CO2 Efflux From a Subtropical Oxisol Under Long-Term Tillage Systems.

Agricultural soils could be sources or sinks of atmospheric C, according to soil management. Effects of tillage, crop residue, and macroaggregate disruption on soil C-CO₂ efflux were evaluated on a subtropical Hapludox in Southern Brazil. Long-term (25 years) conventional tillage (CT) and no-till (NT) plots under intensive crop rotation were selected for this study. SOC stocks at the 0-0.30 m soil layer were 66.2 and 73.2 Mg C ha^-1 for CT and NT, respectively. The treatments were: (T1) bare soil; (T2) soil + crop residues; (T3) disrupted bare soil (1 m² sub-plot (0-0.15 m depth) disrupted with a shovel to crush all visible (~> 2 mm) soil aggregates). Crop residues (black-oat) levels were similar on CT and NT plots. Soil C-CO₂ efflux was assessed using a LI-COR LI-8100 analyzer and a 20 cm survey chamber with six replications at the 9, 13, and 17 h, during 15 days (Dec/04-Dec/18/2009). Tillage operations were performed at the 1^st (disk plow) and 5^th (disk tandem) evaluation days. Soybean was seeded on the 11^th day. C-CO₂ efflux was highly affected by environmental conditions (rainfall, temperature). Average C-CO₂ efflux from CT and NT soils (T1) were 21.5 and 19.0 kg C ha^-1 day^-1, respectively, 13.1% higher on CT even with 9.5% lower SOC stocks, showing that NT was more C conservative than CT. Aggregate disruption (T3) increased by 0.4 and 3.2% the average C-CO₂ efflux under CT and NT, respectively, reflecting the larger POC stocks and intra-macroaggregate C stabilization under NT. Presence of crop residues (T2) increased 42.6 and 49.6% the C-CO₂ efflux under CT and NT, respectively. Crop residues decomposition had larger impact on soil C-CO₂ efflux than CT on this subtropical clayey Oxisol, contrasting to Reicosky and Lindstrom (1993) which reported high C-CO₂ efflux after tillage on a temperate clay loam Mollisol.