Bacterial necromass decomposition and priming effects in paddy soils depend on long-term fertilization

Long-term fertilization alters nutrient availability and microbial community composition in soil, thereby modulating the decomposition of microbial necromass and its influence on soil organic carbon (SOC) turnover. However, the microbial taxa that drive necromass recycling and how their activity translates into positive or negative priming effects (PEs) on SOC mineralization in rice paddies remain unknown. We combined ¹³C isotope probing and high-throughput sequencing to investigate the microbial groups involved in necromass decomposition and their associated PEs on SOC mineralization in paddy soils subjected to 34 years of mineral fertilization or chicken manure application as compared to unfertilized control soil. Following the addition of ¹³C-labeled bacterial necromass, 50–60 % of the ¹³C was mineralized to CO₂ within 210 days, with fertilized soils releasing 15 % more ¹³C–CO₂ compared to unfertilized soils. Microbial uptake of ¹³C from necromass occurred sequentially: Gram-positive (Gram⁺) bacteria dominated initial incorporation (within 5 days), followed by uptake by Gram-negative (Gram⁻) bacteria and thereafter by actinomycetes and fungi after 40 days. In unfertilized carbon-limited soils, K-strategist taxa, such as Gram⁺ bacteria, Gamma-proteobacteria, Patescibacteria, and Basidiomycota, mined recalcitrant SOC to fulfill their nutrient demands, thus generating a strong positive PE. Conversely, in soils receiving combined mineral and organic inputs, r-strategist taxa, including Gram⁻ bacteria, Alpha-proteobacteria, and Ascomycota, preferentially decomposed newly formed microbial necromass rather than SOC, resulting in a negative PE and net SOC accumulation. These findings demonstrate that fertilization-driven shifts in microbial life-history strategies as well as SOC availability govern necromass turnover and its priming consequences, highlighting necromass recycling as a key lever to raise SOC stabilization. Thus, managing fertilizer regimes to favor targeted microbial guilds offers a promising pathway to increase carbon sequestration and sustain soil health in paddy ecosystems.