BayCEER Workshop 2023

Priming effects explains the greater soil carbon induced by the long-term combined application of manure and mineral fertilization

Organic and synthetic fertilizers not only increase soil fertility and crop productivity but also enhance soil organic carbon (SOC). However, the priming effect (PE) increases 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 in agricultural systems subjected to long-term annual synthetic and/or organic fertilizer application. This aim was achieved by incubating soil samples with and without ¹³C-glucose from four long-term fertilization practices, i.e., unfertilized, synthetic supplemental (+s), cattle farmyard manure (+m), and synthetic fertilizer with farmyard manure (+s +m). Overall, +m increased the cumulative SOC-derived CO2 by 107, 74, and 24 % compared to the unfertilized, +s and +s +m, respectively. The higher SOC-derived CO2 in +m treatment was associated with the greatest priming effect, 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), in +m compared to +s +m, thus enhancing microbial mining for additional N via increasing SOC mineralization. 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.