1
AVAILABLE NITROGEN FOR CORN AND WINTER CEREAL IN SPANISH
2
SOILS BY EUF, CaCl2 AND INCUBATION METHODS
3
Miguel Quemada(1) and Jose Antonio Díez(2)
4
(1)
5
Madrid, Spain. miguel.quemada@upm.es. Tel.: +34 915491122, Fax: +34 915449983
6
(2)
7
ABSTRACT
8
Comparison of methods is necessary to develop a quick and reliable test that can be
9
used to determine soil available nitrogen (N) in an attempt to increase the efficiency of
10
N fertilizers and reduce losses. The objective of this research was to compare the
11
fractions extracted by the CaCl2 and the electro-ultrafiltration (EUF) methods and to
12
correlate them to the mineralization rate (k) obtained from a 112-d incubation of 61 soil
13
samples. Thirty-five soil samples were collected from corn fields and 26 from winter
14
cereal fields. Subsamples were either aerobically incubated to calculate k, or extracted
15
by the EUF and CaCl2 methods to identify three fractions: NO3--N, NH4+-N and Norg-
16
N. The Norg-N extracted by both methods was larger in soils from corn fields than in
17
soils from winter cereal fields. In samples from corn fields, the Norg-N fraction
18
obtained by the EUF method was correlated to the Norg-N measured by the CaCl2
19
method (r = 0.46). Soil N content was related to k in samples from corn fields (r = 0.40),
20
but not in samples from winter cereal fields. Also, k was correlated to inorganic N
21
content extracted by both chemical methods. The CaCl2 method was a reliable
22
alternative for laboratories to determine soil available N for corn, but not for winter
23
cereal.
24
Key words: Available N, N mineralization rate, aerobic incubation, EUF, CaCl2
Dpto Producción Vegetal. Fitotecnia, ETSI Agrónomos, Universidad Politécnica de
Centro de Ciencias Medioambientales, CSIC, Madrid, Spain.
1
1
INTRODUCTION
2
The environmental impact of N fertilizers on ground water and air quality
3
through nitrate leaching and gaseous emissions, has led to revisions of techniques for
4
estimating available N (Diez and Vallejo, 2004). Adjusting N fertilization application
5
not only to soil mineral N content, but to the total amount of available N during the
6
cropping season, might greatly increase the efficiency of N fertilizers and reduce losses
7
(Diez et al., 2000; Vazquez et al., 2005). Several biological and chemical methods have
8
been proposed to quantify N availability, and all of them are based on the estimation of
9
the fraction of total soil N that is mineralized during the growing season (Serna and
10
Pomares, 1992; Jarvis et al., 1996; Sánchez et al. 1998; Picone et al., 2002).
11
Comparison of these methods is necessary to develop a quick and reliable test that can
12
be used to determine soil available N in an attempt to adjust fertilizer application to crop
13
N requirements.
14
One of the most widely used approaches to determine mineralizable N has been
15
aerobic incubation of soil samples within standard conditions for a defined period, and
16
determination of the increases in NO3- and NH4+ concentrations (Jarvis et al., 1996). The
17
protocols for doing this determination are many and varied, but are usually some
18
variation of the method proposed by Stanford and Smith (1972). The objective is to
19
estimate the organic N pool that is available for mineralization, together with a N
20
mineralization rate within optimum conditions characteristic for a certain soil. An
21
incubation period of 16 wk is usually necessary to ensure that all potentially available N
22
has been mineralized (Serna and Pomares, 1992). Comparison between results are often
23
difficult, because the possible combinations of sample pretreatment and incubation
24
conditions are numerous, and all have an impact on N mineralization (Jarvis et al.,
25
1996). Long-term laboratory incubations are considered the most reliable method for
2
1
estimating mineralizable N in soil, but they are not suitable for routine soil testing
2
laboratories because of their time requirement.
3
A chemical approach to the problem of developing a laboratory index of soil N
4
availability is attractive, particularly because chemical methods of analysis are usually
5
more rapid and more precise than incubation methods. The use of electroultrafiltration
6
(EUF) allows determination of not only mineral N (NO3- and NH4+) but also a fraction
7
of organic N that contains readily mineralizable N (Nemeth, 1979). The method has
8
been used in some European countries to study the close relationship between organic N
9
extracted by EUF and soil mineralizable N. this method has, in many instances,
10
provided good correlation with mineralized N and N uptake in pot and field trials for
11
several crops (Wikicky et al., 1982; Appel and Mengel, 1990; Diez and Vallejo, 2004).
12
However, the EUF method is expensive and requires specialized personnel, which limits
13
its application in soil testing laboratories that must process a large number of samples
14
quickly and efficiently. Cheaper and easier alternatives would therefore be useful. The
15
CaCl2 0.01M extraction method (Houba et al., 1986) can be an alternative method to
16
EUF because its simplicity allows CaCl2 extraction to be routinely used in soil
17
laboratories, and because that extraction procedure has shown correlation with EUF
18
results (Diez and Vallejo, 2004). The CaCl2 0.01M extractions contain mineral N (NO3-
19
and NH4+) and a fraction of organic N, that can be compared with the fractions
20
recovered by the EUF method. The CaCl2 0.01M extraction method has been adopted
21
by the ISO (International Organization for Standardization). Chemical methods for
22
estimating mineralizable N are fast, but they need to be better understood and calibrated
23
against biological methods.
24
There is a lack of studies comparing long-term laboratory incubations and
25
chemical N extractions methods. Our objective was to compare the fractions extracted
3
1
by two chemical methods (EUF and CaCl2) and to correlate them to mineralizable N
2
measured in a 112-d incubation of 61 soil samples. This experiment was carried out
3
with soils obtained from different regions of Spain with several climatic conditions.
4
MATERIAL AND METHODS
5
Soil sampling
6
Sixty one soil samples were collected from 19 representative experimental fields
7
located in 7 Spanish regions (Aragón, Cataluña, Castilla La Mancha, País Vasco,
8
Navarra, and La Rioja y Madrid). Soil samples were collected from the control
9
treatment of fertilizer response trials, and each sample was a composite of 25
10
subsamples taken from the upper 30 cm of each trial replication. None of the control
11
treatments had received N fertilizer, either organic or inorganic, for the 2 yr before
12
samples were taken. Climatic conditions in general were fresh and humid in the north
13
and hot and dry in the south. These soils represent five orders: Alfisols, Mollisols,
14
Inceptisols, Entisols and Aridisols. The physico-chemical characteristics were
15
determined in each soil sample, and Table 1 shows the average of the samples taken
16
from the same experimental field.
17
The first 26 soil samples from Table 1 come from eight experimental fields that
18
were used to grow irrigated corn. In these areas maize is the most economically
19
important crop, yielding 14-16 Mg ha-1 when properly fertilized, and is usually sown at
20
the end of March. The rest of the soil samples come from ten experimental fields that
21
were used to grow winter cereals. When properly fertilized wheat and barley achieve
22
yields between 3-6 Mg ha-1, depending on climatic conditions, and both are usually
23
sown at the beginning of November. In all experimental fields, the stubble was
24
ploughed back into the soil at least three months before sowing.
4
1
Soil samples were collected just before sowing. Fresh soil samples were air-
2
dried, sieved (< 2 mm), and stored at room temperature until use.
3
Aerobic incubation
4
Before use, approximately 1 kg of soil was packed into Buchner funnels (14-cm
5
diameter) and leached with 2 L of N-free solution to remove NO3-. The soil was allowed
6
to drain using vacuum until its water content dropped to a tension close to -0.0033 MPa;
7
then the soil was mixed in a plastic bag, and two 5-g subsamples were extracted with 40
8
mL of 1 M KCl for 30 min. The NH4+ and NO3- concentrations of the soil extracts were
9
near zero.
10
Moist soil (60 g oven-dry equivalent) was packed to a depth of 5 cm in acrylic
11
plastic syringes (4 cm diameter, 10 cm long) to achieve 55% water-filled pore space and
12
a bulk density of 1.4 g cm-3. Water-filled pore space was calculated (volumetric water
13
content/porosity) x 100, where porosity = (1 – bulk density/2.65). A nylon screen cloth
14
(screen size 5.3 by 5.3 m) was located between the soil and the bottom of the syringe
15
to minimize soil loss during leaching. The syringes were placed in an incubator at 32
16
ºC, and incubated aerobically during a 16-week period. The air above the soil was kept
17
moist by a large filter paper suspended over the syringes, which had a central tongue
18
immersed in a beaker with distilled water. All samples were run in duplicate.
19
Syringes were removed from the chamber at 14, 42, 60, 91, and 112 d after
20
preparation, and were leached with 180 mL of 0.01 M CaCl2 solution, followed by 30
21
mL of N-free solution. The leachates were made up to 200 mL with CaCl2, and
22
subsamples were saved at -25ºC for later analyses. After the leaching procedure, the
23
cores were allowed to drain with vacuum until a weight within 0.1 g of that measured at
24
the beginning of the experiment was achieved. The leaching procedure took
25
approximately 6 h. The cumulative amount of N mineralized from each soil sample was
5
1
calculated by adding leached inorganic N, and correcting by the amount of inorganic N
2
present in the soil extract at the beginning of the experiment.
3
Electroultrafiltration (EUF) method
4
This method is based on applying an electrical field to soil suspended in water (1:10) to
5
separate nutrients in accordance with a standardized two fraction program: (I) 30 min,
6
200 V, 15 mA, 20ºC, and (II) 5 min, 400V, 150 mA, 80ºC (Nemeth,1979). A 5-g
7
sample of air-dried soil (< 1 mm) was placed in an EUF cell (Vogel S-724). Distilled
8
water (50 mL) was added to the cell to cover the electrode. During the desorption phase,
9
inorganic and organic ions were extracted by the strength of the electric field. With the
10
EUF method, three different N compounds are obtained: NO3--N, NH4+-N and Norg-N.
11
The last fraction of Norg-N, composed of organic compounds with a low molecular
12
weigh, can be used as an index of potentially mineralizable nitrogen in soil (Wiklicky,
13
1982; Sánchez et al., 1998).
14
CaCl2 method
15
Air-dried soil (10 g), pre-treated according to ISO 14255 recommendations, and
16
sieved (< 2mm), was shaken in 100 mL of a 0.01M CaCl2 solution at 20ºC for 2 h at
17
150 rev/min (Houba et al., 1986). The suspension was centrifuged and N parameters
18
determined in the supernatant. Three different N compounds were obtained: NO3--N,
19
NH4+-N and Norg-N. The N values obtained by 0.01M CaCl2 and EUF methods have
20
been compared by several authors (Houba et al., 1986; Díez and Vallejo, 2004).
21
Analytical procedures
22
Total N in EUF and 0.01M CaCl2 extracts was determined by UV radiation
23
digestion and subsequent oxidation with potassium persulfate in an alkaline medium
24
(Diez, 1988). Nitrate determination of the leachates and extracts was performed
25
colorimetrically with N1-naphtylethylenediamine, after reduction of NO3- to NO2-. H3-
6
1
N was measured by employing ion selective electrodes (Orion Reseach AG, USA). UF-
2
Norg was estimated as the difference between EUF-N and EUF-(NO3- plus NH4+). pH
3
was determined in saturated soil paste using a calomel glass electrode (ISO 10390,
4
1994). Total C and N of soil samples were measured by dry combustion with a C and N
5
analyzer (Carlo Elba Instruments, Milan, Italy), and carbonates by gasometry (the CO2
6
released from the soils being treated with HCl 1:1) (ISO 10693, 1995). Organic C was
7
obtained correcting total soil C by carbonates.
8
Statistical analysis
9
In the aerobic incubation samples, a non-linear regression procedure was used to fit a
10
linear model (Nmin = k t) for describing cumulative N mineralized with time. Where
11
Nmin was the cumulative amount of N mineralised at a specific time (t) and k was the
12
mineralization rate.
13
Simple correlation coefficients were calculated to indicate the relationship
14
between the N mineralization rate, soil chemical N availability indices, and total soil N
15
content. Data were analysed using the computer program SPSS (2002).
16
RESULTS
17
The physico-chemical characteristics (Table 1) reveal that all soils where
18
alkaline (pH varied between 7.9 and 8.6) and carbonate content ranged from 27 to 322 g
19
CO3= kg-1. Organic matter content ranged between 11.2 and 25.6 g kg-1, and total N
20
content between 0.70 and 1.8 g N kg-1. Soil texture also showed diversity, with topsoil
21
texture varying from silt loam to sandy clay loam
22
Aerobic incubations
23
The net amount of N mineralized in aerobic incubations increased approximately
24
linearly with time (Table 2). The direct linear (zero-order) relationship was appropriate
25
to fit the net amount of N mineralized with time; therefore, there was not justification
7
1
for trying to fit more complex functions. The regression equation relating cumulative N
2
mineralized during 16 weeks and the mineralization rate (k) was highly significant with
3
a correlation coefficient of 0.98. Therefore, to compare with the other methods of N
4
extraction, we can either use cumulative N mineralized during 16 weeks or k. When
5
considering all soils together, neither the net N mineralized nor the mineralization rate
6
from the aerobic incubations were related to total soil N content.
7
Soils cultivated with corn
8
The net amount of N mineralized during the 16 weeks of aerobic incubation
9
varied from 26 to 91 mg N kg-1 dry soil, representing from 1.9 to 6.5 % of the total soil
10
N (Table 2). Mineralization rates obtained after fitting the linear model ranged from
11
0.28 to 0.87 mg N kg-1 soil d-1.
12
13
14
The mineralization rate from the aerobic incubation presented significant
correlation (p<0.01) with total soil N content (Ns). The regression equation was:
k = - 0.004 + 0.361 Ns
r = 0.46
15
Soils cultivated with winter cereal
16
The net amount of N mineralized during the 16 weeks of aerobic incubation varied from
17
46 to 76 mg N kg-1 soil, representing from 2.6 to 6.2 % of the total soil N (Table 2).
18
Mineralization rates obtained after fitting the linear model ranged from 0.38 to 0.51 mg
19
N kg-1 soil d-1. The mineralization rate from the aerobic incubation did not present
20
significant correlation with total soil N content.
21
Nitrogen extracted by EUF and CaCl2 methods
22
The relative weight of N extracted by both procedures was different in soils
23
dedicated to corn than in soils dedicated to winter cereal (Fig. 1). The CaCl2 method
24
recorded more total N and Norg in the extracts of soils dedicated to corn than on soils
25
dedicated to winter cereal. This difference was not as clearly observed with the EUF
8
1
method, but was still present. In soils dedicated to winter cereal, ammonium extracted
2
by the EUF method was higher than that extracted by the CaCl2 method, while few
3
differences were observed in soils dedicated to corn. Nitrate extracted by both methods
4
was similar in crops dedicated to corn or winter cereals.
5
Soils cultivated with corn
6
In these soils, the CaCl2 method showed more total N in the extracts (42.6 mg N
7
kg-1 on average) compared to the EUF method (26.5 mg N kg-1 on average) (Fig. 1).
8
These results are in agreement with those reported by Diez and Vallejo (2004), but in
9
disagreement with other authors working with sandy soils (Appel and Steffens, 1988).
10
However, there were not noticeable differences in the amount of NO3- extracted by the
11
two procedures (7.9 and 9.0 mg N kg-1 on average for the CaCl2 and EUF methods,
12
respectively). The amount of NH4 extracted by both procedures was small (1.6 and 0.4
13
mg N kg-1 on average for the CaCl2 and EUF methods, respectively). The most marked
14
difference between the two methods was seen for extracted Norg (34.3 and 16.1 mg N
15
kg-1 on average for the CaCl2 and EUF methods, respectively). Similar results were
16
found by Diez and Vallejo (2004) and by Dou et al. (2000).
17
The values of N, NO3-, and Norg extracted by the EUF and CaCl2 methods
18
correlated significantly (p<0.01). The regression equation relating these N fractions as
19
obtained by both methods were:
20
EUF-N (I+II) = 33.110 + 0.358 CaCl2-N
r = 0.58
21
EUF-NO3-- N (I+II) = -1.208 + 1.010 CaCl2-NO3-- N
r = 0.97
22
EUF-Norg-N (I+II) = 31.346 + 0.185 CaCl2-Norg-N
r = 0.46
23
All N fractions, except NH4+-N extracted with CaCl2 were correlated with Ns,
24
while only the NO3--N fraction extracted with EUF was significantly related with Ns.
9
1
The significant regression equation (p<0.01) relating these N fractions as obtained by
2
both methods were:
3
CaCl2-N = 2.514 + 15.504 Ns
r = 0.52
4
CaCl2-NO3--N = - 0.067 + 7.612 Ns
r = 0.49
5
EUF-NO3--N (I+II) = 0.064 + 7.444 Ns
r = 0.50
6
Cl2Ca-Norg-N = 2.505 + 8.227 Ns
r = 0.48
7
Soils cultivated with winter cereal
8
In soils dedicated to winter cereal, the CaCl2 method recorded less total N in the
9
extracts (14.6 mg N kg-1 on average) compared to the EUF method (21.4 mg N kg-1 on
10
average) (Fig. 1). The CaCl2 method extracted less NH4+ than the EUF method (2.4 and
11
8.0 mg N kg-1 on average for the CaCl2 and EUF methods, respectively). The CaCl2
12
method extracted slightly less Norg than the EUF method (2.3 and 3.4 mg N kg-1 on
13
average for the CaCl2 and EUF methods, respectively). However, there were not
14
noticeable differences in the amount of NO3- extracted by the two procedures (9.8 and
15
9.9 mg N kg-1 on average for the CaCl2 and EUF methods, respectively). These results
16
show that the relative weight of N extracted by both procedures was different in soils
17
dedicated to corn and soils dedicated to winter cereal.
18
The only N fraction extracted by the EUF and the CaCl2 methods that correlated
19
significantly was NO3-. The values of total N, NH4+ and Norg extracted by the EUF and
20
the CaCl2 methods did not present significant correlations.
21
None of the N fractions extracted with CaCl2 correlated significantly with Ns.
22
The Norg extracted with EUF was the only N fraction significantly related with Ns
23
(EUF-Norg-N (I+II) = 1.635 – 9.209 Ns; r = 0.56) but the relationship between both
24
parameters were negative.
25
Relationship between N mineralization rate and chemical indices of N availability
10
1
When considering all soils together, the net N mineralization rate from the
2
aerobic incubations was not related to the different N fractions extracted by the
3
chemical methods.
4
Soils cultivated with corn
5
A significant relationship (p<0.01) was observed between the mineralization rate
6
(k) from the aerobic incubations and the N and NO3--N extracted by either the EUF or
7
the CaCl2 method. However, no relationship was observed between k from the aerobic
8
incubations and the NH4+-N or Norg-N extracted by the other procedures. The
9
significant regression equations relating these N fractions as obtained by both methods
10
were:
11
k = 19.083 + 20.214 EUF-N
r = 0.40
12
k = 0.97 + 0.030 EUF-NO3--N
r = 0.57
13
k = 0.192 + 0.013 CaCl2-N
r = 0.50
14
k = 0.159 + 0.27 CaCl2-NO3-- N
r = 0.53
15
Soils cultivated with winter cereal
16
A significant relationship (p<0.01) was observed between the mineralization rate
17
(k) from the aerobic incubations and the total N and NH4+ extracted by the EUF method.
18
However, none of the N fractions extracted with CaCl2 present a significant relationship
19
with k. The significant regression equation relating these N fractions were:
20
k = 0.295 + 0.070 EUF-N
r = 0.60
21
k = 0.345 + 0.130 EUF-NH4+-N
r = 0.46
22
DISCUSSION
23
In many incubation studies, mineralization patterns have been described by first-
24
order kinetics as recommended by Stanford and Smith (1972). However, zero-order
25
kinetics was observed in unamended (Tabatai and Al-Khafaji, 1980; Addiscott, 1983)
11
1
and meadow soils (Simard and N’Dayegamiye, 1993). The results of our incubation
2
experiment with unamended soils showed that, in all soils, the cumulative mineralized
3
N followed a linear increase with time throughout the incubation. In soils amended with
4
organic residues considering first-order kinetics or several N pools might help to
5
describe N mineralized in incubation experiments (Jarvis et al, 1996), but in unamended
6
soils zero-order kinetics might be appropriate to describe N mineralization. The
7
mineralization rates of our incubation experiment are in the range found in the
8
literature: 0.24 to 0.60 mg N kg-1 soil d-1 (Tabatai and Al-Khafaji, 1980), 0.05 to 0.47
9
mg N kg-1 soil d-1 (Addiscott, 1983) and 0.38 to 1.55 mg N kg-1 soil d-1 (Simard and
10
N’Dayegamiye, 1993). Incubation studies evaluate mineralization in soils that are
11
modified by handling, mixing, drying, or rewetting, and it is known that pre-treatments
12
have an impact on mineralization (Cabrera and Kissel, 1988). We agree with Jarvis et
13
al. (1996) when concluding that incubation studies allow comparison of soil types using
14
controlled conditions, but to be of further value for practical applications require
15
standardization and means to extrapolate the information to different cropping systems.
16
In our experiment, soils were pre-incubated to avoid the flush of mineral N caused by
17
soil manipulation, as suggested by Cabrera and Kissel (1988). The mineralization rates
18
we obtained were relatively steady from day 14 until the end of the experiment,
19
suggesting that the observed mineralization rates were characteristics of each soil.
20
In general, when considering all soils together, the N mineralization rate and the
21
chemical indices of N availability were not correlated with total soil N content.
22
However, distinguishing between soil samples taken from the experimental fields
23
dedicated to corn or winter cereal allowed us to identify significant relationships. The
24
mineralization rate from corn soils was related with total soil N content, but this
25
relationship was not observed in winter cereal soils. The difference between these soil
12
1
groups was particularly emphasized by the different relative weight of N fractions
2
extracted by both chemical procedures. The main difference between these soil groups
3
was sampling time, suggesting that N fractions were greatly influenced by climatic
4
conditions. Similar results on the effect of sampling time on the variation of N fractions
5
extracted by chemical methods were obtained by Nemeth and Fürstenfeld (1985) in
6
fallow soils. This sampling-time effect can be explained because conditions prevailing
7
at sampling may influence the content of labile forms of organic materials, and activity
8
of micro-organisms (Jarvis et al., 1996).
9
The best relationships between N fractions extracted by both chemical methods
10
were found in samples taken from soils dedicated to corn. The highest correlation
11
coefficient was obtained with the NO3- fraction that was slightly retained by soil
12
colloids. The significant correlation between the Norg fractions obtained by both
13
methods is relevant, because the CaCl2 extraction method is simpler to perform than the
14
EUF method, and can be a reliable alternative for determining soil available N in
15
laboratory. These results agree with other studies that showed the close relationship
16
between organic N extracted by EUF and CaCl2 (Appel and Steffens, 1988; Appel et al.
17
1990; Diez and Vallejo, 2004). The amount of NH4+ extracted by EUF and CaCl2
18
procedures was not significantly correlated in our experiments, but it was of little
19
importance due to its low soil content.
20
The N mineralization rate was related to mineral N content extracted by
21
chemical methods, but not to NH4+-N or Norg-N. In agreement with our results, Serna
22
and Pomares (1992) found significant correlations between N mineralised during a 16
23
wk incubation and mineral N extracted by several chemical methods (HCl, KMnO4,
24
etc). However, Groot and Houba (1995) found that mineralization rates from a 12 wk
25
incubation were correlated with soluble organic N extracted with CaCl2. The lack of
13
1
relationship between the mineralization rate and the Norg-N fraction observed in our
2
study can be explained by the fact that in aerobic incubations the N is released mainly
3
from mineralization of organic N; while in the chemical indexes we used relative mild
4
extraction methods, suitable to extract only the N in available forms. This explanation
5
agrees with Appel et al. (1996) who tested in a N15 study whether the organic N
6
extracted by EUF or CaCl2 represents a part of the microbial biomass. They concluded
7
that organic N extracted by the chemical methods was derived from the non-biomass
8
soil organic matter, and therefore both extraction methods may provide a suitable index
9
for mineralizable N only in cases where the decomposable organic substrates are
10
derived mainly from sources other than the living soil biota.
11
CONCLUSIONS
12
The results of the incubation experiment showed that in unamended soils zero-
13
order kinetics was appropriate to describe N mineralization. The mineralization rate was
14
related to total soil N content in soils dedicated to corn, but not in soils dedicated to
15
winter cereal. The N mineralization rate from the aerobic incubation was related to
16
mineral N content extracted by chemical methods, but not to total N or Norg-N.
17
In general, the amount of Norg-N extracted by both chemical methods were
18
larger in soils from experimental fields dedicated to corn than from those dedicated to
19
winter cereal; this difference was emphasized by the CaCl2 extraction. A significant
20
correlation between the Norg-N fractions obtained by the CaCl2 method and the EUF
21
method was observed in soils dedicated to corn, but not in soils cultivated with winter
22
cereal. These results showed that the CaCl2 extractant was a reliable alternative for soil
23
laboratories to determine soil available N for corn, but it was not for winter cereal.
24
Additional work is needed to further investigate the effect of sampling time on the
25
variation of N fractions extracted by chemical methods.
14
1
ACKNOWLEDGMENTS
2
The authors are grateful to the Spanish Commission of Science and Technology (Project
3
AGL2001 2214-C06) for financing this research.
4
REFERENCES
5
Addiscot, T.M. 1983. Kinetics and temperature relationships of mineralization and
6
nitrification in Rothamsted soils with differing histories. Journal of Soil Science 34:
7
343-353
8
Appel, T., and D. Steffens. 1988. Comparison of electro ultrafiltration (EUF) extraction
9
and 0.01 M CaCl2 solution in determination of plant available N in soils. Journal of
10
Plant Nutrition and Soil Science 151:127-130
11
Appel, T., and K. Mengel. 1990. Importance of organic nitrogen fractions in sandy
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soils, obtained by electroultrafiltration or CaCl2 , for nitrogen mineralization and
13
nitrogen uptake of rape. Biology and Fertility of Soils 10: 97-101
14
Appel T., B. Schneider, H. Kosegarten. 1996. Extractability of labelled microbial
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biomass N by electroultrafiltration and CaCl2 extraction. Biology and Fertility of
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17
1
Table 1. Soil sample reference number and selected physio-chemical characteristics of soil samples used in the study. Values
2
are given as the average (n = 3 or n = 4) of the soil samples for each experimental field.
3
Soil sample
number
1,2,3
4,5,6
7,8,9
10,11,12
13,14,15
16,17,18,19
20,21,22
23,24,25,26
27,28,29,30
31,32,33
34,35,36
37,38,39
40,41,42
43,44,45
46,47,48
49,50,51
52,53,54
55,56,57
58,59,60,61
County
Cadreita
Tudela
Arganda
Aleix
Cobert
Montañana
Aula Dei
Gimenells
Las Tiesas
Tallada-1
Tallada-2
Gauna
Aranguiz
Beriain
Tafalla
Yarnoz
Oteiza
Valdegón
Tajonar
Soil classification
(USDA)
Xeric Torrifluvent
Xerollic Paleorthid
Typic Xerofluvent
Oxyaquic Xerofluvent
Oxyaquic Xerofluvent
Typic Xerofluvent
Typic Xerofluvent
Petrocalcic Calcixerept
Calcic Xerosol
Oxyaquic Xerofluvent
Oxyaquic Xerofluvent
Vertic Endoaquol
Vertic Endoaquol
Typic Calcixerept
Typic Calcixerept
Fluventic Haploxerept
Fluventic Haploxerept
Typic Xerofluvent
Fluventic Haploxerept
N
Corg
CO3
g N kg-1
g C kg-1
g CO3= kg-1
1.30
1.10
1.20
0.80
1.40
0.70
1.30
1.30
1.00
1.30
1.30
1.70
1.30
1.60
1.40
1.30
1.30
1.50
1.80
9.50
7.70
10.10
6.50
10.40
5.90
9.80
9.50
9.80
10.20
9.90
14.90
10.60
11.60
10.90
9.50
11.30
11.30
14.00
214.0
193.5
27.0
96.5
111.0
248.0
223.0
128.5
276.0
89.5
86.0
39.5
332.0
169.0
246.5
128.5
216.0
199.0
104.0
pH
Sand Silt Clay
Soil texture 4
(USDA)
--------g kg-1-------8.3
8.5
8.1
8.4
8.3
8.3
8.2
8.3
8.6
8.3
8.3
8.1
8.3
8.2
7.9
8.0
8.2
8.4
7.9
213
506
383
497
369
543
525
385
421
489
489
467
211
12
68
171
123
95
146
605
319
475
435
545
325
342
403
240
389
389
254
567
545
643
525
566
499
481
182
175
142
68
86
133
132
212
340
122
122
279
222
335
289
303
311
406
373
Silt loam
Sandy loam
Silt loam
Sandy loam
Silt loam
Sandy loam
Sandy loam
Loam
Clay loam
Loam
Loam
Sandy clay loam
Silt loam
Silty clay loam
Silt loam
Silty clay loam
Silty clay loam
Silty clay loam
Silty clay loam
18
Table 2. Cumulative N mineralized for indicated periods of time in soils aerobically
incubated during 16 weeks, mineralization rate (k) calculated by fitting a linear model,
and coefficient of determination of the model (r2). Values are given as the average (n =
3 or n = 4) of the soil samples for each experimental field plus the standard error.
Soil sample
Cumulative N mineralized for indicated
r2
k
number
periods of incubation
14 d
42 d
60 d
91 d
112 d
---------------------------mg N kg-1 soil---------------------------
mg N kg-1 soil d-1
1,2,3
6.8 ±0.2
14.5 ±3.7
19.7 ±3.1
23.4 ±3.7
30.9 ±2.4
0.283 ± 0.028
0.97
4,5,6
7.7 ±0.6
12.4 ±1.2
18.5 ±3.3
21.4 ±3.7
26.2 ±3.4
0.277 ± 0.019
0.96
7,8,9
10.6 ±0.6
18.3 ±1.0
26.7 ±1.8
31.2 ±1.8
37.4 ±3.0
0.362 ± 0.019
0.97
10,11,12
7.2 ±1.7
13.5 ±2.4
18.3 ±3.8
22.8 ±4.1
28.2 ±4.7
0.266 ± 0.039
0.95
13,14,15
23.5 ±0.4
42.8 ±3.3
62.6 ±3.9
77.1 ±5.4
91.0 ±6.0
0.875 ± 0.047
0.95
16,17,18,19
9.4 ±0.8
16.0 ±0.8
19.9 ±0.7
24.3 ±0.9
30.3 ±1.4
0.288 ± 0.008
0.94
20,21,22
6.4 ±0.4
12.6 ±0.9
16.8 ±0.4
20.3 ±0.3
24.8 ±0.1
0.237 ± 0.003
0.97
23,24,25,26
10.2 ±0.6
19.4 ±1.3
27.6 ±1.7
36.3 ±2.1
42.4 ±2.3
0.404 ± 0.019
0.94
27,28,29,30
16.4 ±2.5
32.4 ±1.6
41.9 ±1.7
52.8 ±1.9
61.9 ±2.1
0.541 ± 0.021
0.97
31,32,33
15.0 ±1.3
32.1 ±4.6
47.6 ±9.1
62.1 ±9.2
75.8 ±9.6
0.513 ± 0.036
0.96
34,35,36
11.5 ±1.1
27.2 ±5.8
43.5 ±9.3
59.1 ±9.0
73.6 ±9.2
0.467 ± 0.014
0.94
37,38,39
17.6 ±1.3
26.4 ±1.4
35.0 ±1.2
47.4 ±1.6
56.2 ±1.4
0.480 ± 0.013
0.95
40,41,42
12.7 ±1.1
21.7 ±1.1
31.5 ±0.4
41.6 ±0.3
51.1 ±0.2
0.427 ± 0.003
0.97
43,44,45
10.4 ±1.1
20.0 ±0.8
30.2 ±1.0
40.1 ±1.5
48.8 ±1.9
0.408 ± 0.013
0.97
46,47,48
7.4 ±0.0
15.7 ±0.1
28.2 ±2.0
39.2 ±2.0
49.3 ±1.9
0.395 ± 0.015
0.96
49,50,51
9.3 ±0.6
17.4 ±0.7
27.6 ±0.7
37.1 ±1.0
45.7 ±0.8
0.377 ± 0.008
0.95
52,53,54
9.4 ±1.4
24.4 ±1.8
40.3 ±2.8
56.9 ±3.7
68.0 ±4.5
0.560 ± 0.032
0.94
55,56,57
8.1 ±0.3
18.3 ±0.7
28.5 ±0.5
38.5 ±0.4
48.2 ±0.4
0.393 ± 0.004
0.97
58,59,60,61
9.1 ±0.4
16.5 ±0.4
26.2 ±1.2
37.7 ±0.8
47.1 ±1.1
0.380 ± 0.008
0.99
19
Figure captions
Figure 1. Values of N, NO3--N, NH4+-N, and Norg-N extracted by EUF and CaCl2
methods from soils cultivated with corn (from 0 to 26) and winter cereal (from 27 to 61)
crops.
20