Direct and indirect impacts of fine root functional traits on decomposition and N loss

Functional traits are crucial drivers of litter decomposition, yet the links between functional traits of fine roots and decomposition remains relatively understudied compared to that of leaf litter. This study aimed to explore how functional traits of fine roots affect decomposition and nitrogen (N) loss.

Functional traits of living plants and litter chemical components of fine roots from 24 species from a desertified grassland were measured. The fine root litter was incubated for 1, 2, 3, 4, 5, 11, 13, 15, 17 and 24 months to investigate fine root decomposition and N dynamics.

The combination of fine root N content (RN), tissue density (RTD) and specific root length (SRL) was positively correlated with N-related substances of fine root litter. Carbon content (RC) was positively correlated with recalcitrant components, while dry matter content (RDMC) was negatively correlated with water-soluble substances. Legumes with higher RN and RTD and lower SRL exhibited greater rates of fine root decomposition and N loss than that of non-legumes. Functional traits had no direct effect on decomposition rates in the whole decomposition period and mass loss in the early decomposition stages, while RC and the combination of RN, RTD and SRL restrained and facilitated fine root decomposition directly by influencing recalcitrant components and N-related substances respectively. SRL directly facilitated mass loss in the later decomposition stages. For N loss, RN directly facilitated fine root N loss, and the combination of RN, RTD and SRL indirectly facilitated it through a negative effect on C:N of fine root litter.

The generality of higher RN and RTD, lower SRL, as well as faster fine roots decomposition and N loss of legumes highlights the importance of legumes in sand fixation, nutrient supply, and ecological restoration in N-limited desertified grasslands. Our findings imply that global changes potentially altering the functional traits of fine roots and functional types of community composition, which may affect below-ground C and nutrient cycle significantly.