Effect Of Conservation Tillage On Weeds In A Rotation System And Its Mechanisms On The Loess Plateau Of Eastern Gansu

Weeds spreading causes tremendous damage to the rapid and healthy development of agricultural production. Conservation tillage technology possesses a wide range of ecological and economic benefits, and its impact on weeds is also very significant. In the course of farming, no-tillage practices can increase weed community diversity, while stubble retention practices can reduce weed density and biomass. A field study on conservation tillage under a rotation system (maize-winter wheat-common vetch) was conducted, which has been established for 12 years on the Loess Plateau of eastern Gansu. Our objectives were to investigate the influence of no-tillage, stubble retention and crop type on weed density, weed species and community shift in the rotation system. We also measured the weed seedbank to elucidate the vertical distribution and community characteristics of weed seeds in the soil under different tillage practices in the early spring before weeds germination. In addition, we tested the inhibition of the cover crop (common vetch) on 3 dominant weeds during maize phase.The main results were as follows:1. Weed species, density, and community characteristics varied from species to species in 3 crop phases under different tillage practices. No-tillage practice significantly increased the weed density at winter wheat phase, while rotation with common vetch decreased the density in the no-tillage field. Stubble retention reduced the weed density in maize phase and the lowest density was observed in the no-tillage plus stubble retention field. One month after maize sowing in early April was conducive to maize emergence and growth due to decreasing weed density during this time. Therefore, no-tillage and stubble retention in certain crop growing season can inhibit the occurrence and distribution of weed effectively. The maize-winter wheat-common vetch rotation has played a significant role in weed control.2. No-tillage practice led to a significant increase in weed species diversity, while stubble retention practice resulted in a decrease in weed species diversity in winter wheat fields. Therefore, no-tillage treatment led to a more uniform and stable distribution of weed community. Weed species diversity under no-tillage treatment was significantly lower than that under conventional tillage in common vetch field. It was quite opposite to what we had found in winter wheat field. This result was probably caused by crop rotation and the different growth periods of weed species since the re-establishment of original weed communities was affected by the change of rotated crops, weed species and the disturbance of common vetch sowing. Stubble retention after winter wheat harvest also had less effect on weed community due to minimal effect of stubble and short duration of the stubble cover effect. The differences of species diversity among different treatments during maize growth period was lower than that during winter wheat and common vetch growth periods, mainly due to minor variations in weed species among different treatments. The Bray-Curtis index and Hierarchical Cluster Analysis of the communities also showed the similar results.3. There was a significantly positive correlation between aboveground weed community and soil weed seedbank in 3 crop fields. No-tillage and stubble retention practices led to greater species number and species individual amount than conventional tillage and no stubble retention respectively during winter wheat and common vetch growth periods, especially at 0-5 cm soil layer. No-tillage and stubble retention practices led to greater species number and less species individual amount than conventional tillage and no stubble retention respectively during maize growth period, especially at 0-5 cm soil layer. These results were also confirmed in the performance of stubble retention to suppress germination and emergence of dominant weeds.4. In the soil ca 0.28%-5.19% of the total soil weed seeds could germinate and grow for a longer period. A large number of weeds could germinate in the field due to the great amount of weed seeds in the soil. No-tillage and stubble retention practices promoted weed seeds germination during winter wheat and common vetch growth periods and inhibited numerous weed seeds germination during maize growth period. The growth of the weeds was suppressed by winter wheat after turning green. Weeds germination was significantly inhibited during maize growth period by no-tillage and stubble retention.5. Germination rates of Chenopodium album and Setaria viridis were significantly suppressed by common vetch aqueous extract, while the extract had less impact on Capsella bursa-pastoris. Therefore, the densities of Chenopodium album and Setaria viridis were significantly reduced by stubble retention practice. This result showed that stubble retention of common vetch had significant effect on weeds. We can use common vetch in multiple cropping to control weeds, protect the bare land and to help accumulating moisture.