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• Fertilization regimes affected straw-C incorporation into amino sugars. • Higher 13C-fungal residues accumulated in NPK plus manure supplied soil. • Higher straw-13C transformed into bacterial C in NPK-treated soil. • NPKM soil sustained greater recovery of 13C in total amino sugars by the end. • Straw-13C amino sugars contributed differentially to 13C-SOC in different soils. Emerging evidences have emphasized the primacy of microorganisms as executor in soil organic C (SOC) formation. Yet, whether and how microbial transformation of crop residue-derived C into necromass is affected by fertilization regimes remains poorly understood. To address this, soils subjected to 3-year applications of chemical fertilizer (NPK) vs manure plus chemical fertilizer (NPKM) were incubated with or without addition of 13C-labelled rice straw for 60 days. The 13C incorporation into amino sugars (microbial necromass biomarker) was identified by using an amino sugar-based stable isotope probing approach. The straw-C were transformed more rapidly into total amino sugars (13C-AS s) in NPK-treated soil, while more pronounced 13C-AS s accumulation occurred in NPKM soil at later stage, indicating temporal microbial C uptake to generate necromass varied depending on fertilization regimes. Moreover, straw-derived fungal C was significantly higher in NPKM over NPK while bacterial counterpart reversed during the incubation. Our results reinforced changes in microbial communities and their C utilization capacities for efficient residue production in different fertilized soils. Additionally, NPKM-treated soil sustained a prolonged but likely more efficient utilization of straw-C with increasing time as evidenced by significantly higher percentage of straw-dervied C enrichment in 13C amino sugar pools by the end of incubation. These findings reveal that fertilization regimes can alter response strategies of microbial transformation of plant C into microbial-derived C. Together with differential contributions of these newly-formed microbial C to SOC pool, our work highlights the key role of microbial anabolisms in building up SOC following crop residue return to soils under different fertilization regimes. [ABSTRACT FROM AUTHOR]

Partial organic fertilizer substitution for chemical fertilizer has been proven to reduce the synthetic nitrogen (N) fertilizer requirement while maintaining or improving rice yield. However, little is known about the underlying physiological mechanisms of such an effect. Field trials were conducted to investigate the effect of organic fertilizer substitution for chemical fertilizer on rice yield and the source – sink relationship in two growing seasons. The treatments were as follows: no N fertilizer application (CK); farmers' fertilizer practice (FFP); optimal chemical N management (OPT); and organic fertilizer substitution based on the OPT treatment (OPT-SN). Compared with treatments of chemical N fertilizer alone, rice yield was maintained in the OPT-SN treatment. The value of the sink – source difference in rice was positive during the grain filling period irrespective of the treatments, implying that the grain yield was source-limited. Although the effects on sink growth were similar among the different N addition treatments, organic fertilizer substitution improved source capacity by extending the duration of source activity when compared with treatments only containing chemical N fertilizer. The source – sink relationship was more balanced (lower sink – source difference) in the OPT-SN treatment relative to treatments of chemical N fertilizer alone, and a significant negative relationship between sink – source difference and grain yield was observed. Overall, organic fertilizer substitution could maintain grain yield by establishing a more balanced source – sink relationship during the grain filling period. Our findings clarify the positive effect of organic fertilizer substitution on source – sink balance and provide a basis for fertilization strategies to improve crop productivity. • Organic substitution extended the duration of source growth. • Organic substitution was beneficial for source capacity. • Organic substitution maintained yield through a balanced source–sink relationship. [ABSTRACT FROM AUTHOR]