沙漠研究 22-1, 279 -282 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－Utilization of Forestry Residue in Erosion Controland Soil Moisture Conservation
Siaw ONWONA-AGYEMAN*1), Shimpei NAKAMURA2) Yoshiko KAWABATA3), Masaaki YAMADA1),
Edward Benjamin SABI4) and Mitsuhiko TANAHASHI2)
Abstract: Chip material obtained from forestry residue was converted into boards by the pressurized steam and compression
technology without using any chemical adhesives. These biomass boards were tested as erosion control materials, and found to bevery effective in reducing soil erosion with extremely high soil retention efficiencies of 94% and92 % on steep and gentle slopes
respectively. Our investigations also revealed that the moisture retention capacities of biomass boards are relatively higher and could
be used to reduce soil moisture losses. The impregnation of urea does not only improve the durability of the boards but also allows
them to retain more moisture. All these benefits suggest that converting forestry residue into boards before applying them as groundcovers in farms could result in more benefits such as yield improvement, soil moisture conservation, elimination of weeding or
herbicide application, erosion control, hill slope stabilization and an overall improvement in ecological services.
Key Words: Biomass boards, Forestry residue, Pressurized steam technology, Soil erosion, Soil moisture conservation

1. Introduction
Soil erosion and soil moisture loss are two major issues thatneed to be seriously considered when dealing with degraded
land restoration especially in arid and semi-arid regions as wellas projects to rehabilitate deserts. Measures to combatdesertification are usually not only time-consuming but also
have higher cost implications. The problem of desertification
is worsening in recent years and arid land areas also expandingat unprecedented rates. To address these issues, manynational and international organizations are drawing programs
to increase farmers’ land conservation awareness (Aqil andMihara, 2010) and promote the use of organic amendments
such as the direct application of forestry or agricultural residue
to farmlands. It was recently reported that wood wastematerial was effectively used to improve soil infiltration andsoil loss (Andy et al., 2011). However, a greater part of thesematerials are likely to be washed away by runoff after heavy
storms or blown away by strong winds. The use of plasticsheets is quite popular in modern agriculture but they are notbiodegradable, act as a barrier to infiltration of water fromrainfall and also their means of disposal poses a threat to theenvironment.
Furthermore, plastic mulches are notsustainable and their prices are likely to go up with thedepletion of oil resources. Meanwhile, large volumes of
biological materials like weeds, forestry residue, agriculturalwaste, etc are generated at places where land is still productive

in many parts of the world but only a small fraction is utilizedwith the rest mostly burnt to waste.
Several research works on waste utilization has been
conducted to convert biomass into particle boards (e.g.
Akaranta, 2000; Odozi et al., 1986; Sampathrajan et al., 1991)
but most of the methods employ chemical adhesives. To
effectively utilize these bioresources, a pressurized steam andcompression technology has been developed and used toproduce mulching materials, hereby referred to as biomass
boards, from forestry residue (Ito et al., 1998a, 1998b,
Onwona-Agyeman et al., 2002). With this technology,
boards can be fabricated from all types of biomass without
using any chemical adhesives. The purpose of this research is
to investigate the possibility of enhancing the functions of thesebiomass boards by testing their abilities to reduce soil erosion,conserve soil moisture, suppress weeds, and most importantly
examine how all these multiple functions of biomass boards
may be integrated into a sustainable agricultural system. Asecond type of boards, hereby referred to as urea-impregnatedboards, were fabricated by first impregnating urea into thechips. These boards would be very useful by slowly releasing
nitrogen to support plant growth in areas where nutrients areeasily washed or leached from soils.

2. Materials and Methods
2.1. Board fabrication process
The main raw material used was wood chips obtained from* Corresponding Author: agyeman@cc.tuat.ac.jp
2-24-16, Nakacho, Koganei, 184-8588, JAPAN. Phone/Fax: +81-42-388-7596
1) Institute of Agriculture, Tokyo University of Agriculture and Technology,3) International Center, Tokyo University of Agriculture and Technology, Japan2) Biomass conversion laboratory, Faculty of Applied Biological Sciences, Gifu University, Japan

4) College of Agriculture and Consumer Sciences, University of Ghana

2.2. Moisture absorption and retention property tests
In accordance with JIS A5908-2003 specifications,
physical property tests such as water absorption capacity andthe internal bonding strength (IB) of both types of boards wasalso determined.
The water retention property wasdetermined by soaking samples in water for 24 hours and thenallowing them to dry in the open air while their moisture
contents were monitored.

Fig. 1. Site for runoff experiments.
Table 1. Physical properties of biomass boards. IB: internal bonding,
MOR: modulus of rupture, MOE: modulus of elasticity, TS:
thickness swelling, HB: Brinell hardness.
Density

IBMOR

MOE

TS

HB

(g/cm 3 )

(KPa)

(MPa)

(MPa)

(%)

(N)

0.64

10.7

0.53

44.6

124.16---- Urea-impregnated boards
Boards90
80
Moisture content （％）

thinned (whole trees) sugi (Crpytomeria Japonica D. Don)
logs. With the high-pressure steam method, dry chips are
initially placed in a metallic frame and compressed to a target
thickness of 2 cm, after which they are placed in an autoclave
where steam at 180°C is injected for a 30-minute duration. The
steaming process, referred to as permanent fixation, is carried out
to ensure that the boards become dimensionally stable afterpressing. The urea-impregnated boards were fabricated by firstsoaking the chips in urea solution for a period of three days after
which the procedure described above was followed.

706050403020100010304050Fig. 2. Moisture absorption and retention of biomass baords.

2520Urea-impregnatedboards15Biomass boards

10Control

5002.4. Erosion control tests
Runoff tests were conducted to compare the sediment
trapping efficiency of biomass boards with wood chips and ricestraw on a gentle (5°) slope and a steep (30°) slope respectively,
using natural storm events (Fig. 1). The site for the runoff
experiment was on the campus of Gifu University and it lasted
for a period of four months, commencing from October 2006
and ending in January 2007. Bare plots were initiallyprepared by clearing weeds on a 30-degree steep slope and a
5-degree gentle slope. Each slope was then divided into equalareas of 120 cm length and 30 cm breadth and corrugatedplastic sheets were used to hydrologically demarcate them intozones. Funnel-shaped stainless plates fitted with mesh were
carefully placed at the bottom edges of each zone to trap
eroded sediment. After every rainfall event, trapped sedimentwere collected from the four different treatments (threereplications) of fully covered biomass boards, rice straw, woodchips and the bare ground area to serve as the control.

20Time (hours)

Evaporation (mm)2.3. Soil moisture conservation tests
The effectiveness of the biomass boards andurea-impregnated boards in conserving soil moisture wasevaluated in the laboratory by the following simple method.
Flowerpots partially filled with equal weights of sandy soil
were covered by test samples of both materials to serve as
mulches. One set of sand-filled flowerpots were not covered
to serve as the control. Next, equal amounts of water wereapplied to all the treatments until saturation point. After that,monitoring of the soil moisture content was periodically done
until all the moisture evaporated.

20406080100

Elapsed Time (hours)

Fig. 3. Soil moisture evaporation under different mulches.

3. Results and Discussion
3.1. Physical properties of biomass boards
The physical properties of the biomass boards used in theexperiments are shown in Table 1.
Although the other properties remained unchanged,
urea-impregnation into the boards resulted in a slight increasein density (0.65 g/cm3) and internal bond strength(Onwona-Agyeman et al., 2007).
3.2. Moisture absorption and retention by biomass boards
After soaking samples in water for 24 hours, it was foundthat the urea-impregnated boards absorbed more water (77%)
than the compost boards (67%) as shown in Figure 2. It canalso be seen from the graph that urea-impregnated boards

Table 2. Erosion control effectiveness of different materials. (SS:Steep slope; GS: Gentle slope)
Erosion

ErosionReduction

Reduction(t/ha)(t/ha)(%)(%)

T reatmentSSGSSSGSBoards

0.2

0.12

94.4

92.3

Rice straw

0.27

0.32

92.4

70.7

Wood chips

0.5

0.48

86.1

80.3

Bare ground

3.6

1.6

00

could retain more moisture for longer durations (over 50 hours)
than the boards (40 hours).
Figure 3 shows the relationship between the amounts ofmoisture loss from the sandy soils covered with the two typesof boards and the control (uncovered). It can be clearly seenthat the moisture loss through evaporation was much higher forthe control compared with the boards. Although there was
not much difference in the evaporation amounts between thepots mulched with the two types of boards, theurea-impregnated boards exhibited a better mulching
characteristic by retaining more soil moisture compared withthe biomass boards.
3.3. Erosion control by biomass boards
Throughout the 4-month duration, a total of 254.3 mm ofrainfall was recorded. The total amount of eroded soil
collected at the end of the experiment from each treatment isshown in Table 2. From the results, the amount of erodedmaterial, estimated in tons per hectare, were found to be least
for the areas fully covered by biomass boards while the bareground showed the highest erosion for both the steep andgentle areas as expected. By taking the amount of erosion on
the bare ground as the reference, the percentage reductions insoil erosion were highest for the boards and estimated as
94.4% and 92.3% for the steep and gentle slopes respectively.
Although raindrop impact and other environmental stresses led
to a slight degradation of the boards after four months, theaverage density had reduced to 0.54 g/cm3 while thickness
swelling changed to 9.24%.

4. Conclusion
Biomass boards are biodegradable materials and with time
will be expected to decompose and return into the soil as asource of organic matter without leaving any traces ofhazardous compounds. Using wood chips will allow weeds
to compete for vital nutrients but converting them into boards
and subsequently using them as mulches in small-scale
farming systems can eliminate or significantly reduce the costof weeding and the need to apply weedicides. Results fromour earlier tests in Ghana indicates that biomass boards will bevery suitable in semiarid agricultural environments, whereannual rainfall is scanty but rainfall events are so intensive that
ordinary wood chips are easily washed and carried away
(Onwona-Agyeman et al., 2008). The idea of impregnating
urea or any type of fertilizer into boards is to render them very
useful in soils where nutrients area easily leached. Our
experiments also revealed that impregnating urea into boards
could improve their moisture holding capacities, retarded soil
moisture loss and serve as a source of nitrogen for crops,
thereby boosting their overall function as mulches. The slightincrease in the strength of boards as a result of urea
impregnation could be due to the bonding between urea andwood constituents, particularly vanillin.
Biomass boards have an added advantage of preventing orlessening on-site erosion. This advantage was also echoed byReeder (2006) who reported that soil management practices
that kept soil particles in place should be preferred to ones thatwere designed to intercept eroded sediment after it had moved
a considerable distance.
In a separate experiment
Onwona-Agyeman et al. (2007) found out that the use of
biomass boards as mulches could reduce weed growth from
0.5 ton/ha/month to 0.05 ton/ha/month.
It is worth noting that the rice straw and wood chips arelighter, decompose within a matter of months and can be easily
washed by runoff and strong winds. The biomass boards on
the other hand, are heavier and can last for nearly two years.
All these benefits suggest that converting wood chips into
boards before applying them as ground covers will result in
more benefits such as yield improvement, soil moisture
conservation, elimination of weeding or herbicide application,
erosion control and an overall improvement in ecologicalservices.
In the near future, we intend to test how these biomass
boards can slow down overland runoff and improve soil
infiltration characteristics. These desirable features of boards
used as mulches can be applied in the stabilization of hillslopes and reduce sediment flow, thereby reducing the loss ofsoil fertility and the threat of flooding. Although the boards
were very effective in limiting the amount of soil eroded withextremely high retention efficiencies of 94.4% and 92.3% on
steep and gentle slopes respectively, their effectiveness should
also be evaluated with respect to sand and silt components ofsoils. As a recommendation, the boards could be laid along
contours to reduce erosion in large-scale agriculture on sloping
lands. In small-scale vegetable production, the boards could
be laid in a much more compact way and then planting thecrops in the gaps between them as shown in Figure 4. This
will ensure weed suppression, moisture conservation, less soil
loss and overall protection of the soil.

Fig. 4. Corn production under biomass board mulching.

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