Carbon and nitrogen in a Ferralsol under zero-tillage rotations based on cover, cash or hay crops

To identify crop rotation systems capable of sequestering C and N to 1 metre depth in a subtropical Ferralsol of Southern Brazil managed under long-term zero-tillage (21yrs), we evaluated six crop sequences: wheat (Triticum aestivum)-soybean (Glycine max) [W-S], the baseline; oat (Avena strigosa, as cover crop)-maize (Zea mays)-wheat-soybean [O-M-W-S]; vetch (Vicia villosa, as legume cover crop)-maize-wheat-soybean [V-M-W-S]; vetch-maize-oat-soybean-wheat-soybean [V-M-O-S-W-S]; ryegrass (Lolium multiflorum, for hay)-maize-ryegrass-soybean [R-M-R-S]; and alfalfa (Medicago sativa, for hay)-maize [A-M]. Compared to W-S and to 1 metre, the hay-based system of A-M showed the largest C and N sequestration rates (0.50 and 0.06 Mg/ha/yr, respectively). Alfalfa, being a perennial legume under cut-regrowth cycles, possibly added more C and N through roots. The other hay system, R-M-R-S, also sequestered C efficiently (0.27 Mg/ha/yr), but not N (0.01 Mg/ha/yr). The legume-based system of V-M-W-S sequestered significant amounts of both C (0.29 Mg/ha/yr) and N (0.04 Mg/ha/yr). The grass-based system of O-M-W-S showed the lowest sequestration of C (0.09 Mg/ha/yr). In all systems, a positive relationship (R-2=0.71) occurred between estimated addition of root C and soil C stock to 1 metre. Whenever C and N sequestration occurred, more than half of that occurred below 20cm depth. Results suggest that adoption of legume-based systems, perennially as A-M or annually as V-M-W-S, is efficient for C and N sequestration in subtropical zero-tillage soils and that roots possibly contribute more to that sequestration than aboveground biomass.