Nutrient loss is sensitive to land cover changes and slope gradients of agricultural hillsides: Evidence from four contrasting pond systems in a hilly catchment

Intensive agricultural cultivation in hilly catchments is subject to significant environmental risks, because the local terrain and exceptionally fragile ecosystem usually promote high levels of nutrient loss. In order to assess such negative effects, we compared long-term (2012–2018) water quality dynamics of four ponds that are fed by runoff from agricultural hillsides with contrasting slopes and land cover near Tianmu Lake in Eastern China. Averaged total nitrogen (TN) levels in ponds increased from 7.0 mg L−1 to 10.0 mg L−1 when the cultivated slopes above them doubled from 8° to 16°. In contrast, total phosphorus (TP) levels varied modestly from 0.05 to 0.07 mg L−1 among the four ponds. Values were lower when drained hillslopes had good vegetation coverage and/or buffer strips. Redundancy analysis (RDA) indicated that relatively high precipitation is the determining factor for N loss from gentle hillsides, but as slope gradients increase, the frequency of rainfall events becomes the major influential factor. TP loss was associated with strong rainfall events and can be greatly reduced if hillsides are well covered with vegetation. When vegetation cover is removed, nutrient loss, especially N, is heightened. Our results underscore the significance of agricultural cultivation of steep hillsides in exacerbating N and P pollution and highlight the value of forests and grasslands in reducing nutrient loss from these sensitive areas. An integrated strategy that includes controlling land exploitation, source control, delivery interception, and pond management should be considered to minimize nutrient loss in hilly areas prone to rapid agricultural expansion.