Incorporation of maize crop residue maintains soybean yield through the stimulation of nitrogen fixation rather than residue-derived nitrogen in Mollisols

Crop residue amendment to soil is recommended as an effective management practice to return nutrients, especially in the maize-soybean rotation system where large amounts of maize residues are produced. Quantifying the utilisation of maize-residue N by the subsequent soybean crop is essential for optimising the N fertilisation strategy for sustainable production. However, whether and how maize residue amendment alters N acquisition in soybean plants are largely unknown. It was hypothesised that maize residue would supply N and enhance N₂ fixation to meet the N requirements of subsequent soybeans. Three treatments, namely: 1) chemical fertiliser (55.2, 35.2 and 22.4 kg ha⁻¹ of N, P and K, respectively), 2) maize residue (8 t ha⁻¹), and 3) non-fertiliser were applied in a maize-soybean rotation system in a Mollisol soil. It was demonstrated that soybean seed yield in the maize-residue treatment was the same as that in the chemical fertiliser treatment, with 2.9 vs. 3.2 t ha⁻¹ in 2014, 2.7 vs. 2.6 t ha⁻¹ in 2016, and 3.0 vs. 3.1 t ha⁻¹ in 2018. A follow-up pot experiment using ¹⁵N-labelled residue indicated that the residue-derived N accounted for 0.5 % of the total N in soybean seeds and the proportion of symbiotically fixed N reached 82 %. The amount of fixed N during the pod-filling period in the residue treatment was 0.66 g plant^-1, which was 49 % and 41 % higher than those in the chemical fertiliser and non-fertiliser treatments, respectively. The stimulation of N₂ fixation was associated with an increase in fixed N per nodule and the enrichment of diazotrophs in the rhizosphere of soybean. With maize residue amendment, the increased N₂-fixing capability of nodules during the reproductive period, rather than residue-derived N, fulfilled the N demand for maintaining seed yield of soybean. In the maize-soybean rotation system, maize residue amendment would facilitate the N₂ fixation to partly substitute for N fertiliser for soybean production in Mollisols.