SOME INTER-RELATIONSHIPS BETWEEN DECOMPOSITION OF VARIOUS PLANT RESIDUES AND LOSS OF SOIL ORGANIC MATTER AS MEASURED BY CARBON-14 LABELLING - PDF

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Report of the FAO/IAEA Technical Meeting Organized by the Food and Agriculture Organization of the United Nations and the International Atomic Energy Agency in cooperation with the International Soil Science
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Report of the FAO/IAEA Technical Meeting Organized by the Food and Agriculture Organization of the United Nations and the International Atomic Energy Agency in cooperation with the International Soil Science Society Brunswick VOlkenrode, 9-14 September PERGAMON PRESS OXFORD LONDON EDINBURGH NEW YORK TORONTO PARIS FRANKFURT SOME INTER-RELATIONSHIPS BETWEEN DECOMPOSITION OF VARIOUS PLANT RESIDUES AND LOSS OF SOIL ORGANIC MATTER AS MEASURED BY CARBON-14 LABELLING J. H. SMITH U.S. Soils Laboratory, Soil and Water Conservation Research Division, Agricultural Research Service, U.S.D.A., Beltsville, Maryland, U.S.A. The addition of fresh plant material to soil has been reported to accelerate the decomposition of indigenous soil organic matter. This claim has been tested at Beltsville, Maryland using C 14-labelled tissue of tops and roots from several crop plants of different maturity stages and C/N ratios. Two treatments were made on a Prairie soil from North Dakota containing 3.5 per cent carbon. In one, the previously stored soil was incubated for two weeks before adding the plant material, and in the second, there was no preincubation. One per cent of C 14-labelled soybean, wheat or corn plant tops or roots was added to the soil and incubated in a closed system. The evolved CO, was absorbed in standard NaOH solution that was sampled and titrated at intervals and the C 14 counted by liquid scintillation. In experiments where the soil was not preincubated, a slight increase in soil carbon loss was observed with the addition of mature corn leaves, 28-day-old soybean tops, mature soybean tops or roots. A reduction in soil organic matter loss was found with the addition of young corn tops or roots, mature corn stalks or roots, wheat straw or roots (both young and mature), 28-day-old soybean roots, and 44- day-old soybean tops or roots. When the soil was preincubated before addition of the plant material all plant parts reduced soil carbon loss significantly except immature soybean roots. The addition of corn roots and wheat roots reduced soil organic matter loss about 50 per cent. Adding fertilizer nitrogen at rates of up to 400 p.p.m. N in the soil, reduced, but did not eliminate, the protecting effect shown by the corn roots and wheat roots. This indicates that C/N ratio is not a major factor. These results show that fresh plant residue frequently has a protective effect on indigenous organic matter and that this is not directly related to the maturity or N content of the plant tissue. The strong suppression of decomposition by root residues of wheat and corn suggests that some toxic compound is involved. Scion certains chercheurs, l'adjonction au sol de substances végetales fraiches accelererait la decomposition des matiêres organiques naturalles. Pour controler cette hypothese, on a procódó a Beltsville (Maryland) a diverses experiences avec des tissus, marques, au carbone-14, de parties aeriennes et de racines de plusieurs plantes se trouvant a des stades de maturite differents et presentant divers rapporsts C/N. Deux traitments ont etó effectues sur un sol de Prairie du Dakota du Nord, renfermant 3.5 pour cent de carbone. Dans un cas, les echantillons de sol preleves depuis quelque temps déjà ont átó in- 223 224 J. H. SMITH cubes pendant deux semaines avant l'adjonction de substances vegetates; dans le second cas, it n'y a pas eu d'incubation prealable. On a ajoute au sot 1 pour cent de parties aeriennes ou de racines de soja, de WO ou de mais marquees au carbone-14 et l'on a ensuite mis le sot a incuber en systeme ferme.. Le CO, libere a ótó absorbe dans une solution standard de NaOH, dont des echantillons ont ete titres a intervalles et oil l'on a mesure le carbone-14 avec un scintillateur liquide. Dans les experiences sans incubation prealable, -on a constató que!'addition de feuilles de males mitres, de parties aeriennes de soja de 28 jours et de parties aeriennes ou de racines de soja mar entrainait un legére augmentation des pertes du sot en carbone. On a observe une diminution des pertes en matieres organiques dans les cas d'addition de parties aeriennes ou de racines de jeune tri gs, de tiges ou de racines de mais mar, de chaume ou de racines de ble (aussi bien jeune que mar), de racines de soja de 28 jours et de parties aeriennes ou de racines de soja de 44 jours. Dans le cas des echantillons de sot pre-incubes avant!'addition de substances vegetates, toutes les adjonctions, sauf celle de racines de soja non encore mar, ont reduit de maniere significative les pertes en carbone. L'adjonction de racines de mais et de ble a fait diminuer d'environ 50 pour cent les pertes en matieres organiques. L'addition d'engrais azote dans des proportions allant jusqu'a 400 mg d'azote par kg de sot a attenué sans eliminer l'effet protecteur des racines de mais et de ble. On peut en deduire que le rapport C/N ne constitute pas un facteur determinant. Ces experiences montrent que les debris vegetaux frais exercent frequemment un effect protecteur sur les matieres organiques du mil et que ce phenomene n'est pas directement lie au degre de maturite ou a la teneur en azote des tissus vegetaux. L'inhibition marquee de la decomposition par les debris de racines de ble et de mais donne a penser qu'un compose toxique intervient. Se ha referido que la adici6n de vegetates frescos al suelo acelera la descomposicion de las materias orginicas existentes en este. Para ver si esto es cierto se han hecho ensayos en Baltsville, Maryland (EE. UU.), utilizando tejidos, marcados con C , de las partes aereas y de las raices de diferentes plantar de cultivo, en distintas faces de madurez y con diversas relaciones C/N. Un suelo de pradera, procedente de Dakota del Norte, que contenia 3.5 por ciento de carbono. se someti6 a dos tratamientos. En uno. el suelo. previamente almacenado. se incub6 durance dos semanas antes de afiadirle los vegetates, y en el segundo, no hubo incubacion previa. Se agrego 1 por ciento de partes aereas o de raices de soja, trigo o maiz. marcadas con carbono-14 al suelo, y despues se incub6 este en un sistema cerrado. El CO, desprendido se absorbio en solucion tipo de NaOH, de la que de vez en cuando se sacaron muestras que se titularon y cuyo contenido de C se determin6 con un contador de centelleo en liquido. En los experimentos realizados sin incubaci6n previa del suelo, se observ6 que la adicion de hojas maduras de maiz, de partes aereas de soja de 28 dias, y de partes aereas o de raices maduras de soja producia un ligero aumento de la perdida de carbono en el suelo. Se hall6 que la adicion de partes aereas o raices jovenes de maiz, de tallos o raices maduros de maiz, de paja o raices de trigo (tanto jovenes como maduras), de raices de soja de 28 dias, y de partes aereas o raices de soja de 44 dias reducia la perdida de materia orginica del suelo. Cuando el suelo se incubi!) antes de afiadirle los vegetates. todos, excepto las raices de soja sin madurar, redujeron notablemente la perdida de carbono del suelo. La adicion de raices de maiz y de raices de trigo redujo en 50 por ciento aproximadamente, la perdida de materia organica del suelo. La adici6n de abonos nitrogenados a razon de hasta 400 p.p.m. de N, redujo. pero no elimino, el efecto protector de las raices de trigo y de maiz. Esto indica que, en este caso. la relacion C,'N no constituye un factor principal. Los resultados obtenidos en estos experimentos demuestran que los residuos vegetates frescos ejetcen frecuentemente una action protectora so bre las materias orgänicas existentes en el suelo, y que esta action no esta directamente relacionada con la madurez ni con el contenido de N de los tejidos vegetates. La fuerte inhibicion de la descomposicion que ocasionan los residuos radicales del trigo y del maiz hace pensar que en este fenomeno entra en juego algtin compuesto toxico. SOME INTER-RELATIONSHIPS MEASURED BY C-14 LABELLING 225 INTRODUCTIONS With the development of tracer techniques using 04, it has become possible to measure the decomposition of indi genous organic matter in soil concurrently with the decomposition of added fresh plant material. Several reports of such studies( ) have shown an acceleration in the soil organic matter decomposition rate as a result of fresh plant material additions. This possibility was indicated by LOHNIS 1 ) many years ago when he suggested that the addition of fresh plant material to soil increased the availability of soil nitro gen for plant use. Other reports. particularly in the case of or ganic soils,( 1.15) show no such effect. PINCK and Awsort. 111) in a critical review of Liihnis' work, made the followin g statement : In view of the somewhat erratic results with this greenhouse soil, due to its hi gh content of available nitrogen, it seems that the data lend little support r LOhnis' statements. They also concluded m experiments that were run without tracers that this sug gested effect of addition of plant material to soil could not be measured accurately without tracers and that the magnitude of the effect could be of only minor importance. It is apparent from the foregoing that the addition of plant materials to soil does not always increase the rate of or ganic matter decomposition. It is desirable to investigate the factors controlling this response in more detail. The experiments reported in this paper concern the influence of decomposition of several plant species and plant parts on the decomposition of soil organic matter when the plant materials are incorporated in the soil. EXPERIMENTAL PROCEDURES The C14-labelled plant materials used in these experiments were grown in the biosynthesis chamber located at the U.S. Soils Laboratory, Beltsville, Maryland. A complete description of this facility with procedures for growing the plants is published elsewhere. (12' 13) Species and maturity of the plant materials used and their nitrogen and carbon contents are listed in Table 1. The soil used in these experiments was a Barnes clay loam from the State of Minnesota with per cent carbon and ph For the decomposition studies, some of the soil samples were preincubated in a moist condition for two weeks and some were not preincubated. This dual approach was used because the artificial stimulation of soil micro-organisms that occurs as a result of drying, storing and rewetting soil could affect decomposition of both indigenous and added organic matter. After the pretreatment of soil, the Cu-labelled plant material was added to the soil, and the system was incubated for either 28 or 59 days. In addition, two incubation controls were included, a control for the soil and a control for the C 14-labelled plant material. The soil control (without added plant material) provided decomposition rates of indigenous soil organic matter, and the C u-labelled plant material control (incubated in inoculated sand) provided decomposition rates and specific activity data. In all the experiments, except one, additional mineral nitroeen was added to the soil at the rate of 100 p.p.m. N to prevent nitrogen deficiency. In the other experiment , and 400-p.p.m. increments of nitrogen were added with corn roots to investieate the influence of nitroeen and corn roots on soil oreanic matter decomposition. In another experiment, the influence of particle size of the added plant residue was studied. In addition to the usual 20- mesh grindin g, a series of plant materials were ground to pass throu gh a 60-mesh sieve and incubated in the same manner. The soil and sand were incubated at 27 e C with 1 per cent additions of the respective plant 226 J. H. SMITH materials. Moisture content at the be ginning of the incubation was 20 per cent for the soil and 131 per cent for the sand-plant material mixtures. A soil suspension was added to the sand to inoculate it with a microflora similar to that inhabiting the soil. In all cases, 75-g samples of soil, in duplicate, were incubated in 500-m1. Erlenmeyer flasks in a closed system with CO2- free air passing over the soil. The air was washed free of CO. by passa ge through a sintered-glass air dispersion tube to produce small bubbles, through NaOH solution in a lar ge flask, and 0.25 N NaOH solution in air-washing vessels equipped with sintered-glass air dispersion tubes. The CO 2 was quantitatively recovered in the first vessel, but for radiological safety, a second absorption vessel was attached. The absorption vessels were changed at intervals throughout the incubation period and the contents transferred to volumetric flasks. An aliquot was titrated to determine total CO 2 and a second aliquot used to determine comparative radioactivity levels by means of liquid scintillation counting. Table 1. Composition of C 14-labelled Plant Parts used in Decomposition Studies* Plant material Nitrogen Carbon 0 51-day-old corn tops day-old corn roots Mature corn stalks Mature corn leaves Mature corn roots Immature wheat tops /.0 Immature wheat roots / 0 Mature wheat straw Mature wheat roots day-old soybean tops day-old soybean roots day-old soybean leaves day-old soybean roots day-old soybean leaves t day-old soybean roots t * The wheat plants were labelled at about 40 pc/g of carbon. All other plant materials were labelled at about 16 pc/g of carbon t Mature. then washed with water in the same manner. The air flow was controlled by means of a calibrated capillary tube in the air line leading to each Erlenmeyer flask. The CO 2 evolved durin g respiration of the soil micro-organisms was trapped in standard One millilitre samples of NaOH solution containing C14 were transferred to 20-m1 counting vessels and the vessels filled with scintillation phosphor. The scintillation phosphor was of the following composition: 385 ml xylene, 385 ml dioxane, 230 ml absolute ethyl alcohol, SOME INTER-RELATIONSHIPS MEASURED BY C-14 LABELLING g naphthalene, 5 g PPO (2,5-diphenyloxazole) and 50 mg POPOP (1, 4-di 12-(5-phenyloxazolyl)] benzene). This scintillation phosphor is miscible with about 7 per cent of water but will become slightly turbid if more than about 0.06 meq of NaOH is present in the counting sample. The turbidity will not materially reduce the counts if they are counted immediately after preparation, but precipitation will lower the counts if the samples are allowed to stand overnight. For experiments in which the soil was not preincubated, the incubation intervals were 0. 5, 1, 1. 5, 2, 4, 8, 14, 28, and 59 days. Since rapid decomposition had subsided by the end of 28 days, later studies on preincubated soils were limited to incubation periods of 2, 4, 8, 14, and 28 days. RESULTS The results of the incubation experiments in Rhich the soils were not preincubated before addition of the C 14-labelled plant material are given in Tables 2 and 3 for 28 and 59 days, respectively. The control soil carbon loss fi gures are from soil that was incubated without plant material addition. The three different sets of values represent three experiments. With plant material added to the soil, the losses that were found from soil and from the added plant material are reported in the second and third columns. These values were calculated from the tracer evaluation. The control soil carbon loss minus the soil-carbon loss is a simple subtraction of the second column values from the first column values. Any negative values in the fourth column represent acceleration of decomposition of the indigenous soil or ganic matter, while positive values represent a decrease of soil organic matter decomposition induced by the addition of fresh plant material. The fifth column gives the per. cent of the plant carbon remaining in the soil at the termination of the incubation experiments. These results show that the majority of plant materials protected the indigenous soil organic matter from decomposition rather than accelerated its loss. Although five cases of acceleration of organic matter loss are shown in Tables 2 and 3, statistical analysis of the data shows them to be no different from the soil control. The results of incubation experiments in which the soil was preincubated for 14 days before addition of the 04-labelled plant material are given in Table 4. The reported results were obtained in three incubation experiments. The preincubation soil carbon loss ranged from 83 to 88 mg per 75 g of soil. The control soil carbon loss values varied somewhat more with one set at 66 mg, another at 77 mg, and the third at 80 mg carbon loss per 75 g of soil. This degree of variability seems to be inherent in soil incubation experiments. In these experiments, as was also observed in the earlier experiment without preincubation, the plant tops decomposed more rapidly than their respective roots. Considerably less soil or ganic matter decomposition was found in a comparable time after addition of the plant material in the preincubation experiments than in the nonpreincubation experiments. This, of course. resulted from the disappearance of the readily decomposable fraction during the preincubation. The preincubation experiments furnished even stron ger evidence that the added plant material protected the soil or ganic matter from decomposition. Increased loss of soil organic matter was obtained only in the case of 44- day-old soybean roots. In contrast, the mature cereal residue markedly retarded soil or ganic matter decomposition, with the reduction ranging from 28 to 53 per cent. The single ne gative value reported that would su ggest acceleration of soil organic matter decomposition was not si gnificantly different from the soil control. 228 J. H. SMITH The data presented in Table 5 are from another experiment similar to those reported in Table 4 except that the plant material was ground to pass a 60-mesh sieve. The results are substantially the same as those reported in Table 4 and again the evidence supports the contention that added plant material protected soil organic matter from decomposition. The negative values for soybeans are again not significantly different from the soil control. The cereal plant parts, except for corn stalks. showed a significant soil organic matter protection. Table 6 shows the results of an experiment conducted to determine the influence of nitro gen rates on the decomposition of youn g and mature corn roots in soil and on loss of soil organic matter. When mature corn roots were added to soil, the increments of nitroaen increased the rate of soil carbon loss. Plant carbon loss did not chan ge except for a slight increase with the last nitrogen increment. The mature root material remaining at the end of the incubation period was nearly constant, but decreased with the last increment of nitrogen. When the young corn roots were added to the soil, the addition of 100 p.p.m. of nitrogen decreased both the soil carbon and the plant carbon losses. The values reported in Table 4 where 100 p.p.m. N was added to the soil, correspond fairly closely to the values shown in Table 6 for both the young and mature corn roots at the 100- p.p.m. N rates. The failure of nitroaen to increase the decomposition rate of the added plant roots is not surprisin g, because the mature corn roots Table 2. Carbon Loss and Balance from Soil and Carbon -labelled Plant Material Incubated for 28 da ys with the Soil not Preincuhated* Control soil C loss to soil Soil C loss Plant C loss Soil control C loss Plant C minus soil C loss remaining me mg mg mg 51-day-old corn tops bcd 19 Mature corn stalks bcd 15 Mature corn leaves cd 14 Mature corn roots a 21 Mature wheat straw '70 4 bed 13 Mature wheat roots bed 27 Immature wheat tops be 4 Immature wheat roots be 27 2S-day-old soybean tops d 4 28-day-old soybean roots bcd day-old soybean leaves b S 44-day-old soybean roots be day-old soybean leavest bed day-old soybean rootst be 32 LSD 19 * According to Duncan multiple-range comparisons, means followed by an identical letter do not differ significantly.
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