Long‐Term Organic Inputs Effects on Soil Carbon and Nitrogen Sequestration in a Temperate Agroecosystem

ABSTRACT Long‐term conservation agriculture with organic inputs is a promising strategy to enhance soil carbon (C) and nitrogen (N) sequestration, yet few multi‐decadal studies quantify these dynamics. Thus, the present study was aimed at evaluating the effects of compost, manure, cover crops, and fallow on soil organic carbon (SOC) and total nitrogen (TN) stocks and dynamics in a 27‐year no‐tillage (NT) cropping system in central Ohio, USA. The experimental layout followed a randomised complete block design with four replications to account for spatial variability and ensure statistical robustness. In spring 2024, soil samples were collected from 0 to 20 and 20 to 40 cm depths by taking three cores per plot in each of four replicated plots per treatment. SOC and TN contents were measured by the dry combustion method, and stocks were calculated using fixed‐depth and equivalent soil mass (ESM) approaches. Sequestration rates (Mg/ha/year) were derived relative to those in the fallow treatment. Compost and manure significantly increased SOC (%) and TN (%) compared with those in soil under fallow, with ESM‐adjusted SOC stocks being 67% and 66% higher, respectively in the 0–20 cm layer. Manuring produced the greatest TN gains (0.061 Mg N/ha/year), whereas compost promoted stable SOC accumulation (0.85 Mg C/ha/year). Cover crops modestly improved SOC and TN contents (21%–27% above fallow) but less than those in soil under organic amendments. The SOC content was positively correlated with aggregate stability ( R 2  = 0.76) and inversely with bulk density ( R 2  = 0.32), confirming functional coupling of organic matter and soil structure. Depth stratification showed that the use of compost concentrated C and N near the surface, while manuring and cover cropping redistributed inputs deeper through soluble pathways and root‐created bio‐pores. Composting resulted in the highest EC, with cover cropping intermediate and manure modestly elevated in the surface layer, underscoring salinity trade‐offs that vary by amendment and soil depth. These findings demonstrate complementary stabilisation mechanisms of organic inputs and highlight the value of ESM‐based accounting. Long‐term organic inputs under NT improve soil health, create climate‐resilient agriculture, and improve ecosystem functions.