Kirsten S. Hofmockel

Increases in the flux of carbon belowground stimulate nitrogen uptake and sustain the long-term enhancement of forest productivity under elevated CO2

The earths future climate state is highly dependent upon changes in terrestrial C storage in response to rising concentrations of atmospheric CO₂. Here we show that consistently enhanced rates of net primary production (NPP) are sustained by a C-cascade through the root-microbe-soil system; increases in the flux of C belowground under elevated CO₂ stimulated microbial activity, accelerated the rate of soil organic matter decomposition and stimulated tree uptake of N bound to this SOM. This process set into motion a positive feedback maintaining greater C gain under elevated CO₂ as a result of increases in canopy N content and higher photosynthetic N-use efficiency. The ecosystem-level consequence of the enhanced requirement for N and the exchange of plant C for N belowground is the dominance of C storage in tree biomass but the preclusion of a large C sink in the soil.

Plant diversity predicts beta but not alpha diversity of soil microbes across grasslands worldwide

Aboveground-belowground interactions exert critical controls on the composition and function of terrestrial ecosystems, yet the fundamental relationships between plant diversity and soil microbial diversity remain elusive. Theory predicts predominantly positive associations but tests within single sites have shown variable relationships, and associations between plant and microbial diversity across broad spatial scales remain largely unexplored. We compared the diversity of plant, bacterial, archaeal and fungal communities in one hundred and forty-five 1xa0m(2) plots across 25 temperate grassland sites from four continents. Across sites, the plant alpha diversity patterns were poorly related to those observed for any soil microbial group. However, plant beta diversity (compositional dissimilarity between sites) was significantly correlated with the beta diversity of bacterial and fungal communities, even after controlling for environmental factors. Thus, across a global range of temperate grasslands, plant diversity can predict patterns in the composition of soil microbial communities, but not patterns in alpha diversity.

Integrating microbial ecology into ecosystem models: challenges and priorities

Microbial communities can potentially mediate feedbacks between global change and ecosystem function, owing to their sensitivity to environmental change and their control over critical biogeochemical processes. Numerous ecosystem models have been developed to predict global change effects, but most do not consider microbial mechanisms in detail. In this idea paper, we examine the extent to which incorporation of microbial ecology into ecosystem models improves predictions of carbon (C) dynamics under warming, changes in precipitation regime, and anthropogenic nitrogen (N) enrichment. We focus on three cases in which this approach might be especially valuable: temporal dynamics in microbial responses to environmental change, variation in ecological function within microbial communities, and N effects on microbial activity. Four microbially-based models have addressed these scenarios. In each case, predictions of the microbial-based models differ—sometimes substantially—from comparable conventional models. However, validation and parameterization of model performance is challenging. We recommend that the development of microbial-based models must occur in conjunction with the development of theoretical frameworks that predict the temporal responses of microbial communities, the phylogenetic distribution of microbial functions, and the response of microbes to N enrichment.

Regional Contingencies in the Relationship between Aboveground Biomass and Litter in the World's Grasslands

Based on regional-scale studies, aboveground production and litter decomposition are thought to positively covary, because they are driven by shared biotic and climatic factors. Until now we have been unable to test whether production and decomposition are generally coupled across climatically dissimilar regions, because we lacked replicated data collected within a single vegetation type across multiple regions, obfuscating the drivers and generality of the association between production and decomposition. Furthermore, our understanding of the relationships between production and decomposition rests heavily on separate meta-analyses of each response, because no studies have simultaneously measured production and the accumulation or decomposition of litter using consistent methods at globally relevant scales. Here, we use a multi-country grassland dataset collected using a standardized protocol to show that live plant biomass (an estimate of aboveground net primary production) and litter disappearance (represented by mass loss of aboveground litter) do not strongly covary. Live biomass and litter disappearance varied at different spatial scales. There was substantial variation in live biomass among continents, sites and plots whereas among continent differences accounted for most of the variation in litter disappearance rates. Although there were strong associations among aboveground biomass, litter disappearance and climatic factors in some regions (e.g. U.S. Great Plains), these relationships were inconsistent within and among the regions represented by this study. These results highlight the importance of replication among regions and continents when characterizing the correlations between ecosystem processes and interpreting their global-scale implications for carbon flux. We must exercise caution in parameterizing litter decomposition and aboveground production in future regional and global carbon models as their relationship is complex.

Community composition and photosynthesis by photoautotrophs under quartz pebbles, Southern Mojave Desert

We used 16s rDNA sequences to identify novel species of cyanobacteria beneath translucent quartz pebbles in the desert pavement on an alluvial piedmont of the Coxcomb Mountains in the southern Mojave Desert, California, USA. Transmission of light, as measured with an integrating sphere, was about 0.08% beneath the thickest pieces of quartz (25 mm) harboring these hypolithic autotrophs. The photosynthetic rate ranged from 0.1 to 1.0 μmol·m−2·s−1 in the linear range of its response to light (PAR of 0–50 μmol·m−2·s−1), over which the apparent quantum-use efficiency was 0.019. Light-saturated rates of 1.7–2.7 μmol·m−2·s−1 were recorded at light intensities of 200–400 μmol·m−2·s−1. The hypolithic community had an upper thermal tolerance of >90°C in laboratory conditions. The quartz pebbles confer a modest greenhouse effect that may be important for photosynthetic activity during cool, wet, wintertime periods that prevail in the Mojave Desert.

Demonstrating microbial co-occurrence pattern analyses within and between ecosystems

Co-occurrence patterns are used in ecology to explore interactions between organisms and environmental effects on coexistence within biological communities. Analysis of co-occurrence patterns among microbial communities has ranged from simple pairwise comparisons between all community members to direct hypothesis testing between focal species. However, co-occurrence patterns are rarely studied across multiple ecosystems or multiple scales of biological organization within the same study. Here we outline an approach to produce co-occurrence analyses that are focused at three different scales: co-occurrence patterns between ecosystems at the community scale, modules of co-occurring microorganisms within communities, and co-occurring pairs within modules that are nested within microbial communities. To demonstrate our co-occurrence analysis approach, we gathered publicly available 16S rRNA amplicon datasets to compare and contrast microbial co-occurrence at different taxonomic levels across different ecosystems. We found differences in community composition and co-occurrence that reflect environmental filtering at the community scale and consistent pairwise occurrences that may be used to infer ecological traits about poorly understood microbial taxa. However, we also found that conclusions derived from applying network statistics to microbial relationships can vary depending on the taxonomic level chosen and criteria used to build co-occurrence networks. We present our statistical analysis and code for public use in analysis of co-occurrence patterns across microbial communities.

A long-term nitrogen fertilizer gradient has little effect on soil organic matter in a high-intensity maize production system

Global maize production alters an enormous soil organic C (SOC) stock, ultimately affecting greenhouse gas concentrations and the capacity of agroecosystems to buffer climate variability. Inorganic N fertilizer is perhaps the most important factor affecting SOC within maize-based systems due to its effects on crop residue production and SOC mineralization. Using a continuous maize cropping system with a 13 year N fertilizer gradient (0-269 kg N ha(-1) yr(-1)) that created a large range in crop residue inputs (3.60-9.94 Mg dry matter ha(-1) yr(-1)), we provide the first agronomic assessment of long-term N fertilizer effects on SOC with direct reference to N rates that are empirically determined to be insufficient, optimum, and excessive. Across the N fertilizer gradient, SOC in physico-chemically protected pools was not affected by N fertilizer rate or residue inputs. However, unprotected particulate organic matter (POM) fractions increased with residue inputs. Although N fertilizer was negatively linearly correlated with POM C/N ratios, the slope of this relationship decreased from the least decomposed POM pools (coarse POM) to the most decomposed POM pools (fine intra-aggregate POM). Moreover, C/N ratios of protected pools did not vary across N rates, suggesting little effect of N fertilizer on soil organic matter (SOM) after decomposition of POM. Comparing a N rate within 4% of agronomic optimum (208 kg N ha(-1) yr(-1)) and an excessive N rate (269 kg N ha(-1) yr(-1)), there were no differences between SOC amount, SOM C/N ratios, or microbial biomass and composition. These data suggest that excessive N fertilizer had little effect on SOM and they complement agronomic assessments of environmental N losses, that demonstrate N2 O and NO3 emissions exponentially increase when agronomic optimum N is surpassed.

Nitrogen addition changes grassland soil organic matter decomposition

Humans have dramatically increased the deposition and availability of nutrients, such as nitrogen (N), worldwide. Soil organic matter (SOM) is a significant global reservoir of carbon (C); however, the effects of N enrichment on this large, heterogeneous C stock are unclear. Nitrogen has variable effects on the biological, chemical, and physical factors that determine SOM pool mean residence time; consequently, we predicted that N enrichment would have distinct effects on SOM pools, including the pool that is readily available for microbial decomposition, as well as the pools that have been stabilized against microbial decomposition via aggregate occlusion and mineral association. We addressed this gap in knowledge by measuring the effects of N addition on different SOM pools at five grassland experiments in the US Central Great Plains that participate in the Nutrient Network and have been fertilized for three or fivexa0years. Overall, N addition decreased microbial respiration of unoccluded OM by as much as 29xa0% relative to control plots, and consequently, decreased C loss from this pool. Furthermore, N addition tended to increase soil aggregation and C occlusion in large macro-aggregates. These results suggest that N addition will increase C sequestration by slowing the decomposition of SOM, as well as stabilizing SOM against microbial decomposition in aggregate-occluded pools. However, the effects of N on all pools studied varied among sites, possibly due to site variation in soil texture. Consequently, increased sequestration of soil C in response to N enrichment may not be universal across grasslands.

Physiological shifts in the microbial community drive changes in enzyme activity in a perennial agroecosystem

Perennial agroecosystems have the potential to promote plant–microbial linkages by increasing the quantity of root carbon entering the soil. However, an understanding of how perennial cropping systems affect microbial communities remains incomplete. The objective of this study was to determine the potential for a fertilized perennial bioenergy cropping system to impact microbial growth and enzyme activity. Three times throughout the growing season we examined the activity of four enzymes involved in decomposition (ß-glucosidase, ß-xylosidase, cellobiohydrolase, and N-acetyl glucosaminidase) in replicated plots of an annual (corn) and perennial-based (switchgrass) cropping system. We also took simultaneous measurements of microbial biomass and potential rates of microbial respiration and net N mineralization. Microbial biomass was unaffected by cropping system. Mid-summer, however, we observed increases in enzyme activity and potential microbial respiration in the perennial system that were independent of microbial biomass, likely in response to labile carbon inputs. Further, we observed lower net N mineralization, higher microbial biomass nitrogen and higher activity of nitrogen liberating enzymes, which are indicative of a community with high nitrogen demands. Overall, our research demonstrates that perennial agroecosystems can affect the physiological capacity of the microbial community, yielding communities with greater nitrogen retention and greater rates of decomposition as a result of allocation of resources towards enzyme production and nitrogen mining. These results can inform biogeochemical models with respect to the importance of temporally dynamic changes in carbon and nitrogen availability and microbial carbon use efficiency as drivers of enzyme production.

Invaded Grassland Communities Have Altered Stability-Maintenance Mechanisms But Equal Stability Compared to Native Communities

Theory predicts that stability should increase with diversity via several mechanisms. We tested predictions in a 5-year experiment that compared low-diversity exotic to high-diversity native plant mixtures under two irrigation treatments. The study included both wet and dry years. Variation in biomass across years (CV) was 50% lower in mixtures than monocultures of both native and exotic species. Growth among species was more asynchronous and overyielding values were greater during and after a drought in native than exotic mixtures. Mean-variance slopes indicated strong portfolio effects in both community types, but the intercept was higher for exotics than for natives, suggesting that exotics were inherently more variable than native species. However, this failed to result in higher CVs in exotic communities because species that heavily dominated plots tended to have lower than expected variance. Results indicate that diversity-stability mechanisms are altered in invaded systems compared to native ones they replaced.