C. M. Mundie

THE ORIGIN OF SOIL POLYSACCHARIDE: TRANSFORMATION OF SUGARS DURING THE DECOMPOSITION IN SOIL OF PLANT MATERIAL LABELLED WITH 14C

Summary Incubation of soil with 14C-rye straw for 448 days resulted in the evolution of about 50 per cent of the carbon of the substrate as CO2 The two main sugars of the straw, glucose and xylose, were degraded to approximately the same extent (70 per cent). The same results were obtained whether the soil was derived from granitic or basic igneous parent material. There was very little transformation of the substrate to galactose, mannose, arabinose, rhamnose, or fucose, and a much slower rate of degradation than with soil incubated with 14C-glucose over a similar period. Hydrolysis of the soil samples by a preliminary treatment with 5 N H2SO4, before treatment with 24 N H2SO4, followed by heating with N H2SO4 did not release significantly greater amounts of sugar than treatment with 24 N H2SO4 and N H2SO4 alone. Separate analysis of the hydrolysates showed that 90 per cent of each of galactose, mannose, arabinose, xylose, rhamnose, or fucose had been extracted by 5 N H2SO4, but only 50 per cent of the glucose. Fractionation of the straw-soil mixture after 224 days incubation showed that the specific activity of the glucose was higher in the humin fraction than in the fulvic acid, as would be expected if the remaining 14C were still in the form of unchanged plant material. This evidence that plant polysaccharide persists in soil could explain the presence of much of the xylose in the soil organic matter.

Transformation of 14C glucose and starch in soil

Abstract To gain information about the breakdown and synthesis of carbohydrate, incubations of soil with 14C-stareh and 14C-glucose were studied under laboratory conditions. Carbon dioxide was liberated equivalent to 60–80 per cent of the substrate during incubation of soil with glucose (14 days) and with starch (84 days). The carbohydrate content of the hydrolysate of the substrate-soil mixture returned almost to the initial soil value within 35 days of incubation with 0.5 per cent starch, and within 7 days with 1 per cent glucose. The total glucose in the mixtures rapidly decreased but the amounts of other sugars present showed little change during incubation. To obtain the specific activities of sugars present in the hydrolysate various methods of separation were used including charcoal column, Celite column, and paper chromatography of the free sugars, and resin column chromatography of their borate complexes. With both substrates there was a rapid redistribution of 14C amongst the sugars. Galactose and mannose acquired considerable activity in all cases, rhamnose and fucose became labelled in one experiment, but arabinose and xylose were not labelled. This pattern of distribution remained unchanged with further incubation and glucose remained the most highly labelled sugar (70–80 per cent of total sugar radioactivity) for as long as the incubations were studied.

Influence of soil type, crop and air drying on residual carbohydrate content and aggregate stability after treatment with periodate and tetraborate

The relationship between the water stability of microaggregates and the residual carbohydrate content of soil was examined in 15 soils from 7 soil series under various cultivations. The carbohydrate was progressively removed by increasing the time of treatment with 0.02 M periodate and 0.1 M tetraborate. The resulting decrease in reducing sugar content was significantly correlated with an increased disruption of microaggregates (> 45 μm) as determined by a turbidimetric method. The most effective treatment removed about 80% of the soil carbohydrate and caused an increase of about 75% in the fraction of microaggregates (< 45 μm) compared to untreated soil.

Organic matter contributed to soil by plant roots during the growth and decomposition of maize

Maize (Zea mays var. Caldera) plants were grown under sterile and not sterile conditions in soil in an atmosphere continuously enriched with 14CO2 for 36 days. At harvest the above ground parts of the maize were cut off and the roots were separated from the soil by washing with water. The soil was dispersed using ultrasonics and separated into soluble clay silt and sand fraction. Roots were included in the coarse sand fraction. 25% of the total label present in the soil ≡ 5.5% of that in the soil-plant system, was water soluble. Very little label was present in the clay and silt fractions (5% in each) and most (65%) was in the sand fraction as root material.Rapid extraction of soil after the removal of roots without ultrasonic treatment released soluble matter which amounted to <0.5% of the total activity in the soil-plant system.Isolated roots steeped in water released about 18% of their activity. Much of the soluble fraction may therefore be root lysate.The soil and roots accounted for 22% of the total activity in the soil-plant system. Glucose accounted for 89% of the sugars in the soluble fraction of the soil.78% or more of the 14C present in glucose, arabinose and xylose constituents of the root-soil mixture occurred in the coarse and fine sand fractions, which also included root material. For mannose and galactose the value was 70% and for rhamnose, 50%.After reinoculation of the soil-root mixture and decomposition for 56 weeks, the water soluble material obtained on fractionation of the soil decreased to less than 1% of the total activity. A much greater proportion, 25%, was present in the clay fraction as a result of decomposition.

Effect of various procedures on the yield of methylated carbohydrate substances obtained from soil

Abstract The molecular shape of a polysaccharide, which determines its cohesive and water retention properties, depends on the types of bonding present between sugar residues. These may be determined by permethylation and hydrolysis. Neither acetylation, nor reduction of soil prior to methylation, increased the overall yield of methylated product, although acetylation resulted in a greater proportion being released in the first methylation which may avoid some s‐elimination reactions on remethylation. Reduction by NaBH4 before each methylation step did not result in any large differences in the proportions of different sugar residues, suggesting that s‐elimination reactions did not have a prominent effect in the remethylation of the soil residue. Grinding and freeze‐milling of the soil prior to methylation gave a greater release of methylated product in the first three methylations and the yield was about 15% greater overall.