Do we really need large spectral libraries for the assessment of soil organic carbon at local scale?

Do we really need large spectral libraries for the assessment of soil organic carbon at local scale?

1 University Miguel Hernandez of Elche (Spain); 2 SLU (Sweden); 3 University Rey Juan Carlos (Spain); 4 CSIRO – Land & Water (Australia); 5 Cranfield University (UK); 6 University Polytechnic Cartagena (Spain);

7 University of Málaga (Spain).

César Guerrero1, Johanna Wetterlind2, Bo Stenberg2, Fernando T. Maestre3, Raphael A. Viscarra Rossel4, Abdul M. Mouazen5, Boyan Kuang5, Raúl Zornoza6, J.D. Ruiz-Sinoga7

Introduction

* Many times, we need to analyze a large number of samples collected at local scale (i.e., collected in a target site). * Options and approaches for local-scale assessment with NIR spectroscopy:

i) LOCAL models (spectrum-specific model) • Models constructed with similar spectra • Expensive: a large library is needed for select enough similar spectra

ii) Geographically local model (site-specific model) • Models constructed with similar samples (similar matrix) • Expensive approach: these models are only valid for the target site

iii) Large-scale models (regional, national, continental…) • A large model is not a guarantee of accurate predictions (large variability) • Prone to include non-linear patterns: biased predictions • We can apply “spiking”

Introduction

SOC NIR spectra

Y X

SOC NIR spectra

Y X

Kennard-Stone algorithm

Spiked Calibration

Spectral Library

Target Site

Prediction Set

Calibration R2, RMSEP, bias,

SEP, RPD

Spectral Library

Target Site

Prediction Set

ŷ

R2, RMSEP, bias,

SEP, RPD ŷ

Spiking Subset

PLS-R PLS-R

Target site (unknown)


SOC ? SOC ?

Spiking

- Useful approach when large-scale models (regional or national) are used at local scale (in a target site). - This approach ensures that model contains similar samples (to those to be predicted). - Implies analytical efforts: the spiking subset must be analyzed with the reference method. - The spiking subset should be small: small contribution on the model (especially if the calibration is big).

PLS-R PLS-R

Unspiked Spiking

X X

Spiking subset

Kennard-Stone algorithm

Spiked Calibration with extra-weighting

Spectral Library Target

Site

Prediction Set

Copies

Spiking Subset

Spiking subset extra-weighted

Spiking Subset

Spiking Subset

Spiking Subset

Spiking Subset

PLS-R


Spiking

SOC NIR spectra

Y X

Copy Paste Paste Paste Paste

copies

Library samples

Spiking subset

cross-validation

0

2

4

6

8

10

0 2 4 6 8 10

Spiking

cross-validation

0

2

4

6

8

10

0 2 4 6 8 10


Spectral library Target

Site

Prediction Set

Copies

Kennard -Stone algorithm

0

2

4

6

8

10

0 2 4 6 8 10


SOC NIR spectra

Y X

Copy Paste Paste Paste Paste

copies

Library samples

Spiking subset

cross-validation

the calibration is forced to fit better the spiking subset

Spiking

cross-validation

0

2

4

6

8

10

0 2 4 6 8 10

Spiking + extra-weighting



Site

Prediction Set

Copies

Kennard -Stone algorithm

SOC NIR spectra

Y X

SOC NIR spectra

Y X



SOC ?

SOC ?

PLS-R

PLS-R X

X

Spiking subset (n=8)

SOC NIR spectra

Y X Target site

(unknown)

SOC ?

PLS-R X

Spiking subset (n=8)

Copies = Extra weight

Unspiked calibration

Spiked calibration

Spiked calibration with Extra-weight

Unspiked Calibration

Spectral library

Target Site

Prediction Set

Spiked Calibration

Spectral library

Target Site

Prediction Set



Site

Prediction Set

Copies

Calibration types

Unspiked Spiking Spiking & Extra-weighting

Unspiked Calibration (UC)

Spectral library

Target Site

Prediction Set

Spiked Calibration (SC)


Site

Prediction Set

Spiked Calibration with extra-weighting (SC+EW)


Site

Prediction Set

Copies

Material & methods

Unspiked

National set Spain n=1096


Spectral library

Target Site

Prediction Set

Spiking Spiking & Extra-weighting



Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Unspiked


Sweden n=125


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Unspiked


Valencia n=121


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Unspiked


Forest, Alicante n=130


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Unspiked


UK n=104


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Unspiked


Forest, Murcia n=156


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Unspiked


Gypsipherous n=95


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Unspiked


Almería (mica schists) n=60


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Unspiked


Cuenca n=55


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Unspiked


Palencia n=55


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Unspiked


Total n=901


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

R2, RMSEP, bias,

SEP, RPIQ, RPD


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Sweden n=125


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Valencia n=121


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods



Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

UK n=104


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods



Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Gypsipherous n=95


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods



Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Cuenca n=55


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Palencia n=55


Total n=901

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

R2, RMSEP, bias,

SEP, RPIQ, RPD


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Sweden n=125


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Valencia n=121


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods



Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

UK n=104


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods



Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Gypsipherous n=95


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods



Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Cuenca n=55


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Palencia n=55


Total n=901

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

R2, RMSEP, bias,

SEP, RPIQ, RPD


Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Predictions: national set (n=1096)

0.85 0.87

0.96

Unspiked Spiked Spiked EW

2.59

2.22

0.71


1.34 1.56

4.89


1.65

1.23

0.21


R2 RMSEP (%SOC) |Bias| (%SOC) RPIQ

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20

Unspiked Spiked Spiked & extra-weight

n=901


0.85 0.87

0.96


2.59

2.22

0.71


1.34 1.56

4.89


R2 RMSEP (%SOC) RPIQ

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20

Spiked Spiked & extra-weight

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20

Unspiked

n=901 n=901

1.65

1.23

0.21


|Bias| (%SOC)


0.85 0.87

0.96


2.59

2.22

0.71


1.34 1.56

4.89



Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20


n=901 n=901 n=901

1.65

1.23

0.21


|Bias| (%SOC)

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

33 % National set Spain n=362

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

1.56

0.95

0.20


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20

Predictions: 33% national set (n=362)


0.85

0.90

0.96


2.57

1.85

0.71


1.35

1.87

4.90



n=901

|Bias| (%SOC)

1.56

0.95

0.20


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20



0.85

0.90

0.96


2.57

1.85

0.71


1.35

1.87

4.90


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20


n=901 n=901

|Bias| (%SOC)

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20



0.85

0.90

0.96


2.57

1.85

0.71


1.35

1.87

4.90


1.56

0.95

0.20


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20


n=901 n=901 n=901

|Bias| (%SOC)

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Provincial set (Alicante) n=147

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

2.88

0.11 0.04


Predictions: provincial set (n=147)


0.18

0.96 0.97


4.97

0.66 0.55


0.70

5.28

6.30


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20


n=901

|Bias| (%SOC)

2.88

0.11 0.04




0.18

0.96 0.97


4.97

0.66 0.55


0.70

5.28

6.30


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20


n=901 n=901

|Bias| (%SOC)



0.18

0.96 0.97


4.97

0.66 0.55


0.70

5.28

6.30


2.88

0.11 0.04


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20


n=901 n=901 n=901

|Bias| (%SOC)

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Geograph. local set (Alicante) n=88

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

1.80

0.06 0.01


Predictions: Geographical local set (n=88)


0.17

0.96 0.96


4.45

0.65 0.60


0.78

5.34 5.75


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20


n=901

|Bias| (%SOC)

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20



0.17

0.96 0.96


4.45

0.65 0.60


0.78

5.34 5.75


1.80

0.06 0.01


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20


n=901 n=901

|Bias| (%SOC)



0.17

0.96 0.96


4.45

0.65 0.60


0.78

5.34 5.75


1.80

0.06 0.01


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20


n=901 n=901 n=901

|Bias| (%SOC)

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

50% Provincial set n=73

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

2.59

0.02 0.03


Predictions: 50% provincial set (n=73)


0.36

0.96 0.97


3.82

0.60 0.55


0.91

5.81 6.25


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20


n=901

|Bias| (%SOC)

2.59

0.02 0.03




0.36

0.96 0.97


3.82

0.60 0.55


0.91

5.81 6.25


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20


n=901 n=901

|Bias| (%SOC)



0.36

0.96 0.97


3.82

0.60 0.55


0.91

5.81 6.25


2.59

0.02 0.03


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20


n=901 n=901 n=901

|Bias| (%SOC)

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

Geograph. local set #2 (Guadalajara)

n=40

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

6.59

0.04 0.01


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20

40

80

120

Predictions: Geographical local set #2 (n=40)


0.00

0.95 0.96


19.92

0.70 0.66


0.17

4.94 5.28



n=901

|Bias| (%SOC)

6.59

0.04 0.01




0.00

0.95 0.96


19.92

0.70 0.66


0.17

4.94 5.28


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20

40

80

120

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20


n=901 n=901

|Bias| (%SOC)



0.00

0.95 0.96


19.92

0.70 0.66


0.17

4.94 5.28


6.59

0.04 0.01


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20

40

80

120

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20


n=901 n=901 n=901

|Bias| (%SOC)

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

25% Provincial set n=36

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

4.02

0.09 0.01




0.03

0.96 0.97


8.02

0.61 0.55


0.43

5.70

6.34


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

10

30

45


n=901

|Bias| (%SOC)

4.02

0.09 0.01




0.03

0.96 0.97


8.02

0.61 0.55


0.43

5.70

6.34


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

10

30

45

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20


n=901 n=901

|Bias| (%SOC)



0.03

0.96 0.97


8.02

0.61 0.55


0.43

5.70

6.34


4.02

0.09 0.01


Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

10

30

45

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)0

5

10

15

20

Measured SOC (%)

0 5 10 15 20

Pre

dic

ted S

OC

(%

)

0

5

10

15

20


n=901 n=901 n=901

|Bias| (%SOC)

Unspiked


Spectral library

Target Site

Prediction Set




Site

Prediction Set



Site

Prediction Set

Copies

Material & methods

0.96 0.95 0.96 0.96 0.96 0.90 0.87

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

0.97 0.96 0.97 0.96 0.97 0.96 0.96

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

0.03 0.00

0.36

0.17 0.18

0.85 0.85

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

Determination coefficient (R2)


+ Spectral library size - + Spectral library size - + Spectral library size -


Spectral library

Target Site

Prediction Set



Site

Prediction Set



Site

Prediction Set

Copies

SEP (%SOC)


6.94

18.81

2.82 4.07 4.06

2.04 2.00

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

0.60 0.70 0.60 0.65 0.65 1.60 1.85

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

0.55 0.66 0.55 0.60 0.55 0.68 0.68

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96


Spectral library

Target Site

Prediction Set



Site

Prediction Set



Site

Prediction Set

Copies


|Bias| (%SOC)


4.02

6.59

2.59

1.80

2.88

1.56 1.65

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

0.09 0.04 0.02 0.06 0.11

0.95 1.23

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

0.01 0.01 0.03 0.01 0.04 0.20 0.21

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96


Spectral library

Target Site

Prediction Set



Site

Prediction Set



Site

Prediction Set

Copies


RMSEP (%SOC)


8.02

19.92

3.82 4.45 4.97

2.57 2.59

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

0.61 0.70 0.60 0.65 0.66 1.85 2.22

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

0.55 0.66 0.55 0.60 0.55 0.71 0.71

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96


Spectral library

Target Site

Prediction Set



Site

Prediction Set



Site

Prediction Set

Copies


RPD


0.39 0.16

0.83 0.71 0.63

1.23 1.22

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

5.20

4.50

5.30 4.87 4.81

1.70 1.42

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

5.79

4.81

5.70 5.24

5.75

4.47 4.46

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96


Spectral library

Target Site

Prediction Set



Site

Prediction Set



Site

Prediction Set

Copies


RPIQ


0.43 0.17

0.91 0.78 0.70

1.35 1.34

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

5.70

4.94

5.81 5.34 5.28

1.87 1.56

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

6.34

5.28

6.25 5.75

6.30

4.90 4.89

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96


Spectral library

Target Site

Prediction Set



Site

Prediction Set



Site

Prediction Set

Copies


RPIQ


0.43 0.17

0.91 0.78 0.70

1.35 1.34

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

5.70

4.94

5.81 5.34 5.28

1.87 1.56

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96

6.34

5.28

6.25 5.75

6.30

4.90 4.89

Size

36

Size

40

Size

73

Size

88

Size

14

7

Size

36

2

Size

10

96


Spectral library

Target Site

Prediction Set



Site

Prediction Set



Site

Prediction Set

Copies


Conclusions and recommendations

• When a large-sized calibration is spiked, the spiking subset should be extra-weighted. Otherwise, the impact of the spiking subset will be low (especially if the spiking subset is small). • For local scale assessment, we don’t need the development of large spectral libraries: once spiked, the small-sized performed even better than the large-sized calibrations. • Do not focus your analytical efforts exclusively in the development of large-sized libraries and large-sized models. Split your analytical efforts in:

• the development of small spectral libraries (a few regional samples) • the analysis of the spiking subsets

• This approach (spiking + extra-weight) gives accurate results and small analytical efforts are needed (new users attracted, more soils would be studied, more benefits for soil management)

Thank you very much for your attention!!

Criticisms welcomed!!

Acknowledgements This work is part of the research project (Ref. CGL2011-27001) “Estimación de carbono orgánico en suelos mediante espectroscopía de infrarrojo cercano (NIR): estrategias para el desarrollo de modelos con aplicabilidad a escala nacional” financed by the Spanish “Ministerio de Economía y Competitividad” (Plan Nacional I+D+i)

R2 |bias|

RMSEP SEP

RPD RPIQ

Education

Do we really need large spectral libraries for the assessment of soil organic carbon at local scale?