Straw mulch can induce greater soil losses from loess slopes than no mulch under extreme rainfall conditions

- Erosion and runoff from bare and mulched soils exposed to simulated rain were compared.
- Erosion could be higher from a fully covered silt loam soil than from bare soil.
- Erosion, reduced by mulch in a clay loam, may increase if storms or slopes were longer.
- Lack of a surface seal and concentration of flow under mulch caused this phenomenon.
- Future field studies is needed to confirm that this occurs under field conditions.

Mulching a soil has long been considered an effective way of reducing soil and water losses as compared to an un-mulched soil. However, under certain conditions of extreme rainfalls, a soil may be more susceptible to rill erosion under a straw mulch cover, where the mulch can concentrate overland flow, than without such a cover if the soil developed a resistant surface seal. This study used two typical soils of the Loess Plateau of China (a silt loam and a clay loam) to study mulch induced erosion under intense simulated rainstorms of 100, 140 and 180 mm h−1 amounting to a rainfall depth of 60 mm on 5-m slopes set at different gradients. Under three mulch rates tested (0, 0.2, and 0.8 kg m−2), application of mulch always reduced runoff from the soils. No rills were formed in the silt loam soil without a mulch cover under any of the experimental rainfall conditions due to the formation of a seal that resisted rill formation. However, under the 0.8 kg m−2 mulch treatment, rills were initiated under all but the least severe experimental conditions combinations and soil losses exceeded those from the soil without mulch. In one case, the soil loss under the mulch was almost three times that from the soil without mulch. In the clay loam soil, which developed a more erodible seal than the silt loam soil, rills were formed under all mulch treatments and rainfall intensity and slope conditions, with the exception of the mulch covered soil under one set of conditions. At the end of the 60-mm rainstorm and under the most intense conditions, the total soil loss from the clay loam soil under mulch (0.8 kg m−2) was less than 50% of that from the soil without mulch and in all cases total soil loss was reduced by mulch; however, towards the end of the storm, the soil loss rates, which had been increasing, exceeded those of the un-mulched soil. These phenomena were attributed to the relatively high resistance to rill formation of the surface seal developed on the silt loam soil as compared to the soil without a seal under the high mulch rate, which protected the soil from raindrop impact. Rill initiation was more likely to occur under the mulch due to the increased surface roughness of a soil without a seal and the presence of the mulch, and to individual straws of the mulch laying directly on the soil surface diverting and concentrating soil surface runoff. The results suggest that, with a greater amount of runoff, the same phenomenon may occur on the clay loam soil, which was more susceptible to rill formation under all mulch treatments and developed a less resistant seal. Reducing the mulch rate when severe runoff events are likely from soils similar to the silt loam should avoid such large soil losses. However, for soils more susceptible to rill formation under mulch, such as the clay loam in this study, this may not be effective. Future studies, and especially field studies, should address this issue.