Characteristics of runoff and nitrogen, phosphorus, and carbon loss in sloping cultivated lands in the typical hilly mountainous region of the Upper Yangtze River Basin
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摘要:
持续的坡面降雨可能引发地表径流和壤中流的形成。其中，地表径流可通过溶蚀、侵蚀、运输等方式将大量富集在表土（通常为0~20 mm厚度）的养分从土壤中转移到受纳环境。降雨强度和坡度的变化可以影响坡面径流强度，从而改变径流中总氮（TN）、总磷（TP）、溶解性有机碳（DOC）等养分浓度与通量过程。通过模拟降雨试验，设置3种典型降雨强度（40、60、90 mm/h）和坡度（6°、12°、18°），探讨不同条件下丘陵山区紫色土坡面径流及坡面TN、TP、DOC流失特征。结果表明：1）产流临界坡度不是一个定值，降雨强度在60 mm/h及以下时，临界坡度介于6°~18°；降雨强度达到90 mm/h时，未出现明显的临界坡度。2）相同降雨强度下，径流中TN和DOC浓度与坡度呈正相关（18°>12°>6°）；降雨强度为90 mm/h时，径流中TP浓度与坡度的关系为6°>12°>18°，其余降雨强度下，则为12°>18°>6°。除12°坡面外，DOC流失量随降雨强度增加呈上升趋势；TP最大流失量出现在90 mm/h降雨强度下的6°坡面，为0.91 mg/m2，表现为来源限制，其余坡度坡面的TP流失表现为携带限制。3）坡面产流量和产沙量主要受降雨强度的影响，产流量和降雨强度与径流中TP流失量和DOC流失量显著相关，坡度与碳、氮、磷流失量的相关性均不显著，径流中TN流失量与TP和DOC流失量显著相关。研究显示，高强度降雨和小坡度组合下，径流量、TP流失量、DOC流失量显著高于其余降雨强度和坡度的组合，而在高强度降雨和大坡度组合下，TN流失量达到峰值。因此，需要特别关注强降雨、小坡度下紫色土中TP、DOC流失及强降雨、大坡度下TN流失问题。
Abstract:Continuous rainfall on the slope may yield surface runoff and subsurface flow. Surface runoff can transfer a large amount of nutrients in the topsoil (usually 0-20 mm thick) from the soil to the receiving environment through dissolution, erosion, and transportation. The change of rainfall intensity and slope can affect the intensity of slope runoff, thus changing the concentration and flux process of total nitrogen (TN), total phosphorus (TP), dissolved organic carbon (DOC) and other nutrients in runoff. A rainfall experiment considering three typical rainfall intensities (40, 60, 90 mm/h) and slopes (6°, 12°, 18°) was conducted, and the characteristics of runoff and the loss of TN, TP, and DOC on purple soil slopes in hilly areas under different conditions were explored. The results showed that: (1) The critical slope gradient for runoff was not a fixed value. When the rainfall intensity was 60 mm/h or less, the critical slope gradient ranged from 6° to 18°. However, no distinct critical slope was observed when the rainfall intensity reached 90 mm/h. (2) Under the same rainfall intensity, the concentration of TN and DOC in runoff was positively correlated with the slope gradient (18°>12°>6°). However, at a rainfall intensity of 90 mm/h, the relationship between TP concentration in runoff and slope was 6°>12°>18°, whereas under other rainfall intensities, it was 12°>18° >6°. With the exception of the 12° slope, the loss of DOC increased as rainfall intensity increased. The maximum loss of TP occurred on the slope of 6° under a rainfall intensity of 90 mm/h, reaching 0.91 mg/m², belonging to the source limitation type. In contrast, TP losses on other slopes belonged to the transport limitation type. (3) The runoff and sediment yield on the slope were primarily influenced by rainfall intensity. Significant correlations existed between runoff and rainfall intensity, as well as between runoff and the losses of TP and DOC. The correlation between the slope gradient and the losses of carbon, nitrogen, and phosphorus was not significant. However, a significant correlation was observed between the loss of TN in runoff and the losses of TP and DOC. Comprehensive research showed that under the combination of high-intensity rainfall and a small slope gradient, the runoff, TP and DOC loss were significantly higher than the other combinations of rainfall intensity and slope. In contrast, in the combination of heavy rainfall intensity and a steep slope gradient, TN loss reached its peak. Therefore, special attention needs to be paid to the loss of TP and DOC in purple soil on small slopes under heavy rainfall and the loss of TN on steep slopes under heavy rainfall.
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Key words:
- purple soil /
- rainfall intensity /
- slope gradient /
- simulated rainfall /
- nutrient loss
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图 2 不同降雨和坡度条件下坡面产流产沙变化特征
Figure 2. Varying characteristics of runoff and sediment production on slope surfaces under diverse rainfall intensities and slope gradients
图 3 不同降雨强度和坡度下紫色土坡面碳、氮、磷流失浓度随产流历时的变化过程
Figure 3. Changes of carbon, nitrogen and phosphorus loss concentration of purple soil slope with runoff duration under different rainfall intensities and slopes
图 4 不同降雨强度和坡度下TN、TP、DOC流失量和产流量变化趋势
Figure 4. Trends of TN, TP, DOC loss amount and runoff yield under different rainfall intensities and slopes
表 1 供试土壤基本理化性质
Table 1. Basic physical and chemical properties of experimental soil
土壤类型 颗粒组成占比/% 容重/（g/cm3） TOC浓度/（g/kg） TN浓度/（g/kg） TP浓度/（g/kg） 黏粒
（<0.002 mm）粉粒
（0.002~0.05 mm）砂粒
（0.05~2 mm）紫色土 1.31 28.50 70.19 1.25 3.75 0.69 0.21 
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表 2 碳、氮、磷流失量与影响因素相关性分析
Table 2. Correlation analysis of carbon, nitrogen and phosphorus loss and influencing factors
项目 TN流失量 TP流失量 DOC流失量 坡度 降雨强度 径流量 产沙量 TN流失量 1 TP流失量 0.717* 1 DOC流失量 0.700* 0.630 1 坡度 0.369 0.053 −0.053 1 降雨强度 0.632 0.738* 0.783* 0 1 径流量 0.650 0.967** 0.633 0 0.843** 1 产沙量 0.767* 0.800** 0.667 0.422 0.843** 0.833** 1 注：*表示在0.05水平上显著相关；**表示在0.01水平上显著相关。
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