Cosmic-ray neutron sensors provide scale-appropriate soil water content and vegetation observations for eddy covariance stations in agricultural ecosystems

Abstract

Continuous information on soil water content (SWC) and plant development is crucial for environmental monitoring, agricultural management, and beyond. Cosmic-ray neutron sensors (CRNS), widely used to estimate SWC, also have the potential to monitor field-scale variations in vegetation properties. In this study, a CRNS measured both epithermal ($E_N$) and thermal ($T_N$) neutron intensities over a 10-year period at an ICOS Class 1 ecosystem station in Selhausen (Germany). Compared to nearby point-scale sensors, the CRNS provided more representative SWC estimates within the monitoring area of the adjacent eddy covariance (EC) station. A general co-development was observed between $T_N$ and gross primary productivity (GPP), but differences during senescence and desiccation suggest that factors beyond plant water content can influence $T_N$. An extensive dataset of plant height (PH), leaf area index (LAI), and dry above-ground biomass (AGB) was used to evaluate the ability of $T_N$ to monitor plant development. $T_N$ was found to be more closely related to vegetation dynamics than to changes in SWC. CRNS estimations of PH, LAI, and AGB yielded relatively good agreement with reference data (RMSE of 0.13 m, 1.01 m²/m², and 0.27 kg/m², respectively). The RMSE obtained with a leave-one-out cross validation generally confirmed these findings. Although CRNS estimates generally had lower accuracy than traditional methods, they have the key advantages of being continuous, non-invasive, and non-laborious. Combined with simultaneous estimation of SWC at a relevant spatial scale, CRNS becomes a particularly interesting tool among long-term monitoring platforms with further potential in modelling, remote sensing, and decision-making in agriculture.