Chapter 8. Determination of nitrate in soils using mid-infrared spectroscopy

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Chapter 8. Determination of nitrate in soils using mid-infrared

spectroscopy

R. Linker

Civil and Environmental Engineering,

Technion-Israel Institute of Technology, Haifa 32000, Israel

Email: [email protected]

ABSTRACT

Excessive fertilization causes significant environmental damage, both in water

sources and in soils. The "precision agriculture" concept, whose goal is to adjust

fertilization based on crop needs and soil properties, has the potential of reducing the

amount of fertilizer used without diminishing yield. However, the lack of adequate

sensors, and in particular the lack of real-time sensors for soil nitrate, is preventing

large-scale implementation of this approach.

This paper presents several approaches based on mid-infrared spectroscopy that

have been recently developed for determination of nitrate in agricultural soils. The

first method is based on Attenuated Total Reflectance (ATR) spectroscopy. The main

drawbacks of this direct method is that the soil sample must be close to water

saturation and that soil constituents, and in particular calcium carbonate, may

interfere with nitrate determination. Several mathematical procedures that have been

developed to minimize the effect of the interferences and increase accuracy are

reviewed and compared. The two other methods are based on the combined use of

mid-infrared spectroscopy and ion-exchange membranes, which are considered to

mimic nutrient uptake by plants and have been used to estimate nutrient availability

in numerous studies. It is shown how mid-infrared spectroscopy can replace

standard chemical analysis to estimate the amount of ions sorbed onto the

membrane, using either transmittance (if membrane absorbance is sufficiently low) or

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Introduction

Modern chemical fertilization, and in particular nitrogen fertilization, has led to

significant increase of crop yield. However, fertilizers tend to be applied in excess,

causing considerable pollution to both soil and ground water. In particular, nitrate has

been associated with various health and environmental problems (Castelnuovo,

1995; Addiscott et al., 1991). Fertilizer application could be significantly reduced by

implementing the “precision fertilization” concept, which aims at adjusting the

fertilization rate to the actual crop demand. One of the main obstacles to large-scale

implementation of this concept is the lack of reliable and fast sensors for direct

monitoring of soil nutrients (Roblin and Barrow, 2000; Robert, 2002). Promising

results toward the development of such sensors for nitrate have been reported with

various modern methods such as ion selective electrodes (Adsett et al., 1999), ion

sensitive field effect transistor (Birrell and Hummel, 2000), and near- and mid-infrared

spectroscopy (Ehsani et al., 1999, 2001; Shaviv et al., 2003). In addition to those

direct approaches, an indirect method based on ion-exchangers (membranes or

resins) has been developed. Ion-exchangers are believed to mimic closely the

physical process of nutrient uptake by plants, and can thus serve as reliable tools for

evaluating nutrient availability in agricultural fields or environmental systems (Qian

and Schoenau, 2002a,b). In particular, Qian and Schoenau (2005) have shown that

ion exchange membranes in the form of so-called “plant root simulator probes” could

be used to asses soil nitrogen-supply power (NSP). Nutrient availability is determined

by letting the ion-exchanger interact with the soil for a known period of time, after

which it is removed and eluted with strong extractants. Standard chemical methods

are used for quantitative determination of the nutrients in the extracts. Such

procedures are time consuming and ion-dependent, and require the use of chemicals

both for de-sorbing the ions and analyzing the extracts.

The present paper reviews recent developments related to the use of mid-infrared

spectroscopy for fast and accurate determination of nitrate in agricultural soils. Three

approaches are briefly described and compared: Attenuated Total Reflectance (ATR)

for direct determination of nitrate in saturated pastes, combined use of ion-exchange

membranes and transmittance spectroscopy, and combined use of ion-exchange

membranes and photoacoustic spectroscopy.

Attenuated total reflectance of soil pastes

Attenuated total reflectance (ATR) is commonly used for analysis of liquids and

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Aochi, 1996). The method relies on a crystal with a high refractive index that serves

as a waveguide in which the IR radiation propagates. During the measurement, the

crystal is in contact with the sample and the beam is directed in such a way that it

hits the crystal/sample interface several times before being directed to the detector

(Fig. 1). Whenever it hits the interface, the beam penetrates a few microns into the

sample, so that the signal that reaches the detector contains information about the

absorbance of the sample. Since the penetration depth is limited to a few microns,

very good contact between the sample and the crystal is required, which for nitrate

determination in soil can be achieved by working with samples close to water

saturation (Shaviv et al., 2003, Linker et al., 2004, 2005, 2006). However, water

absorbance in the mid-infrared range is very strong and, as shown below, the

presence of water interferes with nitrate determination.

IR source Detector Focusing

mirror Focusing

mirror Sample ATR crystal

Fig. 1. Schematic description of the attenuated total reflectance (ATR) spectroscopy

approach

Figure 2 shows typical spectra of de-ionized water and de-ionized water and sandy

soil with 1000 mg[nitrate N]/kg[water]. Two absorbance bands characteristic of water

(e.g. Libnau et al., 1994) are clearly visible in all three spectra. The water band

centered around 1640 cm-1 is much larger than the nitrate band visible around 1350

cm-1. It can also been seen the spectra are slightly biased or tilted relative to each

other (see the circled regions in Fig. 2), and the magnitude of these changes is

similar to the size of the nitrate band itself despite the unrealistically high nitrate

concentration used in the Figure. For realistic nitrate concentrations, these changes

are an order of magnitude larger than the nitrate band itself. As a result, the spectra

need to be properly corrected prior to quantitative analysis, which can be achieved by

careful subtraction of the water contribution to the spectrum and baseline correction

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Fig. 2. Typical ATR spectra of de-ionized water (bold), de-ionized water with 1000

mg[nitrate N]/kg[water] (thin) and sandy soil paste with 1000 mg[nitrate N]/kg[water]

(dashed).

Figure 3 shows typical water-subtracted spectra of saturated soil pastes. The nitrate

band (around 1350 cm-1) is clearly visible in the spectra of the sandy soils (Soils

#1-3), but for calcareous soils (Soils #4-6) a strong interference band due to calcium

carbonate appears around 1450 cm-1 and the nitrate band is hardly visible. As a

result, although a strong correlation exists between the nitrate absorbance band and

the nitrate concentration, this correlation is soil dependent. Straightforward

quantitative analysis, such as using the area under the nitrate band as a predictor

regardless of the soil type, leads to very poor results (not shown). Quantitative

analysis of the spectra must therefore be conducted with chemometrics tools such as

partial least squares (PLS) (Linker et al., 2004), neural networks (NN) (Linker et al.,

2005) or wavelets (Jahn et al., 2006). Determination accuracy can further be

improved by using a two-step approach according to which the soil type is

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be used (Fig. 4). Soil identification can be achieved by comparing the ATR spectrum

of the sample to a reference library, using either simple correlation calculations or

more sophisticated classifiers such as neural networks (Linker et al., 2005, 2006).

800 1000 1200 1400 1600 -0.1 0 0.1 0.2 0.3 0.4 0.5 Soil #6 800 1000 1200 1400 1600 -0.1 0 0.1 0.2 0.3 0.4 0.5 Soil #2 800 1000 1200 1400 1600 -0.1 0 0.1 0.2 0.3 0.4 0.5 Soil #4 A b s o rb a n c e u n it s 800 1000 1200 1400 1600 -0.1 0 0.1 0.2 0.3 0.4 0.5 Soil #1 A b s o rb a n c e u n it s 800 1000 1200 1400 1600 -0.1 0 0.1 0.2 0.3 0.4 0.5 Soil #5

Wavenumber, cm-1

800 1000 1200 1400 1600 -0.1 0 0.1 0.2 0.3 0.4 0.5 Soil #3 NO 3 CO 3

Fig. 3. Water subtracted and baseline corrected spectra of six soils commonly used

in Israeli agriculture. In each frame, the various lines correspond to increasing nitrate

concentrations. The top frames correspond to light sandy soils and the bottom

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Fig. 4. Schematic representation of the two-step approach proposed by Linker et al.

(2005, 2006)

Typical results are presented in Fig. 5, which shows the actual vs. estimated nitrate

concentration in five soil types that differ in terms of calcium carbonate, clay and

organic matter content. The determination errors range from 5mg[N]/kg[dry soil] for

light sandy soils (soil type #1) to 8mg[N]/kg[dry soil] for clay soils with high calcium

carbonate concentration (soil type #3). Similar results were reported in Linker et al.

(2004, 2005, 2006), and consistently indicated that high carbonate content is the

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0 50 100 150

0 50 100 150

Actual nitrate concentration, mg[N]/kg[dry soil]

E s ti m a te d n it ra te c o n c e n tr a ti o n , m g [N ]/ k g [d ry s o il ]

Soil type #1 Soil type #2 Soil type #3 Soil type #4 Soil type #5

Fig. 5. Actual vs. estimated nitrate concentration for five soil types. Nitrate

determination based on ATR spectra of saturated soil pastes, soil identification, and

soil-dependent partial last squares models.

Transmittance analysis of ion exchange membranes

As mentioned in the Introduction, ion-exchange membranes are commonly used in

studies dealing with nutrient availability, and standard chemical methods are used to

determine the amount of nutrients sorbed onto the membranes. Recently, Linker and

Shaviv (2006) showed that transmittance spectroscopy could replace such chemical

analyses. Figure 6 shows the spectra of unloaded and loaded membranes (BDH

55164-2S). The membranes were loaded by placing them into one-to-one soil pastes

for 30 minutes, after which they were rinsed with water and wiped dry with sterile

wipes. The spectrum of the loaded membrane shows two regions that can be

associated with nitrate loading (indicated by arrows). Linker and Shaviv (2006)

investigated the use of these two regions for quantitative analysis and concluded that

the small but well-defined band centered at 1040 cm-1 yields more accurate

predictions. Typical results are shown in Fig. 7, and the high correlation between

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error is approximately 1.3 µeq, which for the pastes used corresponds to 3.6

mg[N]/kg[dry soil]. Such determination errors are significantly lower than those

achieved with the ATR approach. In addition, contrary to the ATR results, the

determination accuracy is not affected by calcium carbonate content or other soil

constituents, and the method is very robust.

800 900

1000 1100

1200 1300

1400 0.5

1 1.5 2 2.5

Wave number, cm-1

A

b

s

o

rb

a

n

c

e

u

n

it

s

Unloaded Loaded with nitrate

Fig. 6. Transmittance spectra of unloaded and loaded BDH membrane. The arrows

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-10 0 10 20 30 40 50 60 70 80

-10 0 10 20 30 40 50 60 70 80

Actual nitrate concentration, mg[N]/kg[dry soil]

P re d ic te d n it ra te c o n c e n tr a ti o n , m g [N ]/ k g [d ry s o il ]

H1; RMSE: 3.20 mg[N]/kg[soil] H2; RMSE: 2.59 mg[N]/kg[soil] H3; RMSE: 1.73 mg[N]/kg[soil] H4; RMSE: 3.36 mg[N]/kg[soil] H5; RMSE: 5.03 mg[N]/kg[soil] G1; RMSE: 5.39 mg[N]/kg[soil] G2; RMSE: 3.89 mg[N]/kg[soil] L; RMSE: 2.29 mg[N]/kg[soil] R; RMSE: 5.62 mg[N]/kg[soil]

Fig. 7. Actual vs. estimated charge on BDH membrane for four soil types: light sandy

soils (H); clay soils with high calcium carbonate and medium (G) and high (L) clay

content; and soil with very high calcium carbonate content (R).

Photoacoustic analysis of ion exchange membranes

The main limitation of the transmittance-based method presented in the previous

section is the requirement that the membrane be sufficiently transparent in the mid-IR

range. Unfortunately, a wide variety of commercially-available ion exchange

membranes, and in particular those used for water purification, are too thick for

transmittance measurements. In such a case, photoacoustic spectroscopy (PAS) can

be used instead. Photoacoustic spectroscopy is based on the absorption of

electromagnetic radiation by the sample and non-radiative relaxation that leads to

local warming of the sample. Pressure fluctuations are then generated by thermal

expansion, which can be detected by a very sensitive microphone (Fig. 8). In this

fashion, highly absorbing solid samples can be analyzed without any special

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Fig. 8. Schematic description of photoacoustic spectroscopy

Typical photoacoustic spectra of ion-exchange membranes (AR103, Ionics Inc.)

unloaded and loaded with nitrate are shown in Fig. 9 and the nitrate band is clearly

visible in the 1300-1500 cm-1 interval. Quantitative results obtained by applying PLS

to the 1200-1550 cm-1 interval are shown in Fig. 10. The average determination error

is about 2.2 µeq (6.3 mg[N]/kg[dry soil]). Although this is about 50% higher than the

error achieved with the transmittance technique, such errors are similar to the ones

obtained with ATR and show that for ion exchange membranes for which

transmittance measurements are not possible (such as the AR103 used in this

study), reasonably accurate quantitative analysis can be achieved by photoacoustic

spectroscopy.

Modulated IR radiation

Sample

KBr

Heliu

Compute

IR

128 500 1000 1500 2000 2500 3000 3500 40000 0.1 0.2 0.3 0.4 0.5 0.6

Wavenumber, cm-1

P A S u n it s Unloaded membrane Loaded membrane

Fig. 9. Photoacoustic spectra of unloaded and loaded AR103 membrane.

-5 0 5 10 15 20 25 30 35 40

-5 0 5 10 15 20 25 30 35 40

Actual charge in soil paste, µeq

E s ti m a te d m e m b ra n e l o a d in g

, µ

e

q

Fig. 10. Actual vs. estimated charge on AR103 membrane. The various symbols

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Conclusion

Mid-infrared spectroscopy appears to be a very promising tool for rapid determination

of nitrate concentration in agricultural soils, either directly or in combination with ion

exchange membranes. Direct determination by ATR spectroscopy requires minimal

sample preparation, but advanced data processing is necessary to minimize

interferences due to water and soil constituents, in particular calcium carbonate that

has an absorbance band that overlaps with the nitrate one. Typical determination

errors range from 5 to 8 mg[N]/kg[dry soil], with the lowest errors corresponding to

light sandy soils.

When used together with ion-exchange membranes, mid-infrared spectroscopy can

replace the standard chemical methods that are traditionally used to estimate the

ions sorbed onto the membrane. If the membrane is sufficiently transparent in the

mid-IR range, transmittance spectroscopy can be used and very accurate

determination can be achieved (average determination error less than 4 mg[N]/kg[dry

soil]). For membranes that are not transparent in the mid-IR range, photoacoustic

spectroscopy can be used. Although the determination errors are larger (about 6

mg[N]/kg[dry soil]), such errors would still be acceptable for most agricultural

applications.

Acknowledgments

This study has been supported by the US-Israel Binational Research and

Development Fund (BARD Project US-3293-20c) and by the Grand Water Research

Institute (GWRI) at the Technion-ITT.

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