Long-term continuous farmyard manure application increases soil carbon when combined with mineral fertilizers due to lower priming effects

Organic and synthetic fertilizers not only increase soil fertility and crop productivity but also enhance soil organic carbon (SOC). However, the priming effect (PE) leads to increased soil carbon (C) loss through native SOC mineralization. To date, the mechanisms by which long-term (>66 years) synthetic and/or organic fertilization alters net SOC sequestration remain a matter of debate. This study aimed to assess the effects of different fertilization practices on SOC decomposition and PE in agricultural systems subjected to long-term annual synthetic and/or organic fertilizer application. This aim was achieved by collecting topsoil samples (0–20 cm) from four long-term fertilization practices, i.e., unfertilized, synthetic supplemental (+s), cattle farmyard manure (+m, similar nutrient amount to +s), and synthetic fertilizer with farmyard manure (+s +m, the highest nutrient amount). The soil samples were incubated for 33 days with and without ¹³C-glucose addition, and a CO₂ isotope analyzer combined with a modeling approach was used to establish a real-time method to monitor CO₂ and ¹³CO₂ production rates during the incubation period. Overall, +m increased the cumulative SOC-derived CO₂ (SOC-CO₂) by 107, 74, and 24 % compared to the unfertilized, +s and +s +m, respectively. The higher SOC-CO₂ in +m treatment was associated with the greatest priming effect (PE, 390 ± 21 mg C kg soil⁻¹), which corresponded to a 30 % increase compared to the average of the treatments that involved synthetic fertilizer (+s and +s +m) and a 137 % increase compared to the unfertilized control. The results were explained by the lower dissolved nitrogen (N), a proxy of available mineral N, in +m compared to +s +m, thus enhancing microbial mining for additional N via increasing SOC mineralization. However, the combined application of synthetic fertilizer and manure in the +s +m treatment provided enough easily accessible nutrients for microbial growth and activities from the applied synthetic fertilizer, leading to lower SOC mineralization than manure (+m) alone. Nevertheless, the treatments with manure application (i.e., +m and +s +m) significantly increased net SOC compared to the synthetically fertilized treatment and unfertilized control, suggesting greater C inputs than outputs and leading to high SOC accumulation over time. These results indicated that organic manure has a great potential to mitigate climate change by increasing SOC over time, which can be fostered by the addition of synthetic fertilizer; however, caution still needs to be taken regarding the quality and quantity of the added fertilizer.