effect on key enzymes involving the nitrogen metabolism of the plant.

  Conclusion  With a 15% reduction on chemical fertilizer, addition of SRH boosted the photosynthetic efficiency and storage of photosynthetic products of corn leaves without any significant effect on the nitrogen metabolism of the plant.