Crops are usually planted on sloping land in mountainous areas due to limited suitable land area. This results in serious soil erosion and loss of nitrogen (N) and phosphorus (P) to land degradation and water eutrophication. It is important to adopt appropriate cultivation practices to change this situation. However, few long-term in situ measurements are available to assess the magnitude of effects of combined cultivation patterns on soil erosion and nutrient loss from sloping farmland with red soil, as well to quantify N and P losses through runoff and sediment transport. A field trial with the cash crop (CC) Nicotiana tabacum was carried out under natural rainfall conditions on sloping farmland with red soil in Yunnan, China during 2014–2017. Four cultivation patterns were applied. They included NVF (No fertilizer application + Vertical ridge + Film covered), OVF (Optimizing fertilizer application + Vertical ridge + Film covered), OHF (Optimizing fertilizer application + Horizontal ridge + Film covered), and OHFR (Optimizing fertilizer application + Horizontal ridge + Film removed). The first two treatments belonged to the vertical ridge (VR) group, and the remaining treatments belonged to the horizontal ridge (HR) group. Results indicated the HR group performed significantly better than the VR group, especially the OHFR treatment, in terms of the HR group producing average runoff (177.12–182.27 mm), sediment loss (2673.33–3309.17 kg·ha−1), and nutrient loss of total nitrogen (TN) (7.58–7.93 kg·ha−1), total phosphorus (TP) (1.00–1.09 kg·ha−1) through runoff, TN (3.53–4.72 kg·ha−1), TP (2.59–2.76 kg·ha−1) through sediment. TN was lost mainly through runoff transport, while TP was lost mainly through sediment transport. On average, the HR group decreased runoff, sediment, total N and P loss by 39% to 73% relative to the OVF treatment, whereas NVF treatment increased 3% to 30% of those (p < 0.05). Under four cultivation patterns, total dissolved nitrogen (TDN) was the dominant form, which accounted for 71–77% of TN. The average percentage of NO3−-N/TN was about 45–52%, much higher than NH4+-N/TN of around 8–10% in runoff. Total dissolved phosphorus (TDP) made up about 48–59% of TP in runoff. Redundancy analysis (RDA) showed that sediment, runoff, and soil pH were the three key factors controlling nutrient loss. In conclusion, OHFR is recommended because it consistently outperformed other patterns in terms of reducing runoff, sediment, and nutrient losses.
Effects of Spatial Scales on Runoff/Sediment Transport in Mountain Catchments (4): Avenues for Prediction Improvement
