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Appendix C
Nitrogen Transformation in the Decomposition of Natural Organic Materials at Different Stages of GrowthS. A. Waksman and F. G. Tenney, New Jersey Agricultural Experiment Station, U.S.A.
(Proceedings and Papers of the first International Congress of Soil Science, Washington, D C, 1927, p. 209.)
To be able to understand the reasons for the rapidity of liberation of nitrogen from the decomposition of plants at different stages of growth, we must know the composition of the plant at these various stages and the nature of decomposition of the various plant constituents. Although the plant continues to assimilate nutrients, including nitrogen, until maturity, the percentage of nitrogen in the plant reaches a maximum at an early stage, then gradually diminishes, reaching a minimum at maturity or a little before maturity. This is true not only of nitrogen but also of certain other elements.
Plant materials decompose more rapidly and the nitrogen is liberated more readily (in the form of ammonia) at an early stage of growth and less so when the plant is matured. Two causes are to be considered here: (1) the rapidity of decomposition of the various plant constituents; (2) the relation of the nitrogen to the carbon content of the plant tissues.
At an early stage of growth, the plant is rich in water-soluble constituents, in protein and is low in lignins. When the plant approaches maturity, the amount of the first diminishes and of the second increases. The water-soluble constituents, the proteins and even the pentosans and celluloses decompose very rapidly provided sufficient nitrogen and minerals are available for the micro-organisms. The lignins do not decompose at all in a brief period of time of one or two months. More so, their presence has even an injurious effect upon the decomposition of the celluloses with which they are combined chemically or physically. The larger the lignin content of the plant the slower does the plant decompose even when there is present aufficient nitrogen and minerals.
It has been shown repeatedly that the organisms (fungi and bacteria) decomposing the celluloses and pentosans require a very definite amount of nitrogen for the synthesis of their protoplasm. Since the cell substance of living and dead protoplasm always contain a definite, although varying, amount of nitrogen and since there is a more or less definite ratio between the amount of cellulose decomposed and cell substance synthesized, depending of course upon the nature of the organisms and environmental conditions, the ratio between the cellulose decomposed and nitrogen required by the organisms is also definite. This nitrogen is transformed from an inorganic into an organic form. Of course in normal soil, in the presence of the complex cell population, the cell substance soon decomposes, a part of the nitrogen is again liberated as ammonia and a part remains in the soil and is resistant to rapid decomposition. The amount of nitrogen which becomes available in the soil is a balance between the nitrogen liberated from the decomposition of the plant materials and that absorbed by the micro-organisms which decompose the non-nitrogenous and nitrogenous constituents. The younger the plant, the higher is its nitrogen content and the more rapidly does it decompose, therefore the greater is the amount of nitrogen that becomes available. The lower the nitrogen content of the plant the less of it is liberated and the more of it is assimilated by micro-organisms.
These phenomena can be brought out most clearly when the same plant is examined at different stages of growth. The rye plant was selected for this purpose. The seeds were planted in the fall. The samples taken on April 28th (I), May 17th (II), June 2nd (III), and June 30th (IV). In the third sampling the plants were divided into (a) heads, (b) stems and leaves. The fourth sample was divided into (a) heads, (b) stems and leaves, (c) roots. The plants were analysed and the rapidity of their decomposition determined, using sand or soil as a medium and 2 g. of the organic matter. In the case of sand some inorganic nitrogen and minerals were added and a soil suspension used for inoculation. The evolution of carbon dioxide and accumulation of ammonia and nitrate nitrogen was used as an index of decomposition. Tables I and II show the composition of the plant and the amount of nitrogen made available after 26 days of decomposition.
Table I
Composition of Rye Straw at Different Stages of Growth on a Dry BasisNo. of sample Moisture content at time of harvest Ash Nitrogen Cold water soluble fraction Pentosans Cellulose Lignin % % % % % % % I 80.0 7.3 2.39 32.6 15.9 17.2 9.9 II 78.8 5.7 1.76 22.0 20.5 26.1 13.5 IIIa 57.4 4.9 1.01 18.2 22.7 30.6 19.0 IIIb 60.2 5.9 2.20 20.3 22.7 20.1 16.0 IVa 15.0 3.2 1.22 4.7 11.9 4.6 13.4 IVb 15.0 3.7 0.22 9.5 21.7 34.6 18.8 IVc ? ? 0.55 4.7 26.6 37.7 21.0
Table 2
Decomposition of Rye at Different Stages of Growth(2 g. of dry material added to 100 g. of sand or soil medium) Date of sampling No. of sample Nitrogen content of material CO2 given off in 27 days Available Nitrogen (NH2-N-NO2-N) absorbed (+) or liberated (-) Sand medium Soil medium % mg.C. mg.C Sand medium N. Soil medium N. April 28 I 2.39 337.7 286.8 +10.1 +22. 2 May 17 II 1.76 280.5 280.4 +0.8 +3.0 June 3 IIIa 1.01 215.7 199.5 -12.1 -7.5 June 3 IIIb 2.20 261.9 244.8 +5.7 +7.5 June 30 IVa 1.22 269.9 273.7 -4.4 -2.1 June 30 IVb 0.22 221.4 187.9 -16.0 -8.9 June 30 IVc 0.55 187.0 158.4 -8.1 -9.4 Root material used in the decomposition was equivalent to 1.67 g. of moisture free and ash-free organic matter.
When a plant material contains about 1.7 per cent nitrogen, as in the rye of the second sampling, there seems to be sufficient nitrogen for the growth of micro-organisms which decompose this material more or less completely. When the plant material contains less than 1.7 per cent of nitrogen, as in the case of the stems and leaves of the third preparation, additional nitrogen will be required, before the organic matter is completely decomposed (speaking, of course, relatively, since if a long enough period of time is allowed for the decomposition, less additional nitrogen will be needed). If the organic material contains more than I.7 per cent nitrogen, as in the case of the plants in the first planting and the heads of the third sampling, a part of the nitrogen will be liberated as ammonia, in the decomposition processes. The difference between the nitrogen content of the heads and this hypothetical figure = 0.5 (2.2 - 1.7) per cent. or 10 mg. nitrogen for the 2 g. of organic matter; actually 5.7 mg. and 7.5 mg. of nitrogen were liberated as ammonia in the sand and soil media respectively. The difference between the hypothetical figure and the nitrogen content of the stems and leaves was 0.69 (1.7 - 1.01) per cent or 13.8 mg. nitrogen for the 2 g. of plant material used. Actually 12.1 and 7.5 mg. of nitrogen were consumed in the sand and soil media. Had the decomposition been allowed to proceed further, the results would have approached from both directions the hypothetical figure and, with prolonged decomposition (of synthesized substances), would have exceeded it.
The decomposition of 10g. dry portions of the second sampling and 20g. dry portions of the stems and leaves of the fourth sampling was studied separately in a sand medium containing available nitrogen and minerals. Only the data for the organic matter portion, insoluble in ether and water, are reported. The results show that the pentosans and celluloses are rapidly decomposed, while the lignins are affected only to a very inconsiderable extent. The nitrogen figures are of direct interest here. Just about as much insoluble protein was left in the first as in the second experiment: in the first the protein is considerably reduced, in the second increased. This tends to explain the activities of the micro-organisms in the soil.
Table 3
Composition of Organic Matter at Beginning and End of DecompositionSample II Organic matter (free from ether and water-soluble substances and ash) At beginning of experiment At end of experiment mg. mg. 7,465 2,015 Pentosan 2,050 380 Cellulose (calculated) 2,610 610 Lignin 1,180 750 Protein (insoluble in water) 816 253 Unaccounted for 8.6% 1%
Table 4
Composition of Organic Matter at Beginning and End of DecompositionSample IV -- Stems and Leaves Organic matter present (free from ether and water-soluble substances and Ash) At beginning of experiment At end of experiment mg. mg. 15,114 8,770 Pentosan 3,928 1,553 Cellulose 6,262 2,766 Lignin 3,403 3,019 Protein 181 519 Unaccounted for 10.25% 10.41%
The results show that since there is a very definite ratio between the energy and nitrogen consumption of the microorganisms decomposing the organic matter, it is easy to calculate, given a certain amount of plant material and knowing its nitrogen content, whether nitrogen will be liberated in an available form or additional nitrogen will be required within a given period of time. Calculations can also be made as to how much of this nitrogen is required for the decomposition of the plant material and how long it may take before the nitrogen is again made available.
Next: Appendix D. An Experiment in the Management of Indian Labour
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