Influence of prolonged arable cropping on lignin compounds in sandy soils of the South African Highveld

The preservation of plant residues is important for sustainable arable cropping. Lignin is a marker for plant residues in soils. We have investigated influences of the length of cultivation on the dynamics of lignin. Composite samples were taken from the top 20 cm of soils that have been cropped for periods varying from 0 to 98 years in each of three different agro-ecosystems in the Free State Province of South Africa. Lignin-derived phenols were determined in the < 2 μm (clay), 2-20 μm (silt), 20-250 μm (fine sand) and 250-2000 μm (coarse sand) size separates. With increasing length of cultivation, the concentration of such phenols decreased to 36% of that in the grassland. The lignin contents as proportions of the total carbon did not change during cultivation, suggesting that there was no selective enrichment of lignin moieties as C was lost as a result of cultivation. The loss rate constants of lignin concentrations in particle-size fractions increased in the order clay (0.17 year(-1)) ≤ silt (0.18 year(-1)) < fine sand (0.20 year(-1)) < coarse sand (0.22 year(-1)). Increasing ratios of phenolic acids to aldehydes in bulk soil, silt and fine sand fractions with increasing length of cultivation indicated that side chains were being oxidized. The ratios in the silt fraction, however, decreased after 10-20 years. We attribute this to a loss of lignin together with silt by wind erosion, resulting in a rejuvenation of lignin compounds in the remaining silt-sized pools of C.