Fertilization alters microbial community composition and functional patterns by changing the chemical nature of soil organic carbon: A field study in a Halosol

- Organic fertilization increased hydrolase activity and depleted labile C.
- Long-term (11 years) organic fertilization application resulted in higher alkyl-C contents and alkyl C/O-alkyl C ratio.
- Alkyl-C, carbonyl-C and ketone-C explained most of the variation (> 89%) in soil microbial community.
- Alkyl-C and ketone-C explained most of the variation (> 91%) in enzyme activity.

Insight into the effects of fertilization on soil organic carbon (C) content and composition, and on microbial community and functions related to C decomposition are being gained, although the linkage between them remains elusive. To address this knowledge gap, a field experiment was conducted under different fertilization regimes, no fertilization (CL), chemical fertilization for 11 years (CF), organic fertilization for 6 years (OF6) and organic fertilization for 11 years (OF11) in an alkaline sandy loam soil. Changes in soil C chemistry using 13C nuclear magnetic resonance (NMR) spectroscopy, microbial community using phospholipid fatty acid (PLFA) analysis and microbial functions using six hydrolase and three oxidoreductase enzyme activities were assessed. Fertilization significantly increased soil carbon contents, microbial biomass, and the abundance of fungi and bacteria. Nevertheless, CF induced a higher fungal-to-bacterial ratio due to a lower response in bacterial than fungal growth. In contrast, OF11 increased bacterial abundance, hydrolase activity, and consequently depleted labile C, resulting in higher alkyl-C contents and alkyl C/O-alkyl C ratios compared to other fertilizer treatments. Changes in microbial community composition and enzyme activity were tightly linked to soil C forms. Alkyl-C, carbonyl-C and ketone-C explained most of the variation (> 89%) in soil microbial community, while alkyl-C and ketone-C explained most of the variation (> 91%) in enzyme activity. Our results indicate that C composition rather than C quantity shaped soil microbial community composition and enzyme activity, restricting soil C decomposition. The practice of long-term organic fertilizer use is suggested to increase the quantity and recalcitrance of soil organic C.