Introduction
Application of organic materials as soil amendments is an important
management strategy that can improve and uplift soil-quality characteristics
and alter the nutrient cycling through mineralization or immobilization
turnover of added materials (Khalil et al., 2005; Campos et al., 2013; Baldi
and Toselli, 2014; Novara et al., 2013; Hueso-González et al., 2014;
Oliveira et al., 2014). Use of local organic materials derived either from
livestock or plants have been attaining worldwide support for improving the
fertility and productivity potential of degraded and nutrient-poor soils
(Huang et al., 2004; Tejada and Benítez, 2014). Indeed, plant residues
and animal manures are potentially important sources of nutrients for crop
production in smallholder agriculture. However, the Hindu Kush Himalayan
regions, including the state of Azad Jammu and Kashmir, have a wide
diversity of leguminous species and non-leguminous plants compared to the
livestock production. Hence, use of plant residues as organic nutrient
source is relatively simple for the farmers compared to the
application of manure. Incorporating plant residues into agricultural soils can
sustain organic carbon content, improve soil physical properties, enhance
biological activities and increase nutrient availability (Hadas et al.,
2004; Cayuela et al., 2009). In the short-term, incorporation of plant
residues provides the energy and nutrients for microbial growth and
activity, acts as a driving force for the mineralization–immobilization
processes in the soil and is a source of nitrogen (N) for plants (Jansson and
Persson, 1982). In the long-term, incorporation of crop residues is
important for the maintenance of organic carbon (C) and N stocks in the
nutrient pool of arable soils (Rasmussen and Parton, 1994).
Incorporation of crop residues provides readily available C and N to soils
depending upon the decomposition rates and synchrony of nutrient
mineralization (Murungu et al., 2011). The N availability from these
residues depends on the amount of N mineralized or immobilized during
decomposition. However, previous studies demonstrated that the decomposition
and nutrient release rates of residues are often regulated by environmental
factors, such as temperature and soil moisture, and biochemical composition
of plant materials and their interaction (Abiven et al., 2005; Khalil et al.,
2005). The biochemical composition or quality parameters such as total N
concentration, lignin (LG), polyphenols (PP), carbon : nitrogen (C / N) ratio,
LG / N, PP / N and (LG + PP) / N ratios are considered useful indicators that control
decomposition and N release of added residues (Nakhone and Tabatabai, 2008;
Vahdat et al., 2011; Abera et al., 2012). However, it has not been clearly
established which of these variables correlate best with N
mineralization of plant residues (Nakhone and Tabatabai, 2008), as
contrasting results have been reported in the literature (Nourbakhsh and
Dick, 2005). On the one hand, it has been reported that N released from leguminous tree
leaves indicated that the (lignin + polyphenol) : N ratio was the most
important factor in predicting N mineralization (Mafongoya et al., 1998). On
the other hand, Frankenberger and Abdelmagid (1985) suggested that lignin
content of the legumes is not a good predictor of the N mineralization.
Handayanto et al. (1994) suggested that the N concentration or lignin : N
ratio of the leaves were not good indicators of N release for agroforestry
materials. Palm and Sanchez (1991) attributed the differences in N
mineralization rates of various tropical legumes to polyphenols.
Handayanto et al. (1994) found, however, that the total N content of plant
residues was not correlated with rates of N released under non-limiting N
conditions.
Earlier studies clearly demonstrated the beneficial effects of plant
residues on soil–plant systems (Huang et al., 2004; Cayuela et al., 2009;
Khalil et al., 2005; Baldi and Toselli, 2014). However, there is still a
scope to explore the possibilities for achieving maximum benefits in term of
rate, time and amount of N released. For example, the synchronization of net
N mineralization with plant/crop growth is desirable to maximize N delivery
for the crop and minimize N losses. Abiven et al. (2005) reported that one
of the tools to achieve synchronization is the use of plant residues with
different natures and qualities. Application of residues with a high C / N ratio
results in immediate net N immobilization while residues with a low C / N ratio
result in net N mineralization, showing that mineralization–immobilization
turnover (MIT) can be influenced differently by chemical components of added
plant materials. To achieve this target, the combination of legumes and
non-legumes plant materials or different plant components of the same plant
species, i.e., root, shoot and leaves, can be tested.
Keeping in mind the beneficial effects of plant residues on soil–plant
systems, especially in the mountainous upland soils vulnerable to soil
(water) erosion, the present work aims to (i) examine the initial biochemical
composition and quality characteristics of on-farm available plant residues
and to (ii) quantify the N-release potential (mineralization) of these
residues added to a soil incubated under controlled laboratory conditions
(25 ∘C) in Rawalakot, Azad Jammu and Kashmir, Pakistan.
Materials and methods
Soil sampling
The soil used in this study was collected from an arable field located at
the research farm of the Faculty of Agriculture of the University of Poonch,
Rawalakot, Azad Jammu and Kashmir, Pakistan. The study site is located at
latitude 33∘51′32.18′′ N, longitude 73∘45′34.93′′ E and an
elevation of 1638 m above sea level. The climate of the region is
subtemperate. Mean daily maximum and minimum air temperatures ranged from
27 to 29 ∘C (June–July) and 1.0 to -3.5 ∘C (January–February). The
mean annual rainfall ranged between 1100 and 1500 mm with more than 50 % of
the total precipitation during monsoon each year. The soil in the study site
was clay loam in texture, classified as Humic Lithic Eutrudepts
(Inceptisols; Ali et al., 2006). The field was bare at the time of sampling
but previously maize (Zea mays L.) and wheat (Triticum aestivum L.) were cultivated. The selected
field was divided into 10 subplots to ensure proper and representative
soil sampling. Soil samples were collected from a depth of 0–15 cm at random
from three points in each plot using a soil auger of 5 cm in diameter.
The soil samples from all the selected plots were thoroughly mixed to get a
composite sample. The field-fresh soil was passed through a 4 mm sieve to
eliminate coarse rock and plant material, thoroughly mixed to ensure
uniformity and stored at 4 ∘C before use (not more than 2 weeks). A
subsample of about 0.5 kg was taken, air dried, passed through a 2 mm
sieve and used for the determination of physical and chemical
characteristics. The original soil analysis is presented in Table 1.
Selected physicochemical properties of the soil used in the
study.
Soil properties
Values
Bulk density (Mg m-3)
1.20
Particle density (Mg m-3)
2.48
Porosity (%)
48.3
Sand (g kg-1)
241
Silt (g kg-1)
394
Clay (g kg-1)
365
Texture class
clay loam
pH
7.2
CEC (cmol kg-1)
7.3
Organic matter (g kg-1)
10.4
Organic C (g kg-1)
6.03
Total N (g kg-1)
0.58
C : N ratio
10:1
Total mineral N (mg kg-1)
8.7
Total organic N (mg kg-1)
591.0
P (mg kg-1)
3.4
K (mg kg-1)
88.0
Fe (mg kg-1)
15.7
Mn (mg kg-1)
17.0
Cu (mg kg-1)
1.02
Zn (mg kg-1)
1.16
Collection of plant residues
Six predominant on-farm available plant species were selected. These
included Glycine max shoot, Glycine max root, Trifolium repens shoot,
Trifolium repens root, Zea mays shoot, Zea mays root, and leaves of
Populus euramericana, Robinia pseudoacacia and Elaeagnus umbellata. Plant
samples/residues were collected at different times during the year 2012.
Glycine max and Trifolium repens samples were collected from the field before flowering (summer) while
Zea mays samples were taken 1 week before crop harvest. The tree leaves were
sampled in late fall. Plant residues were washed with running tap water,
rinsed three times with distilled water, dried at 65 ∘C for 48 h, milled
and passed through a 1 mm sieve. Triplicate samples of plant residue were
taken and analyzed for their C, N, lignin and polyphenol concentrations.
Total N contents of the residues were determined by Kjeldhal digestion,
distillation and the titration method (Bremner and Mulvaney, 1982). Wet
digestion method was used for organic C analysis (Nelson and Sommers, 1982).
The lignin content was determined using Van Soest methods (Van Soest et al.,
1991). Soluble polyphenols were extracted in hot water (100 ∘C, 1 h) and
determined by colorimetry using a Folin–Denis reagent (Folin and Denis,
1915).
Laboratory incubation
The incubation methods used in this study were followed by the methods used
in our previous studies (Abbasi et al., 2011; Abbasi and Khizar, 2012).
Briefly stated, about 100 g of soil already stored in the refrigerator at
4 ∘C was weighed and transferred into 200 mL glass jars. The initial
moisture content of the soil was 28 % (w/w), which was increased by adding
distilled water to achieve a final water-filled pore space of 58 %.
The treatments were comprised of a control (no N) and nine plant residues
sources, i.e., Glycine max shoot, Glycine max root, Trifolium repens shoot, Trifolium repens root, Zea mays shoot, Zea mays root, and leaves of
Populus euramericana, Robinia pseudoacacia and Elaeagnus umbellata; 10 incubation timings, i.e., 0, 7, 14, 21, 28, 42, 60, 80, 100 and 120
days; and three replications. Altogether, a total of 300 jars (10
treatments × 10 incubation timings × 3 replications) were
arranged in a completely randomized design. Plant residues were weighed and
added into the jars at a rate equivalent to 200 mg N kg-1. After
adding residues, all the jars were weighed and their weights were recorded.
The soil was then incubated under controlled conditions at 25 ∘C. Soil
moisture was checked/adjusted after every 2 days by weighing the glass
jars and adding the required amount of distilled water when the loss was
greater than 0.05 g.
Soil extraction and analysis
Samples of all 10 treatments were analyzed for total mineral nitrogen (TMN)
as described previously (Abbasi and Khizar, 2012). Initial concentration of TMN
(NH4+-N+NO3--N) on day 0 was determined by
extracting soil samples with 200 mL of 1 M KCl added directly to the flask
immediately after incorporation of each N source. Thereafter, triplicate
samples from each treatment were removed randomly from the incubator at
different incubation timings and extracted by shaking for 1 h with 200 mL of 1 M KCl followed by filtration. The total mineral N of the extract
was determined by using the steam distillation and titration method (Keeney
and Nelson, 1982). Net cumulative N mineralized (NCNM) from different plant-residue treatments was calculated following the method described previously
(Sistani et al., 2008).
Mean biochemical composition of the plant residues used in
the experiment (n= 3).
Plant residues
Plant organs
Total N
Total C
Lignin
Polyphenols
C / N
LG / N
PP / N
LG+PP / N
(treatments)
(LG)
(PP)
g kg-1
Glycine max
shoot
35.2a
447c
11f
13.1f
12.7
0.3
0.4
0.7
Glycine max
root
12.8e
466b
29d
26.9d
36.4
2.3
2.1
4.4
Zea mays
shoot
9.6f
472ab
41b
29.5cd
49.2
4.3
3.1
7.3
Zea mays
root
4.0g
486a
48a
31.4c
121.5
12.0
7.9
19.9
Trifolium repens
shoot
27.4b
397g
13f
18.0e
14.4
0.4
0.6
1.1
Trifolium repens
root
16.0d
423de
21e
20.2e
26.4
1.3
1.2
2.5
Populus euramericana
leaves
20.8c
435cd
34c
53.8a
20.9
1.6
2.6
4.2
Robinia pseudoacacia
leaves
33.3a
404fg
28d
32.3c
12.1
0.8
1.0
1.8
Elaeagnus umbellata
leaves
34.7a
418ef
32cd
38.7b
12.1
0.9
1.1
2.0
LSD (p≤ 0.05)
–
3.14
14.16
4.53
3.77
–
–
–
–
Note: different letters in each column show significant differences among
treatments with p≤ 0.05
Statistical analysis
All data were statistically analyzed by multifactorial analysis of variance using the software package MSTATC Version 3.1 (1990).
Least-significant differences (LSD) were used as a post hoc test to indicate
significant variations within the values of either treatments or time
intervals. Correlation (r) between initial quality characteristics of the
plant residues (total nitrogen, LG, PP and their ratios) and net N mineralization and
the correlation among quality traits were also conducted using SPSS
Statistics version 20.0 for Mac (IBM Corp., 2011). A probability level of p≤ 0.05 was considered significant (Steel and Torrie, 1980).
Results and discussion
Chemical composition of the residues – residue quality
A significant difference (p≤ 0.05) among different residue treatments was
observed for different components of the plant residues presented in Table 2. The total N ranged from a minimum of 4.0 to a
maximum of 35.2 g kg-1. Shoots of Glycine max and leaves of Robinia pseudoacacia and Elaeagnus umbellata displayed the highest N compared
to the remaining treatments (Table 2). The total C contents varied between
397 g kg-1 in the Trifolium repens shoot and a maximum of 486 g kg-1 in the Zea mays root.
Zea mays (both shoot and root) displayed the highest C contents compared to the
remaining plant-residue treatments. The C : N showed a similar trend
recorded for residue C content. The LG content varied between a minimum of
11 g kg-1 in the Glycine max shoot and a maximum of 48 g kg-1 in the Zea mays roots.
Similarly, a minimum PP content (13.1 g kg-1) was recorded in the
Glycine max shoot while a maximum PP (52.8 g kg-1) was found in the Populus euramericana leaves. The
LG / N, PP / N and LG+PP / N ratios were highest in the Zea mays root while the lowest
values were recorded in the Glycine max shoot. Generally, total N contents of the
legume residues were higher compared to the non-legumes. Similarities could
be observed between the same organs of the different species, i.e., all the
roots were characterized by high C, LG and PP contents and lower N
concentration. Leaves were particularly rich in PP and total N. The
differences in the concentration of quality characteristics of residues
according to plant components, i.e., shoot, root and leaves, have been reported
previously (Abiven et al., 2005; Nourbakhsh and Dick, 2005). It has been
reported that high lignin content in root was due to the presence of suberin in
the roots and its ability to form complex barriers when associated with
lignin (Abiven et al., 2005). Plant residues used in this study provided a
wide range of contrasted chemical composition and significant variation in
quality characteristics because of the difference in (i) type of species,
i.e., leguminous and non-leguminous, trees and crops, and (ii) plant
components/organs, i.e., shoot, root and leaves.
Mean changes in the concentration of total mineral N of
a soil amended with different plant residues and incubated at 25 ∘C
under controlled laboratory conditions during a 120-day period (n= 3).
Days after plant-residue addition
Treatments
0
7
14
21
28
42
60
80
100
120
LSD (p≤ 0.05)
mg N kg-1 soil
Control
13.7
13.9
12.9
17.1
30.9
65.9
63.1
75.6
77.7
51.7
2.88
T1
14.8
39.2
49.2
76.8
96.7
158.1
165.2
174.1
188.7
160.9
7.90
T2
13.7
8.1
5.2
8.3
11.8
13.8
28.4
50.4
49.4
27.7
8.15
T3
13.7
7.4
6.2
6.9
10.5
23.1
21.2
36.1
46.7
21.0
5.34
T4
14.3
7.4
9.4
7.7
8.8
15.3
22.2
21.4
32.4
26.4
4.30
T5
14.1
19.0
21.6
55.5
62.5
86.8
127.6
150.8
145.8
93.3
7.31
T6
15.5
8.2
5.2
23.9
34.0
85.3
98.0
149.9
130.2
85.8
9.46
T7
13.0
5.7
4.1
8.6
22.6
55.5
73.1
106.8
87.3
66.9
8.39
T8
13.9
7.4
9.2
23.6
46.6
91.3
111.0
138.9
127.8
93.7
7.83
T9
12.9
9.4
14.5
25.3
51.1
80.1
92.7
140.0
116.4
93.5
6.88
LSD (p≤ 0.05)
2.43
4.77
3.12
5.11
7.63
8.23
6.87
9.23
8.27
7.34
T0 is the control; T1 is Glycine max shoot, T2 is Glycine max root; T3 is Zea mays
shoot, T4 is Z. mays root; T5 is Trifolium repens shoot; T6 is Trifolium repens root; T7 are Populus euramericana leaves;
T8 are Robinia pseudoacacia leaves; T9 are Elaeagnus umbellata leaves. LSD represents
the least significant difference (p≤ 0.05) among incubation periods
(within rows) and among the treatments (within column).
Nitrogen mineralization
Analysis of variance showed that N mineralization was significantly (p≤ 0.05) affected by the treatments and the incubation timings, while the
interaction between the treatments and the timings was also significant.
Results indicated that the control soil without any amendment released a
maximum of 77.7 mg N kg-1 on day 100 compared to 13.7 mg kg-1 at
the start, showing a substantial release of N into mineral N pool (Table 3).
Expressed as the total N initially present, the net N mineralized during the
incubation was 14 %. The mineralization of native soil N observed here
was in accordance with our previous study where a maximum of 90 mg kg-1
mineral N was released from the control soil, representing 16 % of the
initial N of the soil (Abbasi and Khizar, 2012). Among different plant
materials added, the legumes, i.e., the shoot of Glycine max and shoot and root of
Trifolium repens, exhibited significantly higher TMN compared to the non-legumes. The maximum
TMN released from these amendments varied between 150 and 189 mg kg-1. The mean values indicated that these legumes were collectively
able to release 85 mg N kg-1 compared to 20 mg kg-1 by maize and
58 mg N kg-1 by leaves of the non-legumes trees. As expected, the plant
organs also affected N mineralization and, in general, roots displayed
significantly lower TMN compared to the shoot and leaves. Incorporation of
Glycine max root and Zea mays shoot and root resulted in a constant decrease in TMN, and the
maximum values ranged between 32 and 49 mg kg-1 compared to 78 mg kg-1 in the control treatment. However, after initial
negative values until day 14 and 21, leaves of Populus euramericana, Robinia pseudoacacia and Elaeagnus umbellata continuously increased
TMN until reaching between 107 and 140 mg kg-1 (highest values).
Net cumulative N mineralized from the added plant
residues at different incubation periods. Legend: T1 is Glycine max shoot, T2 is Glycine max root; T3 is Zea mays shoot,
T4 is Zea mays
root; T5 is Trifolium repens shoot; T6 is Trifolium repens root; T7 are Populus euramericana leaves;
T8 are Robinia pseudoacacia leaves; T9 are Elaeagnus umbellata
leaves.
Net cumulative N mineralization
Nitrogen mineralization of added plant residues was determined on the basis
of net cumulative N mineralized. The N mineralization from Glycine max and
Trifolium repens shoot showed positive values throughout the incubation, ranging from 24
to 110 mg kg-1 for Glycine max and 5 to 75 mg kg-1 for Trifolium repens (Fig. 1). Considering
the NCNM at the end day 120, the net N mineralized as percentage of total N
applied from Glycine max and Trifolium repens shoot was 54 and 21 %, respectively. The percent of
N mineralized from Glycine max shoot had been reported previously and ranged
from
39 to 43 % of applied N residues (Nakhone and Tabatabai, 2008). However, the NCNM from Glycine max roots, Zea mays shoot and Zea mays roots exhibited negative
values throughout the incubation, indicating net immobilization. Among the
three residues, Zea mays roots displayed higher negative values leading to higher
immobilization. Roots of Glycine max and leaves of Populus euramericana, Robinia pseudoacacia and Elaeagnus umbellata showed four phases of
mineralization–immobilization turnover: initial negative values from
days 7 to 21, slow mineralization from days 21 to 60, a rapid mineralization
between days 60 and 80 and a decline in net between days 100 and 120. The net N
mineralized as percentage of total N applied from roots of Glycine max and leaves of Populus euramericana, Robinia pseudoacacia and
Elaeagnus umbellata was 16, 8, 21 and 21 %, respectively. Net nitrogen mineralization (% of
added N) from different organic materials during 110 days of incubation was
in the range of -35 % in Triticum aestivum (wheat) residues to 81 % in Trifolium repens (white
clover) residues (Kumar and Goh, 2003). Similarly, a 44, 38 and 35 % of N
added had been released from the leaves of peanut, pigeon pea and hairy
indigo, respectively (Thippayarugs et al., 2008).
The mineralization–immobilization turnover of added
plant residues representing three phases during 120 days incubation.
All legumes (except Glycine max root) exhibited the highest NCNM (average 30 % of
added plant N residues) compared to non-legumes (17 %). Similarly, the
cereal crop Zea mays shoot and root exhibited net immobilization compared to net
mineralization observed in the legumes and tree leaves. The plant components
also showed variation in NCNM. For example, shoots of Glycine max and Trifolium repens mineralized an
average of 74 mg N kg-1 compared to 4 mg N kg-1 from the roots.
Likewise, leaves of forest trees showed higher NCNM compared to the roots of
legumes and non-legumes crop.
The shoots of Glycine max and Trifolium repens exhibited the highest NCNM without any negative value during
incubation because of high N concentration and a low C / N ratio. However, it is
interesting to note that the total N concentration of the leaves of Robinia pseudoacacia and
Elaeagnus umbellata was higher and C / N ratio was lower compared to the Trifolium repens shoot, but the net
mineralization (averaged) of Trifolium repens shoot was higher (47 and 58 %) compared to
the leaves of Robinia pseudoacacia and Elaeagnus umbellata, respectively. The low mineralization in leaves in spite
of high N content and low C / N ratio was attributed to higher concentration
of LG, PP, LG / N, PP / N and LG+PP / N. These results demonstrated the effect of
other factors in addition to total N and C / N ratio on plant-residue
decomposition and N mineralization kinetics. As indicated in a previous
study (Trinsoutrot et al., 2000), the net accumulation (whether positive or
negative) of mineral N in soil during decomposition of organic residues is
directly related to the residue N content. However, our results clearly
indicated that N was not the only factor affecting the mineralization of
added residues; some additional quality characteristics also influenced
MIT of plant residues. Likewise, the total N content and C / N ratio of the
leaves of Robinia pseudoacacia and Elaeagnus umbellata were on par with Glycine max shoot but the net mineralization of Glycine max shoot
was 3-fold higher. It had been reported that organic materials with similar
C / N ratios may mineralize different amounts of N because of differences in
composition that are not reflected by the C / N ratio (e.g., different lignin
contents) (Mohanty et al., 2011).
Similarly, roots of Glycine max and Zea mays showed net immobilization while roots of Trifolium repens displayed
fast decomposition and net N-release pattern. This discrepancy in root MIT
was mainly due to high N concentration, low C / N ratio and low LG and PP
contents of the roots of Trifolium repens. The N turnover shown by Trifolium repens roots confirmed the
strong below-ground N dynamics and residual effect of Trifolium repens when grown in the soil.
Mineralization trend of added plant residues across timings
(a) and soil organic matter (SOM) turnover of different plant residues
recorded at the start of the experiment on day 0 and at the end of
incubation on day 120 (b). The hanging bar on each major line represents
the LSD (p≤ 0.05) between incubation periods and between each treatment.
Pearson linear correlation coefficients between initial
quality characteristics of the plant residues and net N mineralization and
correlation within plant-quality characteristics.
Nmin
TN
LG
PP
C : N
LG : N
PP : N
TN
0.89**
LG
-0.84**
-0.66*
PP
-0.42ns
-0.10ns
0.62*
C : N
-0.69*
-0.80**
0.73*
0.07ns
LG : N
-0.68*
-0.76**
0.77**
0.14ns
0.99**
PP : N
-0.73*
-0.77**
0.82**
0.29ns
0.99**
0.98**
LG + PP : N
-0.70*
-0.76**
0.79**
0.19ns
0.99**
1.00**
0.99**
** and * represent significant levels at p≤0.01 and p≤0.05, respectively; the
correlation significance and non-significance level was calculated at
p≤ 0.05. The abbreviations represent N mineralization (Nmin,), total nitrogen (TN), lignin (LG) and polyphenols (PP).
Among the leaves of different trees tested, leaves of Robinia pseudoacacia and Elaeagnus umbellata released a
substantial amount of N into the mineral N pool. Leaf residues have been
described as high-quality litter materials in terms of high N and low lignin
contents (Thippayarugs et al., 2008) and have been found to decompose easily
and release mineral N substantially (Mtambanengwe and Kirchmann, 1995) as
observed in our study. However, Populus euramericana leaves exhibited higher net immobilization
(for a longer period) and lower net mineralization. The variation was again
due to disparity in the biochemical composition. The low N content, high C / N
ratio and high PP content may have been largely responsible for the slow
decomposition and low net mineralization of Populus euramericana leaves. These results inferred
that the same plant components may not necessarily show similar
decomposition and mineralization turnover because of the variation in
biochemical composition.
In general, the added plant residues increased organic matter stock in soil
and
thereby increased N mineralization and N transformation processes in soil.
Plant or crop residues, when added or incorporated into the soil, increase
the organic matter (avoid the climate change), reduce the soil and water
losses and increase the biological activity in the soils. Such changes bring
a substantial improvement in the physical, chemical and microbial
properties of soil and eventually in the soil quality (Giménez
Morera et al., 2010; Jiménez et al., 2013; Zhao et al., 2013; Singh et
al., 2014; Prats et al., 2014)
Pattern and trend of N mineralization
The patterns of N mineralization varied among plant residues and plant
components. After incorporation into soil and during incubation, the added
residues exhibited three main patterns of cumulative net mineralization
(Fig. 2): (i) a pattern of the continuous and rapid release of net N
throughout the incubation without showing any negative value indicating net
mineralization, shown by the Glycine max shoot and
Trifolium repens shoot; (ii) a pattern shown by the Trifolium repens roots
and Populus euramericana, Robinia pseudoacacia and Elaeagnus umbellata leaves indicated
initial negative values of net cumulative immobilization for variable
periods followed by slow and then a rapid release of N, indicating
immobilization–mineralization turnover; (iii) a pattern of continuous
negative values throughout the incubation, indicating net N immobilization as
seen in the case of the Glycine max root and the Zea mays shoot and root. The MIT and
N-release patterns by plant residues observed here were in accordance with
those
reported previously in both leguminous and non-leguminous plant residues (Kumar
and Goh, 2003).
The N mineralization trend over time showed wide variation (Fig. 3a).
These results highlighted the time taken for releasing N into the mineral N pool
by the added plant residues. Results showed an initial lag phase where most of
the applied residues endured immobilization with little mineralization; only
the Glycine max and Trifolium repens shoots showed mineralization during 0 to 21 days of incubation. The rapid
mineralization phase occurred from day 28 to day 80. Thereafter a declining
phase of mineralization started toward the later part of the incubation from day
100 to day 120.
Changes in soil organic matter
In order to examine the changes in soil organic matter (SOM) in response to
added plant residues, a comparison between the SOM at the start of day 0 and the end of incubation on day 120 has been shown (Fig. 3b). Soil organic matter contents of all the treatments recorded on day 120
were lower than those recorded on day 0. The unaccounted SOM ranged between 32
and 67 % compared to that recorded on day 0. The decreasing trend of SOM
was substantially higher for the treatments showing mineralization
(54–67 %) compared to those showing immobilization (32–38 %). By the
end of day 120, the loss of SOM was in the following order: Trifolium repens shoot > Elaeagnus umbellata leave > Trifolium repens root = Robinia pseudoacacia
leaves > Populus euramericana > Glycine max shoot > Zea mays shoot > Zea mays root = Glycine max root.
The SOM turnover observed here coincided with net mineralization. In the
initial lag phase when mineralization was either very low or displayed
negative values, on average only 8 % of the initial SOM had been utilized
(7–21 days). The SOM utilization during days 28–80 when mineralization was
rapid was 31 % of the initial amount, while 43 % of initial SOM was
utilized in the later part of incubation (between days 100 and 120) when
mineralization start showing a declining trend.
Relationship between cumulative N mineralization and residue-quality characteristics
Results of the study showed highly significant positive correlation between
N mineralization and plant-residue N concentrations (r= 0.89;
p≤ 0.01) (Table 4). In contrast, a negative significant correlations
existed between net cumulative N mineralized and LG (r=-0.84;
p≤ 0.01), NCNM and C / N ratio (r=-0.69; p≤ 0.05), NCNM and LG / N
ratio (r= –0.68; p≤ 0.05), NCNM and PP/N ratio (r=-0.73; p≤ 0.05) and NCNM and LG + PP / N ratio (r=-0.70; p≤ 0.05). The correlation
between N mineralization and PP was nonsignificant with p≤ 0.05. The
significant positive correlation between net rates of N mineralization and
residue N concentration observed is consistent with other studies
(Nourbakhsh and Dick, 2005; Vahdat et al., 2011). It has been reported that
N availability may control the decomposition of plant residues, particularly
those with low N content such as cereals, when the N requirements of the soil
decomposers are not met by the residue or soil N contents (Vahdat et al.,
2011). A negative correlation was also observed between net N mineralization
and C / N ratio of the plant materials. Previously, total N contents and C / N
ratio were considered adequate for predicting the net N mineralization of
crop residue. However, the latest studies, including the present work,
highlight the role of other quality characteristics, including LG and PP, that
affect net mineralization of plant residues. The closer relationship
between net mineralization and residue lignin contents (r=-0.84;
p≤ 0.01) than that of the C / N ratio (r=-0.69; p≤ 0.05) recorded in
this study was in accordance with previous findings (Vahdat et al., 2011).
The highly significant positive correlation between net N mineralization and
the residue N content (r= 0.89; p≤ 0.01) confirms the previous results
(Nourbakhsh and Dick, 2005; Vahdat et al., 2011), indicating that residue N
concentration can be considered a better tool to predict mineralization of
added organic residues compared to the C / N ratio.