Residue quality does not influence long-term soil carbon stabilization

Background/Question/Methods Concern about increasing atmospheric carbon dioxide concentrations warrants the need for an improved understanding of mechanisms that drive soil carbon stabilization. While residue quality influences short-term mineralization rates, its impacts on long-term carbon cycling are unknown. Soil aggregates provide structural units for the physical protection of organic matter and carbon stabilization. As aggregate dynamics are linked to microbial activity, the quality of residue substrate may govern the rates of aggregate formation and breakdown. The objectives of our study were 1) to determine how residue quality controls aggregation, and 2) to assess how this relationship influences carbon stabilization. We examined these interactions using a short-term laboratory incubation and a long-term agricultural field trial in Embu, Kenya. The field trial was initiated in March 2002 with different quality residue inputs (no input, high quality Tithonia diversifolia, medium quality Calliandra calothyrsus, and low quality maize stover), applied at a rate of 4 Mg C/ha/yr alone and in combination with 120 kg N/ha mineral fertilizer. Maize was grown each season. Soil samples (0-15 cm) were collected in March 2005 to measure long-term aggregation and carbon stabilization. Additionally, short-term carbon dynamics were assessed by incubating soil from the site with 13C-enriched residues. Residue and fertilizer inputs were applied at the same rates to 200 g of soil and incubated for 3, 6, and 18 months. Results/Conclusions Adding residue, regardless of quality, increased soil aggregation in both the short- and long-term. In the incubation experiment, inputs of high quality residue resulted in the stabilization of 50% more residue-derived carbon than low quality residue after 3 months. However, after 18 months of incubation there were no differences in the amount of carbon stabilized between the residue qualities. After 3 years of inputs in the field plots, adding residue significantly increased total soil carbon by an average of 6 g C/kg soil over the no input control. Similar to the last sampling of the incubation, there were no differences in the amount of carbon stabilized among residue qualities in the field. Nitrogen fertilizer additions did not significantly alter soil carbon contents in either experiment. We conclude that residue quality does not affect the long-term stabilization of soil organic carbon; only increased residue quantities lead to a long-term stabilization.