Seasonal changes of soil organic matter composition in separate carbon pools of a cultivated Chernozem: The role of crops and fertilization

Soil organic matter (SOM) is a crucial property affecting most soil functions; thus, sampling and investigation are essential. However, the SOM composition and content may be less constant than assumed, even in crop fields. This study investigated whether fertilization and crop type affect the effect of sampling time on soil organic carbon (SOC) content and SOM composition. A cultivated Haplic Chernozem (Loamic) topsoil in Martonvásár, Hungary, was studied in a long-term experiment to compare the effects of fertilization on soil properties under different crops. Soil samples were collected during summer (August 2018) and spring (April 2019) to study SOC content and SOM composition in the bulk soil and two soil carbon (C) pools, such as the mineral phase-associated organic matter (OM) or slow pool (attached to particles <63 μm) and aggregate-associated OM or fast pool (occluded by particles >63 μm). SOM composition was estimated using Fourier transform infrared spectroscopy. Neither fertilization nor crops affected the SOC content and SOM composition. The slow pool had a higher SOC content and more stable SOM composition than that of the fast pool. Furthermore, the labile SOM component (amide nitrogen, and polysaccharides) ratio was higher during spring than during summer in the fast pool, whereas the phenolic lignin content decreased. Varying plant residual versus microbial contributions to total SOC may explain these temporal variations. Consequently, the soil sampling date may affect the SOM content and composition results; thus, repeated seasonal sampling is suggested to estimate possible fluctuations in SOM properties and calculate the annual average.