Saturday, 15 July 2006

Effects of Tillage on Soil Aggregate Distribution and Its Content in Glomalin and Carbohydrates.

Fernando Borie1, Alfredo Morales1, Rosa Rubio1, María Aguilera2, and Gilda Borie2. (1) Universidad de La Frontera, Avenida Francisco Salazar 01145, Temuco, Chile, (2) Universidad de Chile, Olivos 1007, Independencia, Santiago, Chile

Cultivation reduces carbon content and changes soil stability. Glomalin and carbohydrates have been reported as having an important role in the stability of soil aggregates. We investigated the effect of tillage system on some soil characteristics such as pH, C, N and S levels, total glomalin and carbohydrates contents in the whole soil as well as associated to aggregate-size classes (microaggregates:< 0.1, 0.10-0.25mm and macroaggregates: 0.25-1.0, 1.0-2.0, 2.0-5.0, > 5.0 mm). Measurements were in the sixth year of an on-going tillage-rotation experiment conducted on an Ultisol from Southern Chile under no-till (NT) and conventional tillage with stubble burnt (CT). In the whole soil pH was 0.1 pH units higher under NT than CT (5.43 and 5.33, respectively). Also in the whole soil under no tillage C, N and S contents accounted for 61.1, 5.2 and 0.88 g kg-1 respectively, being 21%, 24% and 24% higher than CT. Glomalin levels accounted for 7.18 and 5.76 g kg-1 for NT and CT treatments, and carbohydrates contents also decreased from 3.99 g kg-1 under NT to 3.40 under CT. This represents an increase of 24.6% and 17.4% for glomalin and carbohydrates, respectively when soil is not disturbed. In relation to the nature of C in the SOM, carbon associated to glomalin represented 5.06% and 4.92% of total organic C for NT and CT, respectively. On the other hand, C associated to carbohydrates represented 2.61 and 2.69, respectively. In relation to soil aggregation, microaggregates accounted for 15% on weight of total soil aggregates under NT in comparison with 9% observed under CT. On the contrary, the percentage of macroaggregates > 5mm were higher under CT (30%) than under NT suggesting that tillage favors the soil macroaggregate formation at the expenses of microaggregates. Glomalin and carbohydrates were higher in all soil aggregates-size classes under NT than CT observing the highest difference in soil aggregates > 5mmn. In summary, in spite of glomalin and carbohydrates contents were concentrated in soil microaggregates than in macroaggregates both cementing compounds were found in higher amounts in the last ones. In soil aggregates > 5 mm the increase of glomalin and carbohydrates under NT in comparison with CT were 73.5% and 71.8% respectively. It is concluded that glomalin and carbohydrates are important compounds involved in the aggregation of the volcanic soil used.

Acknowledgements: Fondecyt Project Nº 1020306

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