Plant-microbial responses to reduced precipitation depend on tree species in a temperate forest.

Given that global change is predicted to increase the frequency and severity of drought in temperate forests, it is critical to understand the degree to which plant belowground responses cascade through the soil system to drive ecosystem responses to water stress. While most research has focused on plant and microbial responses independently of each other, a gap in our understanding lies in the integrated response of plant-microbial interactions to water stress. We investigated the extent to which divergent belowground responses to reduced precipitation between sugar maple trees (Acer saccharum) versus oak trees (Oak spp.) may influence microbial activity via throughfall exclusion in the field. Evidence that oak trees send carbon belowground to prime microbial activity more than maples under ambient conditions and in response to water stress suggests there is the potential for corresponding impacts of reduced precipitation on microbial activity. As such, we tested the hypothesis that differences in belowground C allocation between oaks and maples would stimulate microbial activity in the oak treatment soils and reduce microbial activity in in the sugar maple treatment soils compared to their respective controls. We found that the treatment led to declines in N mineralization, soil respiration, and oxidative enzyme activity in the sugar maple treatment plot. These declines may be due to sugar maple trees reducing root C transfers to the soil. By contrast, the reduced precipitation treatment enhanced soil respiration, as well as rates of N mineralization and peroxidase activity in the oak rhizosphere. This enhanced activity suggests that oak roots provided optimal rhizosphere conditions during water stress to prime microbial activity to support net primary production. With future changes in precipitation predicted for forests in the Eastern US, we show that the strength of plant-microbial interactions drives the degree to which reduced precipitation impacts soil C and nutrient cycling.

Multiplex qPCR assays to distinguish individual species of arbuscular mycorrhizal fungi from roots and soil.

Currently, root colonization measurements of arbuscular mycorrhizal fungi (AMF) require staining and microscopy, and species-level identification of the fungi by such observations is not possible. Here, we present novel multiplex real-time PCR assays targeting the glomalin genes of 11 different species of AMF commonly found in temperate agricultural soils, which independently detect and measure the abundance of these fungi using DNA extracts from soil and or root tissue. The availability of these tools will not only increase throughput in determining levels of root colonization but can provide species-specific levels of root colonization from a single sample. This will help to establish which AMF species, or combinations of different species, provide the most benefits to crops, and will aid in the development of AMF for use as biofertilizers.

Altered plant carbon partitioning enhanced forest ecosystem carbon storage after 25 years of nitrogen additions.

Decades of atmospheric nitrogen (N) deposition in the northeastern USA have enhanced this globally important forest carbon (C) sink by relieving N limitation. While many N fertilization experiments found increased forest C storage, the mechanisms driving this response at the ecosystem scale remain uncertain. Following the optimal allocation theory, augmented N availability may reduce belowground C investment by trees to roots and soil symbionts. To test this prediction and its implications on soil biogeochemistry, we constructed C and N budgets for a long-term, whole-watershed N fertilization study at the Fernow Experimental Forest, WV, USA. Nitrogen fertilization increased C storage by shifting C partitioning away from belowground components and towards aboveground woody biomass production. Fertilization also reduced the C cost of N acquisition, allowing for greater C sequestration in vegetation. Despite equal fine litter inputs, the C and N stocks and C : N ratio of the upper mineral soil were greater in the fertilized watershed, likely due to reduced decomposition of plant litter. By combining aboveground and belowground data at the watershed scale, this study demonstrates how plant C allocation responses to N additions may result in greater C storage in both vegetation and soil.

Differences in microbial community response to nitrogen fertilization result in unique enzyme shifts between arbuscular and ectomycorrhizal-dominated soils.

While the effect of nitrogen (N) deposition on belowground carbon (C) cycling varies, emerging evidence shows that forest soils dominated by trees that associate with ectomycorrhizal fungi (ECM) store more C than soils dominated by trees that associate with arbuscular mycorrhizae (AM) with increasing N deposition. We hypothesized that this is due to unique nutrient cycling responses to N between AM and ECM-dominated soils. ECM trees primarily obtain N through fungal mining of soil organic matter subsidized by root-C. As such, we expected the largest N-induced responses of C and N cycling to occur in ECM rhizospheres and be driven by fungi. Conversely, as AM trees rely on bacterial scavengers in bulk soils to cycle N, we predicted the largest AM responses to be driven by shifts in bacteria and occur in bulk soils. To test this hypothesis, we measured microbial community composition, metatranscriptome profiles, and extracellular enzyme activity in bulk, rhizosphere, and organic horizon (OH) soils in AM and ECM-dominated soils at Bear Brook Watershed in Maine, USA. After 27 years of N fertilization, fungal community composition shifted across ECM soils, but bacterial communities shifted across AM soils. These shifts were mirrored by enhanced C relative to N mining enzyme activities in both mycorrhizal types, but this occurred in different soil fractions. In ECM stands these shifts occurred in rhizosphere soils, but in AM stands they occurred in bulk soils. Additionally, ECM OH soils exhibited the opposite response with declines in C relative to N mining. As rhizosphere soils account for only a small portion of total soil volume relative to bulk soils, coupled with declines in C to N enzyme activity in ECM OH soils, we posit that this may partly explain why ECM soils store more C than AM soils as N inputs increase.

Interactions among plants, bacteria, and fungi reduce extracellular enzyme activities under long-term N fertilization.

Atmospheric nitrogen (N) deposition has enhanced soil carbon (C) stocks in temperate forests. Most research has posited that these soil C gains are driven primarily by shifts in fungal community composition with elevated N leading to declines in lignin degrading Basidiomycetes. Recent research, however, suggests that plants and soil microbes are dynamically intertwined, whereby plants send C subsidies to rhizosphere microbes to enhance enzyme production and the mobilization of N. Thus, under elevated N, trees may reduce belowground C allocation leading to cascading impacts on the ability of microbes to degrade soil organic matter through a shift in microbial species and/or a change in plant-microbe interactions. The objective of this study was to determine the extent to which couplings among plant, fungal, and bacterial responses to N fertilization alter the activity of enzymes that are the primary agents of soil decomposition. We measured fungal and bacterial community composition, root-microbial interactions, and extracellular enzyme activity in the rhizosphere, bulk, and organic horizon of soils sampled from a long-term (>25 years), whole-watershed, N fertilization experiment at the Fernow Experimental Forest in West Virginia, USA. We observed significant declines in plant C investment to fine root biomass (24.7%), root morphology, and arbuscular mycorrhizal (AM) colonization (55.9%). Moreover, we found that declines in extracellular enzyme activity were significantly correlated with a shift in bacterial community composition, but not fungal community composition. This bacterial community shift was also correlated with reduced AM fungal colonization indicating that declines in plant investment belowground drive the response of bacterial community structure and function to N fertilization. Collectively, we find that enzyme activity responses to N fertilization are not solely driven by fungi, but instead reflect a whole ecosystem response, whereby declines in the strength of belowground C investment to gain N cascade through the soil environment.

Inoculation of black turtle beans (Phaseolus vulgaris) with mycorrhizal fungi increases the nutritional quality of seeds.

The use of arbuscular mycorrhizal fungi (AMF) as biofertilizers has proven successful in boosting the yield and nutritional quality of a variety of crops. AMF associate with plant roots and exchange soil nutrients for photosynthetically derived C in the form of sugars and lipids. Past research has shown that not all AMF species are equal in their benefit to nutrient uptake and crop health, and that the most beneficial AMF species appear to vary by host species. Although an important human food staple, especially in developing regions where nutrient deficiency is a prevalent threat to public health, little work has been done to test the effectiveness of AMF in enhancing the nutritional quality of common bean (Phaseolus vulgaris L.). Therefore, our objective was to determine the most beneficial AMF species for inoculation of this important crop. We inoculated black beans (Phaseolus vulgaris black turtle beans) with eight individual AMF species and one mixed species inoculum in an outdoor pot trial over 3 months and assessed the extent to which they altered yield, mineral nutrient and anthocyanin concentration of seeds and leaf tissues. Despite seeing no yield effects from inoculation, we found that across treatments percent root length colonized by AMF was positively correlated with plant tissue P, Cu, and Zn concentration. Underlying these broad benefits, seeds from plants inoculated with three AMF species, Claroideoglomus claroideum (+15%), Funneliformis mosseae (+13%), and Gigaspora rosea (+11%) had higher P concentration than non-mycorrhizal plants. C. claroideum also increased seed potassium (K) and copper (Cu), as well as leaf aluminum (Al) concentration making it a promising candidate to further test the benefit of individual AMF species on black bean growth in field trials.