Testing the effects of chemistry of organic matter inputs on soil carbon dynamics in an un-confounded experiment

Background/Question/Methods  

The effects of quality of organic matter (OM) inputs on soil carbon (C) dynamics are difficult to determine because quantity and quality are usually confounded in observational studies. To disentangle these effects, we conducted an experiment in which measured quantities of pure chemicals (cellulose, chitin, and lignin) were added to sub-plots within a replicated field experiment in which tree species was the main treatment. This study was conducted in 20-yr-old mono-dominant plantations of Pentaclethra macroloba and Vochysia guatemalensis growing under similar environmental conditions at La Selva Biological Station, Costa Rica. Natural detrital inputs for the two species were ~1000 g C m^-2 yr^-1; we doubled inputs, adding 1000 g C m^-2 yr^-1 of each of the three chemicals (except in the control) to separate 1-m² subplots within each 0.25-ha plot, at 6-mo intervals over two years. In the last 6 months, separate cylinders within all subplots were amended with nitrogen (N) to adjust cellulose and lignin treatments to the C:N of chitin, 9.6. The design was: 2 Species x 4 Chemicals x 2 Nitrogen x 4 Replicates. Response variables included changes in: soil C and N stocks; soil respiration; fine-root biomass; potential C mineralization; and carbohydrates, including cellulose-derived glucose.

Results/Conclusions   

With baseline soil C flux ~2000 g C m^-2 yr^-1, responses to the chemical additions were highly detectable. Soil respiration rates ranged from 1087(±135) to 4033 (±720) g C m^-2 yr^-1 under lignin and chitin amendments, respectively. The surprising result was that cellulose was not immediately mineralized, as evidenced by lack of soil respiration response and high extractable glucose six weeks post-addition. Cellulose and lignin have higher C:N ratios than does chitin, so N was hypothesized to limit decomposition of cellulose, even in this site with high soil N stocks. Addition of N in incubation experiments stimulated mineralization of cellulose to the rate in chitin, but did not increase lignin decomposition. This suggests that molecular complexity of lignin is more limiting to mineralization than C:N stoichiometry. Higher fine-root biomass, root-rhizosphere respiration, and total soil carbohydrates in chitin treatments highlight the active role of roots in this field experiment. Ratios of (Galactose + Mannose) to (Arabinose + Xylose materials) indicated that chitin and lignin, and to some extent N addition, promoted microbial activity. Decomposition of the model substrates was thus driven by a complex interplay of stoichiometry and molecular complexity, in interaction with microbial and fine-root dynamics.