Long-term Fertilizer Application Induces Changes in Carbon Storage and Distribution, and the Consequent Color of Black soil

This study investigated the impact of different fertilizer treatments on soil color and humic substances (HSs) in a maize cropping system over a 33-year long-term field trial on black soil. Five treatments were included: (1) Fallow (uncultivated, natural evolution), (2) CK (no fertilizer application), (3) NPK (only chemical fertilizer application), (4) NPKM (livestock manure partially replacing chemical fertilizer), and (5) NPKS (straw partially replacing chemical fertilizer). Soil organic carbon (SOC) in the Fallow, NPKM, and NPKS treatments increased significantly by 68.2%, 95.5% and 42.4% compared to the initial SOC, respectively (P < 0.05), while SOC content in NPK declined by 9.8%. Humic acid (HA), fulvic acid (FA) and humin (HM) were all significantly higher by 15.4%, 15.8%, 58.8% in Fallow treatment, 53.9%, 21.1%, 153.9% in NPKM, 19.2%, 15.8%, 54.0% in NPKS compared to initial soil (P < 0.05), respectively, while HA and FA were significantly lower in the NPK treatment, decreased by 7.7% and 11.0%, respectively. Average soil spectral reflectance (SSR) values decreased in Fallow, NPKS, and NPKM treatments by 7.6%, 7.8% and 11.1%, respectively, but increased in NPK treatment by a 22.4%. Linear regression analyses showed significant negative correlations between SSR with HA ( r = − 0.462, P < 0.01 ), FA ( r = − 0.445, P < 0.01 ), HM contents ( r = − 0.462, P < 0.01 ). Chemical fertilizer application decreased SOC, HA and FA contents of black soil and resulted in the “faded” of black soil, while organic fertilizer and straw addition elevate SOC content and contribute to soil darkening.