Influence of mixed cropping rotations (pasture-arable) on organic matter content, water stable aggregation and clod porosity in a group of soils

The effects of previous cropping history on soil organic matter content, aggregate stability and clod porosity were investigated on three soil types commonly used for mixed cropping in the Canterbury region of New Zealand. An index of previous cropping history (number of years under pasture or arable cropping immediately prior to sampling) and the hot water-extractable carbohydrate fraction were closely correlated with aggregate stability. Total N, organic C, acid-hydrolysable and cold water-extractable carbohydrate were moderately well correlated with aggregate stability whilst HCl- and NaOH-extractable carbohydrates were poorly correlated with aggregate stability. Aggregate stability was correlated with clod porosity when arable and pasture samples were analysed separately, but not when all soils were included. In many of the typical short-term mixed cropping rotations used in the study area the total soil organic matter content remained relatively unchanged yet microbial biomass C, hot water-extractable carbohydrate and aggregate stability increased markedly during the pasture phase and declined during the arable period. It is suggested that the increase in aggregate stability during the short-term pasture is due principally to production of binding carbohydrates (which are hot water-extractable) by the large microbial biomass present in the pasture rhizosphere. When the pasture is ploughed under the microbial biomass declines as does aggregate stability. The increase in aggregate stability following pasture establishment is considerably more rapid than the increase in clod porosity. Thus, at a similar clod porosity, pasture aggregates had a higher aggregate stability than their arable counterparts. © 1991.