Abstract
Nitrogen (N) and phosphorus (P) in topsoils are critical for plant nutrition. Relatively little is known about the spatial patterns of N and P in the organic layer of mountainous landscapes. Therefore, the spatial distributions of N and P in both the organic layer and the A horizon were analyzed using a light detection and ranging (LiDAR) digital elevation model and vegetation metrics. The objective of the study was to analyze the effect of vegetation and topography on the spatial patterns of N and P in a small watershed covered by forest in South Korea. Soil samples were collected using the conditioned latin hypercube method. LiDAR vegetation metrics, the normalized difference vegetation index (NDVI), and terrain parameters were derived as predictors. Spatial explicit predictions of N/P ratios were obtained using a random forest with uncertainty analysis. We tested different strategies of model validation (repeated 2-fold to 20-fold and leave-one-out cross validation). Repeated 10-fold cross validation was selected for model validation due to the comparatively high accuracy and low variance of prediction. Surface curvature was the best predictor of P contents in the organic layer and in the A horizon, while LiDAR vegetation metrics and NDVI were important predictors of N in the organic layer. N/P ratios increased with surface curvature and were higher on the convex upper slope than on the concave lower slope. This was due to P enrichment of the soil on the lower slope and a more even spatial distribution of N. Our digital soil maps showed that the topsoils on the upper slopes contained relatively little P. These findings are critical for understanding N and P dynamics in mountainous ecosystems.
Highlights
Nitrogen (N) and phosphorus (P) are the most important nutrients for primary productivity in terrestrial ecosystems [1,2]
Soil nutrient content varies during long-term soil development, such that N increases while P declines during the course of pedogenesis
The study area has a size of 9.84 km2 and is located in the downstream area of the Soyang lake watershed, Gangwon province, South Korea (Fig 1)
Summary
Nitrogen (N) and phosphorus (P) are the most important nutrients for primary productivity in terrestrial ecosystems [1,2]. Soil nutrient content varies during long-term soil development, such that N increases while P declines during the course of pedogenesis. This is because N enters the ecosystem via N-fixing microorganisms, whereas P is derived from the weathering of minerals. The annual average wet input of N ranged from 12.9 to 24.9 kg ha-1year-1 from 2005 to 2010 [6], and is markedly higher than that during pre-industrial times. This might have effects on the productivity, biodiversity, and community composition of plants [9]
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