Nitrogen Mineralization: Challenges of a Changing Paradigm

Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, Santa Barbara, California, United States
Ecology (Impact Factor: 4.66). 03/2004; 85(3):591–602. DOI: 10.1890/03-8002


Until recently, the common view of the terrestrial nitrogen cycle had been driven by two core assumptions—plants use only inorganic N and they compete poorly against soil microbes for N. Thus, plants were thought to use N that microbes ‘‘left over,’’ allowing the N cycle to be divided cleanly into two pieces—the microbial decomposition side and the plant uptake and use side. These were linked by the process of net mineralization. Over the last decade, research has changed these views. N cycling is now seen as being driven by the depolymerization of N-containing polymers by microbial (including mycorrhizal) extracellular enzymes. This releases organic N-containing monomers that may be used by either plants or microbes. However, a complete new conceptual model of the soil N cycle needs to incorporate recent research on plant–microbe competition and microsite processes to explain the dynamics of N across the wide range of N availability found in terrestrial ecosystems. We discuss the evolution of thinking about the soil N cycle, propose a new integrated conceptual model that explains how N cycling changes as ecosystem N availability changes, and discuss methodological issues raised by the changing paradigm of terrestrial N cycling.

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Available from: Joshua P Schimel, Oct 12, 2015
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    • "Nitrate leaching represents a resource loss and can threaten drinking water quality. Nitrate concentrations in soil solution and nitrate leaching depend on the relation between uptake by plants and soil organisms, atmospheric N 2 fixation, N mineralization (ammonification and nitrification), N deposition from the atmosphere, denitrification, and volatilization (Corre et al., 2002; Schimel and Bennett, 2004). Leaching of nitrate from soil is mainly driven by land-use type, management (e.g., fertilization), land-use change, climate, and soil properties (Dijkstra et al., 2007; Lilburne and Webb, 2002; Perego et al., 2012; Schilling and Spooner, 2006; Strebel et al., 1989). "
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    ABSTRACT: In biodiversity-ecosystem functioning experiments, plant diversity increases biomass production mainly because of complementary resource use. We determined the influence of seasonality and time since conversion from fertilized arable land to unfertilized grassland on the plant diversity-nitrate leaching relationship. NO3-N concentrations in soil solution, water contents in the main rooting zone, and climate data were measured between 2003 and 2006 in a grassland plant diversity experiment in Jena, Germany which consists of 82 plots with 1–60 plant species and 1–4 plant functional groups (legumes, grasses, non-leguminous tall herbs, and non-leguminous small herbs). To cope with data gaps and uneven sampling intervals, water contents were simulated with Bayesian statistical models, based on the measured data. Downward water fluxes were modeled with a deterministic water balance model. Monthly NO3-N fluxes were calculated as NO3-N concentration times downward water flux and statistically analyzed. The statistical results were confirmed with the help of a completely simulated NO3-N leaching data set without any data gaps. Plant species richness quantitatively decreased NO3-N leaching in winter, when leaching was highest, more than in summer. The presence of legumes increased and the presence of grasses decreased NO3-N leaching. The presence of small herbs decreased NO3-N leaching and this effect strengthened with time. We conclude that especially shortly after land-use change from fertilized arable land to unfertilized grassland, NO3-N leaching can be reduced if species-rich mixtures without legumes are established.
    Agriculture Ecosystems & Environment 12/2015; 211. DOI:10.1016/j.agee.2015.06.002 · 3.40 Impact Factor
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    • "Subsequently , it became more widely known that the plants also take up organic N (N€ asholm et al., 1998, 2009) and that various forms of organic N may dominate N uptake in ecosystems with a low soil N supply (Schimel and Bennett, 2004). The occurrence of inorganic N could be viewed as a supply of organic N in excess of the current biological demands of ecosystems (Schimel and Bennett, 2004), which is also dependent on the supply of carbon to the organisms (Hart et al., 1994a). Like peptides and amino acids, inorganic forms of N are rapidly taken up when the supply is low, and may, therefore not be detected when their pool sizes are measured. "
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    ABSTRACT: Abstract Plant growth in boreal forests is commonly limited by a low supply of nitrogen, a condition that may be aggravated by high tree below-ground allocation of carbon to ectomycorrhizal (ECM) fungi and associated microorganisms. These in turn immobilise N and reduce its availability to plants as boreal ecosystems develop. Here, we studied a boreal forest ecosystem chronosequence created by new land rising out of the sea due to iso-static rebound along the coast of northern Sweden. We used height over the ocean to estimate ecosystem age and examined its relationship to soil microbial community structure and the gross turnover of N. The youngest soils develop with meadows by the coast, followed by a zone of N2-fixing alder trees, and primary boreal conifer forest on ground up to 560 years old. The young soils in meadows contained little organic matter and microbial biomass per unit area. Nitrogen was turned over at low rates when expressed per area (m−2), but specific rates (per gram soil carbon (C)) were the highest found along the transect. In the zone with alder, the amounts of soil C and microbial biomass were much higher (bacterial biomass had doubled and fungal biomass quadrupled). Rates of gross N mineralisation (expressed on an area basis) were highest, but the retention of added labelled {NH} 4 + was lowest in this soil as compared to other ages. The alder zone also had the largest extractable pools of inorganic N in soil and highest N % in plant foliage. In the older conifer forest ecosystems the amounts of soil C and N, as well as biomass of both bacteria and fungi increased. Data on organic matter 14C suggested that the largest input of recently fixed plant C occurred in the younger coniferous forest ecosystems. With increasing ecosystem age, the ratio of microbial C to total soil C was constant, whereas the ratio of microbial N to total soil N increased and gross N mineralization declined. Simultaneously, plant foliar N % decreased and the natural abundance of 15N in the soil increased. More specifically, the difference in δ15N between plant foliage and soil increased, which is related to relatively greater retention of 15N relative to 14N by {ECM} fungi as N is taken up from the soil and some N is transferred to the plant host. In the conifer forest, where these changes were greatest, we found increased fungal biomass in the F- and H-horizons of the mor-layer, in which {ECM} fungi are known to dominate (the uppermost horizon with litter and moss is dominated by saprotrophic fungi). Hence, we propose that the decreasing availability of N to the plants and the subsequent decline in plant production in ageing boreal forests is linked to high tree belowground C allocation to {ECM} fungi, a strong microbial sink for available soil N.
    Soil Biology and Biochemistry 12/2015; 91:200-211. DOI:10.1016/j.soilbio.2015.08.041 · 3.93 Impact Factor
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    • "Total soil N availability alone was not sufficient to explain variation in the dominant chemical form of soil N, in contrast with conceptual models (Schimel & Bennett, 2004). Our results partially support the MANE model (Phillips et al., 2013): soil N cycling was mediated by shifts in fungal community composition , which in turn were associated with changes in plant communities across the 18 plots. "
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    ABSTRACT: Rates of ecosystem nitrogen (N) cycling may be mediated by the presence of ectomycorrhizal fungi, which compete directly with free-living microbes for N. In the regenerating tropical dry forests of Central America, the distribution of ectomycorrhizal trees is affected by succession and soil parent material, both of which may exert independent influence over soil N fluxes. In order to quantify these interacting controls, we used a scale-explicit sampling strategy to examine soil N cycling at scales ranging from the microsite to ecosystem level. We measured fungal community composition, total and inorganic N pools, gross proteolytic rate, net N mineralization and microbial extracellular enzyme activity at multiple locations within 18 permanent plots that span dramatic gradients of soil N concentration, stand age and forest composition. The ratio of inorganic to organic N cycling was correlated with variation in fungal community structure, consistent with a strong influence of ectomycorrhiza on ecosystem-scale N cycling. However, on average, > 61% of the variation in soil biogeochemistry occurred within plots, and the effects of forest composition were mediated by this local-scale heterogeneity in total soil N concentrations. These cross-scale interactions demonstrate the importance of a spatially explicit approach towards an understanding of controls on element cycling.
    New Phytologist 09/2015; DOI:10.1111/nph.13654 · 7.67 Impact Factor
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