Peer review

Appropriate N management can improve soil fertility and favour crop growth and production. Plant activities (e.g. N uptake and root exudates) also have feedback effects on soil N transformations and influence soil N supply and retention capacity. However, the inherent mechanisms between the feedback effect intensities and soil fertilizer are not fully elucidated. Thus, we carried out 15N tracing studies with arable soils at various long-term N fertilizer applications, including unfertilized control, synthetic N fertilizer, biochar, pig manure with and without synthetic N fertilizer, crop residues with and without synthetic N fertilizer. Soil gross N transformation rates, maize N uptake rates, and rhizosphere priming effects (RPE) on soil gross N transformations were quantified to explore mechanisms of soil N dynamics. Results showed that long-term N fertilizer applications significantly stimulated soil mineral N production rates, i.e. gross rates of N mineralization (M) and autotrophic nitrification (ONH4), and further enhanced maize N uptake and yield. Negative RPE on M were observed following long-term N fertilizer applications, ranging from −6.71 to −17.44 mg N kg−1 d−1. Furthermore, the strongest negative RPE on M were found for the combinations of chemical and organic N fertilizers. RPE on ONH4 (ranging from −3.10 to −31.91 mg N kg−1 d−1) also showed a similar trend to RPE on M in different treatments. Microbial N immobilization rates (ITN) and dissimilatory NO3− reduction to NH4+ (DNO3) were stimulated by the presence of maize, showing an obvious positive RPE on ITN and DNO3. Therefore, N retention capacity in the rhizosphere was enhanced via a strong negative RPE on ONH4, and a positive RPE on ITN and DNO3 in presence of maize following long-term N fertilizer applications. The stronger capacity of soil N supply in response to appropriate N management leads to higher maize activities and stronger feedbacks between plant and soil N transformation to improve N retention in rhizosphere, suggesting that the “Matthew effect” also applies to nutrient dynamics in the rhizosphere. Our results show that the combined applications of chemical and organic N fertilizers are the best way to enhance yield, NUE and N retention, and reduce the risks of soil N losses in arable soils.

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