Soil respiration and net carbon flux response to long-term reduced/no-tillage with and without residues in a wheat-maize cropping system

Conservation tillage is not only beneficial to the improvement of integrated soil fertility and crop yield, but also plays a pivotal role in the achievement of ecological agricultural production. Based on a continuous 10-year conservation tillage experiment in the North China Plain, this paper aimed to investigate the effects on soil respiration and net carbon (C) flux in the wheat-maize cropping system, and to identify the physicochemical controls of soil respiration C emission under different tillage and residue managements. Results showed that soil respiration was generally determined by soil temperature, with the lowest and highest rates of 0.50 and 6.54 mu mol CO2-C m(-2) s(-1) in January and July, respectively. Compared with continuous tillage, the reduced/notillage without residue significantly reduced soil respiration rate and the cumulative CO2 emissions, which was principally due to the increased bulk density and decreased effective gas diffusivity according to the redundancy analysis. Whereas, over 90 % increases in soil respiration C emission could be ascribed to the accumulation of organic C, especially for the labile fraction, under residue returning than under residue removing. Additionally, the increased organic C stock in topsoil possibly accounted for the accelerated respiration C emission under reduced/no-tillage with residues. From the perspective of net C flux, it was suggested that decreasing tillage intensity generally reduced the C emissions from agricultural inputs by 11.0 %, while those were increased on average by 7.7 % through implementing residue crushing under residue returning relative to residue removing. Residue returning also increased the mean annual organic C accumulation rate by 115.2 % at the 0-20 cm depth. Collectively, each of the tillage and residue management served as small net C source, but reduced/no-tillage with residues significantly decreased the net C flux while increasing the sustainability and C productivity indexes for wheat-maize cropping system.