Mapping the Forage Nitrogen-Phosphorus Ratio Based on Sentinel-2 MSI Data and a Random Forest Algorithm in an Alpine Grassland Ecosystem of the Tibetan Plateau

Jinlong Gao,Wenlong Li,Jie Liu,Mengjing Hou,Caixia Wu,Tiangang Liang,Qisheng Feng,Jing Ge

doi:10.3390/rs12182929

Abstract

Nondestructive and accurate estimating of the forage nitrogen–phosphorus (N:P) ratio is conducive to the real-time diagnosis of nutrient limitation and the formulation of a management scheme during the growth and development of forage. New-generation high-resolution remote sensors equipped with strategic red-edge wavebands offer opportunities and challenges for estimating and mapping forage N:P ratio in support of the sustainable utilization of alpine grassland resources. This study aims to detect the forage N:P ratio as an ecological indicator of grassland nutrient content by employing Sentinel-2 multispectral instrument (MSI) data and a random forest (RF) algorithm. The results showed that the estimation accuracy (R2) of the forage N:P ratio model established by combining the optimized spectral bands and vegetation indices (VIs) is 0.49 and 0.59 in the vigorous growth period (July) and the senescing period (November) of forage, respectively. Moreover, Sentinel-2 MSI B9 and B12 bands contributed greatly to the estimation of the forage N:P ratio, and the VIs (RECI2) constructed by B5 and B8A bands performed well in the estimation of the forage N:P ratio. Overall, it is promising to map the spatial distribution of the forage N:P ratio in alpine grassland using Sentinel-2 MSI data at regional scales. This study will be potentially beneficial in implementing precise positioning of vegetation nutrient deficiency and scientific fertilization management of grassland.

Highlights

The biochemical parameters of forage in natural alpine grasslands, such as nitrogen (N), phosphorus (P) and chlorophyll, are important aspects of forage nutrition conditions and feed value and play an important role in the life cycle of plants [1,2]
The forage N:P ratio is significantly correlated with the N and P contents at different growth stages (Table 4), which further supports using the vegetation indices (VIs) related to N, P and chlorophyll to estimate the forage N:P ratio
Based on high-resolution multispectral satellite data and an random forest (RF) machine learning algorithm, this study evaluates the feasibility of using Sentinel-2 multispectral instrument (MSI) spectral bands and VIs to retrieve the forage N:P ratio of alpine grassland on a regional scale

Summary

Introduction

The biochemical parameters of forage in natural alpine grasslands, such as nitrogen (N), phosphorus (P) and chlorophyll, are important aspects of forage nutrition conditions and feed value and play an important role in the life cycle of plants [1,2]. The division scheme of threshold developed by Tessier and Raynal can be applied to natural alpine grassland ecosystems [10]. In this scheme, an N:P ratio < 14 is likely plant N limitation, an N:P ratio > 16 tends to indicate P deficiency, and an N:P ratio ranging from 14 to 16 indicates that the growth of vegetation is limited by both N and P [10]. Investigating the spatial distribution characteristics of the N:P ratio at a regional scale can provide useful information for the rational utilization of alpine grassland resources and the diagnosis of forage growth status

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Conclusion