林木落葉の分解について : 落葉の化学的組成とその分解速度との関係について

Abstract

京都近郊で採取したハンノキ, ヒメヤシアブシ, コナラ, ヒノキ, スギ, アカマツについて一定の条件下で CO2 発生量を指標として分解速度を測定し落葉の有機組成分との関係を求めた｡ 1. ハンノキ, コナラ, スギ, ヒノキの30日間の分解速度は CO2 発生量の大きかつたものから順にハンノキ>コナラ>ヒノキ>スギとなつた｡ またヒメヤシアブシ, コナラ, アカマツでは54日間のCO2発生量で大きかつたものから順にヒメヤシアブシ>コナラ>アカマツとなつた｡ 各有機組成分分析結果によるとスギ, アカマツ, ヒノキはコナラ, ヒメヤシアブシ, ハンノキに比してアルコール ･ ベンゾール抽出物, またエーテル及びアルコール抽出物の量が多く約2倍に達したほか, ハンノキ, ヒメヤシアブシで窒素量が他より明らかに多く約2.5%を示した｡ このほかの組成分については多少の差があつてもその量的な比較から分解速度の難易を説明することはできないようである｡ 2. アカマツ, コナラでは可給態窒素の不足によつて初期の分解速度がかなり抑制されていたようであるが, ヒメヤシアブシでは少なくとも初期の分解に窒素及びその他の無機養分の不足が制限因子となつていたとは考えられない｡ しかし充分の無機養分を与えた条件のもとでもなお分解速度は樹種別に差異がありその順位も無機養分を添加しないときと変化がなかつた｡ 3. アカマツ, スギのエーテル抽出物を除去しても分解速度は促進されずむしろ CO2 発生量が低下する場合があつたので, エーテルで抽出される部分は針葉樹落葉の分解を直接妨げているようには思われない｡ しかしこの部分の分解速度ははやかつたとは考えられないので, この部分を広葉より多量に含むことは初期の分解速度に不利な条件と考えられる｡ またこのエーテル抽出物をセルローズや広葉に添加すると初期の分解開始がおくれ一時的に分解抑制の効果を示したが, 一且分解が始まるとその後の分解速度には何の有害作用を与えていなかかつた｡ 4. ヒメヤシアブシの水溶性物質はエーテル抽出物をも含めて分解がはやかつたようでこの樹種落葉の初期の CO2 発生量のかなりの部分を占めていたが, アカマツやスギでは水溶性物質を除去しても分解速度に著しい変化はなかつた｡ 2%HCl処理材料では各樹種ともに分解速度の低下が著しく, とくにスギ, アカマツでは初期の CO2 発生量の約60%はへミセルローズ部分の分解によつていたもののようである｡ ヒメヤシアブシの2%HCl処理材料はセルローズが少なくリグニンが多かつたが初期の分解速度は最もはやく30日間の CO2 発生量もよりセルローズに富むコナラとほぼ同様であつた｡ スギ, アカマツではこの材料の分解速度もまたおそかつた｡ 5. 2%HCl 処理材料を 4%NaOHである程度リグニンを除去するとその分解速度は明らかに増大し, セルローズの分解が多量のリグニンの存在によつて抑制されていたことを示した｡ 従つて少なくとも同一樹種においてはリグニン含有量が少ない程分解速度ははやくなるが, 樹種が違うとこの関係は成立しない｡ 6. 単離された硫酸リグニン, 混酸リグニンの分解速度はその他の組成分に比して著しくおそく, 樹種間にも明らかな差異を認め難い｡ またセルローズと混合してもその分解に有害な作用は全く与えなかつた｡ すなわち, リグニンは極めて分解され難く, 細胞膜中ではセルローズと密接に結合するから, 機械的にセルローズの分解に対し保護的な作用を与えていると考えられる｡ 7. ヒメヤシアブシのように分解速度のはやい落葉ではその水溶性物質や細胞膜構成部分の分解速度もまた他に比してはやく, アカマツ, スギのように分解のおそいものではこれら諸成分の分解速度がおそかつた｡ 従つて樹種による分解速度の差異は単にその組成分の量的な比較だけから判断することは困難で各組成分の分解に対する難易性を考慮する必要がある｡

This report deals with the relation between the rate of decomposition of forest litter and their chemical composition. The index used for the rate of decomposition was the development of CO2, under controlled and uniform laboratory condition. (1) The tests proved that there was a greater or lesser variation in the rate of decomposition among the kinds of litter. The order of the decomposingrate of litter in the period of 30 days was following, that which decomposed most rapidly being mentioned first (25℃) ; (i) Alnus japonica (Hannoki) (ii) Quercus serrata (Konara) (iii) Chamaecyparis obtusa (Hinoki) (iv) Cryptomeria japonica (Sugi). And, the other series, in the period of 54 days the order was following (25℃) : (i) Alnus pendula (Himeyashabusi) (ii) Quercus serrata (Konara) (iii) Pinus densiflora (Akamatsu). According to the results of proximate analysis of fresh litter, in the case of needles (Sugi, Akamatsu and Hinoki) the contents of the alcohol-benzol soluble fraction or ether and alcohol soluble fraction were almost double that of broad leaves (Konara, Hannoki and Himeyashabusi), and Hannoki and Himeyashabusi had much more nitrogen than other litter (about 2.5%). Among the kinds of litter investigated, there were some differences in the relative amount of the remaining chemical composition such as water soluble substances, cellulose, hemicellulose, and lignin, but there was no direct correlation between their relative amoumt and the rate of decomposition, if different kinds of litter were compared with one another. (2) When inorganic nitrogen was introduced, there was a strong increase in the development of CO2 in Akamatsu, and Konara, but in Himeyashabusi, it seemed that there were sufficient nutrients salts and nitrogen for early period of decomposition. Under the sufficient nutrients salts and nitrogen were added, however, there were differences in the rate of decomposition between the kinds of litter, and the order of decomposingrate was the same to that of nutrient salts and nitrogen were not added. The low content of nitrogen in Akamatsu was distinctly the cause of its slow decomposition, but it was not the primary factor causing the difference in the rate of decomposition between Himeyashabusi or Konara. (3) It considered that the slow decompostion of needles was due to the high content of alcohol-benzol soluble fraction, which contain waxes, resins and fats. When the needles of Sugi and Akamatsu treated with ether, there were not the increase in the development of CO2, but slightly dicreased, therefore, it seemed that the ether soluble fraction of needles were greatly not retarded the decomposition. But it appeared that this fraction were not so rapidly decomposed, during the early periods, so it may be the unfavorable condition for needles that the contents of the ether soluble fraction, which somewhat resistant for decomposition, were almost double to broad leaves. When the ether extracts of Sugi and Akamatsu needles added to cellulose or broad leaves (Konara. Himeyashabusi), the decomposition were retarded in the early period, in the case of cellulose about I month, and in Konara and Himeyashabusi about 2 ~ 3 days. After this period, the rate of decomposition were as rapidly as that of the untreated matter, so that the ether extracts of Sugi and Akamatsu needles had no injurious influence to the rate of decomposition of cellulose and broad leaves on this period. (4) It seemed that the water soluble fraction of Himeyashabusi, including the ether extacts were very rapidly decomposed, but in Akamatsu and Sugi needles, the remove of water soluble fraction, including the ether extracts had no influence for the CO2 evolution in 30 days. The development of CO2 were great extent decreased in the materials which treated with 2%HCl. Particularly, in Sugi and Akamatsu about 60% of the amount of CO2 evolution from original materials were due to the fraction which soluble in 2 % HCl. In spite of the lowest cellulose contents and the highest lignin content in the material of Himeyashabusi which treated with 2% HCl, it decomposed most rapidly in the period of 30 days. In the case of Sugi, Akamatsu the cellulose content was almost same as Konara leaves, but their decomposition were slower than Himeyashabusi or Konara in those materials. (5) When certain amount of lignin removed from the materials treated with 2%HCl by the treatment with 4% NaOH, the evolution of CO2 were greatly increased, and their decomposition gradually resembled to the decomposition of pure cellulose in proportion to the decrease of the ratio between cellulose and lignin. This tends to indicate that the high lignin contents of forest litter were the one of the chief factor of their slower decomposition than the other plant residues such as woods, or rice strow. In this experiments, it could not make clear the difference of the depressing effect of lignin on cellulose decomposition between tree species. (6) The isolated lignin (Sulfric acid lignin, Wenzel-Urban lignin) decomposed very slowly compared with the other composition such as cellulose, and it seemed that there was no difference in the rate of their decomposition between the kinds of litter. The mixed materials with cellulose and isolated lignin decomposed as rapidly as cellulose, therefore the isolated lignin had no injurious influences upon the cellulose decomposition. Therefore the lignin which in cell membrane protects the cellulose from rapid decomposition only mechanically. (7) In the case of the rapidly decomposable litter such as Himeyashabusi, its water soluble substances and substances composing cell-membrane were also decomposed easier than Akamatsu and Sugi. If different species of litter were compared with one another, it seemed difficult to assume the rate of decomposition in proportion to their relative amounts of the chemical composition. For determing the rate of decomposition of forest litter, it seemed the more important factor that there were the difference in the nature of each chemical composition such as water soluble fraction, hemicellulose and lignin which is characteristics for tree species between the kinds of litter.