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Changes in Tree Nutrient Stocks and Soil Fertility Characteristics under Smallholder Rubber Cultivation in a West Sumatran Lowland of Indonesia

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年 本籍 名 月日

(国籍)

学位論文審査結果の報告書

学位の種類

学位記番号

IRWIN MIRZA UN仏MI

1987年7 インドネシア国

学位授与の条件

(博士の学位) 博 論文題目 月 士(

234

Under sma11holder Rubber cultivation in a w'est sumau'an Lowland of lndonesia 18

(インドネシア国西スマトラ州低地の小規模農家栽培ゴム園における樹木

学位規程第5条該当

養分ストックと士壌肥沃度特性の変化)

ChangesinTreeNutrientstocks and soilFertiliw characteristics

学)

学位論文審査終了日

審査委員

平成31年 平成31年 、、. 1 5日 4日 (主査) (副主査) (副主査) (副査) 儲導教員) 川崎 飯嶋 努 盛雄 松本光朗 松本光朗 13 ^^.",エ. 〆エ'、 i.、} .゛,^'、^^' ム.}レ・ー, コ @

学位論文受理

農号

氏生

日 室 1 2

月月

(2)

Rubbertree (Heveαみrasilie11Sis ) is a11industrial crop with hi8h ecohomic value because ofits nature to Produce mi11弓,1atex asthe primary source ofnaturalrubber. cunently,1ndonesia produceS 節nua11y 3 mi11ion

tons ofnatural Nbber 丘'om 3.6 mi11ion ha ofNbber gardens, most(859る) ofwhich are cultivated by Sma11holder fanners, and has more th釦 2 mi11ion households enjoying the rubber・derived income. TO

SuccesS員111y estimate the capaciw ofecosystems seNices provided by Nbber plantation and sustainably enhance the productivi匂,, ofrubber f帥ning,the impact ofNbber cultivation on environments needsto be assessed in this counり,. Although the study on the impacts ofla11d use change from natural forests orjungle Nbber agoforeststo monoculture 川bber plantation on the stocks ofnutrients (inc. carbon [CD and soil fe丘ili匂,, has been ce丘ainly accumulated, much less is available how the nU廿ient stocks and soil fe此ility Parameters change during Nbber pla11tations, especia11y those m幼aged by sma11holder fanners with limited

Use ofextemalresources such as fertilizers a11d agrochemicals. Therefore,the aim ofthe present study was to

exalnine changes in 廿ee nU廿ientstocks a11d soil fe此ili司,, characteristics under sma11holder Nbber farm血g during an economic life time ofNbber h'ee (ca.25 years)in a low land area of訊11est sumatra,1ndonesia.

言△ 文内 J'",

T訊lenty、five Nbber gardenS 訊lith different ages ofNbbertrees(3 t027・year・old) managed by different Sma11holder fanners were selected forthis study hlthe center ofeach rubber garden, soilsamples were taken from 負・eshly exposed soilprofile at o.1 m 加teNalup to o.8 m in depth. Generalsoil feπiliw paralneters ofthe C011ected soilsalnples (PH, EC,total c a11d N, available [Bray 21 P, exchangeable ca, Mg, K, and acidiり,in

addition to available [Nlehlich 3] micronutrients (Fe, N111, cu a11d zn) were done f0110wing the routine

methods ofthe laboratory analysis. Above・ and belowgroundtree biomasses were sampled in 12 Nbber gardens, which were selected from the above 部Ven 25 gardensto coverthe 川bber's economic lifespa11, by the 10gging and trenching method, respectively Abovegound biomasS 圦las fudherseparated into t11ree

Components; stem, branches a11d foliage (1eaves and twigs). undergroW仕1(weed) biomass, and land・surface Htter 圦lere also sampled. The contents ofselected nutrients(C, N, P, K, ca, Mg, Fe, Mn, cu and zn) ofeach

tree componentwere detemlined bythe digestion method.

の ι二ι

The increasing pattems ofboth biomass and c stock in the 川bbertree atthe study site were mostly 丘tted We11by the Gompertz model.1he di釘neter at breast height(D) was identified as a fairly good estimator, Which is easily measurable and ca11Predict accurately both biomasS 釦d c stock in the Nbbertree based on the linearregession model. These models were also adoptable to many ofthe other nutrients、vith S1電ht

modifications.圦lith these 訂'ee growth and c accumulation pattems,this study revealed that c stock in both

above- a11d belo、N部'ound 訂'ee biomasses increased consecutively, butthose in the other pools (i.e., soil orga11ic matter, undergowth biomass and land sudace litter)remained on a relatively constantlevelduring the

economic life time ofthe rubbertree. The major contributorsto the c stock 加 the Nbber gardens were soil (522士18.0%) and abovegoundtree biomass(42.8士16.9%), and belowgound biomasses(4.9士13%)

Contributed to a much less extent. undergowth (0.10士0.039る) and la11d surface litter(0.05士0.02ツ0) accounted

for only the ne容ligible p0πion (<0.2% in totaD. These results imply that Nbbertree cU此ivation have a Potentialrole in improving the regionalc sequestration due to c assimilation by the rubbertree.

On 壮le other hand,there are no cleartrends ofsoilfe此iliり,, decline during the economic life cycle ofNbber tree, regardless ofno- or .10W・input practice ofthe sma11holder Nbber farmers This resU此 Suggeststhatthe amounts ofmacronutrientstaken up by the Nbbertree are notlarge enough to overbalancethe removalof these nutrients from the soil and/or naturalreplenishment mechanisms ofsoilnutrients, e.g., nutrient-rich

depositle丑 a丑er aash 丑ood, may existin the study site. hlcontrast, soilmicronutrients, especia11y cu and zn,

(3)

-exhibited decreasing trends oftheir availability in the same period, suggesting si即ificant mining ofsoil micronutrientsthough the rubber cultivation atthe study site. Although the cunent level ofthese

macronU廿ient availabiHty is st辺 high enough to suppod the Nbber cultivation because ofthe inherenay high

availabiliり, ofthe micronutrients due to the low pH and high level oforga11ic ma廿er in the studied soils but

the 会lture risk ofcu and zn deficiencies remains in the study site,1n this regard, soil organic ma杜er is

Considered the key component ofsustainable soilmanagementin this site because most ofthe examined soil nutrients were positively correlated 圦lith soil organic c.

(4)

-インドネシア国は生産量において世界シェア約ν4を占める世界有数の天然ゴム生産地で ある。とりわけスマトラ島には多くのゴム園が分布しているが、その大部分が小規模栽培 農家によって管理されているのが特徴である。ゴムノキの生育は環境条件や管理手法に よって異なるため、士地固有の条件を考慮した栽培手法の確立が必要不可欠である。本研 究は、スマトラ島低地部(西スマトラ県ダーマスラヤ郡)を調査対象として、この地にお けるゴムノキの生育条件と炭素・養分貯蔵能、士壊肥沃度の変化を明らかにしている。 最初の項目(第二章)では、調査地におけるゴムノキの成長特性を明らかにし、成長モ デルを用いてバイオマスと炭素の蓄積パターンを明らかにするとともに、アロメトリー式 を構築し、胸高直径や樹高データからバイオマスと炭素ストックを予測することが可能に なった。また、次の項目(第三章)では、ゴムノキにおける養分の蓄積パターンについて 明らかにしている。 続く項目(第四章)では、ゴム園における5つの炭素プール(樹木地上部、樹木地下部、 士壌有機物、林床植生、地表りツター)の経時変化を明らかにし、士壌有機物、林床植 生、地表りツターの三つのプールは栽培期間中を通じてほぼ一定であることが示された。 最後の項目(第五・六章)では、ゴムノキ栽培期間中における士壌肥沃度の変化につい て調査し、士壌の一般理化学性と多量養分については大きな変化は見られないが、微量養 分(とりわけCUとZn)については減少傾向が認められた。 以上、本研究で得られた知見はインドネシア国スマトラ島低地帯におけるゴムノキの生 育予測とバイオマス・炭素量の予測を通じて、ゴム園の炭素貯蔵能について多角的に評価 しただけでなく、士壌肥沃度の変化を明らかにすることで、持続可能なゴムノキ栽培手法 の構築ヘ寄与できる基本情報を提供している。 よって、本論文は博士(農学)の学位論文として価値あるものと認める。なお、審査に あたっては、論文に関する専攻内審査および公聴会など所定の手続きを経たうえ、平成31 年2月4日、農学研究科教授会において、論文の価値ならびに博士の学位を授与される学力 が十分であると認められた。 上L 16 -の を二' 払、 Jハ

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