Lignin patterns in soil and termite nests of the Brazilian Cerrado

Organic matter is generally enriched in termite nests. Since lignin resists degradation by most termite species, it could be a prominent tracer of the organic matter incorporated into termite nests and released into nest surroundings. Our objective was to quantify the content and alteration degree of lignin in termite nests and the surrounding soils, and to analyze the development of the lignin status from young to older termite nests. We sampled Cornitermes silvestrii earth mounds and the surrounding soil in an extensive grid scheme in a Cerrado savanna in Central Brazil. Older mounds were secondarily inhabited by Nasutitermes kemneri and other inquilines; those mounds and reference soil sites were also sampled. Lignin contents were estimated by the sum of lignin-derived phenols and additionally by mid-infrared spectroscopy followed by a partial least squares regression. Nests inhabited by primary termites had a lignin content of 1952 mg kg−1, which was 15 times more than at 10 cm depth of the reference soils. In comparison, organic carbon was only enriched by a factor of 2 in primarily inhabited nests. Our accompanying microcosm experiment showed that the lignin composition was not changed during incorporation into termite nests. Elevated lignin contents could still be found at 60 cm distance from the nest border as well as up to 60 cm soil depth beneath the nests. The lignin content in older nests was only half that of younger nests, and the influence on the nest surroundings was less prominent. Higher acid-to-aldehyde ratios in those older nests indicated that the remaining lignin was oxidized and thus partly degraded during nest aging. We conclude that the savanna termite C. silvestrii enriches lignin in its earth mounds, but that only a minor part enters the soil-protected lignin fraction when nests decay, as most of the lignin is lost during nest aging.

Research Highlights
► Grass-feeder termites accumulate lignin in preference to other organic compounds in their nests.
► Termite activities elevate also the lignin contents in the underlying and lateral adjoining soil.
► Lignin degradation occurs in older, but still inhabited, termite nests, and in the surrounding soil.
► Only a minor part enters the soil lignin reservoir when nests decay.