Nutrient Status Of Ecosystems Research Articles

Microbial phosphorus recycling in soil by intra- and extracellular mechanisms

Rising global stoichiometric imbalance between increasing nitrogen (N) availability and depleting phosphorus (P) resources increases the importance of soil microbial P recycling. The contribution of extra- versus intracellular P (re-)cycling depending on ecosystem nutrient status is vastly unclear, making soil microorganisms a blind spot in our understanding of ecosystem responses to increasing P deficiency. We quantified P incorporation into microbial DNA and phospholipids by 33P labeling under contrasting conditions: low/high P soil × low/high carbon (C)NP application. By combining 33P and 14C labeling with tracing of microbial community biomarkers and functional genes, we disengaged the role of DNA and phospholipids in soil P cycling. Microorganisms in low P soil preferentially allocated P to phospholipids with an acceleration of phospholipids metabolism driven by C addition, which was strongly related to high abundances of microbial community members (e.g. some G-) with a fast phospholipids turnover. In high P soil, however, more P was allocated to DNA with a microbial functional shift towards DNA synthesis to support a replicative growth when sufficient C was supplied, which was coupled with a strong enrichment of fungal copiotrophs and microbial genes coding DNA primase. Consequently, adaptation to low P availability accelerated microbial intracellular P recycling through reutilization of the P stored in phospholipids. However, microorganisms under high P availability commonly adopted extracellular P recycling with release and reuse of DNA P by microbial death-growth dynamics. These results advance our understanding on microbial adaptation to P deficiency in soil by regulating component-specific P pathways and reflect the specific functions of phospholipids and DNA for P recycling.

Responses of carbon, nitrogen, and phosphorus contents and stoichiometry in soil and fine roots to natural vegetation restoration in a tropical mountainous area, Southern China.

The stoichiometry of key elements such as C, N, and P is an important indicator of ecosystem nutrient status and biogeochemical cycling. Nevertheless, the responses of soil and plant C:N:P stoichiometric characteristics to natural vegetation restoration remain poorly understood. In this study, we investigated C, N, and P contents and stoichiometry in soil and fine roots along vegetation restoration stages (grassland, shrubland, secondary forest, and primary forest) in a tropical mountainous area in southern China. We found that soil organic carbon, total N, C:P ratio, and N:P ratio significantly increased with vegetation restoration and significantly decreased with increasing soil depth, whereas there was no significant effect on soil total P and C:N ratio. Furthermore, vegetation restoration significantly increased the fine root N and P content and N:P ratio, whereas soil depth significantly decreased the fine root N content and increased the C:N ratio. The increasing average N:P ratio in fine roots from 17.59 to 21.45 suggested that P limitation increased with vegetation restoration. There were many significant correlations between C, N, and P contents and their ratios in soil and fine roots, indicating a reciprocal control of nutrient stoichiometric characteristics between them. These results contribute to our understanding of changes in soil and plant nutrient status and biogeochemical cycling during vegetation restoration and provide valuable information for restoration and management of tropical ecosystems.

Frequent Fire Reduces the Magnitude of Positive Interactions Between an Invasive Grass and Soil Microbes in Temperate Forests

Fire activity is increasing in many regions. Although increased fire activity is expected to promote plant invasion, over longer time periods, frequent fire can shift the nutrient status of ecosystems, which may alter interactions between invasive plants and soil microbial decomposers. Here, we applied a fire treatment to plots embedded in deciduous forests under regimes of either fire exclusion or frequent fire and invaded by the most widespread invasive grass in the eastern USA (Microstegium vimineum) to determine how frequent fire affects plant–soil interactions while controlling for time after fire. We predicted that frequent fire would increase microbial nitrogen (N) limitation, leading to lower Microstegium productivity and reduced soil carbon (C) and N loss. We found that Microstegium leaf, root, and microbial biomass C/N ratios were significantly wider under frequent fire than fire exclusion. Consistent with these patterns, dissolved organic N concentration was 22% lower and the activity of an exoenzyme targeting N acquisition was 59% higher under frequent fire, indicating that frequent fire increased microbial N limitation. Higher surface soil C and N and fire-induced increases in particulate soil organic matter N suggest that frequent fire in these grass-invaded forests enhanced microbial N limitation through the accumulation of microbially resistant pyrogenic N. A structural equation model confirmed that these changes were interrelated and associated with 87% lower Microstegium aboveground biomass and 85% higher fine root biomass. These findings provide new insights into how frequent fire impacts plant–soil microbial interactions and thus may feedback to plant invasion over longer time periods.

Nitrogen cycle responses to elevated CO2 depend on ecosystem nutrient status

Nitrogen (N) limitation of terrestrial ecosystems is a crucial factor for predicting how these ecosystems respond and feedback to climate change. Nitrogen availability for plants in terrestrial ecosystems depends on the internal soil N cycle and inputs to the ecosystem via biological N2 fixation. We reviewed the effect of elevated atmospheric CO2 concentrations (eCO2) on gross soil N transformations to advance our understanding of ecosystem responses to eCO2. Overall, neither gross mineralization nor gross nitrification was altered by eCO2. However, emerging from ecosystem specific analysis, we propose a new conceptual model for eCO2 effects on gross mineralization based on ecosystem nutrient status: gross mineralization is only stimulated in N limited ecosystems, but unaffected in phosphorus limited ecosystems. Moreover, the ratio of ammonium oxidation to immobilization is decreased under eCO2, indicating a tighter N cycle with reduced ecosystem N losses. This new conceptual model on N cycle responses to eCO2 should be tested in the future in independent experiments and it provides a new concept for refining mechanistic models of ecosystem responses to climate change.

Slow recovery of High Arctic heath communities from nitrogen enrichment.

Arctic ecosystems are strongly nutrient limited and exhibit dramatic responses to nitrogen (N) enrichment, the reversibility of which is unknown. This study uniquely assesses the potential for tundra heath to recover from N deposition and the influence of phosphorus (P) availability on recovery. We revisited an experiment in Svalbard, established in 1991, in which N was applied at rates representing atmospheric N deposition in Europe (10 and 50kgNha(-1) yr(-1) ; 'low' and 'high', respectively) for 3-8yr. We investigated whether significant effects on vegetation composition and ecosystem nutrient status persisted up to 18yr post-treatment. Although the tundra heath is no longer N saturated, N treatment effects persist and are strongly P-dependent. Vegetation was more resilient to N where no P was added, although shrub cover is still reduced in low-N plots. Where P was also added (5kgPha(-1) yr(-1) ), there are still effects of low N on community composition and nutrient dynamics. High N, with and without P, has many lasting impacts. Importantly, N+P has caused dramatically increased moss abundance, which influences nutrient dynamics. Our key finding is that Arctic ecosystems are slow to recover from even small N inputs, particularly where P is not limiting.

Carbon mineralization is promoted by phosphorus and reduced by nitrogen addition in the organic horizon of northern hardwood forests

Limitations to the respiratory activity of heterotrophic soil microorganisms exert important controls of CO2 efflux from soils. In the northeastern US, ecosystem nutrient status varies across the landscape and changes with forest succession following disturbance, likely impacting soil microbial processes regulating the transformation and emission of carbon (C). We tested whether nitrogen (N) or phosphorus (P) limit the mineralization of soil organic C (SOC) or that of added C sources in the Oe horizon of successional and mature northern hardwood forests in three locations in central New Hampshire, USA. Added N reduced mineralization of C from SOC and from added leaf litter and cellulose. Added P did not affect mineralization from SOC; however, it did enhance mineralization of litter- and cellulose- C in organic horizons from all forest locations. Added N increased microbial biomass N and K2SO4-extractable DON pools, but added P had no effect. Microbial biomass C increased with litter addition but did not respond to either nutrient. The direction of responses to added nutrients was consistent among sites and between forest ages. We conclude that in these organic horizons limitation by N promotes mineralization of C from SOC, whereas limitation by P constrains mineralization of C from new organic inputs. We also suggest that N suppresses respiration in these organic horizons either by relieving the N limitation of microbial biomass synthesis, or by slowing turnover of C through the microbial pool; concurrent measures of microbial growth and turnover are needed to resolve this question.

Implications of seasonal variation in nitrate export from forested ecosystems: a review from the hydrological perspective of ecosystem dynamics

AbstractIn recent decades, the seasonal variation in NO3− discharge from forest ecosystems has been used increasingly by ecologists and hydrologists as a diagnostic indicator of the nutrient status of ecosystems. Major factors underlying the seasonal patterns of stream NO3− concentration include seasonal variations in (1) ecosystem nutrient demand, (2) solute transport capability of the hydrological condition, and (3) in‐stream nutrient usage and supply. In this article I show how case studies have helped elucidate the dominant controlling factors by comparing data from Japanese catchments with previously compiled data from studies in North America and Europe. Moreover, I explain the different influences that hydrological and biogeochemical controls exert in regions with rainy (Japan) and dry (northeastern United States and Europe) summers. The seasonal variation in hydrological conditions is the predominant controlling factor in Japanese forests, whereas nutrient demand may predominate in the northeastern United States and Europe. I emphasize the importance of comparing seasonal patterns among regions with differing climates to obtain more universal explanations of seasonal variations in NO3−‐related biogeochemical and hydrological dynamics in a forest ecosystem. In addition, multi‐scale investigations will be needed to provide insight into the relative contributions of hillslope biogeochemical effects and in‐stream biological activities.

Effects of soil mesofauna and microclimate on nitrogen dynamics in leaf litter decomposition along an elevation gradient

The nutrient dynamics in decomposing litter has proved to play a crucial role in regulating the nutrient status of ecosystems and vegetation productivity. Little is known, however, about the effect mechanism of decomposer and its environment on the nitrogen (N) mineralisation of litter along an elevation gradient in such sites. We investigated the effects of soil mesofauna using litter bags on N dynamics in decomposing Castanopsis carlesii litter along an elevation gradient across four types of ecosystems in southeastern China: evergreen broadleaf forest (EVB), coniferous forest (COF), dwarf forest (DWF) and alpine meadow (ALM). The mean contribution of soil mesofauna to N concentration was 12.6% at EVB, 10.6% at COF, 5.4% at DWF, and 3.1% at ALM. The N concentration of litter with mesofauna was significantly related to H′ (Shannon-Wiener) and GN (group number) across the four sites. The effects of soil mesofauna on N concentration of litter were significantly linked to some special faunal groups, including Oribatida, Mesostigmata and Collembola. The N concentration in litter bags were positively related with mean annual air temperature, soil temperature and litter moisture along the elevation gradient. We concluded that the rapid accumulation of N in lower elevation sites during the first few months can result in the retention of mobile N in soils and the effects of soil mesofauna on N dynamics may be intimately associated with microclimate (warm and humid) and faunal diversity along the elevation gradient. Key words : Nitrogen concentration, mesofauna, faunal diversity, elevation gradient, Wuyi National Nature Reserve, China.

Nutrient limitation during long-term ecosystem development inferred from a mat-forming moss

Extant models of long-term ecosystem nutrient status, largely formulated using data from mid- and low-latitude settings dominated by vascular vegetation, predict nitrogen (N) limitation in the early stages of primary succession and phosphorus (P) limitation in the latter stages. The aims of this study were a) to examine the use of mosses to establish ecosystem nutrient status; b) to test an existing model of nutrient limitation during long-term primary succession and c) to assess the connection between nutrient dynamics and the emergence of spatially structured vegetation in a high-latitude, moss-dominated community. Spatiotemporal changes in nutrient availability were inferred using tissue nutrient concentrations in a moss sampled from a long (850-year) chronosequence of lava flows in Iceland. This is the first use of bryophyte nutrient concentrations, even though they should be a good indicator of nutrient status in high-latitude habitats. Samples of the moss, Racomitrium lanuginosum, from different topographic positions were collected from seven lava flows of different ages (26-848 years) and analyzed to establish concentrations of N, P and major mineral nutrients. Concentrations of N were low on all sites and evidence of progressive N accumulation is lacking; increasing concentrations of phosphorus indicated increasing N limitation with terrain age. This pattern contrasts with other studies of long-term nutrient limitation, where N limitation reduces and P limitation increases with increasing terrain age. Concentrations of mineral nutrients did not vary significantly with terrain age. Nutrient concentrations were only weakly related to spatial patterning in the vegetation. It is likely that element concentrations were strongly influenced by inputs of allochthonous sediment. These findings suggest that mat-forming, pleurocarpous mosses may be used to infer long-term changes in N & P limitation during primary succession and that existing models of nutrient limitation may not be robust over century - millennial timescales in low productivity, high-latitude habitats.

Seasonal Patterns of Nitrate Discharge from Forested Catchments: Information Derived from Japanese Case Studies

AbstractIn recent decades, the seasonal variation of NO3− discharge from forest ecosystems has been increasingly used by ecologists and hydrologists as a diagnostic indicator of the nutrient status of ecosystems. Major factors controlling the seasonal patterns of stream NO3− concentrations include seasonal variations in (i) nutrient demands of the ecosystem, (ii) solute transport capability of the hydrological condition, and (iii) in‐stream nutrient usage and supply. In this article, we show how case studies have helped elucidate the dominant controlling factors by comparing data from Japanese catchments with previously compiled data from studies in North America and Europe, and explain the different influences hydrological and biogeochemical controls exert in rainy summer regions ( Japanese catchments) and dry summer regions (sites in the northeast United States and Europe). The seasonal variation of hydrological conditions is a predominant controlling factor in Japanese forests, whereas nutrient demand may predominate in the northeast United States and Europe. We emphasize that it is important to compare seasonal patterns among different climate regions to obtain more universal explanations of the seasonal variation in stream NO3− concentration. In addition, multi‐scale investigations will be needed to provide insight into the relative contribution of hillslope biogeochemical effects and the influences of in‐stream biological activities.

Deposition of atmospheric constituents and its impact on nutrient budgets of oak forests ( Quercus petraea and Quercus robur) in Lower Austria

Geochemical processes in central European oak ecosystems ( Quercus petraea and Quercus robur) suffering stand decline were studied in two oak stands of the Weinviertel, Lower Austria, about 30 km north of Vienna. Stores of chemical elements were determined by soil and biomass inventories. Deposition input was monitored over a 2 year period by bulk sampling of throughfall. Soil solution chemistry was studied by tension lysimetry over a 1 year period. Mineral nutrition of oak was judged by foliar analysis. Bulk deposition rates were 10–12 kg N ha −1 year −1, and 15–20 kg S ha −1 year −1. Total annual nitrogen gain is high. Both systems lose calcium and magnesium. Foliar nutrient levels indicate sufficient nutrition with main mineral nutrients, except for magnesium, which is in moderately low supply. Based on these findings, the hypothesis that pollutant deposition has been the cause of a sudden and severe appearance of decline symptoms in the second half of the 1980s must be dismissed. The data on deposition rates and ecosystem nutrient status, however, indicate that the soil of both systems is acidifying, nitrogen stores are increasing, and magnesium pools are depleted. If deposition of pollutants continues at current rates, a slow but steady degradation of many oak ecosystems in the Austrian Weinviertel is inevitable.