Effects of long-term repeated mineral and organic fertilizer applications on soil nitrogen transformations

Repeated applications of mineral and/or organic fertilizer will probably affect gross nitrogen (N) dynamics in soils in the long term but only a limited number of observations are available. Here we present results of a 15N tracing study with soil from the various fertilizer treatments of the Huang‐Huai‐Hai Plain experiment that has been in operation for more than 17 years. Mineral fertilizer in various combinations of N, phosphorus (P) and potassium (K), organic manure (OM) or a mixture of mineral fertilizer and manure had been repeatedly applied for 17 years. The gross N transformation rates were quantified with a 15N tracing model, which uses a parameter optimization routine based on Bayesian principles. Mineralization of soil organic matter was at least 2.7 times greater in all fertilizer treatments compared with the untreated control (0.67 µg N g−1 day−1). While application of mineral N enhanced mineralization from recalcitrant organic N, the application of organic fertilizers stimulated the mineralization of labile organic N. Gross nitrate (NO3−) production solely resulted from ammonium (NH4+) oxidation. Compared with the gross NO3− production in the control treatment (2.22 µg N g−1 day−1), long‐term N applications stimulated gross nitrification by more than 5.3 times. The largest gaseous N emissions were associated with the organic manure treatments. The ratio of gross NO3− production to total mineral N consumption, a ratio proposed previously to determine potential NO3− loss, was a good indicator except for the treatment without N application. This ratio increased from 0.8 in the control to 2.7 in the mixture of mineral fertilizer and manure treatment. The largest gaseous N emissions (N2O + NO) (P < 0.05) were generally found at greater ratios. Results clearly showed that various fertilizers have a differential effect on N dynamics and potential gaseous N losses in the long term.