Carbon dioxide enrichment and nitrogen fertilization effects on cotton (Gossypium hirsutum L.) plant residue chemistry and decomposition

Increased atmospheric carbon dioxide (CO2) concentration will likely cause changes in plant productivity and composition that might affect soil decomp osition processes. The objective of this study was to test to what extent e levated CO2 and N fertility-induced changes in residue quality controlled d ecomposition rates. Cotton ( Gossypium hirsutum L.) was grown in 8-l pots a nd exposed to two concentrations of CO2 (390 or 722 mu mol mol(-1)) and two levels of N fertilization (1.0 or 0.25 g l(-1) soil) within greenhouse cha mbers for 8 wks. Plants were then chemically defoliated and air-dried. Leaf , stem and root residues were assayed for total non-structural carbohydrate s (TNC), lignin (LTGA), proanthocyanidins (PA), C and N. Respiration rates of an unsterilized sandy soil (Lakeland Sand) mixed with residues from the various treatments were determined using a soda lime trap to measure CO2 re lease. At harvest, TNC and PA concentrations were 17 to 45% higher in resid ues previously treated with elevated CO2 compared with controls. Leaf and s tem residue LTGA concentrations were not significantly affected by either t he elevated CO2 or N fertilization treatments, although root residue LTGA c oncentration was 30% greater in plants treated with elevated CO2. The conce ntration of TNC in leaf residues from the low N fertilization treatment was 2.3 times greater than that in the high N fertilization treatment, althoug h TNC concentration in root and stem residues was suppressed 13 to 23% by t he low soil N treatment. PA and LTGA concentrations in leaf, root and stem residues were affected by less than 10% by the low N fertilization treatmen t. N concentration was 14 to 44% lower in residues obtained from the elevat ed CO2 and low N fertilization treatments. In the soil microbial respiratio n assay, cumulative CO2 release was 10 to 14% lower in soils amended with r esidues from the elevated CO2 and low N fertility treatments, although trea tment differences diminished as the experiment progressed. Treatment effect s on residue N concentration and C:N ratios appeared to be the most importa nt factors affecting soil microbial respiration. The results of our study s trongly suggest that, although elevated CO2 and N fertility may have signif icant impact on post-harvest plant residue quality of cotton, neither facto r is likely to substantially affect decomposition. Thus, C cycling might no t be affected in this way, but via simple increases in plant biomass produc tion.