Chapter One - Current sensor technologies for in situ and on-line measurement of soil nitrogen for variable rate fertilization: A review

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Abstract

Recommendations for nitrogen (N) fertilization are based on the analysis of the soil mineral N (SMN) content of one composite sample that is assumed to represent the average concentration, but that almost by definition fails to properly represent the inherent spatial variability of the field. Improvement in N management can thus be achieved by high-resolution measurements of the soil N status. However, soil sampling, transportation, sample preparation and laboratory analysis are laborious, costly and time-consuming. Additionally, errors associated with all these steps (particularly sampling and sample preparation) are notorious sources of inaccuracy. In situ and on-line measurement modes could overcome these drawbacks. This paper presents a state-of-the-art review of proximal soil sensing technologies for in situ and on-line measurements of total nitrogen (TN) and mineral N in soils. Visible and near-infrared spectroscopy (vis-NIRS) and mid-infrared spectroscopy (MIRS) have been successfully used in in situ and on-line measurement platforms for determining TN. The review also focuses on data fusion (DF) approaches not only to improve the prediction performance of TN by vis-NIRS and MIRS but also to delineate management zones for site-specific N management. Electrochemical sensors such as ion-selective electrodes (ISE) or ion-selective field-effect transistors (ISFET) were found to be the most suitable to measure SMN. However, it was found that both sensor technologies are difficult to use for on-line measurements because they require the preparation of a soil solution and sufficient time delay is needed to reach a stable plateau. These limitations restrict the implementation of electrochemical sensing tools for site-specific N management, although they are the best methods to measure SMN, a needed parameter as a direct measure to determine N recommendations. Research on proximal soil sensing (PSS) for N measurement in soils should focus on the development of robust and accurate in situ and on-line sensors for SMN, whereas improvement in the performance of vis-NIRS and MIRS for TN prediction by using advanced modelling and DF techniques is recommended. This development is essential to promote the implementation of variable rate N fertilization (VRNF). The review suggested the fusion of on-line collected soil data with crop data, present and historical yield, topography and weather conditions as the optimal solution for map-based and sensor-based VRNF scenarios. Furthermore, a new approach for VRNF, designated as the map-sensor-based approach, is proposed. In this approach, management zones (MZ) maps for the soil is developed in advance at the beginning of the cropping season, and this information is combined with current crop growth status [e.g., normalized differential vegetation index (NDVI)], measured with a proximal crop sensor to calculate the recommendation and implement VRNF in real-time.

Introduction

Nitrogen (N) is the most crucial nutrient for plant growth and crop yield in agriculture, and its availability to a crop is mainly determined by the soil mineral N (SMN) concentration at any given time during the growing season. However, SMN is traditionally viewed as the most difficult nutrient to manage because of the very large spatial and temporal variability. Because of this, N is uniformly applied across the entire field, based on an N recommendation that takes into account a number of factors, of which SMN is probably the most important one. Traditionally, SMN is measured on one composite sample taken from 0 to 30 cm (or sometimes 0–60 or 0–90 cm) depth, and the N recommendation may or may not take into account other factors related to specific soil and management factors. With uniform N fertilization, parts of the field with high fertility or N content will receive over-application of fertilizers leading to inefficient N use and environmental pollution risk including N leaching, whereas other parts will receive sub-optimal fertilizer rates, leading to reduced crop performance and yield (Koch et al., 2004). One way to modify traditional practices and contribute to the environmental remediation is to implement precision agriculture (PA) for within-field variability management (Delin and Stenberg, 2014). PA includes the use of technology for site-specific application of farm input resources that helps to manage production inputs (e.g., N fertilizers), site-specifically, in a manner and levels specific to the site, with optimal goals of maximizing yield and nutrient use efficiency (Bongiovanni and Lowenberg-Deboer, 2004). However, for the implementation of precision management of N fertilization, data on SMN is required at high sampling resolution to allow assessment of the spatial and temporal variability in available N to the crop. Traditional methods of laboratory analyses to determine SMN are not suitable, as they are costly, time-consuming, slow, require experience operator and dispose of chemicals to the environment. Therefore, a new paradigm in data collection using new sensing technologies is required to overcome the shortcomings of traditional laboratory methods. This can be achieved by means of remote sensing, proximal sensing or the fusion of data from both sensing technologies to acquire information not only on soil N but other affecting parameters and crop attributes.

Data collected by proximal sensors are valuable to farmers and land managers to build site-specific databases that show the association between soil nutrient concentration, plant reaction and crop development during the growing season with resulting yield (Viscarra Rossel and Bouma, 2016). Over the years, soil sensors have become smaller, more rugged, faster, more accurate, more energy-efficient, wireless and more intelligent (Viscarra Rossel and Bouma, 2016). Proximal soil sensors (PSS) enable the measurement of key soil properties at high sampling resolution, to allow the quantification of within-field spatial and temporal variability necessary for PA applications. They concern the measurement of soil attributes while the sensor is in direct contact with the soil or within 2–3 m distance from the soil surface (Viscarra Rossel et al., 2011). Proximal soil sensing can be divided into laboratory, in situ (non-mobile), or on-line (mobile) measurement modes. While the laboratory mode requires soil samples to be transferred from its natural position to the laboratory for pre-treatment and laboratory chemical analysis, in situ and on-line modes refer to the measurement of soil in their natural position. Although the laboratory measurement with PSS has advantages over the chemical analyses, they are still demanding extra sample transportation and preparation (e.g., drying, grinding and sieving). Advancements in the sensing technology have led to the development of in situ and on-line sensors. While the former group of sensors allows performing field measurements without the need to take soil samples to a laboratory, it is still time-consuming and does not allow to produce high sampling resolution data in a short period of time. The use of on-line sensors enables a greater amount of data to be obtained (> 500 samples per ha) and allows to generate more detailed maps for better and science-based within-field soil and crop management. Perhaps one of the main advantages of the on-line sensors in addition to the high-resolution maps, is that they can also be used for real-time control of inputs (Adamchuk and Jasa, 2002), such as the case of the sensor-based variable rate (VR) applications.

Kuang et al. (2012) categorized PSS into five main categories: (1) reflectance, (2) conductivity, resistivity and permittivity, (3) passive radiometric, (4) soil strength and (5) electrochemical. Each of these categories targets some specific soil parameters. Among these different PSS technologies, the reflectance and electrochemical-based sensors were proven to accurately quantify total nitrogen (TN) and SMN in soils, respectively (Kuang et al., 2012). For in situ and on-line applications, the visible and near-infrared (vis-NIR) spectroscopy (vis-NIRS) technique seems the best candidate to measure TN, because it is robust against vibration, less sensitive to moisture compared to mid-infrared (MIR) spectroscopy (MIRS), which has not been implemented in on-line measurement mode so far. However, they can only measure TN. On-line measurement of SMN with electrochemical sensing has got limited research attention because of difficulties associated with preparing a soil solution, while travelling with a speed of 3–5 km/h, in addition to the time required for electrodes exchange reaction. Although few studies have reported the development of on-line sensing platforms (Adamchuk et al., 2005; Adsett et al., 1999; Sibley et al., 2008, Sibley et al., 2009), problems associated with duration of extraction and analysis, noise and achievable level of accuracy are the most affecting factors hindering the development of robust electrochemical sensing system for on-line measurement of SMN.

The development of multivariate and machine learning calibration models to assess key soil fertility parameters including TN using sensor data (e.g., spectra, images, output voltage) have greatly improved measuring the complexity of the N-cycle in agricultural processes. Moreover, data fusion (DF) analysis has increased the accuracy of prediction and opened new horizons for nutrient management. DF involves complex processes related to data collection, storage and analysis. Different sources of information are merged using mathematical, statistical and geo-statistical techniques to create relationships between parameters that yield robust prediction models. In addition, DF can be used to combine soil and crop data to obtain a better view of the N dynamics in soil and plants (De Benedetto et al., 2013; Pantazi et al., 2015). It has been used for creating fertility and recommendation maps for variable-rate nitrogen fertilization (VRNF), e.g., by means of management zones (MZ) maps (Nawar et al., 2017). This solution is optimal when no data on SMN is available; hence, a field should be divided into zones having different fertility classes, necessary for VRNF.

This paper provides a critical review of the current state-of-the-art of technologies for the measurement of TN and SMN in soils. The focus will be on field applications, including the in situ and on-line sensing modes. The review will present spectra modeling approaches for the prediction of key soil fertility parameters associated with N fertilization and the potential of DF approaches for the delineation of MZs in deriving VRNF recommendations based-on decision support systems. Finally, the review introduces for the first time a new approach for VRNF by combining in advance developed soil fertility maps obtained from PSS with real-time collected crop growth data using proximal crop sensors to calculate recommendations and implement VRNF in real-time.

Section snippets

Traditional methods

Traditional laboratory methods for TN measurement based-on wet chemistry include wet-oxidation or dry-combustion techniques. Both Kjeldahl-digestion and Dumas-combustion are the most common methods, respectively. The Kjeldahl digestion method relays on the conversion of organic and inorganic N into NH4+ followed by the measurement of its concentration. The dry-combustion method involves initial oxidation followed by a reduction of NOx, where N2 gases are measured (McGill and Figueiredo, 1993).

Spectral data analysis

In order to obtain quantitative or qualitative information about the soil TN status from the vis-NIR and MIR spectral data, spectra pre-processing and modelling are required. Researchers observed from vis-NIR spectral analysis that higher reflectance in the entire spectral range (350–2500 nm) is associated with lower TN content and vice versa (Fystro, 2002; Gillon et al., 1999; Udelhoven et al., 2003). Calibration techniques of spectra can be divided into linear and nonlinear approaches. The

Integration and future prospects

One of the most important challenges for VRNF is the on-line measurement of SMN, necessary to create high-resolution recommendation maps. As discussed above that the electrochemical sensors are proven technologies for SMN measurement under in situ (non-mobile) and laboratory conditions (Table 2). However, less success was reported for the on-line measurement mode. From the literature review above, it can be concluded that ISEs can be used for successful measurement of SMN. However, the majority

Conclusions

This review shows that optical reflectance and electrochemical sensors have been widely used for soil nitrogen (N) assessment. These sensor technologies can measure soil total nitrogen (TN) and soil mineral nitrogen (SMN), respectively, with high accuracy under laboratory conditions, despite less accuracy is to be expected under both the in situ and on-line measurements modes. When electrochemical sensors are adopted for on-line measurement of SMN, a challenging task of preparing a soil

Acknowledgment

Authors acknowledge the financial support received from the Research Foundation—Flanders (FWO) for Odysseus I SiTeMan Project (Nr. G0F9216N).

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