Microbial responses to nitrogen addition in three contrasting grassland ecosystems

Abstract

The effects of global N enrichment on soil processes in grassland ecosystems have received relatively little study. We assessed microbial community response to experimental increases in N availability by measuring extracellular enzyme activity (EEA) in soils from three grasslands with contrasting edaphic and climatic characteristics: a semiarid grassland at the Sevilleta National Wildlife Refuge, New Mexico, USA (SEV), and mesic grasslands at Konza Prairie, Kansas, USA (KNZ) and Ukulinga Research Farm, KwaZulu-Natal, South Africa (SAF). We hypothesized that, with N enrichment, soil microbial communities would increase C and P acquisition activity, decrease N acquisition activity, and reduce oxidative enzyme production (leading to recalcitrant soil organic matter [SOM] accumulation), and that the magnitude of response would decrease with soil age (due to higher stabilization of enzyme pools and P limitation of response). Cellulolytic activities followed the pattern predicted, increasing 35-52% in the youngest soil (SEV), 10-14% in the intermediate soil (KNZ) and remaining constant in the oldest soil (SAF). The magnitude of phosphatase response did not vary among sites. N acquisition activity response was driven by the enzyme closest to its pH optimum in each soil: i.e., leucine aminopeptidase in alkaline soil, beta-N-acetylglucosaminidase in acidic soil. Oxidative enzyme activity varied widely across ecosystems, but did not decrease with N amendment at any site. Likewise, SOM and %C pools did not respond to N enrichment. Between-site variation in both soil properties and EEA exceeded any treatment response, and a large portion of EEA variability (leucine aminopeptidase and oxidative enzymes), 68% as shown by principal components analysis, was strongly related to soil pH (r = 0.91, P < 0.001). In these grassland ecosystems, soil microbial responses appear constrained by a molecular-scale (pH) edaphic factor, making potential breakdown rates of SOM resistant to N enrichment.

Chronic nitrogen fertilization and carbon sequestration in grassland soils: evidence of a microbial enzyme link

“…Positive N addition effects on cellulose-degrading enzymes were also found in forest (Carreiro et al 2000;Saiya-Cork et al 2002;Allison et al 2008;Keeler et al 2009;Cusak et al 2011) and grassland studies (Ajwa et al 1999;Stursova et al 2006;Keeler et al 2009). Other studies, however, found that N addition can have variable effects on BG activity along a soil age gradient (Zeglin et al 2007) or that N fertilization does not have any significant effect on BG activity (Mineau et al 2014). Further research could address whether and how these variable effects of N fertilization on BG activity may depend on the spatial scale considered (i.e.…”

“…Recent studies show that N inputs can have positive effects on total soil C stocks (Fornara et al 2013) and on the C content of both organic (Song et al 2014) and mineral soils (Liu and Greaver 2010;Fornara and Tilman 2012). Other studies show that N fertilization has no effect on the C content of bulk soils (Zeglin et al 2007) or on the C content of organic and mineral soils (Lu et al 2011). Such high soil C variability in response to N fertilization may depend on several environmental factors including differences in climate, soil and litter biochemical composition, management history, N addition rates and the time interval (years) to which soils have been exposed to chronic N additions; it may also depend on soil mineralogy and the reactivity of soil minerals (Sulman et al 2014).…”

Extracellular Enzyme Activity and Microbial Diversity Measured on Seafloor Exposed Basalts from Loihi Seamount Indicate the Importance of Basalts to Global Biogeochemical Cycling

“…The nutrient chemistry of background water from the sites sampled here (Table 1) indicates that inorganic N and P are likely not in limited supply, and therefore, the enzyme activities at Loihi might be functioning to liberate organic C for microbial consumption. LAP hydrolyzes leucine from the N terminus of polypeptides and has been shown experimentally to be responsive to the addition of amino acids (55). However, there is no simple relationship between bulk measures of N availability and microbial N-acquiring LAP activity (6).…”

Responses of soil enzyme activity and microbial community compositions to nitrogen addition in bulk and microaggregate soil in the temperate steppe of Inner Mongolia

“…As soil P and moisture would influence the shifting of soil C in plant litter decomposition through increase N-P ratios in organisms or soil moisture by limiting the diffusion of substrates and enzymes, what's more, they will be also altered when N added [26,76]. N enrichment can also have negative effect on soil microbial growth by changing carbon, water or phosphorus [31,53].…”