Dissolved organic carbon chemical composition controls the rate of CO2 release from rewetted soil

The CO2 flush from dried and rewetted soil has been proposed as an indicator of soil health. Drying and rewetting releases mineralizable carbon (C) due to effects on microbial populations and aggregate stability. It is a complex, integrative process, and its relationship to soil health is incompletely understood. In order to increase our understanding of this process, we investigated changes in the quantity and chemistry of dissolved organic carbon (DOC) in a laboratory incubation study during 48 h after rewetting dried soil. A LI-COR infrared gas analyzer was used to quantify CO2 released. DOC was extracted by water and quantified by a Shimadzu TOC-V-CPH. Changes in chemical composition of the DOC were followed during decomposition using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR-MS). The initial DOC was 67% lignin-like, and 6.5% N-containing aliphatic material (number-averaged formula distribution). The contribution of N-containing aliphatics to total DOC declined to 0.2% by 48 h, although the amount of DOC declined only 34% by 48 h. The lignin-like material increased to 76% of the total DOC by 48 h of incubation, suggesting it is less microbially available than other DOC constituents. The decline in CO2 release rate was related to the loss of readily mineralizable N-containing compounds rather than to total DOC, supporting the use of the CO2 flush as an indicator of mineralizable N. Further studies designed to elucidate the biological and abiotic mechanisms controlling the CO2 flush will improve its usefulness as a soil health indicator.