Remotely detected plant function in two midwestern prairie grassland experiments reveals belowground processes

Imaging spectroscopy provides the opportunity to incorporate leaf and canopy optical data into ecological studies, but the extent to which remote sensing of vegetation can enhance the study of belowground processes is not well understood. In grassland systems, aboveground and belowground vegetation quantity and quality are coupled, and both influence belowground microbial processes and nutrient cycling, providing a potential link between remote sensing and belowground processes. We hypothesized that ecosystem productivity, and the chemical, structural and phylogenetic-functional composition of plant communities would be detectable with remote sensing and could be used to characterize belowground plant and soil processes in two grassland biodiversity experiments—the BioDIV experiment at Cedar Creek Ecosystem Science Reserve in Minnesota and the Wood River Nature Conservancy experiment in Nebraska. Specifically, we tested whether aboveground vegetation chemistry and productivity, as detected from airborne sensors, predict soil properties microbial processes and community composition. Imaging spectroscopy data were used to map aboveground biomass and green vegetation cover, functional traits and phylogenetic-functional community composition of vegetation. We examined the relationships between the image-derived variables and soil carbon and nitrogen concentration, microbial community composition, biomass and extracellular enzyme activity, and soil processes, including net nitrogen mineralization. In the BioDIV experiment—which has low overall diversity and productivity despite high variation in each—belowground processes were driven mainly by variation in the amount of organic matter inputs to soils. As a consequence, soil respiration, microbial biomass and enzyme activity, and fungal and bacterial composition and diversity were significantly predicted by remotely sensed vegetation cover. In contrast, at Wood River, where plant diversity and productivity were consistently higher, remotely sensed functional, chemical and phylogenetic composition of vegetation predicted belowground extracellular enzyme activity, microbial biomass, and net nitrogen mineralization rates, while aboveground biomass did not. The strong, contrasting associations between the quantity and chemistry of aboveground inputs with belowground soil processes and properties provide a basis for using imaging spectroscopy to understand belowground processes across productivity gradients in grassland systems. However, a mechanistic understanding of how above and belowground components interact among different ecosystems remains critical to extending these results broadly.

Soil microbial community responding to moderately elevated nitrogen deposition in a Japanese cool temperate forest surrounded by fertilized grasslands

In order to examine the hypothesis that the soil microbial community in a nitrogen-limited forest responds to moderately elevated nitrogen deposition (< 10kg N ha-1 yr-1), correlations between nitrogen deposition and soil microbial properties were analyzed in a cool temperate forest surrounded by normally fertilized pasture grasslands in northern Japan. Three experimental plots were established in forest edges adjacent to the grasslands and the other three plots were in forest interiors at least 700 m away from the grasslands. Nitrogen deposition in each plot was measured from May to November 2018. In August 2018, litter and surface soil samples were collected from all plots to measure net nitrogen mineralization and nitrification rates as indicators of microbial activity, and microbial biomass and various gene abundances (i.e., bacterial 16S rRNA, fungal ITS, and bacterial and archaeal amoA genes) as indicators of microbial abundance. Nitrogen deposition in forest edges was 1.4-fold greater than that in forest interiors, whereas maximum deposition was 3.7 kg N ha−1. Nitrogen deposition was significantly correlated with net nitrogen mineralization and nitrification rates and 16S rRNA and bacterial amoA gene abundances. Microbial community structures analyzed for bacterial 16S rRNA and fungal ITS gene amplicons were different between litter and soil samples, but were similar between the forest edge and interior. Nitrogen deposition was also correlated with the soil carbon-to-nitrogen ratio and nitrate and ammonium contents. Thus, it was suggested that some soil microbial activities and abundances in a nitrogen-limited forest likely responded to moderately elevated nitrogen deposition. These findings provide primary information on soil microbial response to moderately elevated nitrogen deposition.

Does Soil Microbial Community Respond to Moderately Elevated Nitrogen Deposition? A Correlation Analysis in a Cool Temperate Forest Surrounded by Pasture Grasslands in Northern Japan

We analyzed relationships between nitrogen deposition (deposition of nitrate and ammonium ions) and soil microbial properties, which were spatially varied in a cool temperate forest surrounded by normally fertilized pasture grasslands in northern Japan. The aim of the present study was to gain the primary information on soil microbial response to moderately elevated nitrogen deposition (< 10 kg N ha−1 y−1). We established three experimental plots in the forest edge adjacent to the grasslands and other three plots in the forest interior at least 700 m away from the grasslands. During May to November 2018, nitrogen deposition in each plot was measured. In August 2018, litter and soil (0−5 cm depth) samples were collected from all plots to measure net nitrogen mineralization and nitrification rates as indicators of microbial activity, and microbial biomass carbon and nitrogen and various gene abundances (i.e. bacterial 16S rRNA, fungal ITS, bacterial amoA, and archaeal amoA genes) as indicators of microbial abundance. Nitrogen deposition in the forest edge was 1.4-fold greater than that in the forest interior, even while the maximum deposition was 3.7 kg N ha−1. Nitrogen deposition was significantly correlated to the net nitrogen mineralization and nitrification rates and the 16S rRNA and bacterial amoA gene abundances. Microbial community structures in litter and soil samples were also analyzed using a high throughput DNA sequencer for the bacterial 16S rRNA and fungal ITS gene amplicons. Microbial community structures were different between litter and soil samples but were similar between the forest edge and interior. Significant correlations of nitrogen deposition to the soil carbon-to-nitrogen ratio, and the nitrate and ammonium contents were also observed. Thus, our results show that moderately elevated nitrogen deposition in nitrogen-limited forest edges likely stimulate microbial activities and abundances in soils.