Free light fraction carbon and nitrogen, a physically uncomplexed soil organic matter distribution within subtropical grass and leucaena–grass pastures

Quantifying the size and turnover of physically uncomplexed soil organic matter (SOM) is crucial for the understanding of nutrient cycling and storage of soil organic carbon (SOC). However, the C and nitrogen (N) dynamics of SOM fractions in leucaena (Leucaena leucocephala)–grass pastures remains unclear. We assessed the potential of leucaena to sequester labile, free light fraction (fLF) C and N in soil by estimating the origin, quantity and vertical distribution of physically unprotected SOM. The soil from a chronosequence of seasonally grazed leucaena stands (0–40 years) was sampled to a depth of 0.2 m and soil and fLF were analysed for organic C, N and d13C and d15N. On average, the fLF formed 20% of SOC and 14% of total N stocks in the upper 0.1 m of soil from leucaena rows and showed a peak of fLF-C and fLF-N stocks in the 22-year-stand. The fLF d13C and fLF d15N values indicated that leucaena produced 37% of fLF-C and 28% of fLF-N in the upper 0.1 m of soil from leucaena rows. Irrespective of pasture type or soil depth, the majority of fLF-C originated from the accompanying C4 pasture-grass species. This study suggests that fLF-C and fLF-N, the labile SOM, can form a significant portion of total SOM, especially in leucaena–grass pastures.