Combining Models And Data To Assess Options For Soil C Sequestration

Two of the biggest constraints for improving household food security in West Africa are retention of rainwater in the field and improvement of soil quality (Kaya, 2000; Lal, 1997a, 1997b; Ringius, 2002; Bationo et al., 2003). The practice known as ridge tillage or aménagement en courbes de niveau (Gigou et al., 2000) was designed to address these issues concurrently, and is thought to have potential for sequestering SOC. This is logical since ridge tillage increases crop biomass production and grain yield in Mali (Gigou et al., 2000). Unfortunately, data for evaluating SOC sequestration potential in West Africa are scarce (Lal, 1997a, 1997b; Pieri, 1992; Ring-ius, 2002). Estimates of SOC sequestration potential are needed to help guide research and to give donors confidence that their investments will succeed. Soil C measurements taken by Yost and colleagues (Yost et al., 2002; Neely and Uehara, 2002) show that SOC levels in Mali are very low (ranging from 0.13% to 0.88% of soil mass) in the top 20 cm, and that fields that have been under ridge tillage for several years tend to have higher SOC levels than fields under conventional tillage. However, few measurements have been made to date, and thus no conclusions can be made regarding how much SOC will increase under RT, nor how long it might take to achieve that increase. Our hypothesis was that RT, coupled with other soil management practices, could increase soil C in the top 20 cm of soil by 5 metric tons ha-1 over 10 years. Objectives were (1) to adapt the DSSAT-CENTURY maize model for simulating RT vs. CT management systems in Mali using available data, and (2) to conduct a 10-year computer experiment to make preliminary estimates of potential SOC sequestration amounts under CT vs. RT management systems.

This study demonstrates the adaptation of a cropping system model for studying management options for increasing soil carbon in Oumarbougou, Mali (Lat 12.18 N, Long 5.14

W). Rainfall in the region is 900 to 1000 mm per year, falling unimodally from June to October (Roncoli et al., 2002). The cultivated soils in the area are characterized as red sandy soils (bogo bile), generally alfisols with high sand content and low organic C and N. The area is highly prone to runoff and erosion, as is the case in much of West Africa (Bielders et al., 1996; Daba, 1999; Rockstrom et al., 1998; Zhang and Miller, 1996).

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