Long‐Term Fertilization Practices Alter Aluminum Fractions and Coordinate State in Soil Colloids

Understanding how fertilization practices affect Al fractions is important for the alleviation of soil acidification and the sequestration of soil organic C (SOC). Two selective extraction methods, high‐resolution 27 Al nuclear magnetic resonance (NMR) spectroscopy and Fourier‐transform infrared spectroscopy (FTIR), were used to assess the transformation of Al fractions in Ferralic Cambisol soils under long‐term (22‐yr) treatment with chemical and/or organic fertilizers. The results showed that Al fractions were significantly ( P < 0.05) altered by long‐term fertilization. Compared with chemical fertilization (N and N–P–K), organic fertilization (manure alone and N–P–K with manure) significantly ( P < 0.05) increased amorphous Al and decreased exchangeable Al, while the addition of lime (N with lime and N–P–K with lime) significantly ( P < 0.05) increased weakly organically bound Al and decreased exchangeable Al. Amorphous Al was significantly positively correlated with soil C ( P < 0.01), indicating that amorphous Al could enhance soil C sequestration. In contrast, exchangeable Al was significantly negatively correlated with soil pH ( P < 0.01), indicating that reducing the concentration of exchangeable Al could alleviate soil acidification. The 27 Al NMR and FTIR spectroscopy results of soil colloids further confirmed the presence of amorphous Al as allophane and imogolite in soil colloids under no‐fertilization and organic‐fertilization treatments but not under chemical fertilization, suggesting that the enhancement of soil nanominerals by organic fertilization may be another new mechanism for alleviating soil acidification. Our results provide novel insight into how Al fractions and their coordinate states under long‐term fertilization enhance soil C sequestration while alleviating soil acidification.