How do environmental factors and different fertilizer strategies affect soil CO2 emission and carbon sequestration in the upland soils of southern China?

• The mean value of soil CO2 fluxes from 8:00 to 10:00 am could represent its daily mean value during June–August.
• The mean value of soil CO2 fluxes 9:00 am to 12:00 pm for the rest month of a year (except June–August).
• Soil temperature and moisture combined together could explain 70–83% of the variations of CO2 emission.
• Annual soil CO2 fluxes in manure treatments were significantly higher than those in non-manure treatments.
• Manure applications represented a carbon sink after 19-year fertilizations.

Upland soils have been identified as a major CO2 source induced by human activities, such as fertilizer applications. The aim of this study is to identify the characteristics of soil CO2 emission and carbon balance in cropland ecosystems after continuous fertilizer applications over decades. The measurements of soil surface CO2 fluxes throughout the years of 2009 and 2010 were carried out based on a fertilization experiment (from 1990) in a double cropping system rotated with winter wheat (Triticum aestivum L.) and maize (Zea mays L.) in upland soil in southern China. Four treatments were chosen from the experiment for this study: no-fertilizer application (SR), nitrogen–phosphorus–potassium chemical fertilizers (NPK), NPK plus pig manure (NPKM) and pig manure alone (M). Results showed that the mean value of soil CO2 fluxes from 08:00 to 10:00 am could represent its daily mean value in summer period (June–August) and that from 09:00 am to 12:00 pm for the rest season of a year. Soil temperature and moisture combined together could explain 70–83% of variations of CO2 emission. Annual cumulative soil CO2 fluxes in the treatments with manure applications (8.2 ± 0.8 and 11.0 ± 1.2 t C ha−1 in 2009, and 7.9 ± 0.9 and 11.1 ± 1.2 t C ha−1 in 2010 in NPKM and M, respectively) were significantly higher than those in the treatments with non-manure addition (2.5 ± 0.2 and 3.4 ± 0.2 t C ha−1 in 2009, and 2.1 ± 0.2 and 3.7 ± 0.3 t C ha−1 in 2010 in SR and NPK, respectively). However, the treatments with manure applications represented a carbon sink in the soil (carbon output/input ratio < 1.0), which demonstrated potential for carbon sequestration.