| A field experiment was conducted on a maize-wheat-sotbean rotation system in the westen Loess Pleteau. There were four treatments with concentional tillage (t), concentional tillage with stubble retention (ts), no-till (nt) and no-till with stubble retention (nts). The dynamic of soil total carbon, soil organic carbon, laible carbon, soil organic carbon mineralization, soil total nitrogen under top soil were measured. Following are the main results:1. Soil total carbon content and organic carbon content of 0-5cm layer under three conservation tillage treatments were increased by 4.1%-33.9% compared to the conventional tillage treatment; the soil laible organic carbon of ts treatment was significantly increased by 6.4% and the soil organic carbon density of nts treatment was significantly increased by 24.7% compared to t treatment, no-tilling resulted in soil organic C accumulation in top soil.2. The soil total nitrogen content under the three conservation treatments was 17.8%-38.0% higher than t treatment. The correlation analysis showed that the soil total nitrogen content of 0-10cm layer was significantly and positively correlated with soil organic carbon content.3. Soil microbial carbon content was different under different treatments and with different plants. The microbial carbon content under nts was increased by 6.4% compared to t treatment in 0-5cm layer; in 5-10cm layer, soil microbial carbon of ts was significantly higher than the t treatment, increased by 30.8% and 84% for maize and wheat, respectively.4. After incubated in the lab for 173 days, the organic carbon accumulative mineralization of 0-5cm under three conservation tillage was increased by 14.9%-78.8% compared to t treatment, and the correlation analysis showed that the soil organic carbon accumulative mineralization was significantly and positively correlated with F1 and organic carbon; and First-order dynamic equation y= a x (1-e -kx) well described the soil organic carbon mineralization (R2:0.939-0.989); ts showed the highest soil organic carbon potential mineralization capacity and nt showed the highest soil organic carbon mineraliazation speed constant. After the incubation in the lab, soil F1 reduced and F2 and F3 increased.5. Staw decomposition rate under ts was significant higher than the nts treatment (p< 0.01), increased by 3%,227.8% and 55.1% for maize, wheat and soyabean, respectively. Staw decomposition rate of maize was 39.5%(ts) and 38.9%(nts) at wheat harvest time; Staw decomposition rate of wheat was 44.9%(ts) and 13.6%(nts) at soybean harvest time; Staw decomposition rate of soybean was 78.4%(ts) and 47.4%(nts) at maize harvest time.6. Soil organic carbon sequestration rate under conservation tillage showed that nts (0.19 Mg/hm2-a) >ts(0.16 Mg/hm2·a)>nt(0.08 Mg/hm2·a), and increased by 140% compared with nts treatmentnt. System carbon value showed that nts (5.03 dollar/hm2-a)>ts (4.82 dollar/hm2-a)>t (4.34 dollar/ hm2·a)>nt (2.54 dollar/hm2-a).Conservation tillage significantly increased the soil carbon sequestration, soil carbon under stubble retention was better than no-till, stubble retention combined with no-till showed great advantages compared to stubble retention or no-till alone. Our research provided theoretical direction for extending conservation tillage and establishing sustainable agricalture in Loess Plateau.
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