Effects of long-term fertilization on oxidizable organic carbon fractions on the Loess Plateau, China

The effects of long-term fertilization on pools of soil organic carbon (SOC) have been well studied, but limited information is available on the oxidizable organic carbon (OOC) fractions, especially for the Loess Plateau in China. We evaluated the effects of a 15-year fertilization on the OOC fractions (F1, F2, F3 and F4) in the 0–20 and 20–40 cm soil layers in flat farmland under nine treatments (N (nitrogen, urea), P (phosphorus, monocalcium phosphate), M (organic fertilizer, composted sheep manure), N+P (NP), M+N (MN), M+P (MP), M+N+P (MNP), CK (control, no fertilizer) and bare land (BL, no crops or fertilizer)). SOC content increased more markedly in the treatment containing manure than in those with inorganic fertilizers alone. F1, F2, F4 and F3 accounted for 47%, 27%, 18% and 8% of total organic carbon, respectively. F1 was a more sensitive index than the other C fractions in the sensitivity index (SI) analysis. F1 and F2 were highly correlated with total nitrogen (TN) and available nitrogen (AN), F3 was negatively correlated with pH and F4 was correlated with TN. A cluster analysis showed that the treatments containing manure formed one group, and the other treatments formed another group, which indicated the different effects of fertilization on soil properties. Long-term fertilization with inorganic fertilizer increased the F4 fraction while manure fertilizer not only increased labile fractions (F1) in a short time, but also increased passive fraction (F4) over a longer term. The mixed fertilizer mainly affected F3 fraction. The study demonstrated that manure fertilizer was recommended to use in the farmland on the Loess Plateau for the long-term sustainability of agriculture.