Response of soil organic matter fractions to biochar and organic fertilizers – Results from a nine-year field experiment

We investigated the effects of biochar (BC) at 0 and 20 t ha-1, combined with two organic fertilizers (municipal solid waste compost, MC, and sewage sludge, SS), on soil organic matter (SOM) in a 9-year field experiment. To capture the protection by soil minerals and iron (Fe) against microbial decomposition, we fractionated SOM into particulate (POM) and mineral-associated organic matter (MAOM), and analyzed the fractions by iron (Fe) K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. BC and the two organic amendments increased soil organic carbon (OC) content, but only the organic fertilizers significantly increased total nitrogen (N) content. BC increased particulate OC and total N contents, while the organic fertilizers only increased particulate total N content. BC significantly increased mineral-associated OC content, while the organic fertilizers increased both mineral-associated OC and total N contents. We found no interaction between BC and organic fertilizers on mineral-associated OC and total N contents. The Fe EXAFS data fitting showed that the Fe(III)-SOM content of the Fe phases in POM and MAOM in unamended soils were noticeably different. Hematite represented the main Fe oxide phase in the POM fractions from all the amended soils, and Fe(III)-SOM averaged around 15%. In the amended soils, besides hematite (also present in the unamended soil), ferrihydrite occurred in all MAOM fractions, although at a different proportion.