Stability of soil organic carbon under long-term fertilization: Results from 13C NMR analysis and laboratory incubation

Long-term fertilization has shown a high relevance as regards soil organic carbon (SOC) sequestration, but the degree of stability of the sequestered SOC has not been widely studied up to now. Using physical fractionation combined with laboratory incubation and NMR spectroscopy, we evaluated the differences in SOC stability caused by long-term fertilization. Four SOC fractions were isolated and examined for contents and chemical composition and cumulative amount of CO2-C respired from the fractions under six fertilization treatments: control (CK); balanced inorganic fertilization (NPK); NPK combined with pig manure (MNPK); NPK combined 1.5 times of pig manure (1.5MNPK); and NPK combined with high amount of manure (M2NPK). The highest contents of SOC were recorded for the coarse particulate organic carbon (cPOC) fraction, ranging from 17.25 to 30.47 g kg-1 under CK and M2NPK. The highest cumulative amount of CO2-C was released from the cPOC fraction under manure treatments (M2NPK and 1.5NPKM), which was 56 and 43% higher than that from CK, whereas the lowest amount of CO2-C was released from the mineral associated-C (MOC) fraction under the same treatments, being 65 and 49% higher than that released from CK, suggesting low SOC stability in cPOC and high SOC stability in MOC fractions. However, manure treatments (M2NPK and 1.5NPKM) greatly lowered the specific amount of C-mineralized (C-mineralized per unit total SOC) in fractions and whole soil, suggesting the ability of manure to accumulate more SOC by reducing SOC losses. Moreover, carbonyl-C was found to be the form of SOC experiencing major degree of sequestration under current fertilization practices. The SOC stability indices; aromaticity index (AI), hydrophobicity index (HI) and alkyl-C/O-alkyl-C were found to be higher in manure treated plots further suggesting higher stability of SOC under manure addition. Thus, long-term manure combined with mineral fertilizers would enhance SOC stability through minimizing SOC losses and promoting accumulation of stable C forms in a Chinese Mollisol.