COS 11-4
Spatial and temporal variabilitiy over-shadow the effects of climate manipulation and grazing on soil bacterial communities in the steppe of northern Mongolia
Some of the most diverse bacterial communities are found in soils.Very little is known about temporal and spatial variation in the composition of soil bacterial communities and the drivers of these differences. We undertook a multi-year experiment to test effects of climate change and grazing on soil bacterial communities in northern Mongolia, a region where air temperatures have increased by 1.6 °C since 1960 and grazing patterns are changing due to more sedentary pastoralism.
We examined soil bacterial communities under climate and grazing manipulation regimes. Open-top chambers (OTCs) created warmer and drier conditions and year-round fencing manipulated grazers (i.e. grazed versus ungrazed plots). A total of 68 soil samples were collected in 2010 and 2011 at two elevations (upper and lower slope locations). DNA was extracted and 16S rRNA V4 PCR amplicons were sequenced using the Illumina platform. PRS probes measured root available nitrogen (Western Ag).
Constrained analysis of proximities (CAP) examined effects of year, slope location, climate and grazing on bacterial communities. Genera abundance was examined using Bray-Curtis dissimilarities. A similar analysis used weighted UniFrac distances to account for phylogenetic relatedness among samples. CAP output was overlain with gradients in nitrate and ammonium concentrations.
Results/Conclusions
Bacterial community structure (Bray-Curtis) varied significantly with year and slope location, but not climate, grazing or any interactions. Samples in 2011 had a greater proportion of bacterial genera (P<0.025). Ammonium concentrations were greater on the upper versus lower slope (P<0.002), while nitrate was greater in 2011 than 2010 (P<0.007). This suggests a link between variation in nitrogen and bacterial community composition.
Weighted UniFrac distances revealed significant differences for slope (P<0.030), year (P<0.005) and climate (P<0.005). CAP ordination showed the space occupied by the upper slope samples was completely contained within the space occupied by the lower slope; suggesting greater phylogenetic diversity on the lower slope.
Year, slope and nitrogen content, but not grazing, are major drivers in structuring bacterial community composition, as measured by both genera abundance and phylogenetic relatedness. OTCs, which cause warmer and drier conditions, only affect the phylogenetic relatedness among samples. Variation in nitrate and ammonium may contribute to the observed spatial and temporal differences in bacterial communities. While experimental treatments, like climate manipulation, can affect soil bacterial communities, larger scale temporal and spatial patterns may overshadow or reinforce climate as a driver of community composition.