REPEATED WET-DRY CYCLES DO NOT ACCELERATE THE MINERALIZATION OF ORGANIC C INVOLVED IN THE MACRO-AGGREGATION OF A SANDY LOAM SOIL

Repeated mild wet-dry cycles were imposed on a sandy loam to accelerate the mineralization of organic C involved in stabilising macro-aggregates. Soil maintained continually moist (control soil) was compared to that subjected to a series of 6 wet-dry cycles. Two patterns of rewetting and drying were investigated: (1) incubated dry at 25 degrees C for six days between each wet-dry cycle (dry-incubated), or (ii) incubated moist for six days at 25 degrees C between each cycle (moist-incubated). Changes in the proportion of >2 mm, 1-2 mm, 0.5-1 mm and 0.25-0.5 mm aggregates, and carbohydrate C extracted by hot-water or hot-1.5 M H2SO4, were measured after each wet-dry cycle, or weekly in the continuously moist control soil. Respiration rates (CO2 efflux) were measured during the incubation of the moist soil between the wet-dry cycles and compared with the continually-moist control soil. The wet-dry treatments did not increase soil respiration in soil after re-wetting compared to soil kept continually moist and incubated for the same period of time. Despite this, the treatments caused changes in the amounts of acid- and water-extractable carbohydrate C fractions and substantial changes in aggregation. Macro-aggregation and the proportion of soil in each fraction did not change in the soil maintained continuously-moist for 6 weeks (control). However, effects of the two wet-dry treatments on total macro-aggregation were similar to those in the >2 mm, 1-2 mm and 0.25-0.5 mm aggregate fractions: there was a rapid decline in aggregation by 48-65% over the first two cycles, a sharp recovery to 78-100% of the initial aggregation after three cycles, and a further decline after 4-6 cycles. The resistance of organic C mineralization to mild wet-dry cycles confirmed that the organic C in this soil is very stable and resistant to decomposition. Despite aggregates being disrupted, the organic C stabilising these aggregates was resistant to decomposition as determined by CO2 efflux. When soil was re-moistened and incubated to allow microbial re-colonization, aggregation was similar to that in the soil where microbial re-colonization was limited by rapid drying treatments. Short term changes in the aggregation of this soil appear to be dominated by chemical and/or physical processes.