Dependence of CUE and microbial carbon pools on soil C:N ratios along a tropical altitudinal gradient.

High microbial carbon use efficiency (CUE) indicates greater proportion of metabolized Cconverted to microbial biomass, and thus it increases the potential for C to become stabilisedin soil as dead microbial biomass remains according to the "soil microbial carbon pump"concept. According to the theory of ecological stoichiometry microbial CUE decreases withincreasing soil C:N ratio, or increasing N limitation, but it is not known whether CUE couldalso directly predict the accumulation of microbial residues, such as amino sugars, into thesoil, or whether this process is more affected by other environmental factors than CUE andstoichiometry. In order to test the hypotheses on the linkage between CUE, soil C:N ratios and microbialcarbon pools, we established soil sampling plots along an altitudinal gradient (900 to 2200 mabove sea level) in the Taita Hills, in south-eastern Kenya. We hypothesised that microbialN-limitation will increase with altitude when moving from the low elevation drier forests tothe moist montane forests. Increasing C:N ratios with elevation would allow us to test oursecond hypothesis that according to the theory of ecological stoichiometry microbial CUEdecreases with increasing soil C:N ratio, or increasing N limitation. We will also studywhether CUE is directly linked to accumulation of microbial residues (amino sugars) insoil. Seven sites were investigated along the elevation gradient and three 10x10 m plots weresampled at each site. At each plot, 20 soil cores were taken from 0-10 cm depth and pooled tomake up a composite sample. To test the first hypothesis, we measured C and N concentrations in soil and microbialbiomass. The average soil C% (r=0.92, p=0.003), soil N% (r=0.90, p=0.006) and soil C:Nratios (r=0.90, p=0.006) increased with elevation. There was also an increasingtrend in the average microbial C:N ratios with elevation (r=0.69, p=0.086). Wemeasured enzyme activities using MUF-substrates, and found that there was a trend ofincreasing NAGase activity with elevation (r = 0.68, p=0.096) and C:N ratio (r =0.74, p=0.059), supporting the hypothesis of increasing N limitation with altitude.However, NAGase activity was not related to total soil N%, indicating that totalsoil N content, or bulk soil C:N ratio may not be good indicators for microbial Navailability. Microbial CUE was determined using two methods: 13C glucose tracing (24 h incubationat 15 ˚ C), and the 18O method. We are currently calculating the results from these analyses.Preliminary results indicate that the specific respiration rates (μg CO2-C produces per g soilC) were best correlated with the ratio of cellobiosidase/peroxidase enzyme activities (r=0.95,p=0.001) that can be used as an index of SOM quality. Specific respiration rates werehighest in the two low elevation forest sites that had high cellobiasidase activity,while they were lower at high elevation sites with high peroxidase activity linked todecomposition of low quality, recalcitrant C. It seems therefore likely that along our gradientthe quality of SOM is more important in determining CUE than stoichiometry. [ABSTRACT FROM AUTHOR]