Soil Conditions Rather Than Long-Term Exposure to Elevated CO2 Affect Soil Microbial Communities Associated with N-Cycling

Abstract

Continuously rising atmospheric CO2 concentrations may lead to an increased transfer of organic C from plants to the soil through rhizodeposition and may affect the interaction between the C- and N-cycle. For instance, fumigation of soils with elevated CO2 (eCO2) concentrations (20% higher compared to current atmospheric concentrations) at the Giessen Free-Air Carbon Dioxide Enrichment (GiFACE) sites resulted in a more than 2-fold increase of long-term N2O emissions and an increase in dissimilatory reduction of nitrate compared to ambient CO2 (aCO2). We hypothesized that the observed differences in soil functioning were based on differences in the abundance and composition of microbial communities in general and especially of those which are responsible for N-transformations in soil. We also expected eCO2 effects on soil parameters, such as on nitrate as previously reported. To explore the impact of long-term eCO2 on soil microbial communities, we applied a molecular approach (qPCR, T-RFLP, and 454 pyrosequencing). Microbial groups were analyzed in soil of three sets of two FACE plots (three replicate samples from each plot), which were fumigated with eCO2 and aCO2, respectively. N-fixers, denitrifiers, archaeal and bacterial ammonia oxidizers, and dissimilatory nitrate reducers producing ammonia were targeted by analysis of functional marker genes, and the overall archaeal community by 16S rRNA genes. Remarkably, soil parameters as well as the abundance and composition of microbial communities in the top soil under eCO2 differed only slightly from soil under aCO2. Wherever differences in microbial community abundance and composition were detected, they were not linked to CO2 level but rather determined by differences in soil parameters (e.g., soil moisture content) due to the localization of the GiFACE sets in the experimental field. We concluded that +20% eCO2 had little to no effect on the overall microbial community involved in N-cycling in the soil but that spatial heterogeneity over extended periods had shaped microbial communities at particular sites in the field. Hence, microbial community composition and abundance alone cannot explain the functional differences leading to higher N2O emissions under eCO2 and future studies should aim at exploring the active members of the soil microbial community.

Elevated Atmospheric CO2 Modifies Mostly the Metabolic Active Rhizosphere Soil Microbiome in the Giessen FACE Experiment

“…These results differ from previous reports of the eCO 2 effects on the Gi-FACE soil microbiome, which stated that only subtle or no effect occurred on microbial communities and that the differences were mostly due to soil conditions and the moisture gradient that occurs at this facility [25][26][27]. Similarly, to the aforementioned studies, our data confirmed that samples from ring-pair A1-E1 had lower water content in comparison to A2-E2 and A3-E3 samples (S2), and that water holding capacity (WHC) significantly influenced the soil microbiome (Table 1, Fig.…”

“…Similarly, also de Menezes et al [26] described that increases in atmospheric CO 2 may cause only minor changes in Gi-FACE's soil bacterial community composition and that functional responses of the soil community are due to factors like soil moisture rather than CO 2 concentration. Brenzinger et al [27] reported that the abundance and composition of microbial communities in the topsoil under eCO 2 presented only small differences from soil under aCO 2 (aCO 2 ), concluding that + 20% CO 2 had little to no effect on the overall microbial community involved in N-cycling in the Gi-FACE soil. More recently, Bei et al [28] described that eCO 2 had significant effects on the functional expression associated to both rhizosphere microbiomes and plant roots; and that abundances of Eukarya relative to Bacteria were significantly decreased in eCO 2 as well.…”

Composition and activity of nitrifier communities in soil are unresponsive to elevated temperature and CO2, but strongly affected by drought

“…Likewise, eCO 2 did not affect gross N mineralization and NH 4 + levels, highlighting that substrate availability for nitrifiers did not change in response to eCO 2 . In addition, other studies also reported very few effects of elevated atmospheric CO 2 concentration on belowground N processes in heathlands and temperate grasslands [70][71][72] showing that soil parameters such as pH and inorganic N concentrations likely play a more significant role than increased atmospheric CO 2 on overall nitrification. Elevated temperature alone also did not significantly affect most of the soil processes and variables studied.…”

Explaining the doubling of N2O emissions under elevated CO2 in the Giessen FACE via in‐field 15N tracing

“…An increase in soil moisture under eCO 2 has been suggested to stimulate denitrification (van Groenigen et al., ) caused by a change in the microbial community (Brenzinger et al., ), for example, a reduced abundance of N 2 O reducers (Guenet et al., ; Regan et al., ). We could not detect significant soil moisture differences in this study, but slightly higher soil moisture under eCO 2 occurred only during the autumn and winter months, when N 2 O emissions were low and very similar under the aCO 2 and eCO 2 treatments.…”