INTRODUCTION
High quantities of inorganic fertilizer, particularly nitrogen (N), have been used to increase world food production. By the year 2020, it is estimated that 70% of plant nutrients will have to come from fertilizer (Ayoub, 1999). Commonly, large amounts of applied fertilizers are not taken up by crops and thus could cause negative environmental impact (Bockman et al., 1990). The practice of crop residue incorporation needs a judicious supplementation with nutrients especially N and P (Salvator and Sabbe, 1995). This supplemented fertilizer N is necessary because soil microorganisms consume considerable amounts of soil mineral N needed for crop residue decomposition. Sustainable agriculture became a major issue of global concern during this decade (Lewandowski et al., 1999). Agronomic practices aimed at reducing the dependence on inputs such as chemical fertilizers, e.g., incorporation of crop residues can contribute to sustainability mainly through improvement of soil fertility as judged by organic carbon, available P and potassium (K) content. Moreover, it was suggested that although in the sub-tropics and tropics it is difficult to increase soil Organic Matter (OM) content substantially and therefore, it is highly desirable to add crop residues and other OM to the soil, whenever possible to maintain soil fertility (Nowak et al., 1998). Situations exist where there is an overabundance of agricultural wastes such as rice residues that may pose serious pollution or disposal problems (Bevacqua and Mellano, 1994). Such wastes could be used to advantage as a nutrient source or even to conserve the environment, if they can be shown to have beneficial effects on soil properties (Mubarak et al., 2003). Information regarding the effect of rice residues on yield and nutrient uptake, soil properties is still needed. The aim of this study was to assess the effects of application of organic residues (rice residues) on rice growth and yield, N uptake and on the changes of soil pH, EC and NH3 concentration in the flooded and soil water.
MATERIALS AND METHODS
Site and Experiment Setup
Pot experiment was carried out under greenhouse conditions in 2004-2005
at the Experimental farm, Ehime University, Matsuyama city, Japan, (33°
57 N, 132° 47 E) with the elevation of 20 m above sea level.
The soil used was gray lowland paddy soil (Eutric Fluvisols in FAO/UNESCO) with
characteristics are (Table 1). Air-dried soil was sieved to
pass through a 2 mm mesh. The herbage residues of rice plants (root, hull and
straw) were cut into 5 cm long pieces and used for experiment. The treatments
were set up in a completely randomized design: (1) control pots (no organic
material amendment), (2) 15N-mineral fertilizer pots labeled with
15N (99.7 atom %), (3) rice hull pots {10 g (9.22 g dry matter) pot-1},
(4) rice root pots (5 g pot-1), (5) incorporated straw pots at rate
5 Mg ha-1 {10 g (18.4 g dry matter) pot-1} and (6) incorporated
straw at rate 10 Mg ha-1 (20 g pot-1). N fertilizer was
applied at rate of 80 kg ha-1. Phosphorus and K fertilizer (phosphorus
oxide (P2O5) and potassium chloride) were applied at rate
of 1.0 and 0.3813 g pot-1, respectively, on 12 June. Wagner pots
(0.02 m2) were filled with 2.50 kg air-dry soil mixed with equivalent
volume of amendments. The rice residues added 2 weeks before transplanting.
Three 17-day-old seedlings of rice (Oryza sativa L. cv.) Sakha 102 was
transplanted to the center of each pot with four replicate on 12 June.
Sampling and Analysis
Rice growth parameters (plant height, number of tillers and leaf chlorophyll
content) were measured at 0, 9, 18, 25, 43, 57 and 113 days after transplanting
(DAT). Chlorophyll content was measured with a chlorophyll meter (SPAD-502;
Minolta Co. Ltd., Japan). Rice plants were harvested by cutting above the soil
surface and thoroughly washed, first under running tap water and finally with
distilled water to remove soil particles and plant debris. The plants were then
separated into grain, straw and root and oven dried to constant weight at 70°C.
The dried samples were weighed before grinding and ground into a fine powder
with an electric mill in preparation for chemical analysis. Total N and 15N
content in grain, straw and root were determined.
Ammonia Measurement
Soil samples were collected using porous ceramic cups at 12 and 25 days
after transplanting and ammonia concentrations were using indophenol method.
The cups are buried in the soil at required depth and water samples are collected
by pumping air out of ceramic cup. This sets up a pressure gradient between
the soil and the inside of the cup. If soil water is held at matrix suctions
less than the negative pressure introduced to the cup, water will move from
the soil into the cup until the pressure equilibrates.
Table 1: |
Chemical constituents of soil used in the study |
 |
The ammonia in the flooded and soil water was measured by method of (Scheiner,
1976). The soil solution samples were used directly for determination of pH
and electrical conductivity (EC). The pH of the standing water in the flooded
water was determined at 0, 8, 14, 35 and 45 days after transplanting. The pH
(H2O) and EC of the soil were measured by using a pH and conductivity
meter (D-24; Horiba Ltd., Japan), respectively, using a suspension of soil and
deionized water in a ratio of 1:5 (shaken on a reciprocal shaker for 30 min).
Statistical Analysis
All data was subjected to analysis of variance (ANOVA) at first, then the
significance of differences between the treatments was determined by a multiple
comparison test with the Tukey-Kramer method at (p<0.05) using KyPlot software
packages (Kyenslab Inc., Tokyo, Japan).
RESULTS AND DISCUSSION
Rice Growth
There were no significant differences between treatments in mean plant height.
However, plant growth was marginally faster in the mineral fertilizer than in
the rice residues treatment from 30 to 60 days after transplanting (Fig.
1a). Number of tillers showed similar trend with a significant differences
in maximum and productive tiller numbers observed with mineral fertilizer treatments
compared with rice residue treatments (Fig. 1b). The total
number of tillers pot-1 was higher in mineral fertilizer treatments
than in rice residues. Generally, the number of tillers is determined during
the vegetative growth period and is mainly governed by tillers capacity of cultivars,
planting density and the availability of mineral nutrition, particularly nitrogen
(Yoshida et al., 1981). Changes over time in chlorophyll content are
shown in Fig. 1c. The highest mean value was recorded at 25
DAT and was followed by a rapid decline in chlorophyll content in both mineral
fertilizer and rice residues treatments. These results indicated that the importance
of basal application of readily chemical nutrients (Ebid et al., 2007;
Ghoneim et al., 2006).
Yield and Yield Component
Numbers of panicles, 1000 grain weight, ripening ratio and yield pot-1
were somewhat higher in mineral fertilizer than other treatments. However, the
difference was not significant at p<0.05 between the rice residues in mean
numbers of panicles, numbers of grains pot-1, ripening ratio (Table
2). Rice grain yield were significantly greater in chemical fertilizer and
rice residue compare with zero-N plot.
Biomass Yield and N Uptake
There was a significant increase in the biomass of rice with the application
of amendments as compared to control (Table 3). Mineral fertilizer
accounted for the highest dry weight flowed by rice root treatment. However,
there was no significant difference in dry weight between rice hull and rice
straw. Nitrogen uptake varied between the grain, straw and root in all treatment.
Total N uptake were significantly (p<0.05) higher than that control and other
rice residues.
Table 2: |
Yield and yield components of rice (Oryza sativa L.)
cv. Sakha 102 |
 |
# Mean within a column followed by the same letter(s)
are not significantly different (Tukey-Kramer test; p<0.05), n = 5 |
Table 3: |
Dry weight and total N uptake by rice Sakha 102 amended with
mineral fertilizer and rice residues |
 |
Mean within a column followed by the same letter(s) are not
significantly different (Tukey-Kramer test; p<0.05), n = 5 |
|
Fig. 1: |
Changes in plant height (a), tiller number (b) and leaf chlorophyll
(c) in rice plants grown in soil amended with mineral fertilizer and rice
residue. Bars referred standard deviation of the mean |
The rice residues amendment also caused a significant increase in the total
plant N compared to control treatments but the rate of straw application had
no significant effect. The reasons why most organic amendments supply low amounts
of available N is immobilization after organic matter decomposition and N mineralization.
In addition, the N from organic matter is also involved in other soil processes
such as nitrification and denitrification.
|
Fig. 2: |
Soil pH over a period of 50 days in a soil amended with mineral
fertilizer and rice residues |
The higher N uptake in the mineral fertilizer treatment when compared with
the rice residues can be attributed to higher amounts of available N (Ghoneim
et al., 2006). The lower rate of rice residues decomposition might be
attributed to the presence of highly decomposition resistant compounds (i.e.,
lignin) for bacteria and a high C/N ratio. Moreover, the addition of mineral
nitrogen could not facilitate decomposition in the short time experiment due
to relatively high lignin content of rice straw and especially decomposition
of rice straw by microorganisms is hardly possible. Because of a high C/N ratio,
the plant residues used in this experiment have a low N content and a high C/N
ratio and expected to induce N immobilization during decomposition in soil (Ebid
et al., 2007).
The Effects on Soil pH, EC and Ammonia Content
The effects of rice residues application and mineral fertilizer on the flooded
water pH of rice are shown in Fig. 2. Rice residue treatments
markedly reduced the pH of the flooded water as compared to mineral fertilizer
alone. The pH values of flooded water showed an appreciable increase during
the first two week of the experiment, they stabilized around pH = 6.0-7.20,
then slightly declined during the following days. There were no significant
differences between rice residues treatments in the pH changes. Such pH variations
are common in flooded soil systems and are due to the proton removal from the
system when Fe3+, Mn4+ and SO42¯
are reduced during anaerobic respiration and to the consumption of the dissolved
CO2 by photoynthetising plant. Moreover, addition of organic matter
to flooded soil raises the NH4+ concentration of floodwater
and leads to a pH increase (Sommer and Hutchings, 2001). Decreasing in soil
pH following organic materials can be partially attributed to the high release
of organic aids causing mobilization of native calcium present as CaCO3
in the soil.
During the decomposition of the rice residues, changes in the EC of the soil-organic
matter mixture were observed (Fig. 3). The soil EC in rice
residues displayed high values, ranged between 6.0 and 8.4 dS m-1
at first two weeks after transplanting. However, the EC of the mineral fertilizer
and control plots had the lowest EC values during the experiment period. These
results possibly reflect the concentration of the mineralized nutrients (NO3,
available P, K, Ca, Mg, etc.) and certain water-soluble compounds (organic acids
or low-molecular weight organic compounds) in the soil after rice residue application.
The EC values of rice straw, root and hull were significantly higher than mineral
fertilizer and the peaks reach the plateau after 4 weeks after rice residues
application and then, the EC decreased. Since, we applied the rice residues
two week before transplanting of the rice plant.
|
Fig. 3: |
Soil EC over a period of 50 days in a soil amended with mineral
fertilizer and rice residues |
|
Fig. 4: |
Ammonia content in flooded water (A) and soil water (B) in
a soil amended with mineral fertilizer and rice residues |
It is proposed that the soil solution EC determined at 4 weeks after submergence
can be used as an index of general fertility of wetland soils. This proposition
is based on the finding that the soil solution EC provides a measure of the
concentration of nutrient elements mobilized in solution.
The ammonia concentration in the flooded water and soil water varied with the N sources; in particular the NH3 content in mineral fertilizer was the highest of those in all treatment (Fig. 4). The highest ammonia content was recorded at 12 DAT and was followed by a rapid decline in both mineral fertilizer and rice residues because of rice N uptake. The large difference in the content of NH3 between mineral fertilizer and rice residue was mostly due to the temporary immobilization of N. In addition, rice residue is characterized by their slow decomposability and may be associated with the polyphenol content or the lignin/N ratio (Kumar and Goh, 2003). However, further studies on gross N mineralization, denitrification and immobilization from rice residue should be carried out to clarify the complete details of N dynamics in the soil.
ACKNOWLEDGMENT
Funding of this research by the Ministry of Education, Culture, Sports, Science and Technology, Japan is gratefully acknowledged.