Manure fertilizer divergently affects organic and inorganic carbon in a dryland agricultural soil

Soil carbon (C) is important to support sustainable agriculture, affect global C cycling, and influence the climate system. Manure fertilization is an important and widely used practice to increase agricultural productivity and soil organic carbon (SOC) pools, whereas its effect on soil inorganic carbon (SIC) and total C in deep soils is not reported. This knowledge gap restricts our ability to accurately evaluate C budget in agricultural soils because SIC in deep soils accounts for more than half of the global soil C pools, while current earth system models rarely take them into account. Herein, we examined changes of soil C along 0- to 3.0-m depth after 35 years of application of manure in a dryland agricultural ecosystem. We also measured C concentrations in soil samples (0-0.2 m) from 1985 to 2019 to evaluate C dynamics in topsoils. The objective was to understand how SIC and SOC in deep soils respond to manure fertilization in semiarid ecosystem, where SIC accounts for a large fraction of total C. We showed a divergent effect of 35 years of manure application on SOC and SIC in 0-3.0 m soil from a dryland agricultural ecosystem. Either within or across the two cropping systems examined, manure increased SOC in top 0.8 m layer but decreased SIC in 0.8-3.0 m layer, which offset SOC increase and resulted in 63.8 Mg ha-1 decrease of total C in 0-3.0 m soil layer. Given the importance of soil C for sustainable agriculture and that drylands contain 80% of the global SIC and similar to 50% of world cropland, immediate attention should be paid to such divergent effects in both mechanisms understanding and model prediction. Manure increased soil organic carbon (SOC) (+15.6%) in top 0.8 m layer. Manure decreased soil inorganic carbon (SIC) (-21.5%) in 0.8-3.0 m layer. Decreased SIC offset increased SOC and resulted in total C loss in 0- to 3.0-m depth.