Abstract
Soil properties variability is a factor that greatly influences cereals crops production and interacts with a proper assessment of crop nutritional status, which is fundamental to support site-specific management able to guarantee a sustainable crop production. Several management strategies of precision agriculture are now available to adjust the nitrogen (N) input to the actual crop needs. Many of the methods have been developed for proximal sensors, but increasing attention is being given to satellite-based N management systems, many of which rely on the assessment of the N status of crops. In this study, the reliability of the crop nutritional status assessment through the estimation of the nitrogen nutrition index (NNI) from Sentinel-2 (S2) satellite images was examined, focusing of the impact of soil properties variability for crop nitrogen deficiency monitoring. Vegetation indices (VIs) and biophysical variables (BVs), such as the green area index (GAI_S2), leaf chlorophyll content (Cab_S2), and canopy chlorophyll content (CCC_S2), derived from S2 imagery, were used to investigate plant N status and NNI retrieval, in the perspective of its use for guiding site-specific N fertilization. Field experiments were conducted on maize and on durum wheat, manipulating 4 groups of plots, according to soil characteristics identified by a soil map and quantified by soil samples analysis, with different N treatments. Field data collection highlighted different responses of the crops to N rate and soil type in terms of NNI, biomass (W), and nitrogen concentration (Na%). For both crops, plots in one soil class (FOR1) evidenced considerably lower values of BVs and stress conditions with respect to others soil classes even for high N rates. Soil samples analyses showed for FOR1 soil class statistically significant differences for pH, compared to the other soil classes, indicating that this property could be a limiting factor for nutrient absorption, hence crop growth, regardless of the amount of N distributed to the crop. The correlation analysis between measured crop related BVs and satellite-based products (VIs and S2_BVs) shows that it is possible to: (i) directly derive NNI from CCC_S2 (R2 = 0.76) and either normalized difference red edge index (NDRE) for maize (R2 = 0.79) or transformed chlorophyll absorption ratio index (TCARI) for durum wheat (R2 = 0.61); (ii) indirectly estimate NNI as the ratio of plant nitrogen uptake (PNUa) and critical plant nitrogen uptake (PNUc) derived using CCC_S2 (R2 = 0.77) and GAI_S2 (R2 = 0.68), respectively. Results of this study confirm that NNI is a good indicator to monitor plants N status, but also highlights the importance of linking this information to soil properties to support N site-specific fertilization in the precision agriculture framework. These findings contribute to rational agro-practices devoted to avoid N fertilization excesses and consequent environmental losses, bringing out the real limiting factors for optimal crop growth.
Highlights
Crop nutritional stress is usually assessed considering N deficiency on the base of plant analysis methods with no direct measurements or in consideration of soil properties information
We found a good relationship between Na% and MCARI (R2 = 0.71; root mean square error (RMSE) = 2.48; mean absolute error (MAE) = 2.29), as previously shown for the same crop by [32], but in general, Vegetation indices (VIs) achieved worse results as compared to S2_BV for retrieving biophysical variables (BVs) as W, canopy chlorophyll content (CCC), and plant nitrogen uptake (PNUa)
In the experiment presented here, a limiting factor for optimal crop growth was identified in the soil properties that reduce the capacity of the crop to uptake N, in conditions of high N availability in the soil and supplied by fertilization
Summary
Crop nutritional stress is usually assessed considering N deficiency on the base of plant analysis methods with no direct measurements or in consideration of soil properties information. Crop N demand changes during the season depending on growing conditions related to the presence of limiting factors This requirement is met by soil N supply capacity and N fertilizations. N supply from soil and fertilizations and N accumulation in crops are very dynamic processes that should be considered through an integrated approach [11,12] and a spatial relationship analysis [13]. The uncertainty in both plant N demand and soil N supply due to seasonality and growth potential often leads to non-optimal nitrogen management strategies [14]
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