Organic amendments affect δ 13 C signature of soil respiration and soil organic C accumulation in a long‐term field experiment in Sweden

Summary The contribution of young and old soil organic carbon ( SOC ) pools to soil CO 2 fluxes and specific respiration rates of these fluxes was determined by using δ 13 C signatures in the U ltuna long‐term continuous soil organic matter experiment ( C‐SOME ). Initiated in 1956, the experiment had a range of treatments amended organically and with mineral nitrogen fertilizer under C 3 cultivation until 1999, and thereafter under C 4 (maize) cultivation. In 2011, soil respiration was measured in situ prior to planting, during growth and after harvest. The contributions from C 4 ‐ and C 3 ‐C as well as their specific respiration rates were estimated from δ 13 C differences in SOC and CO 2 fluxes. The contributions from C 4 ‐C sources were further separated into autotrophic and heterotrophic respiration by comparing respiration rates before and after harvest. Between 165 and 385 g C 4 ‐C  m −2 accumulated during 10 years of maize growth, contributing between 4.9 and 8.1% to the total SOC stock. Although recent C 4 ‐C had an average specific respiration rate that was 8.4–22.6 times greater than C 3 ‐C , total soil respiration was generally equally split between C 3 ‐C and C 4 ‐C . Both pools are therefore important sources of CO 2 in the overall C budget, and a crucial factor in accounting for SOC stock change caused by management. Experimental treatments influenced specific respiration rates of C 4 plant material and accumulation of SOC stock, demonstrating how greater SOC accumulation can be favoured by high‐quality C inputs.