The effects of slope length and coverage ratio on nitrogen and phosphorus loss in red soil Regions of southern China

Soil nutrient loss not only reduces soil productivity, but also causes non-point source pollution and accelerates the eutrophication of surface water. In order to understand the effects of slope lengths (2m, 4m), vegetation coverage ratios (15%, 30%, 45%, 60%, 90%) on the mechanisms of soil nutrient loss, the research studied the simulated rainfall experiment with the slope gradient of 20°, the rainfall intensity of 2.0 mm/min and the time of producing runoff for about 30 min. The experimental results showed that the runoff coefficient is mainly affected by vegetation coverage ratio.


Introduction
Nitrogen (N) and phosphorus (P) are not only the required nutrients for the sustainable development of agroecosystem, but also identified as the main sources of non-point pollution because of their losses caused by surface runoff (Domburg P.,2000). Furthermore, in recent years, in pursuit of high-yields in agriculture, excessive amount of fertilizer was applied but had lower utilization ratio (Jiang et al., 2014), and resulted in serious water-body pollution. According to the investigation, the agricultural non-point source pollution accounts for 60-80% of the total amount of non-point pollution in China (Miao J., et al.,2010). As for the pollutants in the Lake Chaohu of China, 69.5% of total nitrogen (TN) and 51.7 % of total phosphorus (TP) come from non-point pollution in 1995 alone (Luo et al.,2007). In the Lake Taihu of China, 40-90 % of total nitrogen (TN) and 38-90 % of total phosphorus (TP) were from the discharge of the agricultural land (Guo et al.,2004).

Different slope lengths and different vegetation ratios on the characteristics of runoff and sediment
Based on values of slope length, coverage ratio,runoff coefficient and sediment yield,the significance level of these factors was calculated using SPSS The significance levels of each factor of sediment yield and runoff coefficient were given in Table 1. Table  1 showed that slope length had significant effects on sediment yield, but their extremely significant level was less than 0.01. The significant level of the effects of the runoff coefficient on sediment yield was less than 0.05. The runoff coefficient was mainly affected by slope lengths, vegetation coverage ratios, meanwhile, while runoff coefficient mainly affected sediment yield but their extremely significant level was less than 0.01.
Based on the observation of experimental results, the infiltration rate of soil was affected not only by soil properties, but also the slope lengths and vegetation coverage ratios.
There are many literatures that pointed out that increasing vegetation coverage ratio could protect soil surface layer from the direct impact of raindrops and to some extent reduce water loss and soil erosion (Smets et al.,2008).Vegetation cover could mainly hold up a certain amount of the rainwater to soil surface layer and reduce the kinetic energy of raindrops. Then soil surface roughness caused by vegetation cover reduced surface flow of the runoff, increased the infiltration time, and thus put off the runoff-producing time, meanwhile, vegetation roots improved the soil infiltration capacity.

The effect of slope lengths and vegetation ratios on the nitrogen and phosphorus concentrations in runoff 4 Conclusion
The present study took slope length, vegetation cover ratio and different fertilization treatments as variable factors, investigated the mechanism of these factors affected runoff volume, sediments yield and N, P loss: 1)There are significant correlation between vegetation coverage ratio, runoff coefficient and sediment yield (P<0.01) and runoff volume and sediment yield decrease with the increasing of vegetation coverage ratio. Sediment yield mainly depends on slope length, and the effects of different fertilizer treatments on runoff coefficient and sediment yield are not obvious.
2)According to the analysis of relationships between variable factors about TN and TP: slope length, vegetation coverage ratio, the results showed that the correlation coefficient of vegetation coverage ratio and TN concentration is slope lengths have no significant impact on the concentration change of the losses of both TN and TP (P>0.05).