Relations between Microtopography and Soil N and P Observed by an Unmanned Aerial Vehicle and Satellite Remote Sensing (GF-2)

Topography is important for soil nutrient loss and critical source area (CSA) identification. Previous studies have primarily used mass soil sampling to explore the relations between topography and soil nutrients (especially N and P) at the coarse scales. The relations at the microtopographic scale, however, remain unclear. This study integrated unmanned aerial vehicle ( UAV) and satellite remote sensing (GF-2) data to create two new indices - NDVI(N) and NDVI(P). Results revealed more pixels with high NDVI(N) values distributed across low elevation difference grades in paddy land; however, this was reversed for dry land. There were more NDVI( P) pixels with large (small) values at high (low) elevation difference grades in the dry land (paddy land). In dry land, the average NDVI(N) was in the range of 0.25-0.33, and NDVI(P) was in the range of 0.47-0.61 for each elevation grade. In paddy land, the average NDVI( N) and NDVI(P) values for each elevation grade were in the range of 0.24-0.32 and 0.31-0.43, respectively. Microtopography can redistribute N and P spatially within the soil because it changes the direction of flow from irrigation and rainfall and of sediment flow from erosion. Furthermore, soil N and P accumulate simultaneously in the soil of agricultural land.