Objectives The availability of soil nitrogen (N) significantly affects plant uptake of the nutrient. Here, we studied the changes of different forms of N in soils under wheat growth to regulate N uptake of crop. The aim was to understand the effect of soil fertility on N supply capacity.
Methods A wheat pot experiment was conducted with soils having low, medium, and high fertility (denoted as F1, F2, and F3, respectively). The tested soils were collected from the long-term experimental station at “National Long-term Monitoring Station of Fluvo-Aquic Soil Fertility and Fertilizer Efficiency” in Xinxiang, Henan Province. The three soil fertility levels were respectively collected from the treatments of no fertilizer (CK), NPK fertilizer (NPK), and 1.5 times of NPK fertilizer combined with organic fertilizer (1.5MNPK). All treatment pots received the same quantity of fertilizer and transferred to the field with the top 5 cm above the soil surface. Both soil and plant samples were collected at the jointing stage, booting stage, and after wheat harvest. Soil organic N, mineral N (ammonium and nitrate), and immobilized N (microbial biomass N and fixed ammonium) were analyzed. Further, the grain yield, N uptake, and N balance were studied. A structural equation model (SEM) was employed to clarify the contribution of various forms of soil N to crop N uptake.
Results Mineral N contents decreased during wheat growth. After wheat harvest, the values decreased by 1.8–6.8 mg/kg compared with those before sowing. From jointing stage to harvesting stage, soil microbial biomass N increased first and then decreased in F1, while decreased first and then increased in F2, and continuously increased in F3. Soil fixed ammonium content was significantly increased in F1, F2 and F3 from jointing stage to booting stage, and did not change significantly before jointing stage and after booting stage. The immobilized N pool increased by 10.6 mg/kg in F1 but decreased by 14.3 mg/kg in F2 and 32.2 mg/kg in F3 before the jointing stage, and significantly increased in all the three fertility fields from jointing to booting stage; and decreased by 2.4 mg/kg in F1 and increased by 8.2 mg/kg in F2 and 8.7 mg/kg in F3 from booting to harvesting stage. The highest wheat yield, N uptake, and the lowest N balance were observed in F3. Conversely, the lowest N uptake and highest N balance were observed in F1. Structural equation model analysis showed that immobilized N could directly and positively regulate N uptake by wheat, and organic N could regulate N uptake by influencing immobilized N indirectly.
Conclusions Immobilized N pool directly and positively regulate wheat N uptake. The organic N pool affects wheat N uptake through the transformation of immobilized and mineral N. As the fixed ammonium content is relatively stable, microbial biomass N plays a crucial role in wheat growth by increasing soil available N. On the other hand, it can hold extra mineral N which is easily lost.
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