Background Considering the problem of "engineering" water shortage in karst areas, understanding the effects of organic mulch on soil moisture of sloping farmland in karst areas is of great significance to improve the utilization efficiency of regional natural precipitation and to increase crop yield.

Methods The maize monoculture was set as control (CK), 5 straw mulch treatments (SM₁ by straw 1 111 kg/hm², SM₂ by straw 2 222 kg/hm², SM₃ by straw 3 889 kg/hm², SM₄ by straw 5 556 kg/hm², and SM₅ by straw 6 944 kg/hm²) and 4 biochar mulch treatments (BM₁ by biochar 1 389 kg/hm², BM₂ by biochar 2 778 kg/hm², BM₃ by biochar 5 556 kg/hm², and BM₄ by biochar 11 111 kg/hm²) were set, and alfalfa (AI), chicory (CI) and ryegrass intercropping (RI) in maize were set as 3 grain and grass intercropping treatments according to the difference of intercropping plants, 13 treatments in total. During the whole growing season, the soil moisture dynamics in the cultivated layer was observed in the field and 46 observations were recorded in the whole year of 2018.

Results 1) With the increase of straw mulch application rate, the soil moisture in tillage layer showed an upward trend. When the straw mulch application rate was up to SM₅, the soil moisture reached the maximum, which was significantly higher than CK and SM₁-SM₃ (P < 0.05), but not significantly higher than SM₄, that was, when the straw mulch application rate increased from 5 556 (SM₄) to 6 944 kg/hm² (SM₅), the soil moisture did not increase significantly. 2) With the increase of biochar mulch application rate, the soil moisture also showed an increasing trend, but did not reach a significant level (P>0.05). 3) Soil moisture under AI was higher than that of CK, CI and RI, but did not reach the significant level (P>0.05). 4) Among the three different organic mulches, the overall effect of SM on soil moisture conservation was better than that of BM and GI, but did not reach the significant level (P>0.05).

Conclusions The straw mulch of about 6 000 kg/hm²presents promising effect on soil moisture conservation, and might be an appropriate reference mulch application rate for karst areas in Southwest China. The effect of biochar mulch and grain and grass intercropping on soil moisture conservation is not significant, however, biochar mulch might be a suitable organic mulch to conserve soil moisture during critical phenological or drought periods.