Conversion of Forest Hillslopes into Tea Fields Increases Soil Nutrient Losses through Surface Runoff

Abstract

The conversion of forest hillslopes into tea fields changes the slope structure, soil properties, and vegetation, which may increase the great variations in the nitrogen (N) and phosphorus (P) losses through the surface runoff and induce agricultural non-point source pollution, particularly during the first years of conversion. Therefore, a natural forest system and a newly reclaimed tea system were compared to determine the land use conversion effects on the surface runoff and resulting N and P losses during 2013–2016. The results showed that the soil bulk density (BD) at all the depths and the saturated soil conductivities (Ks) in the surface soil were higher in the tea system than in the forest system, indicating that great land use conversion impacts soil’s physical and hydraulic properties. The annual surface runoff volumes and ratios were significantly higher in the tea system than in the forest system (p < 0.05), and this increased surface runoff was largely attributed to the slope structure and soil property changes. The total P (TP), dissolved P (DP), and particle P (PP) concentrations in the surface runoff volumes were similar in the two systems, while the total N (TN) concentrations were higher in the tea system than in the forest system due to the high N fertilizer application rates in the tea system. The percentages of dissolved N (DN) in the TN and particle N (PN) in the TN were, respectively, lower and higher in the tea system than in the forest system, which was associated with increased soil erodibility due to the land use conversion. The mean annual N and P losses through the surface runoff were higher in the tea system (11.07 ± 4.03 kg N ha−1 yr−1 and 1.73 ± 0.19 kg P ha−1 yr−1) than in the forest system (4.51 ± 0.66 kg N ha−1 yr−1 and 0.78 ± 0.18 kg P ha−1 yr−1). The N and P losses exponentially correlated with the surface runoff in both systems; however, the coefficients of determination (R2) were higher in the forest system than in the tea system, indicating that the exponential relationship between the surface runoff and the N and P losses should be more stable in the forest system than in the newly reclaimed tea system. Therefore, more effort should be dedicated to reducing the N and P losses though the surface runoff when forest hillslopes are converted into tea fields, particularly during the first three years of conversion.