Rejuvenation of iron oxides enhances carbon sequestration by the ‘iron gate’ and ‘enzyme latch’ mechanisms in a rice-wheat cropping system

Nine‐year organic fertilization enhances poorly crystalline iron hydroxide formation and phosphorus availability in a temperate rice–wheat cropping system

“…Compared with the initial soil, nine‐year rice and wheat rotations increased the Feo in the control soil (Figure 1), implying that the formation of poorly crystalline Fe hydroxides might be caused by seasonal variations in the redox conditions in the rice–wheat cropping system. The increase in Feo content might be attributed to the microbial‐mediated Fe transformations rather than the direct addition of fertilizer because the amount of Feo in the fertilizers was negligible (Huang et al., 2017; Jia et al., 2022), while the decrease in Fed content might have resulted from the reduction and translocation of Fe hydroxides in soil profiles under periodic redox fluctuations (Huang et al., 2015). The poorly crystalline Fe hydroxides are expected to be preferentially reduced to Fe(II) by microorganisms in reductive conditions (Ginn et al., 2017; Huang et al., 2018).…”

“…The shift from anaerobic to aerobic conditions could cause Fe(II) re‐oxidization and subsequently precipitate as poorly crystalline Fe hydroxides (Ginn et al., 2017; Winkler et al., 2018). However, the reduced Fe(II) is more soluble than Fe(III) and can be prone to leaching from paddy soils, which might also result in a loss of the more rapidly dissolving poorly crystalline Fe hydroxides, and thus bring out an underestimation of the Feo content (Jia et al., 2022; Winkler et al., 2018). Therefore, seasonal changes in redox conditions stimulated the redox cycles of Fe hydroxides, and thus promoted the formation of poorly crystalline Fe hydroxides in rice–wheat cropping systems.…”

“…The Feo content was higher in the fertilized soils than that in the control soil, indicating that fertilization could further enhance the formation of poorly crystalline Fe hydroxides in the rice–wheat rotation system. Long‐term organic fertilization increased the content of stubble, roots, and rhizodeposits in soils, which might accelerate the Fe reduction by stimulating microbial anaerobic respiration (Lovley et al., 1996), and in turn promote the formation of poorly crystalline Fe hydroxides (Huang et al., 2022; Jia et al., 2022). The low molecular weight organic acids induced by organic amendments could increase the availability of Fe minerals and promote the formation of poorly crystalline Fe hydroxides (Yu et al., 2017).…”

“…Compared with the initial soil, nine-year rice and wheat rotations increased the Feo in the control soil (Figure 1), implying that the formation of poorly crystalline Fe hydroxides might be caused by seasonal variations in the redox conditions in the rice-wheat cropping system. The increase in Feo content might be attributed to the microbial-mediated Fe transformations rather than the direct addition of fertilizer because the amount of Feo in the fertilizers was negligible (Huang et al, 2017;Jia et al, 2022), while the decrease in Fed content might have resulted from the reduction and T A B L E 2 Correlation between oxalate-extractable iron hydroxides (Feo) and the contents of H 2 O-extractable total phosphorus (Pt), NaHCO 3 -extractable Pt, NaOH-extractable Pt, HCl-extractable phosphorus (P), and residual-P and between Feo and the proportions (%) of labile phosphorus (P), moderately labile P, and non-labile P.…”

“…Organic amendments not only promote the formation of poorly crystalline Fe hydroxides by stimulating the production of organic acids (Yu et al., 2017) and regulating the Fe redox bacterial communities (Wen et al., 2018) but also prevent the transformation of Fe hydroxides into a more crystalline phase (Huang et al., 2018). Chemical fertilization induces soil acidification, which in turn affects the formation of poorly crystalline Fe hydroxides in paddy soils (Ding et al., 2015; Jia et al., 2022). Generally, fertilization enhances P availability in three ways: directly through the inputs of P sources (Hao et al., 2008; Liu et al., 2020), indirectly through the transformation of Fe hydroxides (Zhao et al., 2018) and to a lesser extent through the increased activity of bacteria related to the P cycle (Chen et al., 2018; Wan et al., 2020).…”