Soil organic carbon and aggregate characteristics in a subtropical cotton production field influenced by century-long crop rotation and fertility management

Understanding long-term effects of agricultural management influences on soil organic carbon (SOC) dynamics and aggregate stability is essential for crop production sustainability. In this study, effects of crop rotation, cover crop and nitrogen (N) fertilization on SOC physical and molecular fractions and water-stable aggregate stability were evaluated by characterizing soils of the world’s oldest, century-long (> 120 years) continuous cotton experiment located in the southern USA. Field treatments include: 1) Continuous cotton with no winter legume/no N (CK), 2) Continuous cotton with winter legume (CWL), 3) Corn-cotton rotation with winter legume (CCWL), 4) Corn-cotton rotation with winter legume plus mineral fertilizer N (CCWLN), and 5) Continuous cotton only with mineral N fertilizer (CN). Total organic C (TOC), total nitrogen (TN) as well as acid-hydrolysis C (AHC) and water extractable organic C (WEOC) from both bulk soil and different aggregate fractions were determined. Soil organic matter (SOM) composition was characterized using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Results showed that CCWL and CCWLN increased bulk soil TOC, AHC and TN by 150-165%, 300-315%, and 198-223%, respectively, as well as aggregates-associated C by 180-246% over CK. The CWL and CN treatments also increased TOC, AHC, and TN but at a less degree. The CCWL increased macroaggregates (250-2000 µm) by 92% followed by CCWLN by 46%, whereas CWL and CN had only limited effects by 1-7% compared to CK. Moreover, CCWL and CCWLN soil samples had more and diversified compounds of polysaccharides, aliphatic, aromatic, lignin, and phenols followed by CWL, CN and CK samples. Across different treatments, aggregate stability indices, mean weighted diameter (MWD) and geometric mean diameter (GMD), were positively related to TOC and TN (R2=0.57-0.65), N-containing compounds and phenols (R2 = 0.71-0.89) as well as polysaccharides and aliphatics (R2=0.53-0.71). It was concluded that the diversified inputs of SOM composition brought by synergistic interactions between corn rotation and winter legume inclusion were mainly responsible for the observed TOC accumulation and aggregate formation and stability in these subtropical cotton production systems.