Effects of Chinese milk vetch incorporation and nitrogen reduction on different forms of Fe and Mn in aggregates of paddy soil

Objectives The formation and stability of soil aggregates are closely related to the content of iron and manganese oxides, which are primarily influenced by management practices and field fertilization. Here, we studied the effect of returning Chinese milk vetch to the field on the aggregate composition and various forms of iron and manganese in paddy soil to reveal the stability mechanism of soil aggregates under nitrogen reduction.

Methods In 2015, a field experiment was conducted in Jingzhou, Hubei Province. The paddy soil derived from Yangtze River alluvial sediment was used as the test material. The treatments in the experiment were CK (no nitrogen fertilizer), MV (Chinese milk vetch planting and returning), N_60% (60% nitrogen fertilizer), N_100% (100% nitrogen fertilizer), MV + N_60% (Chinese milk vetch planting and returning + 60% nitrogen fertilizer), MV + N_100% (Chinese milk vetch planting and returning + 100% nitrogen fertilizer). In 2019, the percentages of soil water-stable aggregates and the contents of various forms of iron and manganese in the aggregates were analyzed.

Rseults Compared with CK, all fertilization treatments improved the proportion of macroaggregates (>0.25 mm) and soil aggregate stability. The effect of MV+N_60% was highly significant, and increased the content of macroaggregates by 49.8% and decreased the microaggregates by 71.0% compared with CK. The different forms of iron and manganese content in macroaggregates was (P<0.05) higher than in micro-aggregates. Except for free manganese in >5 mm aggregates, the other iron and manganese contents (P<0.05) were correlated with MWD or GMD (r=0.474–0.704). The free iron and amorphous iron showed a highly (P<0.05) positive correlation.

Conclusions Free and amorphous Fe are the key drivers of aggregate stability. Chinese milk vetch incorporation and reduced N input increased the free and amorphous Fe in aggregates, promoted the formation of large aggregates and changed the particle size distribution of soil aggregates which improved soil aggregate stability.