J. Plant Nutr. Soil Sci. 2003, 166, 31±38
31
Changes in soil properties and humic substances after long-term amendments with manure and crop residues in dryland farming systems
JosØ Dorado, María-Cristina Zancada, Gonzalo Almendros, and Cristina López-Fando*
Centro de Ciencias Medioambientales, CSIC, Serrano 115 B, E-28006 Madrid, Spain
Accepted 29 November 2002
Summary ± Zusammenfassung
After 16 years of periodical applications of either farmyard manure or crop wastes at two levels of mineral N fertilization to a Calcic Haploxeralf
in the semiarid central Spain, we found significant changes in chemical fertility levels and in the concentration, chemical composition, and
carbon mineralization rates of soil organic matter (SOM). The changes in SOM quality were related to significant improvements of soil physical
properties, mainly aggregate stability and water retention. Such changes were related to the increased concentration of humic colloids in soil,
the mineral N dose, and the type of organic matter applied. When compared with the control plots, the organic matter accumulated in the
amended plots tended to be less transformed, and its total concentration and humification degree decreased with increasing external N-inputs.
Humic acids from the amended plots showed a more marked aliphatic character (mainly after N addition) than those from control plots.
Farmyard manure led to a significant improvement of soil physical properties, but had a comparatively small effect in promoting biodegradation
and humification of crop wastes. This could be due to the high biological stability of the manure used which, in semiarid Mediterranean fields,
usually leads to an accumulation of little transformed SOM.
Veränderungen von Bodeneigenschaften und Huminstoffen nach langjähriger Zufuhr von Stallmist und
Pflanzenrückständen im Trockenfeldbau
Zufuhr von Wirtschaftsdünger und Pflanzenresten zu einem Calcic Haploxeralf im semiariden Zentral-Spanien über 16 Jahre führte zu
¾nderungen der Fruchtbarkeit sowie der Konzentration, der chemischen Zusammensetzung und der biologischen Stabilität der organischen
Substanz des Bodens. Der Versuch beinhaltete zwei unterschiedlich hohe Gaben der mineralischen N-Düngung. Veränderungen in der
Qualität der organischen Substanz des Bodens waren von Verbesserungen der physikalischen Bodeneigenschaften, hauptsächlich von der
Aggregatstabilität und Kapazität zur Wasserspeicherung, abhängig. Weiterhin wurden Zusammenhänge zum Gehalt an Huminstoffkolloiden,
zur Höhe der mineralischen N-Düngung und der Art des angewendeten organischen Dünger festgestellt. Die organischen Substanzen in den
behandelten Varianten waren chemisch weniger verändert und durch geringere Gesamtgehalte und Humifizierungsgrade charakterisiert als die
der Kontrollvarianten. Diese Unterschiede waren stärker ausgeprägt in den Varianten mit erhöhter Mineral-N-Gabe. Die Huminsäuren in Böden
der behandelten Varianten hatten spektroskopische Eigenschaften, die auf relativ gröûere Anteile aliphatischer Verbindungen hindeuteten. Die
Anwendung von organischem Dünger führte zwar zu einer bedeutenden Verbesserung der physikalischen Eigenschaften des Bodens, jedoch
waren die Effekte in Hinsicht auf Biodegradation und Humifizierung der Pflanzenreste verhältnismäûig klein. Dieses könnte an der hohen
biologischen Stabilität der angewandten organischen Dünger liegen, die in dem semiariden Mittelmeergebiet zu einer Akkumulation von wenig
veränderten organischen Bodensubstanzen führt.
Key words: organic matter / organic fertilizers / humic acids / fulvic acids / carbon mineralization
1 Introduction
Dryland agricultural fields in continental Mediterranean
environments are fragile ecosystems permanently subjected
to severe risks of erosion and desertification. Low soil organic
matter (SOM) levels in these soils and a dry, hot summer
season in which the transformation rates of crop wastes
decrease to minimal values, lead to a progressive degradation of soil physical properties favored by tillage practices and
machinery traffic frequently associated to irreversible hardsetting processes (Tate, 1987; Mahboubi and Lal, 1998).
In addition, the low performance of biogeochemical processes in ecosystems where the organic matter humification
is limited to a short period may lead to additional problems of
soil nutrient deficiency associated with microbial immobilization of N, P, and microelements. In fact, in some cases the
addition of crop wastes to improve the physical properties of
these semiarid soils may cause a decline in soil productivity
even after additional inputs of mineral N fertilizer.
*Correspondence: Dr. C. López-Fando; E-mail: lopez.fando@ccma.
csic.es
ã 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
PNSS P97/2B
In this case, additional inputs of matured, stabilized organic
amendments such as composts and old manure, or even of
crop wastes is often considered to be a suitable but
expensive alternative in industrialized countries. To evaluate
the economical benefits of these practices (resulting in
improvement of soil structure in addition to continuous
release of plant available nutrients), a previous knowledge
of the optimum application rates in relation to long-term
effects in the soil is required. This is frequently the case in
continental Mediterranean agrosystems, where the distribution patterns and the extent of the rainfall are often
responsible for more than 80 % of total variance of the crop yields
in agrobiological experiments under field conditions (LópezFando and Almendros, 1995). In such a situation, agricultural
practices may result in highly different soil properties and crop
production in successive years, depending on annual meteorological conditions. This is also the reason why most of the
researchaboutdrylandagriculturalpracticeshavefocusedonthe
accumulation of SOM after many years of periodical applications
(Dalal and Mayer, 1986; Francioso et al., 2000). Such long-term
experiments suggest the feasibility of soil remediation practices
involving invasive organic inputs (Rasmussen and Collins, 1991;
Potter et al., 1998).
1436-8730/03/0102-31 $17.50+.50/0
32
Dorado, Zancada, Almendros, and López-Fando
J. Plant Nutr. Soil Sci. 2003, 166, 31±38
In this study, the comparison between the characteristics of
humus fractions in control plots and in plots receiving
periodical organic inputs is conducted to determine useful
indicators of the extent to which soil properties are being
progressively upgraded in terms of resilience and sustainability. In particular, it is expected that, apart from the increase
in SOM contents, the external inputs would produce changes
in the humification mechanisms and in the overall humus
quality (stability or maturity).
the benzene pretreatment by paraffin oil. Water holding
capacity was determined at atmospheric pressure in a
Richards¢ pressure-membrane extractor operating at 0.1,
10, 50, and 1500 kPa. The pH was measured in water (1:2.5
w:w), total N by micro-Kjeldahl digestion and available P by
the Bray and Kurtz (1945) method. Available K, Ca, Mg, and
Na were extracted with 1 M NH4OAc (pH = 7). Oxidizable
carbon (C) was determined by using the Walkley and Black
method (Nelson and Sommers, 1982).
2 Materials and methods
2.3 Humus fractions
2.1 Location of the field and experimental design
The field experiment was carried out at the CSIC experimental farm ªLa Higueruelaº (Toledo), in a cereal-producing
area of central Spain under a semiarid continental climate.
The average annual rainfall, mainly occurring in autumn and
spring, is 400 mm with high summer temperatures (23 C in
average) and a long cold season (6 C average winter
temperature). Soil samples (ca. 500 g) were collected with a
spade from the 0±20 cm layer of a Calcic Haploxeralf (USDA,
1975) with a sandy loam texture (total sand = 783 ± 29 g kg±1,
silt = 82± 11 g kg±1, clay = 135 ± 22 g kg±1).
This work is part of the research performed by 22 European
Centers in the frame of the International Organic Nitrogen
Long-Term Experiment (IOSDV) at Madrid, coordinated by
the Justus-Liebig-University in Giessen, Germany. The
experiment is arranged in a split-plot design with the organic
amendment as the main plot and the N mineral fertilization as
subplots. The treatments were replicated three times (three
blocks). The different organic fertilizing treatments consisted
of: (1) control without organic inputs; (2) addition of 3 Mg ha±1
of barley straw (480 g kg±1 organic C; 8 g kg±1 total N; 2 g kg±1
total P; 18 g kg±1 K; 4 g kg±1 Ca, and 2 g kg±1 Mg) every year,
and approximately 2.5 Mg ha±1 of rape crop (420 g kg±1
organic C; 21 g kg±1 total N; 3 g kg±1 total P; 27 g kg±1 K; 6 g
kg±1 Ca, and 2 g kg±1 Mg) topdressed on soil as green
manure amendment, applied every three years; and (3)
amendment with 30 Mg ha±1 of 2-year old cattle manure
(320 g kg±1 organic C; 24 g kg±1 total N; 5 g kg±1 total P; 23 g
kg±1 K; 36 g kg±1 Ca, and 8 g kg±1 Mg) applied every three
years. Mineral N fertilization was as follows: (1) no mineral N
application (N = 0) and (2) mineral fertilization with 100 kg ha±1
of 33.5 % NH4NO3 (N = 100). When straw was used as
amendment, an additional dose of 30 kg ha±1 of 33.5 %
NH4NO3 was added to prevent microbial N-immobilization
during the decomposition of this residue of high C : N ratio.
Besides the above treatments, the experimental plots received a
background fertilization consisting of 60 kg ha±1 of P2O5, as triple
superphosphate and 80 kg ha±1 of K2O, as K2SO4.
2.2 General analyses
Soil samples were air-dried, homogenized with a wooden
cylinder on a table, and sieved to 2 mm. Particle size analysis
was performed with the pipette method (Bouyoucos, 1936). In
order to assess soil aggregate stability (instability index, Is),
the modified Feodoroff¢s (1960) method was used, changing
The preparative isolation and quantitative analysis of humus
fractions were based on protocols described by Dabin (1971),
Duchaufour and Jacquin (1975), and Merlet (1971). The
procedure used includes a previous physical separation of the
lightsoilfractionconsistingofthenotyetdecomposedsoilorganic
particles (free organic matter; flotation in 2 M H3PO4). The pale
yellowish acid solution remaining after densimetric separation
(free fulvic acids) was subjected to quantitative determination.
Then, the soil residue was extracted with 0.1 M Na4P2O7
(continuous stirring for one hour) followed by centrifugation at
2600 g and further extractions with 0.1M NaOH, repeated five
times. The total humic extract was obtained by aggregating the
successive alkaline supernatants after centrifugation. Aliquots of
this total humic extract were precipitated to pH=1 with H2SO4 (1:1
w:w)and centrifugedat3020 g. Theacid-insolubleorganicmatter
so obtained was used for quantitative determination of the humic
acids(HAs);theacid-solublefulvicacids(FAs)werecalculatedby
difference. Finally, the non-extractable fraction (humin) was
calculated by difference between the total organic C and the sum
of the C in the free organic matter, the total humic extract, and the
free fulvic acid fractions.
The humic extract remaining after the quantitative determinations was acidified to pH = 1 with HCl to precipitate the HAs
which were de-ashed with 1 M HCl-HF treatments and
centrifuged to discard the supernatant solution, then the
residue was re-dissolved in 0.5 M NaOH and centrifuged at
43500 g. The insoluble residue (mainly mineral colloids) was
discarded and the new supernatant solution of Na-humate
was re-precipitated with HCl, centrifuged, and dialyzed in
cellophane bags to remove the salts introduced during the
extraction procedure. The resulting HA suspension was dried
at 40 C and kept for chemical analyses.
2.4 Humic acid characteristics
Visible spectra were obtained from solutions of 200 mg C l±1 in
0.02 M NaHCO3 (Kononova, 1966). Optical densities at 465 nm
(E4) and 665 nm (E6) of HAs were determined from the above
solutions using a Shimadzu UV-240 spectrophotometer, and the
E4 : E6 ratioswere calculated. The E4 : E6 isconsideredto inform
about the polydispersity of the HAs, whereas the E4 would inform
of their aromaticity (Chen et al., 1977; Traina et al., 1990).
Infrared (IR) spectra over the wavenumber range of 4000 ±
600 cm±1 were obtained from KBr pellets (2.0 mg HA in 200
mg KBr) with a Bruker IFS28 Fourier-transform IR spectrophotometer. The spectra were acquired by accumulating 100
J. Plant Nutr. Soil Sci. 2003, 166, 31±38
Soil properties after amendments with manure and crop residues
interpherograms at a 2 cm±1 resolution. For resolution
enhancement, a digital procedure based on subtracting the
original spectrum from a multiple of its second derivative
(Rosenfeld and Kak, 1982; Almendros and Sanz, 1992) was
used. Some semiquantitative data useful for spectral
comparisons were calculated by dividing the absorption
values by the intensity value of the band at 1620 cm±1
(spectral maximum, to a large extent produced by aromatic
structures; Dupuis and Jambu, 1969).
2.5 Soil respiratory activity
To assess the potential biodegradability of the organic matter
from different plots under comparative conditions, the soil
respiratory activity was measured in the laboratory. Whole
soil samples (20 g of air-dried soil homogenized to 2 mm and
moistened to 60 % soil water holding capacity at atmospheric
pressure) were incubated in an oven at 24±1 C inside 250 ml
Erlenmeyer flasks with rubber stoppers provided with inlet
and outlet polyethylene tubes. The CO2 released by the
microbial metabolism was periodically measured for 44 days
with a Carmhograph-12 (Wösthoff) gas analyzer by connecting the flasks to the CO2 analyzer while replacing the gas
phase with CO2-free air provided by a soda lime column
(Almendros et al., 1990). The mineralization of the organic
matter was expressed as mg of C released per 100 g of soil C
per day (mineralization rate) (Chone et al., 1973).
33
2.6 Statistical procedures
Soil data and HA properties corresponding to 3 spatial
replications (split-plot design) were subjected to a two-factor
analysis of variance. The results of this test are shown as
main effects and interactions, corresponding to significant
differences between the treatments to be compared
(untreated plots vs amended ones, either with or without N
fertilization). Non-linear least-square regression analysis was
used to calculate parameters from cumulative C mineralization data (Riffaldi et al., 1996). Simple and multiple linear
regression models (in the latter case using the backward
automatic variable selection) were applied to relate soil organic
fractionstosoilrespiratoryactivity.Theleastsignificantdifference
(LSD) test at P<0.05 was used to compare means of the soil
respiratory activity and of the IR analyses.
3 Results and discussion
3.1 Organic matter contents and properties
Tabs. 1 to 3 show the soil properties and the crop yields, the
SOM characteristics and the statistical analyses of the data,
after 16 years of organic and N inputs in the experimental
plots. The results point to a significant favorable effect of
organic inputs on soil aggregate stability (decreased Is). This
improvement in structural stability with respect to control plots
Table 1: Effect of organic amendments and mineral nitrogen fertilization on soil physical and chemical characteristics and crop yield parameters
Tabelle 1: Einfluss der organischen und der Mineral-N-Düngung auf physikalische und chemische Bodeneigenschaften und Erträge
Crop wastea
Control
0
100
0
Manureb
100
0
100
±1
N input (kg ha )
Is (structural instability index)
WHC (g kg±1)c
4.8
221
4.5
226
2.3
238
4.1
267
2.3
228
4.0
283
Water retention (-0.1 kPa)
31.1
32.4
29.0
39.0
32.9
39.0
Water retention (-10 kPa)
20.8
17.2
13.8
21.0
18.1
23.1
Water retention (-50 kPa)
14.1
11.7
9.5
17.4
12.8
18.0
Water retention (-1500 kPa)
6.0
5.7
3.9
8.5
5.6
8.9
pH
6.9
5.7
5.5
5.9
7.1
7.5
9
P2O5>
>
>
>
>
>
K >
>
=
Ca
(mg (kg soil)±1)
>
>
>
Mg >
>
>
>
>
;
Na
9
Organic C =
(g (kg soil)±1)
Total N ;
C/N ratio
Barley grain yieldd
Wheat grain yieldd
9
>
>
=
>
>
;
Sorghum plant yieldd
195
100
110
170
530
623
245
190
310
340
1250
790
1185
940
650
2190
1220
2900
142
173
175
264
136
246
10
15
10
23
40
63
4.6
5.1
6.8
7.2
7.3
9.6
0.6
0.5
0.5
0.7
1.1
1.1
7.8
3035
(kg ha±1)
10.2
3047
13.6
3396
10.5
2392
6.8
3710
8.7
2642
2236
2603
3172
2133
2982
2072
12232
11777
11246
10415
13754
13028
a
Amendment with barley straw (3 Mg ha±1 per year) plus rape crop topdressed (2.5 Mg ha±1 every three years), b amendment with 2-year old
cattle manure (30 Mg ha±1 every three years), c water holding capacity, d average values for the period 1991±1999.
34
Dorado, Zancada, Almendros, and López-Fando
J. Plant Nutr. Soil Sci. 2003, 166, 31±38
Table 2: Effect of organic amendments and mineral nitrogen fertilization on the carbon contents of the different soil organic matter fractions and
on the spectroscopic characteristics of the humic acids
Tabelle 2: Einfluss der organischen und der Mineral-N-Düngung auf die C-Gehalte der unterschiedlichen organischen Fraktionen des Bodens
und auf die spektralen Eigenschaften der Huminsäuren
Crop wastea
Control
0
100
0
Manureb
100
0
100
±1
N input (kg ha )
d
Free organic matter
0.4
(9.3)c
0.8
(14.7)
1.3
(18.5)
0.6
(8.0)
0.8
(10.7)
0.6
(6.2)
Total humic extractd
2.6
2.9
4.0
4.2
4.2
5.4
Humic acid (HA)
0.7
(14.4)
0.9
(17.4)
1.3
(18.9)
1.2
(16.7)
1.9
(25.5)
2.3
(24.5)
Fulvic acid (FA)d
1.8
(39.6)
1.9
(36.6)
2.5
(37.4)
2.9
(40.5)
2.2
(30.2)
2.9
(30.2)
Free fulvic acidd
0.11
(2.4)
0.09
(1.8)
0.13
(2.0)
0.10
(1.3)
0.13
(1.7)
0.15
(1.5)
Non-extractable (humin)d
1.6
(34.3)
1.5
(29.4)
1.6
(23.2)
2.4
(33.5)
2.3
(31.8)
3.6
(37.6)
0.36
0.48
0.51
0.41
0.84
0.82
0.83
0.84
0.82
0.79
0.72
0.63
5.17
5.56
5.94
6.07
5.81
6.12
d
HA : FA ratio
E4
e
E4 : E6 ratiof
a
±1
±1
b
Amendment with barley straw (3 Mg ha every year) plus rape crop topdressed (2.5 Mg ha every three years), amendment with 2-year old
cattle manure (30 Mg ha±1 every three years), c in brackets, the relative value expressed as g per 100 g of the total C in soil, d g C (kg soil)±1, e
optical density of the HAs (465 nm, 200 mg l±1), f optical density ratio (465 and 665 nm) in the HA spectra.
was not a function of the organic matter type used (crop
wasteorfarmyardmanure),butofthedoseofN.Thiseffectmaybe
explainedintermsofthechangesinhumusqualityafteradditional
N inputs, in particular the lower HA : FA ratio and the higher E4 :
E6 ratio (Tab. 2), both pointing to a lower molecular size of soil
organic colloidal fractions that could be related to the decreased
cementing properties of the organic matrix after external N-inputs
in the organic matter-treated plots.
A positive effect of organic inputs on the water holding
capacity (Tab. 1) is observed mainly in plots receiving N
inputs. The incorporation of organic matter has a significant
positive effect on water retention at relatively low pressures
(e.g., ±0.1 to ±10 kPa) and without N addition, and the
increases became progressively more significant at higher
pressures and in plots with N fertilization (Tab. 3). This could
be due to the above-mentioned changes in the composition of
the soil organic colloidal fractions whose decreased molecular size might favor the stabilization of the microaggregate
fraction in soils where the Ca2+ concentration enables the
bridging of the most soluble and reactive FA-like humic
substances.
Organic inputs also caused changes in the nutrient (N, P, K,
Mg, and Na) status of soil (Tab. 3). This could be explained in
terms of the composition of the original amendment (see 2)
and its effect in soil biomass production and humus quality.
All plots showed a N-dependent increase in organic C
concentration (very significant with manure), suggesting that
the accumulation of stable humic substances from external
inputs significantly increased in the non N-limited plots.
The distribution of total C in the different humus fractions
(Tab. 2) indicates that organic inputs, mainly with N
fertilization, have not accumulated as non-decomposed plant
debris (free organic matter) but as humic-type colloids. Such
an accumulation was also observed in the non-extractable
humin from amended plots, indicating an active insolubilization in the plots with additional N fertilization. On the other
hand, the increased HA : FA ratio in plots amended with
farmyard manure indicates that the soil matrix accumulates
the most insoluble humic colloids.
The optical density (Tab. 2) of the HA solutions adjusted to
the same concentration is traditionally considered to inform
about the extent of the aromatic domain of the HA
macromolecules (Traina et al., 1990). Decreased E4 values
of the HAs (Tab. 2) from amended plots as regards the control
plots (mainly in the case of farmyard manure) may indicate
that, independently of its total concentration, the external
inputs led to soil enrichment in comparatively young aliphatic
substances. Since the optical density of the HAs showed an
additional decrease in the case of the N-supplemented plots,
these substances in the latter plots probably include aliphatic
microbial biomass.
The latter hypothesis agrees with the changes observed (Tab.
3) in the E4 : E6 ratio (a parameter considered to decrease
with the molecular size of humic macromolecules), which
J. Plant Nutr. Soil Sci. 2003, 166, 31±38
Soil properties after amendments with manure and crop residues
35
Table 3: Main effects and interactions between factors [organic amendment (OM) and mineral nitrogen fertilization (N)] influencing soil and
humic acid characteristics
Tabelle 3: Haupteffekte und Abhängigkeiten zwischen den verschiedenen Faktoren [organische Behandlung (OM) und Mineralstickstoffdüngung (N)], die die Boden- und Huminsäureeigenschaften beeinflussen
Manure
OM
N
Crop waste
OM ” N
interaction
average
OM
N
ns
OM ” N
interaction
average
Structural instability index
±1.5**
0.7*
1.0**
3.9
±1.5*
1.0*
3.9
Water retention (-0.1 kPa)
4.2**
3.7**
2.4**
33.8
2.3*
4.3*
5.6**
32.9
Water retention (-10 kPa)
1.6**
0.7*
4.3**
19.8
ns
1.8*
5.4*
18.2
Water retention (-50 kPa)
2.5**
1.5**
3.8**
14.1
ns
2.8*
5.1**
13.2
1.8**
6.5
0.4*
2.5**
6.0
1.3*
6.6
ns
Water retention (-1500 kPa)
1.4**
pH
1.6*
1.5**
ns
2.2*
ns
ns
5.7
P2O5
429**
ns
ns
362
ns
ns
78*
144
K
803**
±258**
±203**
619
ns
ns
ns
271
ns
ns
71*
ns
Na
Mg
C
N
C:N
88.8*
ns
3.6**
0.6**
±1.3*
Total organic matter
1.1**
Humic acid
1.3**
Total humic extract
2.1*
Non extractable (humin)
1.4**
HA : FA ratio
0.4*
E4 : E6 ratio
0.6**
1.4**
ns
2.1**
ns
0.4**
ns
0.6*
ns
0.4*
0.9*
56.9
174
6.6
4.2*
9.2*
3.8*
ns
110*
79*
2.2*
ns
ns
14.4
164
5.9
ns
0.8
ns
ns
ns
0.6
ns
8.4
ns
ns
ns
10.5
ns
0.1*
ns
1.1
ns
ns
ns
0.9
1.4
0.5*
ns
±0.2*
1.0
3.8
1.4*
ns
ns
3.4
2.2
ns
ns
ns
1.8
ns
0.6
ns
ns
±0.1*
0.4
ns
5.7
0.6*
±0.1*
5.7
0.7*
0.3*
The meaning of the parameters analyzed is explained in Tabs. 1 and 2.
ns = non-significant (P>0.05); * significant at P<0.05; ** significant at P<0.01. Those parameters showing non-significant differences as regards
the control plots are not included.
points to microbial fragmentation of the organic colloids with a
N-dependant performance, as suggested by the IR spectra to
be discussed below.
The IR profiles were quite similar in all HAs, the main
difference consisting of the relative intensity of the spectral
bands. Resolution-enhanced spectra (Fig. 1) showed a
typical band pattern, coinciding with that of lignins (i.e.,
peaks at 1510, 1460, 1420, 1380, 1270, 1230, and 1130
cm±1; Farmer and Morrison, 1960; Fengel and Wegener,
1989). The spectra also showed additional amide (1650 and
1550 cm±1) and carboxyl (1720 cm±1) peaks, as a whole
suggesting accumulation of a mixture of oxidized altered
lignin and protein, typical from straw composts (Jenkinson
and Tinsley, 1959).
Figure 1: Resolution-enhanced FT-IR spectra (2000±800 cm±1) of
humic acids from untreated plots, plots amended with barley straw
(3 Mg ha±1 every year) plus rape crop topdressed (2.5 Mg ha±1 every
three years) and plots amended with two-year old cattle manure
(30 Mg ha±1 every three years). The spectra correspond to plots
which received no additional N-fertilization. Each spectrum was
obtained by digital averaging of two spectra from replicated spatial
sampling.
Abbildung 1: Hochaufgelöste FT-IR-Spektren (2000±800 cm±1) der
Huminsäuren aus unbehandelten Kontrollparzellen und aus Parzellen, die mit Gerstenstroh (3 Mg ha±1 pro Jahr) plus Rapsstroh (2,5 Mg
ha±1 alle drei Jahre) und mit zwei Jahre altem Stallmist (30 Mg ha±1
alle drei Jahre) behandelt wurden. Die gezeigten Spektren
entsprechen Parzellen, die keine zusätzliche N-Düngung erhalten
haben. Jedes Spektrum wurde durch digitale Mittelwertbildung der
Spektren von räumlich wiederholten Probenahmen berechnet.
36
Dorado, Zancada, Almendros, and López-Fando
J. Plant Nutr. Soil Sci. 2003, 166, 31±38
Figure 2: Absorbance ratios between diagnostic FT-IR bands of
humic acids. For sample labels see Fig. 1. The results shown
correspond to plots with no additional N fertilization. Error bars
indicate the standard deviations of replicated spatial sampling. Within
a subplot, bars labelled with the same letter are not significantly
different at P<0.05.
Abbildung 2: Verhältnisse der Absorption zwischen diagnostischen
FT-IR-Banden. Für Probenbezeichnungen s. Abb. 1. Die gezeigten
Resultate entsprechen Plots ohne zusätzliche N-Düngung. Fehlerbalken zeigen die Standardabweichungen des wiederholten räumlichen Musterstückes an. Varianten, die mit dem gleichen Buchstaben
beschriftet wurden, unterscheiden sich nicht signifikant bei P<0.05.
Figure 3: Upper panel: Daily rate and
cumulative mineralization (adjusted to the
function Ct=C0 (1±e±kt)+C1) in laboratory
incubation experiments with whole soil
samples. The percentages of the total C
released as CO2 in the different stages of the
44-day incubation are shown under the
curves. Sample codes concerning the
organic amendment refer to Fig. 1. The N
mineral fertilization doses (0 to 100 kg ha±1)
are shown in brackets. Error bars indicate
standard deviations of replicated spatial
sampling. Lower panel: daily mineralization
rate during the different stages of the 44-day
laboratory incubation. The results shown
correspond to plots which received no
additional N-fertilization. Within a stage,
bars labelled with the same letter are not
significantly different at P<0.05.
Abbildung 3: Oberer Teil: Tägliche Rate
und kumulative Mineralisierung (angepasst
an die Funktion Ct=C0(1±e±kt)+C1) in LaborInkubationsexperimenten mit Gesamt-Bodenproben. Die Kurven zeigen die Anteile
des Gesamt-C, das als CO2 in den unterschiedlichen Stadien der 44-tägigen Inkubation freigegeben wurde. Probenbezeichnung
wie in Abb. 1. Die N-Mineraldüngungdosen
(0 bis 100 kg ha±1) sind in Klammern
dargestellt. Fehlerbalken zeigen Standardabweichungen der räumlich wiederholten
Probennahmen. Unterer Teil: Tägliche Mineralisierungrate während der unterschiedlichen Stadien des 44-tägigen Labor-Inkubationsexperimentes. Die gezeigten Resultate
entsprechen Parzellen, die keine zusätzliche
N-Düngung erhalten haben. Innerhalb eines
Stadiums sind die Varianten, die mit dem
gleichen Buchstaben beschriftet wurden,
nicht signifikant verschieden bei P<0.05.
J. Plant Nutr. Soil Sci. 2003, 166, 31±38
Soil properties after amendments with manure and crop residues
A comparison between spectral profiles showed the most
marked lignin band pattern in the soils that received organic
matter inputs, mainly farmyard manure. This was mostly
observed (Fig. 2) in the enhanced intensity of the peaks due
to aromatic ring vibrations at 1510 and 1420 cm±1, the alkyl
bending band (scissoring) at 1460 cm±1, the additional
diagnostic bands of syringyl (1330 cm±1), guaiacyl (1270
cm±1), and phenolic groups (1230 cm±1), and a similar or
lower intensity of the bands to which contribute carboxyl
groups (1720 cm±1).
All the above spectroscopic indices (including those in the
visible range) suggested that farmyard manure was less
suitable than the lignocellulosic crop wastes to increase the
biological stability ± maturity ± of the HAs (i.e., it led to
decreased optical density and to a more marked lignin pattern
in the IR profile). Similar results were observed in the plots
treated with additional N inputs (data not shown) but the
differences between the two organic amendments were
smaller. Considering that the 2-year-old manure was a
more mature, inert material than the crop wastes used in
this experiment, this result could be tentatively explained by
hypothesizing that the latter amendments, comparatively
more biodegradable, have induced the development of
microbial populations with a positive effect on the biodegradation and humification of the crop wastes in the field. On
the other hand, the more stable manure has produced a
significant upgrade in soil physicochemical properties (Tab.
1), probably due to some increase of microaggregate
hydrophobicity, and a less marked secondary effect in
favoring the microbial transformation of the SOM.
3.2 Soil C mineralization curves in laboratory
conditions
Potential changes in the performance of the soil biogeochemical system after organic matter inputs are to a large
extent summarized by the in vitro mineralization curves of the
whole soils under controled laboratory conditions (Fig. 3). For
comparative purposes, the daily respiration curves were
divided into three successive stages. The first stage would
correspond to the early microbial degradation of the readily
degradable substances concomitant with the rapid colonization of the external aggregate surfaces exposed to microorganisms after homogenization of the soil samples. The
subsequent stage would be more representative of a
stabilized respiration after the laboratory mechanical disturbance of the soil material (Almendros and Dorado, 1999).
The successive mineralization stages indicate that the
organic matter accumulated in the soil treated with crop
waste is readily transformed by soil microorganisms (stage I),
whereas that from the manure-treated soil has a similar
stability as the original soil (Fig. 3, lower panel). The highest
stability of soil from the manured plots in the late stage of
mineralization may coincide with a selectively preserved
lignin domain in the HAs.
Cumulative mineralized C showed a curvilinear relationship
with time over the 44-day incubation period (Fig. 3, upper
panel). Thus, comparisons between soils based on these
37
cumulative mineralization data can be made after a non-linear
adjustment. A first-order equation proposed by Riffaldi et al.
(1996) gave the best fit to respiration data: Ct=C0(1±e±kt)+C1.
This model contains a parameter capable of defining a pool of
easily decomposable substrate producing a mineralization
flush (C1) during the first incubation interval. Soils amended
with straw showed values for this rapidly mineralizing C pool
higher than controls. On the contrary, soil samples amended
with manure pointed to lower values of C1 than control soils,
which agreed with the comparatively high initial stability of
this organic input. Fig. 3 also shows the initial potential rate of
C mineralization (C0k), which can be more responsive to soil
activity than the individually examined parameters (Riffaldi et
al., 1996). The C0k values varied considerably, showing the
lowest values in control soils and the highest values in
amended soils (especially those amended with straw)
indicating the extent to which soil biogeochemical cycle
accelerates with the external C inputs.
Table 4: Significant (P<0.05) correlation coefficients between soil
organic fractions properties and soil respiratory activity
Tabelle 4: Signifikante (P<0.05) Korrelationskoeffizienten der
Beziehungen zwischen organischen Fraktionen des Bodens und
der Bodenatmungs-Aktivität
C0
k
C0 k
Organic C
±0.925(0.008) 0.980(0.001)
ns
Humic acid
±0.967(0.002) 0.960(0.002)
ns
Fulvic acid
ns
0.851(0.032) 0.921(0.009)
HA : FA ratio
±0.849(0.033)
Total humic extract
±0.919(0.010) 0.977(0.001)
ns
Non-extractable (humin) ±0.903(0.014) 0.954(0.003)
ns
0.897(0.015) ±0.908(0.012)
ns
E4
E4 : E6 ratio
ns
ns
ns
0.830(0.041) 0.917(0.010)
ns = non-significant (P>0.05). In brackets, the P-value for the model
fitted.
Table 5: Coefficients of multiple regressiona functions between soil
organic fraction properties and soil respiratory activity
Tabelle 5: Koeffizienten der Regressionsgleichungena für Beziehungen zwischen organischen Fraktionen des Bodens und Bodenatmungs-Aktivität
Humic acid
C0
k
C0 k
±507
ns
ns
Fulvic acid
ns
ns
9.3
Total humic extract
ns
0.005
ns
±0.006
ns
E4
E4:E6 ratio
4737
ns
ns
R-squared statistic
0.991
0.994
0.997
P-value
0.001
0.001
0.001
a
3.6
Backward multiple regression. Stepwise variable selection.
ns = non-significant (P>0.05).
38
Dorado, Zancada, Almendros, and López-Fando
In order to forecast to some extent the soil characteristics
related with the intrinsic biodegradability of the organic
matter, Tab. 4 shows the coefficients from the simple linear
regression functions between soil properties and the abovementioned parameters calculated from the soil respiratory
curves. Potentially mineralizable C (C0) was negatively
correlated with the total amount of C and with the
concentration of the most recalcitrant organic fractions (HA
and humin), and positively with the E4. Conversely, k values
were positively correlated with the concentration of the
different humic fractions and with the E4 : E6 ratio, and
negatively with the E4. There was a good correlation between
the initial potential rate of C mineralization (C0k) and those
parameters reflecting labile C, such as concentration of FAs
and high E4 : E6 ratio. The results obtained in the multiple
regression analysis (Tab. 5) confirm some of the previous
considerations, suggesting that the most intense mineralization activity (C0k) was associated with the accumulation of
low molecular weight soil fractions (FAs and HAs with high
E4 : E6 ratio) whereas the C0 values were explained in terms
of parameters pointing to humus maturity (aromaticity and
low amount of organic fractions) which may be analytical
descriptors of intense selective biodegradation in semiarid
soils.
Acknowledgments
Financial support from the Spanish CICyT to grant AGL200204186 is gratefully acknowledged.
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