The Dry-Hot Valley is a unique geographical region in southwestern China, where steep-slope cultivation and accelerating changes in land-use have resulted in land degradation and have aggravated soil erosion, with profound impacts on soil fertility. Soil microbes play a key role in soil fertility, but the impact of land-use changes on soil microbes in the Dry-Hot Valley is not well known. Here, we compared characteristics and drivers of soil microbial community composition and soil fertility in typical Dry-Hot Valley land uses of sugarcane land (SL), forest land (FL), barren land (BL) converted from former maize land (ML), and ML control. Our results showed that BL and SL had reduced soil organic carbon (SOC), total nitrogen (TN), and total potassium (TK) compared to ML and FL. This indicated that conversion of ML to SL and abandonment of ML had the potential to decrease soil fertility. We also found that fungal phyla Zoopagomycota and Blastocladiomycota were absent in SL and BL, respectively, indicating that land-use change from ML to SL decreased the diversity of the bacterial community. Redundancy analysis indicated that the relative abundance of bacterial phyla was positively correlated with TN, SOC, and available potassium (AK) content, and that fungal phyla were positively correlated with AK. Land-use indirectly affected the relative abundance of bacterial phyla through effects on soil moisture, clay, and AK contents, and that of fungal phyla through effects on clay and AK contents. In addition, land-use effects on bacteria were greater than those on fungi, indicating that bacterial communities were more sensitive to land-use changes. Management regimes that incorporate soil carbon conservation, potassium addition, and judicious irrigation are expected to benefit the stability of the plant–soil system in the Dry-Hot Valley.

<p>Despite soil erosion has a strong impact on crop yield, whether soil erosion depth leads to abrupt or gradual crop yield changes is not well understood. To investigate how crop yields respond to soil erosion depth, we conducted a simulated erosion experiment by adopting the cut-and-fill method from 2012 to 2018 in a typical mountain area in the southeastern China. A completely randomized design with five soil erosion depth (5, 10, 20, 30 and 40 cm soil cut) and a control (0 cm soil cut) were used. Each treatment had three replicates. Maize was planted in these simulated erosion plots and maize yields were monitored from 2012 to 2018. Our results showed that the maize yield decreased with erosion depth and with decreasing remaining A<sub>p</sub> horizon depth. Inconsistent with earlier studies, maize yield exhibited a quadratic function rather than linear response to increase in soil erosion depth and decrease in remaining A<sub>p</sub> horizon depth. Soil erosion depth led to abrupt changes in maize yield. Compared with control, maize yield did not decrease significantly at 5 cm erosion depth or ＞25 cm remaining A horizon depth, but its reduction rate per 1cm of soil loss (3.36%) increased sharply at 10 cm erosion depth or 20 cm remaining A<sub>p</sub> horizon. When remaining A<sub>p</sub> horizon left 10 cm, maize yield demonstrated the lowest, which may be irreversible via application of chemical fertilization. Considering high heterogeneity of A<sub>p </sub>horizon in the mountain area, soil erosion-crop yield relationship could be expressed well by remaining A<sub>p </sub>horizon. We also found remaining A<sub>p</sub> depth had a significant direct and indirect (via reduced SOM, soil available water, AP and AK contents) negative effect on maize yield. These results could be useful in identifying allowable soil-loss thickness and highlight the importance of soil nutrient monitoring in different soil erosion levels in designing a fertilization scheme aimed at ensuring food security.</p>