In-season temporal variability of soil carbon and nitrogen pools after half a century of a tillage and crop rotation gradient

Reduced soil disturbance and diversified crop rotations are practices that can enhance organic matter and soil health. Understanding how these practices influence seasonal soil carbon (C) and nitrogen (N) variability during the growing season is critical for agroecosystem sustainability. We assessed temporal dynamics of soil C and N pools at six sampling dates over a maize (Zea mays L.) growing season, in a 55-year tillage and crop rotation experiment on silt loam and clay loam alfisols. Crop rotation had a more consistent effect than tillage on soil C and N pools (0–20 cm depth), with the most diverse rotation increasing soil organic carbon (SOC), permanganate oxidizable carbon (POXC), mineralizable carbon (Min C), total nitrogen (TN), autoclaved-citrate extractable (ACE) protein, inorganic N at both sites. No-Till increased C and N pools in the clay loam, but not in the silt loam soil. In general, fractions of C (POXC and Min C) and N (ACE protein and inorganic N) were more seasonally variable than total pools (SOC and TN). Despite temporal variation, tillage and rotation effects remained mostly consistent throughout the growing season, except for Min C which values decreased, and treatment differences diminished as the season progressed. Our findings suggest that 1) crop diversification with perennials enhances soil C and N regardless of soil type or tillage; 2) long-term No-Till has stronger effects in clay loam than silt loam soils, and 3) although C and N pools vary seasonally, long-term management effects persist throughout the growing season.