Changes in carbon and nitrogen stable isotopic ratios with decomposition of plant residue

Abstract

The transformation of plant residue into soil organic matter (SOM) plays a crucial role in maintaining the function of ecosystems. To elucidate the formation processes of SOM from plant residue, we examined changes in carbon (C) and nitrogen (N) contents, along with δ¹³C and δ¹⁵N values, during the decomposition of Artemisia princeps residue in an incubation experiment using 100 % quartz sand as an artificial soil. The results indicated that plant residue decomposition occurred in two distinct stages: (1) a rapid initial loss of C and N (43–54 % and 36–47 %, respectively) within the early 90 days, followed by (2) a slower loss (4–14 % and 7–15 %, respectively) over the remaining 90–360 days. In the early stage, δ¹³C and δ¹⁵N values increased similarly to trophic fractionation, with a δ¹⁵N/δ¹³C ratio of 1.54. In the latter stage, this ratio increased significantly to 12.8, probably due to increased C resistance to decomposition, while continuous N metabolism. By integrating data from both the literature and our study, we concluded that C and N in plant residue undergo one to two times the metabolic turnover relative to trophic turnover to form stable SOM. These findings are essential for understanding the preservation of plant residue, particularly labile compounds, in soils. This preservation would occur through microbial transformation. Our study offers a theoretical framework for understanding the SOM dynamics based on δ¹³C and δ¹⁵N values, emphasizing the utility of stable C and N isotope analyses in elucidating SOM transformation.