Limited impacts of occasional tillage on dry aggregate size distribution and soil carbon and nitrogen fractions in semi-arid drylands

Abstract

Tillage management that minimizes the frequency and intensity of soil disturbance can increase soil carbon (C) and nitrogen (N) sequestration and improve the resilience of dryland cropping systems, yet the impact of occasional disturbance on soil aggregate formation and the soil organic carbon (SOC) storage within aggregates has not been studied well. We evaluated the effect of four tillage management practices on soil dry aggregate size distribution, aggregate-protected C and N, mineral-associated organic matter carbon (MAOM-C), particulate organic matter carbon (POM-C), and corn (Zea mays L.) and sorghum (Sorghum bicolor (L.) Moench) yields in a semi-arid dryland cropping system. Treatments included conventional tillage (CT), strip-tillage (ST), no-tillage (NT), and occasional tillage (OT) management in a corn-sorghum rotation. Soil macro-aggregates were 51–54% greater under ST, NT, and OT, while small and micro-aggregates were greater in CT. Conventional tillage reduced soil aggregate-associated C by 28–31% in macro-aggregates and 47–53% in small aggregates at 26 months (M) sampling compared to ST, NT, and OT. In clay + silt fraction, CT had 14–16%, 21–26%, and 36–43% less SOC at 7, 14, and 26M samplings, respectively, than ST, NT, and OT. Aggregate associated N was generally similar under ST, NT, and OT, which was greater on average than CT. Soil MOAM-C and POM-C under ST, NT, and OT were generally greater than respective SOC fractions under CT at 19 and 26 M after OT implementation. Corn and sorghum yields were similar among tillage systems in 2020, but greater under ST, NT, and OT than CT in 2021. Our results suggest that while frequent intensive tillage can lower SOC and N storage, a single stubble mulch occasional tillage after several years of NT does not lead to soil C and N losses and soil structural instability in semi-arid drylands.