Nitrogen loss is the main reason for land quality degradation and productivity decline and an important factor in groundwater pollution. Extreme rainfall has occurred frequently in Karst areas of southwest China in recent years. It is of great significance to study the response of soil nitrogen loss to extreme rainfall in Karst areas to prevent and treat land quality degradation and non-point source pollution. In this study, field monitoring and indoor artificial rainfall simulation were used to study the loss characteristics of total soil nitrogen (TN), ammonium (NH4+-N) nitrogen, and nitrate-nitrogen (NO3−-N) in Karst bare slope farmland (slope angles of 5° and 10°) under extreme rainfall conditions. The results showed that: (1) Extreme rainfall (90 mm/h) increased the surface runoff, middle soil runoff, and underground runoff by 1.68 times, 1.16 times, and 1.43 times, respectively, compared with moderate rainfall (60 mm/h), so that nitrogen loss increased with runoff. (2) The loss of nitrate-nitrogen in surface, soil, and underground under extreme rainfall conditions was 223.99, 147.93, and 174.02% higher than that under moderate rainfall conditions, respectively; the nitrate losses were 203.78, 160.18, and 195.39% higher, respectively. Total nitrogen losses were 187.33, 115.45, and 138.68% higher, respectively. (3) The influencing factors of total soil nitrogen and nitrate-nitrogen loss in Karst slope farmland were slope > rainfall duration > rainfall intensity, while the influencing factors of ammonium nitrogen loss were rainfall duration > slope > rainfall intensity. Therefore, in controlling nitrogen loss in Karst slope farmland, changing slope degree and increasing farmland coverage may be useful measures to slow the nitrogen loss caused by extreme rainfall.
Analyzing the ecological stoichiometric characteristics and soil enzyme activity of litter and soil in different vegetation types within karst areas can help to clarify the nutrient cycles and element abundance in those areas, in addition to providing basic data for vegetation restoration and reconstruction. In this study, the carbon (C), nitrogen (N), and phosphorus (P) contents of litter and soil and the alkaline phosphatase (ALP), sucrase (Suc), urease (Ure), and catalase (CAT) activity of soil were measured in grassland (GR), shrubland (SR), arbor and shrub compound forest (AS), and arbor forest (AR). The correlation between litter and soil stoichiometry and soil enzyme activity was analyzed to reveal the effects of different vegetation types on the C, N, and P stoichiometric characteristics of litter and soil, soil enzyme activity, and their driving mechanisms. The results showed that the C, N, and P contents of litter in the study area were 366.2–404.48 g/kg, 12.37–15.26 g/kg, and 0.76–1.05 g/kg, respectively. The C, N, and P contents of soil in the study area were 27.69–42.4 g/kg, 2.38–4.25 g/kg, and 0.56–0.68 g/kg, respectively. The litter N content and soil C and N contents were highest in the arbor forest (p < 0.05), while those in the grassland were the lowest (p < 0.05). The C:P and N:P ratios of the litter and soil in the arbor forest and arbor and shrub compound forest were higher than those in the other two vegetation types; however, the C:N ratio of the litter and soil in the arbor forest was lower than that in the other three vegetation types. The N element had a strong coupling relationship between litter and soil, while the P element had a weak relationship. The activity of the four soil enzymes in the four vegetation types were ranked as follows: arbor forest > arbor and shrub compound forest > shrubland > grassland. In general, the arbor forest communities were more conducive to nutrient cycling and accumulation. This information could help to guide the restoration and management of vegetation in karst areas.
The rainfall intensity, slope, and underground pore density are major factors affecting the soil erosion of maize‐covered karst slopes. We studied the erosion process of maize‐covered karst slopes and the influencing mechanism of soil erosion on these slopes under different rainfall intensities, slopes, and underground pore density via simulated rainfall tests. The results are as follows: (1) No runoff was observed on the slope surface under light rainfall (30 mm hr−1); instead, subsurface flow was predominant. However, runoff from the surface and subsurface flows under moderate rainfall (60 mm hr−1) increased with the rainfall intensity. The average surface runoff under extreme rainfall (90 mm hr−1) was 2.3‐times that under moderate rainfall. In addition, the average subsurface runoff under extreme rainfall was 1.6‐ and 3.5‐times that under moderate rainfall and light rainfall, respectively. The total amounts of surface and subsurface soil losses increased as the rainfall intensity increased. The surface and subsurface soil losses from light to moderate and extreme rainfall events increased by 258% and 151%, respectively. The soil loss from the surface was 5.3‐times greater than that from the subsurface, indicating that the erosion of maize‐covered karst slopes mainly occurs on the surface and that the subsurface loss is relatively small. (2) As the slope angle increased, the runoff, sediment yield, and proportion of erosion on the surface increased, and vice versa. (3) The subsurface runoff and sediment yield increased as the underground pore density increased, while the opposite occurred on the surface. Multiple regression analysis showed that the rainfall intensity is the most critical factor affecting the soil erosion of maize ‐covered slopes in karst areas, followed by the slope, while the influence of the underground pore density is small.
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