Lignin in Particle‐Size Fractions of Native Grassland Soils as Influenced by Climate

Abstract

The turnover of soil organic carbon (SOC) in grasslands can be predicted as a function of climate, plant lignin content, texture, and kinetically defined C pools. Particle‐size fractionation has been used to identify soil C pools. This study was conducted to investigate influences of climate on the dynamics of lignin in particle‐size fractions. Composite samples were taken from the top 10 cm of 18 native grassland sites along temperature and precipitation transects from Central Saskatoon, Canada, to South Texas. Lignin‐derived phenols were determined in the <2 μm (clay), 2‐ to 20‐μm (silt), 20‐ to 250‐μm (fine sand) and 250‐ to 2000‐μm (coarse sand) size separates. With decreasing particle size the concentration of lignin‐derived phenols decreased significantly from 72 g kg −1 SOC in the coarse sand fractions to 12 g kg −1 SOC in the clay fractions. Increasing phenolic acids to aldehyde ratios indicated that side chain oxidation proceeded as particle size decreased. Moreover, these ratios decreased in fractions <250 μm with increasing mean annual temperature (MAT) at the sites. This suggests that the degree of lignin decomposition decreased with increasing MAT, possibly because there was a lack of additional C sources, such as saccharides of root litter, which are needed for the cometabolic decay of lignin.