Assessing the combined effect of tillage and nitrogen fertilizer rate on soil organic carbon sequestration using carbon isotope ratio

Tillage practices affect soil organic carbon (SOC) storage in agroecosystems, but information is limited regarding the interactive effects of tillage methods and nitrogen (N) fertilizer application rates on SOC stocks. It was hypothesized that, by enhancing crop productivity, high rates of N fertilizer would increase crop residue input and ultimately result in higher SOC stock. To assess the merit and limitations of that hypothesis, a study was conducted using soil samples (0–5, 5–10, 10–15 and 15–30 cm depths) from long-term (52 years) experimental plots (Kentucky, USA) under continuous corn (Zea mays L.), managed with either no-till (NT) or moldboard plow (MB), and receiving N fertilizer at rates of 0, 84, and 168 kg N ha⁻¹ y⁻¹. Soil C concentration and its ¹³C abundance was measured using IRMS, and the amount of corn residue-C input was derived from crop yield. Using the isotope mixing model, SOC was partitioned into corn-derived and non-corn-derived SOC pool. Results showed that the effect of tillage and N fertilization on SOC was largely limited to the top 10 cm soil layer. N fertilizer had a marginal effect on SOC under MB but resulted in substantial increase in SOC stock under NT. The SOC stock (0–10 cm) under NT exceeded the SOC stock under MB by 7.32, 15.79, and 18.1 Mg C ha⁻¹ at the 0, 84, and 168 kg N ha⁻¹ application rates. NT was more effective than MB in retaining new C input, storing 4.3 % of the corn residue C input (as opposed to only 0.7 % under MB). Corn-C sequestration rate (0–10 cm; Mg C ha⁻¹ y⁻¹) was between 0.1 and 0.15 under MB and 0.11–0.32 under NT. Further, the pool of pre-existing SOC (non-corn-derived) was 1.6–1.7-fold larger, and its depletion rate slower, under NT than under MB. This study results illustrate the effects of tillage and N fertilization in enhancing SOC stock, namely that N fertilization, when accompanied with NT, can be beneficial to SOC accumulation in agroecosystems.