Two approaches for net ecosystem carbon budgets and soil carbon sequestration in a rice-wheat rotation system in China

Few studies have comprehensively evaluated the method of estimating the net ecosystem carbon budget (NECB). We compared two approaches for studying the NECB components on the crop seasonal scale as validated by the soil organic carbon (SOC) changes measured over the 5-year period of 2009-2014. The field trial was initiated with four integrated soilcrop system management (ISSM) practices at different nitrogen application rates relative to the local farmer's practices (FP) rate, namely, N1 (25 % reduction), N2 (10 % reduction), N3 (FP rate) and N4 (25 % increase) with no nitrogen (NN) and FP as the controls. Compared with the FP, the four ISSMscenarios of N1, N2, N3 and N4 significantly increased rice yields by 9.5, 19, 33 and 41 %, while increasing the agronomic nitrogen use efficiency (NUE) by 71, 75, 99 and 79 %, respectively. The SOC sequestration potentials were estimated to be -0.15 to 0.35 Mg C ha(-1) year(-1) from the net primary production minus heterotrophic respiration approach and-0.32 to 0.67 Mg C ha(-1) year(-1) from the gross primary production minus ecosystem respiration approach for the 2010-2011 rice-wheat annual cycle. Similarly, the annual topsoil carbon sequestration rate over 2009-2014 was measured to be-0.22 Mg C ha(-1) year(-1) for the NN plot and 0.13-0.42 Mg C ha(-1) year(-1) for the five fertilized treatments. Both NECB approaches provided a sound basis for accurate assessment of the SOC changes. Compared to the SOC sequestration rate from the FP, the proposed N3 and N4 scenarios increased the SOC sequestration rates while also improving rice yield and NUE.