Abstract:

Rice straw return and planting green manure in winter are important ways to improve paddy soil fertility and reduce chemical fertilizer input. Chinese milk vetch （Astragalus sinicus） is the most important winter leguminous green manure crop. A typical crop rotation system in Hunan Province is： Chinese milk vetch-early rice-late rice. Many studies have shown that Chinese milk vetch can reduce chemical nitrogen fertilizer input in rice production. If the input of chemical potassium fertilizer in rice production can also be reduced， this will encourage the promotion and use of Chinese milk vetch. Here we report a 2-year field experiment on cultivation of Chinese milk vetch followed by double cropping of rice， in an acidic red yellow soil and an alkaline purple alluvial soil in Hunan， in which the effect of co-utilization of Chinese milk vetch and double cropping rice straw on the soil K cycle and balance were investigated. The experiments had the same treatments in two soil types， including winter fallow-rice-rice without rice straw return （FRR）， Chinese milk vetch-rice-rice without rice straw return （MvRR）， Chinese milk vetch-rice-rice with rice straw return （MvRR+St）. The results indicated： 1） Compared with FRR and MvRR， the content of different soil K forms of the acidic red yellow soil were not influenced in MvRR+St， while the contents of available K and non-exchangeable K of the alkaline purple alluvial soil were significantly increased in MvRR+St. Compared to MvRR， the contents of water-soluble K， non-specifically absorbed K and specifically absorbed K of the alkaline purple alluvial soil in MvRR+St were significantly increased by 134.0%， 93.0% and 73.4%， respectively. 2） Compared with MvRR， the total K content and K uptake of Chinese milk vetch in the acidic red yellow soil in MvRR+St were significantly increased by 68.6% and 91.1%， respectively； the total K content and K uptake of Chinese milk vetch of the alkaline purple alluvial soil in MvRR+St were significantly increased by 56.4% and 81.2%， respectively. Compared with red yellow soil， the K uptake of Chinese milk vetch in alkaline purple alluvial soil was greater than that in acidic red yellow soil under the same treatment. 3） The yield and K uptake of early rice and late rice were not influenced by MvRR+St in the two typical paddy soils. 4） With the current potassium nutrient input in this study， the MvRR+St treatment had a K surplus in the two typical paddy soils （red yellow soil K surplus was 401.15 kg·ha^-1， purple alluvial soil K surplus was 403.42 kg·ha^-1）. This shows that the co-utilization of Chinese milk vetch and rice straw can reduce the input of chemical potassium fertilizer in double cropping rice production. On the whole， the co-utilization of Chinese milk vetch and double cropping rice straw is beneficial to the K cycle and balance of paddy soils.

Key words: Chinese milk vetch, rice straw, co-utilization, red yellow soil, purple alluvial soil, K cycle