Nitrogen and Lignin Content as Predictors of Litter Decay Rates: A Microcosm Test

Decomposition rates of leaf litter have been predicted from the leaves' lignin or nutrient (N or P) contents, the C:N ratio, and more recently the lignin:N ratio. But tests of these predictors have been based on groups of substrates each spanning only part of the natural range of lignin contents, and neglecting low—lignin (<10%) species. We allowed leaf litter from eight species of tree, shrub, or herb, ranging in lignin content from 3.4 to 20.5%, to decompose in laboratory microcosms for up to 4 mo (equivalent to 1.5—2 yr decay in the field) to test two hypotheses: (1) that the lignin: nitrogen ratio would have a better correlation with mass loss rates than would the C:N ratio, nutrient content, or other substrate quality indexes, and (2) that correlations of mass loss with initial N content would decrease, while correlations with lignin content would increase, as decay proceeded. Contrary to the first hypothesis, nitrogen content and the C:N ratio were the best predictors of mass loss rate, and were substantially better than the lignin:N ratio. We could find no better predictor of decomposition rate than the C:N ratio, and no better regression model than the simple linear one. However, when regressions were tested using pine needles (lignin content 26.2%), the C:N ratio and N content badly underestimated mass remaining (by 10—16%), while lignin content and the lignin:N ratio overestimated it by <2%. In accordance with the second hypothesis, regressions of initial lignin content or lignin:N ratio on mass remaining improved (higher R²) from 2 to 4 mo decomposition, while those of N content grew worse, illustrating succession of nitrogen to lignin control of decomposition rate. Reported correlations of the lignin: N ratio with decomposition rate for some litter types arise as a special case of this two—phase mechanism of control by nutrients and lignin. For substrates low in lignin, or where a broad range of lignin contents is being considered, the C:N ratio is a better predictor of decomposition rate than the lignin:N ratio.