Nitrogen fertilizer and landscape position affect soil aggregate size distribution, and intra-aggregate carbon and nitrogen under switchgrass in a marginal cropland

Dedicated bioenergy crops such as switchgrass (Panicum virgatum L.) can be grown on marginally productive lands and positively influence soil properties. However, nitrogen management, and landscape can alter soil structural attributes under bioenergy crop production. This study investigated the impacts of long-term nitrogen fertilization (0-N, 0 kg N/ha; 56-N, 56 kg N/ha and 112-N, 112 kg N/ha) and landscape positions (shoulder and footslope) on soil organic carbon (SOC) and structural attributes under switchgrass production. The 112-N rate enhanced the proportion of 2-4 mm water-stable aggregates by 49%, aggregate associated carbon in 2-4 mm and >4 mm aggregates by 16 and 24%, respectively, aggregate associated nitrogen in >4 mm aggregates by 33% and reduced soil bulk density by 19% compared to the 0-N rate. Footslope position increased the proportion of 2-4 mm water-stable aggregates by 26% and lowered bulk density by 8% compared to the shoulder position. Results showed a significant N-rate x landscape position interaction on SOC and glomalin related soil protein content in bulk soil. Overall, this study showed that nitrogen application to switchgrass planted at footslope on a marginally yielding cropland improved soil structure and physical conditions.