SMALL-SCALE HETEROGENEITY OF SOIL CHEMICAL-PROPERTIES .1. A TECHNIQUEFOR RAPID AGGREGATE FRACTIONATION

Several recent studies have observed physical, chemical and microbiolo gical heterogeneities on the scale of soil aggregates. The publication s emphasize the ecological importance of these small scale gradients. This paper introduces a method for the rapid fractionation of soil str ucture units into a surface fraction and a core fraction. The techniqu e combines a rapid freezing in liquid N-2 and standardized wet sieving of the structure units. During the sieving process the aggregate surf aces begin to thaw and are consequently washed through the sieve. The soil passing the sieve during a specified time is termed aggregate sur face fractions, the remainder is the aggregate core fraction. Samples from 18 forest soil profile were taken to study the precision and accu racy of the method, as well as factors determining the efficiency of f ractionation. The precision of aggregate fractionation as determined b y parallel fractionations yielded coefficients of variability of 5.3 t o 7.5 % and 6.5 to 11.4 % for the amount of core and surface fraction, respectively. Furthermore, the proposed technique yielded results com parable to those obtained by manual razor-peeling of the aggregate sur faces. The variation of concentrations of chemical parameters in water extracts of both aggregate fractions was up to six times greater than in soil solutions obtained from mixed samples. Increasing soil organi c C resulted in a decrease in the proportion of the aggregate surface soil collected under fixed sieving conditions, while clay content corr elated positively with the amount of the aggregate surface fraction co llected.