Soil Eiol. Eiochem. Vol. 15, No. 6, pp. 687-691, 1983 Printed in Great Britain. All rights reserved 0038-0717/83 $3.00 + 0.00 Copyright 0 1983 Pergamon Press Ltd GROWING CROPS AND TRANSFORMATIONS OF 14C-LABELLED SOIL ORGANIC MATTER J. B. REID* and M. J. Goss Agricultural Research Council Letcombe Laboratory, Wantage, Oxon OX12 9JT, U.K. zyxwvutsrqponmlkj (Accepted 20 July 1983) Summary-Transformations of native soil organic materials, previously labelled with 14C, were in- vestigated in soil planted with maize or perennial ryegrass and in fallow controls. There was more “‘C in cold water extracts from planted soils than from fallow controls-an effect apparently caused by suppression of processes that remove labelled organic materials from this fraction of the soil organic matter. Decomposition of the labelled organic matter to “‘CO, was significantly less in the planted soils than in fallow controls. INTRODUCTION The rates of transformation of organic matter in cropped soils has an important influence on both physical and chemical aspects of soil fertility. Reid and Goss (1982a) demonstrated that compared with fallow controls the growth of maize and of perennial ryegrass roots slowed considerably the rate of evo- lution of 14C0, from labelled organic matter incorpo- rated in the soil. These results accorded well with those of Sparling et al. (1982) who also measured the levels of 14C02 evolved from labelled soil organic matter and observed that the evolution from soils supporting barley plants was 70% less than that from control soils supporting no plants. Our purpose was to characterize further the nature of this effect of growing roots on organic matter transformations. Previous research (Reid and Goss, 1981, 1982b) had suggested that in the presence of maize roots the efficacy of native soil polysaccharides as aggregate- stabilizing agents was impaired, hence we have paid particular attention to the polysaccharide com- ponents of the soil organic matter. MATERIALS AND METHODS The details of the incorporation of 14C into the soil organic matter and the techniques for growing plants while collecting the evolved 14C0, were described by Reid and Goss (1982a). In brief, 25 seedlings of barley (Hordeum vulgare cv. Midas) were planted in a tub of sieved (< 2.0 mm) soil taken from the Ap horizon of a Sutton series sandy loam (2% organic C, 0.22% total N). Once established, the plants were pulse-labelled on 6 separate occasions with 14C02 to give a total of 5 mCi per tub. When the plants had been growing for 100 days and were flowering, the shoots were cut off and the roots left to decompose for 67 days. The soil was kept moist and disturbed *Present address: Department of Soil Science, Lincoln College, Canterbury, New Zealand. with a trowel twice to encourage incorporation. The labelled moist soil was then sieved (< 2.0 mm) and wetted to a water content of 22% (w/w). Afterwards the equivalent of 344 g o.d. soil was sealed into each of 20 glass jars adapted to allow continuous passage of air through the soil. Seedlings could be sealed into the top of each pot or the jars left fallow. Throughout the experiment the gravimetric water content of the soil was maintained close to 22% (giving an air-filled porosity of 20%) by adding water with a syringe through a septum. The experiment is scheduled in Table 1. Soil anaIyses All pots were used to provide soil samples during the experiment, the time of sampling depending on treatment. There were 6 treatments. Soil with maize grown for 22 days (Maize soil) and soil in which ryegrass was grown for 42 days (Ryegrass soil) were the two main treatments. Each had two “control” treatments, fallow soil sampled at the time the crop was sown (sowing date control) and fallow soil sam- pled at the same time as the planted soil @llow control 1 for maize andfallow control 2 for ryegrass). The four pots forming the fallow control for the maize soil also served as the sowing date control for the ryegrass soil. The 14C0, evolved from each jar was trapped in ethanolamine and the amount released was measured by liquid scintillation counting every 10 days or so during plant growth. Fractions of the soil organic matter were prepared from the fresh soil from each jar and the 14C contents (expressed in counts min-‘) measured using a Beck- man lowbeta II planchette counting system after the subsamples had been acidified (if necessary) and dried into filter papers. With the use of appropriate stan- dards the total activity in each fraction was calculated as o.d. gg’ soil. The following fractions were pre- pared. Fraction A-O.5 M NaOH extractable organic matter. 73.48 Fresh soil was added to 200mI OSMN~OH and stood for 17 h at 20°C. The sus- pension was then shaken for 30 s and centrifuged at 687