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SOIL & WATER MANAGEMENT & CONSERVATION

Building Up Organic Matter in a Subtropical Paleudult under Legume Cover-Crop-Based Rotations

^a Univ. Federal do Rio Grande do Sul, C.P. 776, 90001-970 Porto Alegre (RS), Brazil
^b One Shields Ave., Dep. of Plant Sciences, Univ. of California, Davis, CA 95616-8780

^* Corresponding author (cimelio.bayer{at}ufrgs.br).

The potential of conservation management systems to ameliorate degraded agricultural soils and mitigate global warming is related to their potential for long-term stabilization of soil organic matter (SOM). This study was performed in a 19-yr-old experiment that was set up on a degraded Paleudult (220 g kg^–1 clay) in southern Brazil to (i) evaluate the effect of seven no-till crop rotations (grass- and legume-based cover crop systems) and mineral N fertilization (0 and 145 kg ha^–1 yr^–1) on soil organic C (SOC) stocks (0–17.5-cm depth) and (ii) estimate rates of SOM dynamics in these systems under subtropical climate conditions. Annual C input (shoot + root) ranged from 2.61 to 7.84 Mg ha^–1, with the highest values in legume-based and N-fertilized cropping systems. The SOC stocks were closely related to C input levels, and a minimum C input of 4.05 Mg ha^–1 yr^–1 was estimated to maintain the original SOC stock of 31.38 Mg ha^–1. Based on the one-compartment model of SOM dynamics, the SOM decomposition rate was estimated to be 1.2% and the humification coefficient was estimated to be 9.6%. After 19 yr, the stock of the original SOC decreased to about 24.78 Mg ha^–1, while accumulation of SOC derived from the crops ranged from 4.26 to 12.79 Mg ha^–1. Our results highlighted the benefits of legume cover crop species in no-till systems for the stabilization of SOC in degraded agricultural soils.

Abbreviations: F/M, fallow/maize • L+M, lablab+maize • O+V/M+C, black oat+vetch/maize+cowpea • O/M, black oat/maize • O+V/M, black oat+vetch/maize • P+M, pigeon pea+maize • SOC, soil organic carbon • SOM, soil organic matter • 0 N, 0 kg N ha^–1 yr^–1 • 145 N, mean of 145 kg N ha^–1 yr^–1