Cover crop effects on soybean residue decomposition and P release in no-tillage systems of Argentina

• Cover crops increase soybean residue decomposition slightly under Argentinean Pampas conditions.
• Cover crops provide substantially more carbon input than is lost through increased soybean residue decomposition under a no-tillage system.
• Increased soybean residue decomposition in the presence of CC was associated with the soil and residue microbial biomass and activity.

Cover crops (CC) provide many benefits to soils but their effect on decomposition of previous crop residues and release of nutrients in continuous no-tillage soybean [Glycine max (L.) Merr.] production are little known. Our objective was to quantify CC effects on decomposition and phosphorus (P) release from soybean residue using litterbags. Three CC species (oat, Avena sativa L.; rye, Secale cereal L.; and rye grass, Lolium multiflorum L.) and a no CC control were evaluated. Temperature, moisture content, microbial biomass and microbial activity were measured in the surface 2 cm of soil and residues. Cover crops increased soybean residue decomposition slightly both years (8.2 and 6.4%). Phosphorus release from soybean residue did not show any significant differences. Cover crops increased microbial biomass quantity and activity in both soil and residue samples (p < 0.001, p = 0.049 for soil and residue microbial biomass; p = 0.060, p = 0.003 for soil and residue microbial activity, respectively). Increased residue decomposition with CC was associated with higher soil and residue microbial biomass and activity, higher near-surface (0–2 cm) moisture content (due to shading) and soil organic carbon enrichment by CC. Even though CC increased soybean residue decomposition (233 kg ha−1), this effect was compensated for by the annual addition of approximately 6500 kg ha−1 of CC biomass. This study demonstrated another role for CC when calibrating models that simulate the decomposition of residues in no-tillage systems.