Organic amendments facilitate soil carbon sequestration via organic carbon accumulation and mitigation of inorganic carbon loss

Both soil organic carbon (SOC) and soil inorganic carbon (SIC) are important carbon reservoirs involved in the global carbon cycle. Fertilization especially organic amendments can increase SOC sequestration, while nitrogen fertilization-induced soil acidification leads to significant SIC loss in alkline soil. How fertilization changes SIC and SOC via altering abiotic and biotic properties remains unclear. Here, we investigated the effects of mineral fertilizer with/without organic amendments (manure, straw, green manure) on SIC and SOC in bulk soil and particle-size fractions, as well as their interrelationship based on a 38-year field trial. Results showed that compared to the unfertilized soil, mineral fertilization significantly decreased SIC by 13.38%-15.69%, primarily due to decreased soil pH. However, mineral nitrogen plus manure (NM) largely increased SIC by 26.55%, likely resulting from the CO32- reprecipitated by Ca2+ and/or Mg2+ compensation. Mineral nitrogen plus manure and straw (NM, NS) increased SOC by 11.12%-100.35%. For bulk soil and particle-size fractions >0.25 mm, SIC content was positively correlated with SOC. Random forest model revealed that SOC was regulated by soil nutrients (total nitrogen [TN], Olsen-P), microbial biomass and beta-1,4-N-acetyl-glucosaminidase. The change in SIC was directly regulated by soil Mg2+, and indirectly by soil nutrients (e.g., TN, Olsen-P) which affected crop-derived C input. Our findings suggested that organic manure amendments can facilitate SOC accumulation and mitigate SIC loss by occluded more SIC in large soil particles. The results are of fundamental significance for understanding the role of optimal manure application played in SOC sequestration and SIC accumulation.