Changes in microbial community characteristics and soil organic matter with nitrogen additions in two tropical forests

Abstract

Microbial communities and their associated enzyme activities affect the amount and chemical quality of carbon (C) in soils. Increasing nitrogen (N) deposition, particularly in N-rich tropical forests, is likely to change the composition and behavior of microbial communities and feed back on ecosystem structure and function. This study presents a novel assessment of mechanistic links between microbial responses to N deposition and shifts in soil organic matter (SOM) quality and quantity. We used phospholipid fatty acid (PLFA) analysis and microbial enzyme assays in soils to assess microbial community responses to long-term N additions in two distinct tropical rain forests. We used soil density fractionation and 13C nuclear magnetic resonance (NMR) spectroscopy to measure related changes in SOM pool sizes and chemical quality. Microbial biomass increased in response to N fertilization in both tropical forests and corresponded to declines in pools of low-density SOM. The chemical quality of this soil C pool reflected ecosystem-specific changes in microbial community composition. In the lower-elevation forest, there was an increase in gram-negative bacteria PLFA biomass, and there were significant losses of labile C chemical groups (O-alkyls). In contrast, the upper-elevation tropical forest had an increase in fungal PLFAs with N additions and declines in C groups associated with increased soil C storage (alkyls). The dynamics of microbial enzymatic activities with N addition provided a functional link between changes in microbial community structure and SOM chemistry. Ecosystem-specific changes in microbial community composition are likely to have far-reaching effects on soil carbon storage and cycling. This study indicates that microbial communities in N-rich tropical forests can be sensitive to added N, but we can expect significant variability in how ecosystem structure and function respond to N deposition among tropical forest types.

Microbial community structure mediates response of soil C decomposition to litter addition and warming

“…2), inconsistent with a mechanism of priming driven by microbial 'mining' of the soil for N after the addition of C (Fontaine et al, 2003;Moorhead and Sinsabaugh, 2006;Craine et al, 2007). However, the litter additions may have impacted the increase in priming in response to soil temperature, as the higher N content of the soil-litter mixtures may have allowed for the production of a larger number of hydrolytic enzymes Cusack et al, 2011), or a suite of more efficient enzymes (Stone et al, 2012), that were capable of degrading more soil-C as the temperature increased. Whether N availability can interact with temperature to influence the extent of soil-C priming warrants further investigation.…”

Nutrient limitations to bacterial and fungal growth during cellulose decomposition in tropical forest soils

“…In this study, we tested whether microbial C metabolism during cellulose degradation was predominantly constrained by P and N availability in lowland and montane tropical forests, respectively (hypothesis 1) (Cusack et al 2011a;Turner and Wright 2014), and whether these patterns can partly be explained by different nutrient constraints to bacterial and fungal growth (hypothesis 2). However, contrary to hypothesis 1, similar biomass increases in fungal growth (Figs.…”

Are nitrate production and retention processes in subtropical acidic forest soils responsive to ammonium deposition?

“…þ additions on mineralization-immobilization turnover Previous studies have indicated that SOM fractions (Swanston et al, 2004;Janssens et al, 2010;Cusack et al, 2011) and their associated mineralization-immobilization turnover (Corre et al, 2010;Zhang et al, 2012a;Koranda et al, 2014) respond differently to N deposition over time. Thus, in this study, we quantified two specific gross N mineralization rates and two specific gross NH 4 þ immobilization rates, related to either a rapid (N lab ) or a slower (N rec ) turnover of organic N pool (Fig.…”

“…Huygens et al (2008) pointed out that DNRA also depends directly on heterotrophic nitrification for substrate generation. Subtropical/tropical forest ecosystems are projected to receive enhanced N deposition (Galloway et al, 2008;Liu et al, 2013), but changes in the processes, rates and controls of soil N-cycling in these ecosystems under anthropogenic N inputs are less well understood (Silver et al, 2005;Corre et al, 2010;Cusack et al, 2011;Baldos et al, 2015;Gao et al, 2015). For example, Corre et al (2010) reported that during 9 years of N addition to an oldgrowth lowland forest with net primary production not limited by N, microbial biomass decreased with increased soil acidity, but gross N mineralization rates increased.…”