Effect of conversion from ryegrass pasture to wheat cropping on aggregation and bacterial populations in a silt loam soil in New Zealand

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Abstract
Water-stable aggregates and soil bacteria were studied from May to January in a field trial comparing soil structure under three treatments: wheat sown in ploughed soil, direct-drilled wheat and long-term grass. During the trial, both the proportion and size of macroaggregates (> 250 µm) tended to increase. These increases were accompanied by decreases in the proportions of particles in all smaller size ranges, including 2000 µm) increased at the expense of particles between 2000 and 63 µm. The highest proportion of larger macroaggregates occurred under grass, in all except the spring sampling, while the lowest proportion occurred in the ploughed soils. Numbers of bacteria, obtained by direct microscopy, were higher in the ploughed soil soon after ploughing than in the direct-drilled soil, but no other treatment effect was detected. In all plots, numbers of bacteria were lowest in November when the soil was the driest. There were up to twice as many bacteria per g of < 20 µm diameter fraction soil compared with the rest of the soil. Seasonal changes in the bacterial population of the <20 µm diameter fraction did not follow the changes in the whole soil.