Trade-off between soil organic carbon sequestration and nitrous oxide emissions from winter wheat-summer maize rotations: Implications of a 25-year fertilization experiment in Northwestern China

- Field measurements suggest that large N2O fluxes were mainly due to denitrification.
- Time stage relative to SOC saturation seemed critical to estimate C sequestration rate.
- About 88-151% of the sequestered C was offset by N2O emissions over 1990-2015.
- All fertilization regimes in this study were greenhouse gas sources over 1990-2040.

The primary aims of this study were to (i) quantify the variations in nitrous oxide (N2O) emissions and soil organic carbon (SOC) sequestration rates under winter wheat-summer maize cropping systems in Guanzhong Plain and (ii) evaluate the impact of organic amendments on greenhouse gas mitigation over a long-term period. We measured N2O fluxes during the maize season in 2015 under four fertilizer regimes in a long-term fertilization experiment. Soil was treated with only synthetic fertilizers in the maize season and with synthetic fertilizers, synthetic fertilizers plus crop residues and synthetic fertilizers plus low and high levels of dairy manure in the winter wheat season from 1990. The SOC content (0-20 cm) was collected annually at the same site between 1990 and 2015. Synthesis of our measurements and previous observations (between 2000 and 2009) within the investigated agricultural landscape revealed that cumulative N2O emissions increased with the SOC content following natural logarithm models during both the maize and winter wheat seasons (r2 > 0.77, p < 0.001), implying a trade-off between N2O emissions and SOC sequestration. The SOC content increased under all fertilizer regimes, and the dynamics were well fitted by the linear and logistic regression models (r2 > 0.74, p < 0.001), indicating that all the fertilizer treatments in this study sequestered SOC. By applying these regression models, we estimated that the two manure-amended treatments accumulated a negative global warming potential (ranging from − 1.9 to − 12.9 t CO2-equivalent ha− 1) over the past 25 years. However, this benefit would most likely be offset by high N2O emissions at saturated SOC levels before 2020. Our estimates suggest that organic amendments may not be efficient for greenhouse gas mitigation in Guanzhong Plain over a long-term period. We recommend efforts to inhibit N2O production via denitrification as being critical to resolving the conflict between SOC sequestration and N2O emissions.

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