3.1 Effects on soils
Regarding soil and water conservation, the well-developed root systems of toxic weeds can fix sand and capture nutrients from soils with coarser textures (Wang 2001; Wong et al. 2004). As we know, grazing and grassland degradation induce reversed vegetation succession with deterioration of plant community structure from palatable grasses to toxic weeds (Wang et al. 2009; Wu et al. 2009). Even so, compared to bare land, grassland covered by toxic weeds is more susceptible to erosion from strong wind and rain (Zhang, Yue & Qin 2004). On the other hand, toxic weeds significantly increase the water content of the soil surface under drought conditions (An et al.2016). The higher coverage of plants shields topsoil from solar radiation and decreases evaporation (Mchunu & Chaplot 2012); moreover, the soil infiltration rate is relatively high as a result of a well-developed root system, stimulating rainfall storage (Song et al. 2018).
In addition to the physical protection that they provide to grasslands, toxic weeds have remarkable effects on soil nutrient pools and can create fertile islands (Sun et al. 2009) (Fig. 4). Toxic weeds produce more litter as a consequence of their increased growth and because they lose less tissue through grazing. Toxic weeds are also more labile and have higher tissue nitrogen and lower lignin nitrogen compared with other species (An et al. 2016). Soil microorganisms also contribute to the turnover rate and nutrient availability. Soil microbial biomass and soil enzyme activities are higher in toxic weed patches than in areas between these patches (An et al. 2016). Overall, the protection and improvement of soil by toxic weeds provide a superior material basis for plant growth and benefit the recovery of degraded grasslands.