Soil microbial responses to nitrogen addition in arid ecosystems

Abstract

The N cycle of arid ecosystems is influenced by low soil organic matter, high soil pH, and extremes in water potential and temperature that lead to open canopies and development of biological soil crusts (biocrusts). We investigated the effects of N amendment on soil microbial dynamics in a Larrea tridentata-Ambrosia dumosa shrubland site in southern Nevada USA. Sites were fertilized with a NO3-NH4 mix at 0, 7, and 15 kg N ha-1 y-1 from March 2012 to March 2013. In March 2013, biocrust (0–0.5 cm) and bulk soils (0–10 cm) were collected beneath Ambrosia canopies and in the interspaces between plants. Biomass responses were assessed as bacterial and fungal SSU rRNA gene copy number and chlorophyll a concentration. Metabolic responses were measured by five ecoenzyme activities and rates of N transformation. By most measures, nutrient availability, microbial biomass, and process rates were greater in soils beneath the shrub canopy compared to the interspace between plants, and greater in the surface biocrust horizon compared to the deeper 10 cm soil profile. Most measures responded positively to experimental N addition. Effect sizes were generally greater for bulk soil than biocrust. Results were incorporated into a meta-analysis of arid ecosystem responses to N amendment that included data from 14 other studies. Effect sizes were calculated for biomass and metabolic responses. Regressions of effect sizes, calculated for biomass, and metabolic responses, showed similar trends in relation to N application rate and N load (rate × duration). The critical points separating positive from negative treatment effects were 88 kg ha-1 y-1 and 159 kg ha-1, respectively, for biomass, and 70 kg ha-1 y-1 and 114 kg ha-1, respectively, for metabolism. These critical values are comparable to those for microbial biomass, decomposition rates and respiration reported in broader meta-analyses of N amendment effects in mesic ecosystems. However, large effect sizes at low N addition rates indicate that arid ecosystems are sensitive to modest increments in anthropogenic N deposition.

Climatic conditions, soil fertility and atmospheric nitrogen deposition largely determine the structure and functioning of microbial communities in biocrust-dominated Mediterranean drylands

“…These communities regulate soil biogeochemical cycles, and thus their modification by ecosystem N enrichment should alter nutrient cycling, particularly the N and C cycles (Phoenix et al 2012), and the ability of soils to store these elements in the form of organic matter (DeForest et al 2005;Treseder 2008). In this sense, increased and/or decreased N mineralization rates (Biudes and Vourlitis 2012;Sinsabaugh et al 2015), reduced N fixation (DeLuca et al 2007), altered function of C-degrading enzymes (Zak et al 2011) and augmented concentrations of different forms of dissolved organic and inorganic N in soils (Phoenix et al 2012) are expected consequences of increased N deposition mediated by soil microorganisms.…”

“…For example, biocrusts contribute to C and N fixation in soils (Belnap 2002;Li et al 2012), and thus increase soil fertility, while they can also modulate N transformation rates (e.g. nitrification) in dryland soils (Castillo-Monroy et al 2010;DelgadoBaquerizo et al 2013;Sinsabaugh et al 2015). However, little is known on how N deposition and climate variability may simultaneously affect the abundance and physiology of both heterotrophic and autotrophic communities associated with biocrusts in drylands at the global scale, where any anthropogenic negative impacts on these communities would likely be followed by a reduction in the amount and quality of goods and services supplied by them (Maestre et al 2012;Reed et al 2012).…”

Amino Acid: Its Dual Role as Nutrient and Scavenger of Free Radicals in Soil

“…Increased soil respiration and CO 2 efflux were observed, along with increased microbial biomass in arid ecosystems upon the addition of nitrogen sources [37] (the increase was more prominent in bulk soil (0-10) cm depth). Amino acids in both and L-and D-forms can be considered as organic nitrogen and carbon sources for soil microbes [38,39] that can lead to an increase in respiration in soil [40].…”

Simple measurements in a complex system: soil community responses to nitrogen amendment in a Pinus taeda forest

“…Sinsabaugh et al. () also presented regression models for metabolic responses to N amendment, including hydrolytic enzyme activities, using data from arid ecosystem studies (Sinsabaugh et al. ).…”