Effect of continuous wheat and maize straw incorporation on soil nitrogen and Fourier transform infrared spectroscopic (FTIR) characterization of soil organic matter

A 6-year field experiment was conducted to study the changes in the forms of soil nitrogen under the treatments of SF (chemical fertilizer application without straw incorporation), T1 (chemical fertilizer application with smashed maize straw incorporation), T2 (chemical fertilizer application with smashed wheat straw incorporation), T3 (chemical fertilizer application with smashed maize and wheat straws incorporation), with no fertilization and no straw incorporation (CK)as the control. Change in FTIR (Fourier transform infrared spectroscopic) characteristics of soil organic matter was also analyzed. The results showed that compared with SF, soil organic nitrogen under T1, T2 and T3 treatments increased respectively by 3.7%, 15.9% and 18.5% and soil inorganic nitrogen decreased by 15.5%, 15.9% and 24.0%. And the ammonium nitrogen content in the treatment of T1, T2, T3 decreased respectively by 11.3%, 6.0% and 12.0% and nitrate nitrogen decreased by 19.3%, 22.9% and 32.1%. The C/N ratio in SOM under T1 and T3 treatments decreased respectively by 2.8% and 1.4%, but the C/N ratio in T2 treatment increased by 1.4%, compared with those under SF treatment. And the ratio of C/O in SOM increased respectively by 9.2%, 12.8% and 12.1% and H/C ratio decreased by 4.6%, 5.5% and 4.6% under T1, T2 and T3 treatments compared with under SF. FTIR spectrum analysis showed that SOM absorption intensity in the 3 5003 200 cm1 band increased under T1, T2 and T3 treatments, with a new band at the 2 924 cm^-1 band. This suggested an increase in aliphatic compounds of SOM. The absorption intensity at the 1 630 cm^-1 band strengthened, suggesting increase in aromatic compounds content. The results verified that fertilization significantly increased the soil contents of organic and inorganic nitrogen as well as the ratios of C/N and C/O in SOM. The incorporation of crop straw residue decreased inorganic nitrogen as well as the ratios of C/N, H/C in SOM. It also increased the content of organic nitrogen and the ratio of C/O in SOM. Meanwhile, the incorporation of straw residue (under T1, T2 and T3 treatments) increased the contents of alcohols, phenols and carboxylic acids, aromatic carbon and amide groups of SOM. The incorporation of both wheat and maize straw residues had more obvious effect.