2.2 | Experimental treatments and field management
This field experiment began in 2008. The site had a long history (> 40 year) of wheat and maize double cropping rotation under the same managements before the study, and the typical rotation of wheat (cv. Mianyang 26) and maize (cv. Nonghua 50) was also chosen for this study. A randomized complete block design experiment using 12.5-m-wide and 56-m-long plots was managed over 8 years with four replicates. Three tillage methods [control with no-tillage and straw removal (CK), no-tillage with straw stubbles 30–40 cm in height (NT), and rotary tillage with straw incorporation (RT)] were applied before maize planting, and two straw treatments [straw return (SR) and no straw return (SR0)] were applied after maize harvest. Thus, there were totally 24 plots with four replicates for each treatment(CK-SR, CK-SR0, NT-SR, NT-SR0, RT-SR, and RT-SR0).Both wheat and maize were harvested using combine-harvesters.Wheat straw was manually removed from the farmland in CK, but was remained with straw stubbles 30–40 cm in height in NT, and was chopped into about 10 cm pieces and incorporated into the top 10 cm soil layer immediately after harvest with ~ 2-3 times of rotary tillage in RT. Maize straw was chopped into about 10 cm pieces and incorporated into the top 20 cm soil layer immediately after harvest with ~ 2-3 times of rotary tillage in SR, and was manually removed from theSR0 plots.
The wheat was sown at a rate of 210–225 kg ha-1yr-1 with a row spacing of 15 cm, and the maize was planted at a density of approximately 63,000 plants ha−1 yr-1 with a row and plant spacing of 60 and 25 cm, respectively. The basal fertilization rates were 120 kg N ha−1 yr-1 and 102 kg P2O5ha−1yr−1for wheat production, and were 67.5 kg N ha−1 yr−1 and 22.5 kg P2O5 ha−1yr−1for maize production, respectively. A rate of 120 kg N ha−1 yr−1 was also applied at the tasseling stage of maize. Urea and diammonium phosphate provided the required N and P2O5 in all treatments. Before sowing wheat, ~ 2-3 times of rotary tillage were applied at a depth of 20 cm in all the plots for the convenience of wheat seeding. Irrigation and pesticide management practices were in keeping with local agronomic practices.