Microbial community composition and species uniformity regulate the mineralization characteristics of organic carbon fractions in red soil

Objective Terrestrial carbon cycle is dependent on the mineralization of organic carbon in soil. We explored the mineralization characteristics of organic carbon components and their microbiological responses to clarify the turnover process of soil organic carbon.

Methods The research was a long-term experiment conducted on the red soil of Qiyang Experimental Station of Chinese Academy of Agricultural Sciences. The tested soil samples were collected from four treatments - fallow (CK₀), no fertilization control (CK), NPK chemical fertilizers (NPK), and NPK with organic fertilizers (NPKM). Firstly, a physical grouping method was used to divide the total organic carbon (OC) into four fractions, as coarse-particle organic carbon (cPOC), fine-particle organic carbon (fPOC), intra-microaggregate-particle organic carbon (iPOC), and mineral-associated organic carbon (MOC). Then, chamber culture method was used to determine the mineralization dynamics of the four fractions, and phospholipid fatty acid (PLFA) method was used to determine the microbial biomass and composition. Finally, redundancy analysis was employed for the contribution of microbial community characteristics to the organic carbon mineralization.

Results Long-term fertilization significantly increased the proportion of active organic carbon fractions (cPOC, fPOC, iPOC). The mineralization rate of all the four fractions was high in the early 1–35 days, and gradually slowed down until stabilized. The maximum mineralization rate and cumulative mineralization amount of each fraction were in order fPOC>cPOC>iPOC>MOC, indicating that the fPOC had the highest activity. Compared with CK, CK₀ increased the maximum mineralization rates of cPOC, fPOC, iPOC, and MOC by 158.0%, 36.4%, 67.3% and 146.0%, respectively; NPK treatment did not affect these indices significantly; while NPKM treatment increased them by 246.0%, 62.9%, 21.4%, and 183.0%, respectively. fPOC exhibited the highest total PLFA biomass at the rapid mineralization stage (94.5 nmol/g), which was about 3 times of other fractions (26.1–35.0 nmol/g). The ratio of Gram-positive bacteria to Gram-negative bacteria (G⁺/G^–) in each fraction was MOC (6.06)>cPOC (5.18)>fPOC (1.26)>iPOC (1.03), and the average ratio of fungi to bacteria (F/B) was iPOC (0.11)>fPOC (0.10)>MOC (0.06)>cPOC (0.03), the fPOC and iPOC had higher F/B ratio and lower G⁺/G^– ratio, and higher diversity index than the other two fractions, indicating a more stable community structure, less nutrient stress and high community diversity. Redundancy analysis showed that microbial factors, total PLFA, fungal, and Gram-negative, accounted for 96.4% of the fractional mineralization differences.

Conclusions The mineralization degree of soil organic carbon is closely related to the microbial community structure and species uniformity of the community. Long-term fertilization, especially with NPKM treatment increased the microbial biomass of active organic carbon fractions, and the extra energy source input (compared to NPK) exaggerated the mineralization of organic carbon. This may be one of the reasons for the complementary application of organic fertilizers to improve soil fertility.