Introduction
Soil water is
essential in successful crop growth and production due to its role in
maintaining cell turgidity, opening and closing of stomata, temperature
regulation, thermal conductivity, nutrient cycling, mineralization, diffusion
and availability of soil nutrients to crops. Australia has uneven rainfall
distribution, mostly concentrated in winter with the occurrence of long spells
of hot and dry weather, which is imposing stresses on plants (Livesley et
al. 2004). In such conditions water availability for successful crops
production becomes more important as many of the agricultural lands are located
in arid to semi-arid region. The drying/wetting influence on the soil
microbiology and nutrients release has been extensively studied. Drying and
rewetting may result in significant lysis of microbial biomass and may affect
microbial composition directly or indirectly. Alterations in bacterial community composition
induced by drying-rewetting may be the reason of the variations in C
mineralization rates (Fierer et al. 2003). Drying and wetting cycles
affect the chemical properties of soil (Williams and Xia 2009), enhances rates
of CO2 production which may persist for more than 15 days after
wetting (Beare et al. 2009) and this response of mineralization to
wetting cycles varies greatly among various studies (Butterly et al.
2010). Soil organic matter contents and texture are important factors affecting
biogenic gas production (CO2 and N2O) during dry wet
cycles (Kirk et al. 2013). Dry/wet events change the equilibrium of soil
C and N transformations relative to the unstressed soil. Soil water contents
determine microbial activity that plays major role in nutrients mineralization
(Paul et al. 2003).
Legumes are rich
natural source of plant nutrients and play an important role in improving soil
physical health (aggregation) and sustaining natural ecosystem by supplying
nutrients like P (Kabir and Koide 2002). Plant residues induced modifications
in soil phosphorus pools and concentration of soil P varies with time, but
there is still need to verify either these changes are soil induced or
applicable in different scenarios (Alamgir et al. 2012). Plant residues
vary regarding their nutrients composition and decomposition rate and the
decomposition can also be affected by environmental factors such as moisture
and temperature. Much work has been done on nitrogen mineralization and residue
quality but research efforts on P availability during decomposition process are
few. Plant residue addition may start mobilization of P depending on C:N ratio
of the added residues and most of the time, the dividing line between
immobilization and mineralization is 20:1. P contents of organic residues
affect decomposition and residues with high P contents decompose more rapidly
(Achat et al. 2012) because higher P contents enhance microbial
activities due to easy fulfilment of their nutrient requirements.
Legumes are much
better in N and P uptake from the soil and are therefore rich sources of these
nutrients due to which decomposition rates of these residues are higher
compared to cereal residues (Nuruzzaman et al. 2005). Under existing
water availability conditions, climate predictions are indicating very severe
droughts in most arid and semi-arid parts of world (Iglesias et al. 2007). Water scarcity is expected to
impose severe limitations on microbial processes, mineralization and C and N
dynamics (Schimel et al. 2007). Drought was shown to cause higher
microbial C:N ratio indicating a shift towards more fungal dominated microbial
community capable of decomposing more complex compounds (Jensen et al.
2003). It is projected that future climate change will impose more pressure on
poor nutrient ecosystems. An increase in seasonal variations and precipitation
patterns may largely affect the microbial activity and nutrients immobilization
(Buckeridge et al. 2013) and thus will affect the nutrients availability
to plants (Michelsen et al. 1999). Under such a scenario, microbial nutrient
mobilization becomes crucial in nutrient-deficient systems (Bargaz et al.
2018). The availability of phosphorus is much more complex and tricky as more
than 80% of applied fertilizer is immediately fixed by iron and/or aluminium in
acidic soils and by calcium in calcareous soils. Phosphatic fertilizers are
manufactured using rock phosphate and its reserves are decreasing. Therefore,
any management system that plays a role in mobilization of phosphorus and other
nutrients in the soil after residue decomposition, will help towards
sustainability in arid and semi-arid cropping systems. The objectives of this
study were to:
i.
determine the role
of alternate wetting and drying in nutrients mobilization as a function of
residue addition and soil water condition.
ii.
evaluate the
effect of residue addition on nutrients availability and wheat growth without
addition of chemical fertilizers.
Materials and
Methods
%20889-898_files/image002.png)
Fig. 1: Percentage of water holding capacity as a
function of time in the control soil without amendment, soil amended with
kikuyu grass residue and soil amended with faba bean residue
Soil preparation
Experimental soil was collected in Spring from 0–0.1 m
depth from Urrbrae permanent pasture (longitude 138˚38’3.2” E Latitude
34˚58’02” S) South Australia. This site is situated in semi-arid region,
with Mediterranean climate, wet and cold winter; and hot and dry summer with
scattered rainfall. The soil used for this experiment was a silt loam (8% sand,
70% silt, and 22% clay) according to FAO classification (FAO 2001). The soil
had maximum water holding capacity (WHC) of 0.34 kg kg-1, pH (1:5)
of 5.6; EC (1:5) of 0.1 dS m-1; total organic carbon of 31 g kg-1;
total organic nitrogen of 1500 mg kg-1; and a bulk density of 1.3 Mg
m-3.
Experiment set up
Soil was air dried and sieved by passing through a 2 mm
sieve. In the first study phase, soil was pre-incubated for 10 days at 50% of
water holding capacity (WHC) at room temperature for microbial activity
activation. The
soil water maintained by weighing the pots daily and adding back the corresponding amounts of water lost
to evaporation. Residue hereafter referred to as amendment were air-dried,
ground and passed through a 2 mm sieve to remove very fine fractions. Two
amendment treatments applied were dried kikuyu grass (Pennisetum clandestinum L.)
residue (C 38.1%, N 1.9%, P 0.42%, C:N 20.1 and C:P 90.7) and dried faba bean
(Vicia
faba) residue (C 40.9%, N 0.68%, P 0.074%, C:N 60.2 and C:P 552.7),
and the third treatment was the control (CO), with no amendment applied. 0.35
kg soil was filled in each plastic pot having 0.5 kg capacity and amendments
were uniformly mixed with the soil at a rate of 1% on weight basis (10 g
amendment in 1 kg of soil).
Different soil water treatments applied were 20 days
moist (20 M), 5 days moist followed by 15 days dry (15 D), 10 days moist
followed by 10 days dry (10 D) and 20 days dry (20 D). All the water treatments
were maintained at 50% of the WHC. The pots were maintained at target WHC by
weighing and adding water daily. The unamended control was maintained at 50% of
WHC throughout the study period. All pots were incubated for 20 days at mean
room temperature of 20 degrees.
In the 2nd phase of the study, which was 21
days after water and amendment treatments applications, all pots were again
brought to 50% of WHC. Seeds of wheat (Triticum
aestivum L.) were pre-soaked, and eight pre-germinated seeds were sown in
each pot and thinned after establishment to maintain four plants per pot. No
inorganic fertilizers were applied to wheat. Wheat plants were harvested 25
days after planting (DAP). During the whole wheat growth period, soil was kept
at 50% of WHC. The timeline sequence for this experiment can be summarized as
follows: i.) initial 10 days of wetting the soil and maintaining the soil water
at 50% of WHC in pots used for this experiment, ii) 20 days of application of
water treatments in the pots, after soil amendment application and iii) 25 days
of growing wheat as an index crop in the pots. This gave a total of 55 days for
this experiment.
Plant measurements and analysis
Measurements taken from each pot during plant harvest at
25 DAP included the root and shoot biomass. The above ground biomass was
carefully cut at the ground and weighed. After the shoot biomass was harvested,
the soil inside of the pot was broken up to isolate the roots, and the roots
were carefully washed under water and weighed. Harvested wheat biomass were
dried and analysed for total N, P, N/pot and P/pot. Nitrogen was determined
using Kjeldhal method for the tissue digestion, after which N was determined
using a colorimetric procedure (Bradstreet, 1965). Phosphorus was determined by
digesting the tissues samples with nitric acid and H2O2
(1:4), after which the quantitative amount of P was determined by colorimetric
procedure (Hanson 1950).
Soil analysis
Soil samples were collected from pots using small cores
at 5, 10, 15, 20 and 45 days after water treatment (DAWT) and were preserved in
cold storage to minimize chemical changes. Soil samples were analysed for
inorganic nitrogen (IN), % ammonium of inorganic nitrogen (%NH4+-N),
available P (AP) at 5, 10, 15, 20 and 45 DAWT. It is worth noting that four of
the samples taken at 5, 10, 15 and 20 DAWT were before the wheat plants were
planted into the pots while the last measurement (45 DAWT) took place at the
harvest of wheat plants. Soil NH4-N were determined by using method
described by Cavagnaro et al. (2006), modification of Miranda et al.
(2001) and Willis et al. (1996). Soil P was determined from extract
collected using anion-exchange resin membranes (Kouno et al. 1995) and P
determination was done calorimetrically (Murphy and Riley 1962).
Statistical analysis
Statistical analysis was performed on the measurements
using the ANOVA procedure in GenStat 15th edition (Payne 2008). Mean separation was performed
using Turkey test (P ≤ 0.05)
after a significant F-ratio.
Results
Soil and water release characteristics
Soil used was a silt loam, with relatively high water
holding capacity. As expected, the soil water was depleted as the soil dried
down over 20 DAWT (Fig. 1). Generally, the water dropped from about 50% of WHC
to under 15% of WHC over a period of 20 days. At 20 DAWT, the KK amendment
(11.7% of WHC) and the control treatment (10.9% of WHC) had a slightly higher
percentage of WHC than the FB residue amendment (8.5% of WHC) [Fig. 1].
Soil inorganic nitrogen
Table 1 presents the average of the soil inorganic
nitrogen (IN) across different moisture treatments. The soil IN decreased over
time with maximum values observed at 5 DAWT and the minimum values at 45 DAWT
(Table 1). At 5 and 10 DAWT, soil water effect was not significant, while its
effect was significant at 15, 20 and 45 DAWT (Table 1). Highest IN was found in
soil water treatment of 15D at 15 DAWT, while at 20 and 45 DAWT, highest IN was
observed in 10D soil water treatment. IN was in sufficient range in 15 D at 15
DAWT while was deficient in 10 D at 20 and 45 DAWT. Amendments affected soil IN
significantly throughout the incubation period, with the highest values found
in KK amended soils, followed by CO, while the least values were observed in FB
amended soils. Across all amendments, the highest value of IN was found at 5
DAWT, which generally decreased with time (Table 1). The interactive effects of
soil water and amendments were also significant at 10, 15 and 20 DAWT, with the
highest values found in KK × 15D at 15 DAWT, while lowest was found in FB × 10D
at 20 DAWT (Fig. 2).
NH4+-N (% of inorganic
nitrogen)
%NH4+-N was significantly affected
by different amendments and soil water treatments throughout the period of crop
growth (Table 2). %NH4+-N contents was highest at 15 DAWT
across all soil water treatments, while 20 M had the lowest %NH4+-N
contents during the incubation period compared to other soil water treatments
(Table 2). Initially higher values of %NH4+-N contents
were found in KK treatment, followed by FB, and the least in CO treatment
(Table 2). For the KK treatment, there was a reduction from 5 to 45 DAWT in %NH4+-N
contents while an increase in %NH4+-N contents for FB was
observed with the highest at 45 DAWT (Table 2). This may be indicative of less
microbial activity that could oxidize NH4-N to NO3-N in
FB treatment due to a higher C:N ratio. The interactive effects of soil water
and amendments were significant, as the highest %NH4+-N
contents were found in FB at 15 D x 45 DAWT and least in CO at 15 D x 20 DAWT
(Fig. 3).
Table 1: Inorganic Nitrogen (N) of soil in unamended
soil (CO) and soil amended with kikuyu (KK) or faba bean (FB) residues
incubated at four soil water treatments before planting: 20 days moist (20 M),
10 days moist+10 days dry (10 D), 5 days moist+15 days dry (15 D), 20 days dry
(10 D) (n=4)
|
|
Inorganic N (mg kg-1) |
||||
|
Water |
5 DAWT* |
10 DAWT |
15 DAWT |
20 DAWT |
45 DAWT |
|
20M |
48.7 |
35.38 |
42.71 b |
21.4 b |
17.69 a |
|
10D |
45.2 |
32.83 |
39.54 c |
26.65 a |
17.89 a |
|
15D |
48.5 |
36.06 |
45.25 a |
21.78 b |
16.69 a |
|
20D |
44.4 |
33.91 |
38.76 c |
22.34 b |
13.94 b |
|
Amendment |
|
|
|
|
|
|
CO |
50.3 b |
34.3 b |
46.54 b |
19.72 b |
6.76 b |
|
KK |
84 a |
64.52 a |
72.04 a |
46.9 a |
38.86 a |
|
FB |
5.8 c |
4.82 c |
6.12 c |
2.51 c |
4.05 c |
|
|
|
|
|
|
|
|
Water |
ns |
ns |
** |
** |
** |
|
Amendment |
** |
** |
** |
** |
* |
|
Water x Amendment |
ns |
** |
** |
** |
ns |
*DAWT – Days after water treatment
%20889-898_files/image004.png)
Fig. 2: Inorganic nitrogen of soil in the unamended
soil (CO) and the soil amended with kikuyu (KK) or faba bean (FB) residues
incubated at four soil water treatments before planting: 20 days moist (20 M),
5 days moist + 15 days dry (15D), 10 days moist + 10 days dry (10 D), 20 days
dry (20 D) (n=4). *DAWT – Days after water treatment
Soil available P (AP)
Overall highest AP was observed in 20 D treatment at 15
DAWT which was 35% higher than at the same level of moisture at 5 DAWT and 59%
higher than at 45 DAWT (Table 3). Soil water effect on AP was significant at 5,
15, 20 and 45 DAWT. AP levels were higher at 15 D, 20 D, 10 D at 5, 10, 15, 20
and 45 DAWT respectively. On the other hand, lower AP levels were observed in
10 D and 20 M at 5, 10, 15, 20 and 45 DAWT (Table 3).
Table 2: Precent ammonium of inorganic N of soil in
unamended soil (CO) and soil amended with kikuyu (KK) or faba bean (FB)
residues incubated at four soil water treatments before planting: 20 days moist
(20M), 10 days moist + 10 days dry (10 D), 5 days moist + 15 days dry (15 D),
20 days dry (10 D) (n=4)
|
|
NH4-N (% of Inorganic N) |
||||
|
Water |
5 DAWT* |
10 DAWT |
15 DAWT |
20 DAWT |
45 DAWT |
|
20M |
33.2 b |
29.69 c |
41.18 c |
15.1 b |
42.9 b |
|
10D |
33.9 b |
35.78 c |
56.18 a |
40.8 a |
46.9 ab |
|
15D |
46.8 a |
38.42 ab |
47.97 b |
23.3 b |
45.5 b |
|
20D |
38.7 b |
42.61 a |
58.44 a |
48.2 a |
51.4 a |
|
Amendment |
|
|
|
|
|
|
CO |
6.7 c |
5.34 c |
27.7 c |
8.1 c |
40.2 b |
|
KK |
58.7 a |
60.49 a |
51.96 b |
48.5 a |
19.7 c |
|
FB |
49.1 b |
44.04 b |
73.17 a |
39 b |
80.2 a |
|
|
|
|
|
|
|
|
Water |
** |
** |
** |
** |
** |
|
Amendment |
** |
** |
** |
** |
* |
|
Water x Amendment |
* |
** |
** |
* |
* |
*DAWT – Days after water treatment
%20889-898_files/image006.png)
Fig. 3: Ammonium nitrogen in the unamended soil
(CO) and the soil amended with kikuyu (KK) or faba bean (FB) residues incubated
at four soil water treatments before planting: 20 days moist (20 M), 5 days
moist + 15 days dry (15 D), 10 days moist + 10 days dry (10 D), 20 days dry (20
D) (n=4). *DAWT – Days after water treatment
In amended soil the AP contents on 5, 15 and 20 DAWT
were highest with KK residues, and lowest with FB residue. Addition of FB
residue did not increase the soil AP contents as compared to that of CO. Higher
values of soil AP were found at 15 DAWT which gradually reduced up to 45 DAWT
(Table 3). KK residue amendment increased soil AP contents from 5 to 20 DAWT,
while there was reduction from 20 to 45 DAWT, which might be due to P uptake by
wheat plants. Regarding interactive effects of soil water and amendments, the
highest values of soil AP were found in KK15D and KK10D at 5, 10, 45 DAWT and
15, 20 DAWT respectively (Fig. 4). All the AP values observed for soil water,
amendments and their interaction effects were in deficient range.
Table 3: Soil available Phosphorus in unamended soil
(CO) and soil amended with kikuyu (KK) or faba bean (FB) residues incubated at
four soil water treatments before planting: 20 days moist (20 M), 10 days
moist+10 days dry (10 D), 5 days moist+15 days dry (15 D), 20 days dry (10 D)
(n=4)
|
|
Soil Available Phosphorus (mg kg-1) |
||||
|
Water |
5 DAWT* |
10 DAWT |
15 DAWT |
20 DAWT |
45 DAWT |
|
20M |
3.38 b |
3.25 |
3.47 b |
3.74 b |
3.13 ab |
|
10D |
3.34 b |
3.25 |
4.27 a |
4.29 a |
2.94 c |
|
15D |
3.65 b |
3.24 |
3.67 b |
3.83 b |
3.27 a |
|
20D |
3.40 b |
3.33 |
4.57 a |
3.84 b |
2.98 bc |
|
Amendment |
|
|
|
|
|
|
CO |
3.39 b |
3.27b |
3.83b |
3.75 b |
2.86 b |
|
KK |
5.28 a |
5.12 a |
6.08 a |
6.13 a |
4.58 a |
|
FB |
1.66 c |
1.42 c |
2.08 c |
1.89 c |
1.80 c |
|
|
|
|
|
|
|
|
Water |
* |
ns |
** |
** |
* |
|
Amendment |
** |
** |
** |
** |
** |
|
Water x Amendment |
* |
ns |
ns |
ns |
ns |
*DAWT – Days after water treatment
%20889-898_files/image008.png)
Fig. 4: Soil available phosphorus in the unamended
soil (CO) and the soil amended with kikuyu (KK) or faba bean (FB) residues
incubated at four soil water treatments before planting: 20 days moist (20 M),
5 days moist + 15 days dry (15 D), 10 days moist + 10 days dry (10 D), 20 days
dry (10 D) (n=4). *DAWT – Days after water treatment
Wheat growth and development
There was no significant increase in shoot weight at
different soil water levels, while residue amendment resulted in significant
differences in shoot weight (Table 4). Amendment with KK increased shoot weight
by 23% compared to the control, while amendment with FB decreased shoot weight
by 275% compared to CO. Higher root biomass was found in 20 D and least in 20 M,
which may be reflecting a negative effect of moisture on root growth and
development due to reduced soil oxygen. Soil amendment had significant effect
on root weight, with the unamended soil having higher root weight than the
amended soils (Fig. 5).
Plant total biomass was significantly affected by soil
water levels with the total biomass of 20 D being the highest, while there was
no significant difference between the total biomass at 15 D, 10 D and 20 M
(Table 4). KK treatment increased biomass by about 4% compared to the CO and by
123% compared to the FB treatment, indicating a negative effect of FB residue
addition on biomass yield.
Root/shoot ratio was the highest at 20 M and the lowest
in 20 D, but effect was not statistically significant. Addition of residue
amendment significantly increased the shoot/root ratio, with KK treatment
having 53% higher ratio than the CO, while the FB treatment resulted in
lowering of shoot/root ratio by 217% compared to the CO. The interaction effect
between moisture levels and amendments was not statistically significant for
the all plant growth parameters measured (Table 4).
Table 4: Shoot, root and total plant dry weight per
pot and shoot/root ratio of wheat in unamended soil (CO) and soil amended with
kikuyu (KK) or faba bean (FB) residues incubated at four soil water treatments
before planting: 20 days moist (20 M), 10 days moist+10 days dry (10 D), 5 days
moist+15 days dry (15 D), 20 days dry (10 D) (n=4)
|
|
Shoot |
Root |
Total Biomass |
Shoot/Root |
|
|
(grams dry weight per pot) |
|||
|
Water |
|
|||
|
20M |
0.25 |
0.12a |
0.37a |
2.07 |
|
10D |
0.24 |
0.13ab |
0.37a |
1.85 |
|
15D |
0.254 |
0.15ab |
0.40ab |
1.83 |
|
20D |
0.26 |
0.16b |
0.42b |
1.71 |
|
Amendment |
|
|
|
|
|
CO |
0.30b |
0.16b |
0.46b |
1.93b |
|
KK |
0.37c |
0.12a |
0.49b |
2.98c |
|
FB |
0.08a |
0.13a |
0.22a |
0.68a |
|
|
|
|
|
|
|
Water |
ns |
* |
* |
ns |
|
Amendment |
** |
** |
** |
** |
|
Water x Amendment |
ns |
ns |
ns |
ns |
%20889-898_files/image010.png)
Fig. 5: Wheat shoot, root and total plant dry
weight per pot and shoot/root ratio in the unamended soil (CO) and the soil
amended with kikuyu (KK) or faba bean (FB) residues incubated at four soil
water treatments before planting: 20 days moist (20 M), 5 days moist + 15 days
dry (15 D), 10 days moist + 10 days dry (10 D), 20 days dry (10 D) (n=4)
Nutrient uptake by the crop
Soil water levels did not have significant effect on the
N and P concentrations in the plant tissues, but the effect of the amendment
was significant at 1% level. Interaction effects of soil water and amendment
were not significant. Generally, the KK treatment had the highest tissue N and
P followed by the CO, while the FB treatment had the lowest N and P
concentrations (Table 5).
Discussion
This study showed that nutrient mineralization was
influenced by the type of residue amendment and to some extent by the length of
time that these amendments were in the soil and treated to different soil water
levels. The response to amendment was well pronounced and dominant, while soil
water level effects were less significant in comparison to the amendment
effects.
Residues were amended under four soil water levels. At
the very first day all the samples were at the same water level and after that,
there was gradual reduction in moisture contents with time. The rate of
reduction of the soil water appeared to be similar under all amendments up till
day 5 before curve for each amendments started separating (Fig. 1).
Soil water levels
at 45 DAWT affected soil IN significantly with the highest amount found in 15 D
and lowest in 20 D. It is generally observed that after rewetting of soil N
mineralization occurs in short-term and rate is often higher than that of the
moist control (Borken and Matzner 2008). Plant growth can be affected not only
by drying/wetting events but by the moist period also (Shi and Marschner 2014).
In general, the highest IN value was found in 10D treatment at 5 DAWT. However,
there was gradual reduction in IN content with time, possibly due to microbial
activities utilizing the nitrogen for metabolism and also due to uptake by the
wheat plants growing for almost for 25 days in the pots. The maximum uptake of
IN was observed in treatments where the soil was kept dry for 20 days after initial
wetting to field capacity. This repeated wetting/drying resulted in more
availability and mobilization of IN and the highest level observed at 15 and 20
DAWT compared with all other intervals of moisture treatments.
Table 4: Shoot, root and total plant dry weight per
pot and shoot/root ratio of wheat in unamended soil (CO) and soil amended with
kikuyu (KK) or faba bean (FB) residues incubated at four soil water treatments
before planting: 20 days moist (20 M), 10 days moist+10 days dry (10 D), 5 days
moist+15 days dry (15 D), 20 days dry (10 D) (n=4)
|
|
Shoot |
Root |
Total Biomass |
Shoot/Root |
|
|
(grams dry weight per pot) |
|||
|
Water |
|
|||
|
20M |
0.25 |
0.12a |
0.37a |
2.07 |
|
10D |
0.24 |
0.13ab |
0.37a |
1.85 |
|
15D |
0.254 |
0.15ab |
0.40ab |
1.83 |
|
20D |
0.26 |
0.16b |
0.42b |
1.71 |
|
Amendment |
|
|
|
|
|
CO |
0.30b |
0.16b |
0.46b |
1.93b |
|
KK |
0.37c |
0.12a |
0.49b |
2.98c |
|
FB |
0.08a |
0.13a |
0.22a |
0.68a |
|
|
|
|
|
|
|
Water |
ns |
* |
* |
ns |
|
Amendment |
** |
** |
** |
** |
|
Water x Amendment |
ns |
ns |
ns |
ns |
Table 5: Tissue nitrogen and phosphorus contents in
soil amended with kikuyu (KK) and faba bean (FB) residues compared with the
control (CO) unamended soil
|
Treatments |
N g/kg |
P g/kg |
|
Water* |
|
|
|
20M |
30.42 |
4.18 |
|
10D |
31.43 |
4.16 |
|
15D |
28.77 |
3.94 |
|
20D |
27.92 |
4.23 |
|
Amendment |
|
|
|
CO |
30.17b |
4.39b |
|
KK |
40.04c |
5.10c |
|
FB |
18.7a |
2.89a |
|
|
|
|
|
Water |
ns |
ns |
|
Amendment |
** |
** |
|
Water x Amendment |
ns |
ns |
*Soil
incubated at four soil water treatments before planting: 20 days moist (20 M),
10 days moist+10 days dry (10 D), 5 days moist+15 days dry (15 D), 20 days dry
(10 D)
%20889-898_files/image011.png)
Fig. 5: Wheat shoot, root and total plant dry
weight per pot and shoot/root ratio in the unamended soil (CO) and the soil
amended with kikuyu (KK) or faba bean (FB) residues incubated at four soil
water treatments before planting: 20 days moist (20 M), 5 days moist + 15 days
dry (15 D), 10 days moist + 10 days dry (10 D), 20 days dry (10 D) (n=4)
Effect of amendments as a function of moisture level
treatment was pronounced on IN, with the highest value observed in KK and
lowest in FB at 5 and 45 DAWT. This might be due to more mineralization in the
KK treatment with lower C:N ratio (20:1), which facilitated a more rapid
microbial decomposition compared to FB treatment with medium C:N (60:1) ratio,
with a more resistant residue to decomposition. Lower IN in FB treatment
compared with the control may be due to the immobilization of the IN due to
resistant insufficient nitrogen FB residue. In addition, poor wheat growth
observed in FB treatment compared to the CO treatment may have been due to the
immobilization of IN due to microbial decomposition of FB residue. Previous
studies have indicated that residues having narrow C:N ratio decompose easily
and can result in nutrient mineralization and with wide C:N ratio result in
immobilization (Hadas et al. 2004). The gradual reduction of IN with
time starting from 5 to 45 DAWT may be due to combined effect of microbial
immobilization and crop uptake. Microbial immobilization would likely be
stronger at the initial phase of crop growth while crop uptake would become
more pronounced as the crop nutrient requirement increases with growth and
development.
Opposite trend was almost observed regarding %NH4+-N
as lower values were found during initial days, and gradually increasing with
time. In general, higher %NH4+-N were found under 20D
treatment with the highest value at 15 DAWT. Higher %NH4+-N
observed in FB treatment compared to KK at 45 DAWT might be due to the fact
that the IN content under low C:N ratio decreased with time, as the N pool
gradually became exhausted over time (Kamkar et al. 2014).
Soil AP significantly varied during all intervals except
at 10 DAWT. Drying/wetting might have resulted in the shattering of aggregates
due to increase in internal pressure upon rewetting (Borken and Matzner 2008)
resulting in higher P availability (Blackwell et al. 2013). Soil AP
contents increased with time, probably be due to P release from crop residues,
and the highest values were observed at 20 DAWT in 10 D treatment. After crop
establishment in the pots, there was significant reduction in soil AP contents,
possibly due to crop uptake. Maximum soil AP contents were found in KK amended
soil at 20 DAWT showing a good release of phosphorus due to low C:N ratio.
Lowest soil AP contents were found in FB amended soils possibly due immobilization
of phosphorus (Braschi et al. 2003).
Plant shoot growth was not significantly affected by
soil water treatments. However, plant root growth was significantly affected by
soil water treatments. With the root growth occurring in the order 20 D >15 D
> 10 D > 20 M, there was an indication that root growth decreased with
increasing number of moist days of incubation after amendment application,
because exposure to drying resulted in increased nutrients release from residue
and thus availability to crops. This might have led to root proliferation
resulting in more aggressive root growth in 20 D treatment. Plant total biomass
was significantly affected by soil water and soil amendments. Highest plant
total biomass observed in 20 D and the least in 20 M, clearly indicated
positive effects of wetting and drying cycles on nutrients mineralization and
thus availability to crops resulting in higher total biomass (Ouyang and Li 2013). Enhanced P uptake by plant after
drying and wetting cycles in a bioassay with different preceding moisture
regimes during plant growth was reported by Bunemann et al. (2013).
Amendment
application also affected total biomass significantly, with the highest total biomass found
in KK amendment followed by CO and least biomass was under FB amendment. This
might be due to the higher nutrient mineralization from KK as evident from soil
IN and AP results.
Soil water
treatments effect was not significant for shoot/root ratio, it was significant
for the soil amendment effect. KK increased this ratio by about 4.4 times
compared to FB and by about 1.5 times than that of the CO. This showed the
higher nutrient mineralization due to low C:N ratio of KK amendment increased
microbial activities, ultimately leading to a better crop growth. Similar to
observations in this study, higher wheat yields were observed in soil amended
with plant residue having low C:N ratio (Bunemann et al. 2013). Nitrogen
mineralization depends upon C:N ratio of added residues and residues with
narrow C:N ratio (high N content) enhance soil microbial activities leading to
increased mineralization during decomposition (Singh and Kumar 2007; Mohanty et
al. 2013).
Soil water levels
effect was not significant on tissue N content while the amendment effect was
significant, with the highest nitrogen uptake observed for KK treatment,
followed by the CO, and the lowest tissue N was found in FB treatment. The
results indicated that KK amendment was releasing more nitrogen into the soil
through mineralization thus promoting a better crop growth and higher tissue N.
Similar findings were observed by Soon and Arshad (2002). The addition of
narrow C:N ratio residue results in mineralization and ultimately N release
while wider C:N ratio results in immobilization and reduced nutrient release
into the soil (Singh and Kumar 2007).
Similar to the
tissue N, the tissue P was not significantly affected by soil water treatments,
however, it was significantly affected by the amendments. Highest tissue P was
found in KK treatment compared with the other treatments and lowest tissue P
was found in FB treatment. The tissue P result showed that higher P
mineralization probably occurred in the KK amended soil, resulting in a better
P release into the soil and higher P uptake by the wheat plants (Venterink et al. 2002).
Conclusion
Organic amendments stimulated nutrients mobilization and uptake by the
wheat crop and thus improved crop growth. KK residue addition significantly
increased soil IN, %NH4+-N, AP and wheat growth and FB
caused immobilization due to wider C:N ratio resulted in poor crop growth.
Microbes also add nutrients in the soil at their turnover. After 20 days, there
was reduction in nutrients status in soil probably due to up taken by the crop.
Incubation has shown positive effects of soil wetting/droning and residue amendments
in enhancing the availability of nutrients to crops uptake, but its effect was
non-significant in most of the cases.
Acknowledgements
Special thanks to the Australian Government for providing funds under
the Endeavour Research Fellowship Award for this study. Support and facilities
provided by the School of Agriculture, Food and Wine, The University of
Adelaide, SA, Australia are highly appreciated.
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