Topsoil placement effect on soil carbon stock improvement of exposed subsoil in Iowa

ABSTRACT:

Management of exposed subsoil after removing the topsoil for road construction presents a soil management challenge and an opportunity to examine potential techniques for improving soil carbon stocks. It is well documented that the construction of roadbeds leaves behind large areas of unproductive exposed subsoil, which is low in soil organic carbon content. The objective of this study was to determine whether a topsoil addition with a corn (Zea mays)—soybean (Glycine max L. Merr.) annual rotation could improve soil carbon in areas that have had topsoil removed. The experimental design was a randomized complete block with three replications. Soil organic carbon fractions, potential C inputs from crop residues, soil CO₂ emission, and microbial biomass carbon of topsoil and exposed subsoil treatments managed under a corn-soybean rotation were measured during the growing seasons of 2003 and 2004. Soil temperature and soil moisture at the 5 cm depth were also measured concurrentlywith soil CO₂ emission readings. The aboveground biomass production and root biomass of corn grown in topsoil was 7.14 Mg ha⁻¹ and 0.8 Mg ha⁻¹ (3.18 ton ac⁻¹ and 0.40 ton ac⁻¹) more than corn grown in the subsoil, respectively. This led to greater potential C inputs from corn grown in topsoil. The improvement in soil organic carbon in the subsoil 0 to 60 cm (0 to 24 in) soil depth with corn and soybean crops over the past 28 years averaged at 0.70 Mg ha⁻¹ yr⁻¹ (0.31 ton ac⁻¹). In contrast, topsoil replacement treatments had greater SOC contents than exposed subsoil, including the 30 to 45 cm (12 to 18 in) soil depth, which is below any added topsoil but within the tillage zone. These findings suggest that topsoil addition can improve exposed subsoil soil-carbon stocks and its crop productivity. It was also observed that microbial biomass carbon contents were 247 and 157 μ g⁻¹ (247 and 157 parts per million) for topsoil and subsoil, respectively. Also, cumulative CO₂ emissions from topsoil were 45% and 47% greater than those from subsoil in 2003 and 2004, respectively.