No abstract

Measuring the stable isotope composition of nitrous oxide (N2O) evolved from soil could improve our understanding of the relative contributions of the main microbial processes (nitrification and denitrification) responsible for N2O formation in soil. However, interpretation of the isotopic data in N2O is complicated by the lack of knowledge of fractionation parameters by different microbial processes responsible for N2O production and consumption. Here we report isotopic enrichment for both nitrogen and oxygen isotopes in two stages of denitrification, N2O production and N2O reduction. We found that during both N2O production and reduction, enrichments were higher for oxygen than nitrogen. For both elements, enrichments were larger for N2O production stage than for N2O reduction. During gross N2O production, the ratio of δ18O‐to‐δ15N differed between soils, ranging from 1.6 to 2.7. By contrast, during N2O reduction, we observed a constant ratio of δ18O‐to‐δ15N with a value near 2.5. If general, this ratio could be used to estimate the proportion of N2O being reduced in the soil before escaping into the atmosphere. Because N2O‐reductase enriches N2O in both isotopes, the global reduction of N2O consumption by soil may contribute to the globally observed isotopic depletion of atmospheric N2O.

The availability of C and N to the soil microbial biomass is an important determinant of the rates of soil N transformations. Here, we present evidence that changes in C and N availability affect the 15 N natural abundance of the microbial biomass relative to other soil N pools. We analysed the 15 N natural abundance signature of the chloroform-labile, extractable, NO 3 -, NH 4 þ and soil total N pools across a cattle manure gradient associated with a water reservoir in semiarid, high-desert grassland. High levels of C and N in soil total, extractable, NO 3 -, NH 4 þ and chloroform-labile fractions were found close to the reservoir. The 15 N value of chloroform-labile N was similar to that of extractable (organic þ inorganic) N and NO 3 -at greater C availability close to the reservoir, but was 15 N-enriched relative to these N-pools at lesser C availability farther away. Possible mechanisms for this variable 15 N-enrichment include isotope fractionation during N assimilation and dissimilation, and changes in substrate use from a less to a more 15 N-enriched substrate with decreasing C availability.

Forest ecosystems assimilate more CO 2 from the atmosphere and store more carbon in woody biomass than most nonforest ecosystems, indicating strong potential for afforestation to serve as a carbon management tool. However, converting grasslands to forests could affect ecosystem-atmosphere exchanges of other greenhouse gases, such as nitrous oxide and methane (CH 4 ), effects that are rarely considered. Here, we show that afforestation on a well-aerated grassland in Siberia reduces soil CH 4 uptake by a factor of 3 after 35 years of tree growth. The decline in CH 4 oxidation was observed both in the field and in laboratory incubation studies under controlled environmental conditions, suggesting that not only physical but also biological factors are responsible for the observed effect. Using incubation experiments with 13 CH 4 and tracking 13 C incorporation into bacterial phospholipid fatty acid (PLFA), we found that, at low CH 4 concentrations, most of the 13 C was incorporated into only two PLFAs, 18 : 1x7 and 16 : 0. High CH 4 concentration increased total 13 C incorporation and the number of PLFA peaks that became labeled, suggesting that the microbial assemblage oxidizing CH 4 shifts with ambient CH 4 concentration. Forests and grasslands exhibited similar labeling profiles for the high-affinity methanotrophs, suggesting that largely the same general groups of methanotrophs were active in both ecosystems. Both PLFA concentration and labeling patterns indicate a threefold decline in the biomass of active methanotrophs due to afforestation, but little change in the methanotroph community. Because the grassland consumed CH 4 at a rate five times higher than forest soils under laboratory conditions, we concluded that not only biomass but also cell-specific activity was higher in grassland than in afforested plots. While the decline in biomass of active methanotrophs can be explained by site preparation (plowing), inorganic N (especially NH 4 1 ) could be responsible for the change in cell-specific activity. Overall, the negative effect of afforestation of upland grassland on soil CH 4 uptake can be largely explained by the reduction in biomass and to a lesser extent by reduced cell-specific activity of CH 4 -oxidizing bacteria.