Effect of aggregated amino acid on the form of iron oxide in paddy soils in North China

The purpose of this study is to characterize the amount of iron oxide in different types of paddy soil and investigate the effect of exogenous polymeric amino acids on their forms transformation. Different types of paddy soil (brown soil, meadow soil and littoral saline soil) in North China were chosen as the research object. An incubation experiment was carried out by adding γ-poly-glutamic acid and poly aspartic acid, according to 0.05% of dry soil weight to these three paddy soils. After constant temperature and anaerobic incubation for 30 days, the content of total iron oxide (T_Fe), free iron oxide (D_Fe), amorphous iron oxide (O_Fe) and complexed iron oxide (C_Fe) were measured, and the activation and complexing degree of iron oxide were calculated in three paddy soils. Simultaneously, no addition of amino acids was set as the control. The results showed that (1) the content of free iron oxide in three typical paddy soils from high to low was:littoral saline soil > meadow soil≥brown soil; the sequence of complex iron oxide content was brown soil > littoral saline soil≥meadow soil; and the sequence of amorphous iron oxide was brown soil > littoral saline soil > meadow soil. (2) The content of amorphous iron oxide and complexed iron oxide increased by 27.72% and 32.25%, respectively, in paddy soil derived from brown soil with γ-poly-glutamic acid application compared with that of control; but there was no significant change for those adding poly aspartic acid. The content of amorphous iron oxide could be significantly increased by adding both γ-poly-glutamic acid and poly aspartate acid in paddy soil derived from meadow soil, and the complexed iron oxide increased by 136.24% and 12.00% compared with that of control, respectively. The addition of γ-poly-glutamic acid effectively increased the content of amorphous and complex iron oxide in paddy soil derived from littoral saline soil. In conclusion, no significant difference in the content of free iron oxide was observed in the three paddy soils with treatment of γ-poly-glutamic acid addition. While the addition of γ-poly-glutamic acid effectively increased the contents of amorphous and complex iron oxide, but decreased the crystallization rate of iron oxide, which is helpful to improve the content of available Fe. Furthermore, it also activated the iron oxide significantly and restrained the crystallization of iron oxide in three paddy soils. While the application of poly aspartic acid had no obvious excitation effect on complexed and amorphous iron oxide.