Long-term organic fertilization has been found to significantly enhance the accumulation and stability of soil organic carbon (SOC) in agricultural ecosystems by altering how microbial and plant-derived residues are stored across different soil particle sizes. The findings offer new insight into soil carbon sequestration mechanisms and provide vital guidance for sustainable fertiliser use and soil health management.

A team led by Dr. HE Hongbo at the Institute of Applied Ecology, Chinese Academy of Sciences, investigated how different sources of organic matter interact with soil particles over three decades of continuous fertilization. The researchers used biochemical markers—amino sugars (indicative of microbial necromass) and lignin phenols (tracers of plant residue)—to distinguish microbial- and plant-derived carbon within soil fractions separated by particle size. Their results, published in Soil & Tillage Research, reveal a marked redistribution of these organic residues due to prolonged manure application. While microbial-derived carbon was originally concentrated in the smallest clay particles, it migrated into coarser sand fractions when high levels of manure were applied. Meanwhile, plant-derived lignin was found to accumulate preferentially in sand fraction, particularly under organic fertilization. This redistribution suggests a decoupling between mineral-associated protection and organic matter retention after clay particles become saturation with microbial necromass. In such cases, the inherent biochemical traits, especially  the decomposition resistance of plant debris, become the primary drivers of SOC persistence. 

Notably, inorganic fertilization alone (NPK) improved microbial necromass stability without substantially increasing total SOC levels, whereas manure led to SOC increases of over 50%, mainly due to enhanced lignin retention in coarser fractions.

The study highlights that long-term soil carbon storage is not only a function of mineral binding but is also increasingly determined by the nature and fate of organic residues. These findings are crucial for carbon-neutral farming, as they underscore the importance of matching fertilization practices to the decomposability of organic inputs to optimize soil carbon sequestration.

The research also stresses the need to reconsider conventional views of mineral protection as the dominant SOC stabilization mechanism, particularly in intensively managed croplands. It offers a practical pathway to enhancing both carbon retention and soil ecosystem resilience.

Figure 1. Long-term fertilization alters the relative accumulation of microbial- and plant-derived residues across soil particle sizes (Image by LI Jie).