UV-B radiation and soil microbial communities: Nitrogen storage (communication arising)

Stark and Hart question our finding that increased UV-B irradiation affects the biomass C:N ratio of Arctic soil microbial communities. Our data for microbial carbon biomass (Cmic; 1.0–3.1 mg per g soil) and nitrogen (Nmic; 0.1–0.3 mg per g soil) fall within the range presented by Stark and Hart (0.2–3.9 and 0.04–0.28 mg per g soil for Cmic and Nmic, respectively), but the C:N ratios in the control plots are greater than those for both similar and contrasting soil types. However, we do not agree that this invalidates our conclusions. Stark and Hart suggest that the high C:N ratios could have occurred as a result of artefacts in the experimental conditions or differences in basic soil properties, such as moisture or carbon content, during fumigation. The finding that basic soil properties can affect fumigation suggests that there is limited value in comparing Cmic and Nmic values from different studies. We agree that this could have important consequences for the results if samples within a study were to vary significantly in these properties. However, we subsequently found no correlation between the microbial C:N ratio and moisture content (P = 0.218) or carbon content (P = 0.300) of the original samples. For statistical analysis, we calculated separate C:N ratios for each pair of Cmic and Nmic values and determined the means of these ratios. However, the C:N ratios presented by Stark and Hart are calculated from a mean Cmic value and a mean Nmic value. Calculation of our C:N ratios using their method gives ratios of 24.1 and 8.9 for ambient and increased UV-B, respectively. These values are much lower for the ambient UV-B treatment, are within the range of published values, and still follow the same pattern as those that we originally reported. Furthermore, we discussed the C:N ratios together with complementary and independent data, which showed significant changes in utilization of carbon compounds by soil bacteria in response to increased UV-B irradiation. These combined results probably reflect alterations in the composition of the microbial community. Our conclusion that increased UV-B irradiation may affect microbial nitrogen storage was based on the increase in Nmic. Stark and Hart's observations highlight the need for more research on the structure and functioning of microbial communities and the methods used to study them. This may be particularly relevant for subarctic heaths, for which there are few comparable data. Previous work from sites close to ours reported Cmic and Nmic values (5.6–12.8 and 0.7–1.3 mg per g soil, respectively) that were highly variable and also greater than those listed by Stark and Hart. Together with our values, these indicate that the microbial biomass and C:N ratio in these extremely organic soils is highly variable and site-specific. We therefore contend that our data, which were obtained under identical experimental conditions, provide evidence that increased UV-B irradiation can affect both the community structure and the amount of nitrogen held within the microbial biomass.