Saturday, 15 July 2006

Aggregate Associated Sulphur Fractions in Soils under Long-Term Fertilization Experiment.

Bal Ram Singh1, Zhihui Yang1, and Sissel Hansen2. (1) Norwegian Univ of Life Sciences, Aas, N-1432, Norway, (2) Norwegian Centre for Ecological Agriculture, Tingvoll, NO-6630, Norway

In recent years, an increased frequency of sulphur deficiency has been observed all over the world. Soil is the main source for sulphur nutrition of crops and sulphur bioavailability is dependent on sulphur fractions. However, little is known about sulphur fraction in soils, especially in aggregate associated sulphur fractions. A long-term experiment established in 1922 on a morainic loam soil in Southeastern part of Norway was chosen to investigate the effects of long-term fertilization on S fractions in bulk soil and those associated with different aggregate sizes. The results showed that Microbial Biomass S (MBS) ranged from 5.1 to 8.6 mg kg-1, averaging 6.8 mg kg-1 and accounting for 1.2% of total S in bulk soils. The higher MBS content was observed in Farm Yard Manure (FYM) plots, and the lowest values in nitrogen and potassium (NK) plots. The aggregate size of 6-20 mm in control (no fertilizer) and chemical fertilizer treatments (NPK and NK) and the aggregate size of 2-6 mm in FYM plots contained the highest content of MBS. The values of MBS decreased with the decreasing size of aggregates. Inorganic S (SO42- -S) varied from 2.2 mg kg-1 in NK plots to 18 mg kg-1 in NPK plots. The differences of inorganic S between aggregate sizes were not pronounced. Organic S was the main fraction and accounted for 97-99% of total S in bulk soils. For organic S, ester S, carbon-bonded S and residual S fractions accounted for 39-52%, 6-10% and 38-51%, respectively. Long-term fertilization resulted in the accumulation of ester S, carbon-bonded S and residual S. The macroaggregate size (>2mm) contained the highest concentration of ester S, particularly in the FYM treatment. But microaggregates (< 1mm) exhibited higher carbon-bonded S than macroaggregates. However, both chemical fertilizers and FYM resulted in accumulation of carbon-bonded S in microaggregates, but the amount C bonded S was higher in FYM than in chemical fertilizers. A significant accumulation of total soil S was observed in the plots receiving either chemical fertilizer or FYM for more than 80 years. Application of FYM generally exhibited a higher total S content than chemical fertilizers (NPK and NK). Between chemical fertilizers, NPK increased the total S by 12% over NK application. On average, the mean total S concentration for all treatment in water stable aggregates of >2 mm, 1-2mm, 0.5-1 mm, 0.25-0.5 mm, 0.106-0.25 mm and <0.106 mm was 612, 591, 459, 472, 441 and 625 mg kg-1, respectively. The macroaggregate sizes (>1 mm) and the finest aggregate size (<0.106 mm) in all treatments showed a significantly greater total S than other microaggregate sizes. The results implied that long-term fertilization contributed to total S build up in soils, particularly in carbon-bonded S and residual S fractions. Formation of macroaggregates from microaggregats resulted in accumulation of ester S.

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