Thursday, February 8, 2007

Soil Aggregation and C Sequestration: Effect of Management Practices.

Karina Fabrizzi1, Charles W. Rice1, Telmo Amado2, Jackson Fiorin3, Pedro Barbagelata4, and Ricardo Melchiori4. (1) Kansas State Univ., 2004 Throckmorton Plant Sciences Center, Manhattan, KS 66506-5501, (2) Soil Department, Federal Unversity of Santa Maria, Santa Maria, Rio Grande do Sul, 97119-900, Brazil, (3) FUNDACEP, RS 342 km 149, Cruz Alta, RS, 10-98100-970, Brazil, (4) Instituto Nacional de Tecnologia Agropecuaria, Estacion Experimental de Parana, Parana, Argentina

Management practices can affect soil aggregation and soil organic carbon (SOC) storage. The objective was to determine the influence of long-term management practices on soil aggregation and its relationship with C sequestration. Six long-term experiments from Kansas (Mollisols, Alfisols), USA,  Brazil (Oxisols), and Argentina (Vertisols) were sampled for SOC and aggregation. Three sites (KS, Brazil, and Argentina) included a native grassland site for comparison.  Tillage systems included conventional tillage (CT), reduced tillage (RT) and no-tillage (NT). N fertilization was evaluated at two sites. Crop rotation (one site) included continuous wheat (W-W), continuous sorghum (S-S), continuous soybean (B-B), wheat-soybean (W-B), and sorghum-soybean (S-B). Soil samples were taken at 0-5 cm and measured water-stable aggregates (WSA), total C and N. Cultivation decreased the amount of macroaggregates and increase microaggregates. Rotations also affected aggregation and the associated C and N, where W-W and S-S under NT produced greater amounts of macroaggregates and soil C, and B-B under CT had the least amounts of macroaggregates and soil C.  In the Oxisols, NT had greater amounts of large macroaggregates (>2000 µm) than CT. In the Vertisols and Mollisols, NT tended to have greater amounts of large macroaggregates than CT. In general, C and N in the native site of the Oxisols were similar or greater than NT but greater than CT. In the Vertisols, NT had greater C and N concentrations in the macroaggregate than the native site. In the Mollisols, the native site had greater C and N concentrations in the aggregates than both tillage systems. Cultivation of native site reduced the mass of macroaggregates and the associated C and N; however NT tended to be more similar to the native grassland sites. The increase in the C associated with macroaggregates reflects the increase in soil carbon related with management in each site.