Microbial Nitrogen Immobilization Research Articles

Soil recalcitrant but not labile organic nitrogen mineralization contributes to microbial nitrogen immobilization and plant nitrogen uptake.

Soil organic nitrogen (N) mineralization not only supports ecosystem productivity but also weakens carbon and N accumulation in soils. Recalcitrant (mainly mineral-associated organic matter) and labile (mainly particulate organic matter) organic materials differ dramatically in nature. Yet, the patterns and drivers of recalcitrant (MNrec) and labile (MNlab) organic N mineralization rates and their consequences on ecosystem N retention are still unclear. By collecting MNrec (299 observations) and MNlab (299 observations) from 57 15N tracing studies, we found that soil pH and total N were the master factors controlling MNrec and MNlab, respectively. This was consistent with the significantly higher rates of MNrec in alkaline soils and of MNlab in natural ecosystems. Interestingly, our analysis revealed that MNrec directly stimulated microbial N immobilization and plant N uptake, while MNlab stimulated the soil gross autotrophic nitrification which discouraged ammonium immobilization and accelerated nitrate production. We also noted that MNrec was more efficient at lower precipitation and higher temperatures due to increased soil pH. In contrast, MNlab was more efficient at higher precipitation and lower temperatures due to increased soil total N. Overall, we suggest that increasing MNrec may lead to a conservative N cycle, improving the ecosystem services and functions, while increasing MNlab may stimulate the potential risk of soil N loss.

Soil microbial necromass regulation of long-term fertilizer N retention influenced by maize stover mulching

Microbial immobilization of nitrogen (N) fertilizer is critical for N retention in soils and is mainly influenced by agricultural management, including crop residue return. However, the underlying relationship between microbially mediated processes and long-term fertilizer N dynamics has not been characterized well in arable soil. Here, by tracing in situ the temporal changes of soil amino sugars initially synthesized from the 15N fertilizer applied in the 1st year, we investigated the dynamic contribution of microbial necromass to long-term fertilizer N retention in soil influenced by maize stove mulching. After the 1st year of fertilizer application, microbial necromass contributed to 36.3–43.9 % of the remaining fertilizer N in the topsoil (0–10 cm), but the remaining fertilizer N in the subsoil (10–20 cm) was predominantly attributed to microbial necromass accumulation, regardless of maize stover mulching. During the following 9 years, the fertilizer-derived microbial necromass N decreased at a slower rate (42.3–52.8 %) than the remaining fertilizer N (73.5–76.7 %) in the topsoil layer, while the former decreased synchronously with the latter in the subsoil layer, indicating that microbial necromass played a critical role in long-term fertilizer N stabilization though it could serve as a transitional N pool for N supply. For the microbial necromass of different origin, the residence time of fertilizer N in fungal necromass (34.7–48.7 y) was longer than that in the bacterial necromass pool (21.4–27.1 y) in the cultivated soil layer (0–20 cm), explicitly indicating that the retention of fungal necromass was inclined to contribute to long-term fertilizer N stabilization while bacterial necromass was associated with fertilizer N turnover. Compared with fertilizer application alone, maize stover mulching enhanced the initial fertilizer N retention by increasing microbial necromass accumulation but had no influence on the subsequent dynamics of both the remaining fertilizer N and fertilizer-derived microbial necromass N, thus improving long-term fertilizer N retention. Specifically, maize stover mulching increased the stability of microbial necromass N by enhancing the retention and prolonging the residence time of fungal necromass over bacterial necromass, thus improving the contribution of microbial necromass to the long-term stabilization of fertilizer N in the cultivated soil.

Enriching organic carbon bioavailability can mitigate soil acidification induced by nitrogen fertilization in croplands through microbial nitrogen immobilization

AbstractExcessive application of ammonium‐based N fertilizers promote nitrification, which is mainly responsible for intensive acidification in cropland soils. Thus, there is an urgent need for the development of control measures to manage soil acidification induced by N fertilization. In this study, we investigated the potential of enriching organic C bioavailability to mitigate acidification induced by N fertilization through microbial N immobilization using an indoor incubation experiment. Results showed that the sole application of ammonium sulfate [(NH4)2SO4] or urea to a cropland soil promoted nitrification of fertilizer N, and subsequently soil pH decreased by 0.48–1.00 units after 45‐d incubation. The combined application of glucose with N fertilizers induced microbial immobilization of almost all fertilizer N within 1 d and gradually induced mineralization of soil organic N after 1 wk of incubation. More than 65% of immobilized total extractable N was returned to soil solution and mainly existed as NH4+–N and extractable organic N, with the pH decrement being <0.10 units after 45‐d incubation. These results indicated that (a) enriching organic C bioavailability in cropland soils intensified microbial N immobilization, (b) intensification of microbial N immobilization induced a buffering effect to regulate soil inorganic N supply, and (c) microbial N immobilization had great potential to inhibit soil acidification and retard aluminum mobilization. The results obtained in this study provide a theoretical strategy to control soil acidification and increase N use efficiency in croplands.

Stover mulching and inhibitor application maintain crop yield and decrease fertilizer N input and losses in no-till cropping systems in Northeast China

Microbial immobilization of nitrogen and mineral fixation of ammonium play important roles in the retention of fertilizer N in soil, and the subsequent microbial mineralization and abiotic release regulate N availability to crops. However, how fertilization management practices temporally influence the biological and abiotic processes of fertilizer N transformation remain unclear. Therefore, we conducted a field micro-plot experiment with 15N-labeled-fertilizer application in a long-term no-till maize cropping system in Northeast China to evaluate the effects of maize stover mulching alone and in combination with inhibitors on the conversion, crop use efficiency, and loss risk of fertilizer N. The following four treatments were established in our trial: (1) conventional NPK fertilization (NPK); (2) NPK plus maize stover mulching (NPKS); (3) 20 % reduced NPK plus maize stover mulching (RNPKS); and (4) RNPKS plus urease and nitrification inhibitors (RNPKSI). The results showed that the retention of fertilizer N was dependent on both soil depth and management practices at the seedling stage of maize. At the soil depth of 0−20 cm, maize stover mulching significantly enhanced the transformation of fertilizer N into fixed NH4+ and organic N pools by 49.9 % and 65.6 % compared with NPK. The reduction of NPK with maize stover mulching further promoted fertilizer N conversion into the organic N pool by 43.2 %, with a reduction in the fixed NH4+ pool by 32.0 %, while the combined with inhibitors application in RNPKSI treatment resulted in the opposite trend due to the extended residence of fertilizer-derived NH4+-N. Meanwhile, at a depth of 20−40 cm, maize stover mulching in NPKS and RNPKS treatments significantly improved the transformation of fertilizer N to the fixed NH4+ pool compared with NPK, and maize stover mulching and inhibitors application in RKPNSI treatment further increased this conversion compares with RNPKS. The subsequent mineralization and release of fertilizer-derived organic N and fixed NH4+ affected crop uptake of fertilizer N. The fertilizer N use efficiency (NUE) in the three maize stover mulching treatments was significantly improved by an average of 16.1 %, while its gaseous losses were remarkably reduced by an average of 32.8 % compared with NPK. These findings suggest that maize stover mulching and inhibitors application in RNPKS and RNPKSI treatments are efficient management strategies in no-till maize cropping system in Northeast China, reducing N input, maintaining crop yield, enhancing NUE, and potentially minimizing environmental pollution risk by regulating fertilizer N conversion between fixed NH4+ and organic N pools.

Nutrient stoichiometry of a plant-microbe-soil system in response to cover crop species and soil type

AimsThe theory of ecological stoichiometry mostly builds on studies of natural terrestrial ecosystems, whereas only limited stoichiometry information is available in response to agronomic practices.MethodsWe designed a greenhouse experiment in order to disentangle the specific role of cover crop identity and soil characteristic in affecting nutrient stoichiometry of a plant-microbe-soil system.ResultsNutrient ratios of cover crop biomass were species-specific and the growth rate explained, for most species considered, the stoichiometric differences in response to soil type. In contrast, the nutrient stoichiometry of soil microbes was more homeostatic and did not respond to either cover crop identity or soil type. Compared to bare soil, the presence of cover crop enhanced microbial phosphorus immobilization in the clay-rich soil, whereas it promoted microbial carbon biomass and microbial nitrogen immobilization in the sandy-rich soil. A greater microbial cumulative respiration in clay soils, where a higher microbial biomass C at the beginning of the incubation was observed, suggested a major role of soil type, compared to cover crop identity, in affecting microbial metabolism.ConclusionsBy understanding the stoichiometric constraints in the plant-microbe-soil system, our findings can help to implement agro-ecological practices by selecting appropriate cover crop species in relation to soil type in order, for example, to avoid nutrient limitation due to microbial nutrient immobilization.

Organic-C quality as a key driver of microbial nitrogen immobilization in soil: A meta-analysis

Microbial nitrogen (N) immobilization in soil can be enhanced by increasing carbon (C) bioavailability, yet the response of microbial N immobilization to the addition of organic matter is uncertain. In the present study, we investigated the effect of organic matter quality on microbial N immobilization. The response ratio (ln R), i.e., the ratio of microbial N immobilization in organic-C amended soil to that in control treatments, was calculated using data from 51 published studies. Overall, the addition of organic-C increased significantly microbial N immobilization by 105% relative to unamended soil. The type of organic-C affected the response of microbial N immobilization to organic-C addition. Glucose (classified as a labile compound) was more effective in stimulating microbial N immobilization than cellobiose and cellulose (classified as intermediately decomposable compounds). Tannin and oxalic acid (classified as recalcitrant compounds) did not significantly affect microbial N immobilization. The C/N ratio of organic materials did not affect the response of microbial N immobilization in the short-term (≤280 d); differences in the effect size (ln R+) were not significant among different C/N ratio groups. The effect of the form of inorganic-N (ammonium [NH4+] or nitrate [NO3−]) on the response of microbial N immobilization to organic-C addition was not significant, with a mean ln R+ of 0.838 (confidence interval [CI]: 0.590–1.099) for NH4+ and 1.642 (CI: 0.951–2.452) for NO3−. Moreover, the ln R of microbial NH4+ immobilization showed a significant positive relationship (P < 0.01) with that of microbial NO3− immobilization. The ln R+ for the experimental period decreased in the order of <30 d, 60–120 d, ≥120 d, and 30–60 d. The chemical quality of organic-C defined by three C pools (i.e., labile, intermediate, and recalcitrant) rather than the C/N ratio is a critical factor regulating the response of microbial N immobilization to organic-C addition.

Altered leaf litter quality exacerbates the negative impact of climate change on decomposition

Abstract Leaf litter decomposition is a key component of global biogeochemical cycles that influence soil carbon storage, nutrient availability and plant productivity. Ongoing climate change will lead to warmer and drier conditions in many dryland regions, potentially affecting litter decomposition and nutrient dynamics. Climate change effects can be direct and/or indirect, for example, through changes in litter quality, yet their relative importance on litter decomposition remains unclear. We conducted a manipulative study in a semi‐arid shrubland to assess the effects of leaf litter quality, forecasted climate change, that is, +2.5°C warming (W), 30% rainfall reduction (RR) as well as their interaction (W + RR) to elucidate their relative effects on litter decomposition. Climatic effects alone reduced decomposition of a homogeneous Control leaf litter collected from Helianthemum squamatum shrubs growing in unmanipulated plots by 23.4%, 18.1% and 29.8% in the W, RR and W + RR treatments respectively. Leaf litter quality was lower in shrubs that had been growing in warmed plots (W and W + RR), as they had lower nutrient concentrations (P, Fe) and higher C:N and C:P ratios than leaf litter produced under ambient (Control) conditions. Lignin concentration was significantly lower in litter from W + RR plots, yet when both climate and litter quality were considered simultaneously, decomposition rates were 32.0%, 26.3% and 39.9% lower in W, RR and W + RR plots compared to Controls. In addition, we found greater microbial N immobilization in leaf litter incubated within warmed (W and W + RR) than within non‐warmed plots (Control and RR). Structural equation modelling showed that higher litter moisture and microbial biomass contents stimulated decomposition. Simulated climate change (W, RR and W + RR) reduced decomposition indirectly by negatively affecting litter moisture contents and litter microbial biomass. Microbial nitrogen immobilization was stimulated by the lower quality (i.e. high C:N ratios) of the leaf litter collected in shrubs from warmed plots (W and W + RR). Synthesis. Our findings indicate that forecasted climate change conditions slow down C and N cycling in a dryland ecosystem, an effect that is further exacerbated by climate change‐induced reductions in litter quality and related reductions in bacterial and fungal biomass in litter.

Two contrasting seasonal patterns in microbial nitrogen immobilization from temperate ecosystems

Ecosystems with seasonal snow cover and soil frost comprise more than half of the terrestrial ecosystems. Recent studies have reported peak microbial biomass carbon (MBC) and nitrogen (MBN) in late winter in alpine and arctic regions, followed by a rapid decline when soil temperatures rise to 0 °C. In these snow-covered ecosystems, the N incorporated into microbes from soil during winter is considered vital for plant nutrient uptake in the following growing seasons. However, evidence from temperate ecosystems has rarely been reported. We studied 24 ecosystems from two forest-steppe ecotones (Saihanba and Saihanwula) with similar climatic conditions, but different soil properties, to comprehensively analyze the seasonal dynamics of microbial biomass and its controlling factors. MBC consistently reached its annual peak in winter, and this seasonal trend was not influenced by different vegetation types or study areas. In contrast, the seasonal dynamics of MBN were strongly influenced by the study area. There were minimum values of winter MBN observed in the Saihanba area while maximum values of winter MBC observed in the Saihanwula area. Soil properties showed poor nutrient status in the Saihanba area compared with the Saihanwula area. Our results suggest winter microbes in the Saihanba area invest N to produce extracellular enzymes, rather than retaining in biomass. We argue that it is a possible strategy to cope with nutrient stress caused by plant competition in the following spring. However, the peak of MBN occurred in the winter in the Saihanwula area, suggesting a potential resource for spring plant N uptake in this area. We also found the magnitude of MBN was positively related to the soil water content under different vegetation types in the Saihanwula area. Our study demonstrates two contrasting seasonal patterns of soil MBN, suggesting that the general conceptual model of N cycle based on the studies from alpine and arctic areas may not be applicable to temperate ecosystems.

Linking microbial immobilization of fertilizer nitrogen to in situ turnover of soil microbial residues in an agro-ecosystem

Understanding long-term microbial immobilization of nitrogen (N) fertilizer is essential for N management in agricultural soils. Evaluating the transformation and accumulation of N fertilizer into microbial residues is critical for developing such an understanding due to the requirement of time-integrated biomarkers and a 15N-labeling technique. By tracing the dynamics of amino sugars derived from annually applied fertilizer over 8 years, we investigated the influence of continuous maize residue mulching on the temporal immobilization of fertilizer N in an agricultural soil and quantified the turnover of microbial residues in situ. We found that the amino sugar transformation rate from fertilizer N was constant over time in both fertilization-only and maize residue mulching managements, but it was significantly higher in the upper cultivation layer (0–10cm) after maize residue mulching. Mulching with maize residue facilitated initial fertilizer N transformation, while the subsequent 7-year application maintained the increased transformation rate. Consequently, the accumulation of fertilizer-derived amino sugars increased linearly in both managements within the 8 years of our field experiment. The mean residence time (MRT) of soil amino sugar-N was estimated by using extrapolation and first-order kinetics approaches, respectively. The calculated MRT of amino sugar-N using first-order kinetics (78 and 154 years at 0–10 and 10–20cm, respectively) was slightly shorter than that estimated by the extrapolation (89 and 165 years at 0–10 and 10–20cm, respectively) in the fertilization-only management. Mulching with maize residue did not change the MRT of amino sugar-N because maize residue addition enhanced the immobilization of maize residue-derived N or the transformation of indigenous soil N in addition to those of fertilizer N, leading to the same proportion of new N assimilated in microbial residues.

Carbon and Nitrogen Mineralization in Soil Combining Sewage Sludge and Straw

ABSTRACT The combined incorporation of sewage sludge (SS) and oat straw (OS) to the soil can increase straw carbon mineralization and microbial nitrogen immobilization. This hypothesis was tested in two laboratory experiments, in which SS was incorporated in the soil with and without OS. One treatment in which only straw was incorporated and a control with only soil were also evaluated. The release of CO2 and mineral N in the soil after organic material incorporation was evaluated for 110 days. The cumulative C mineralization reached 30.1 % for SS and 54.7 % for OS. When these organic materials were incorporated together in the soil, straw C mineralization was not altered. About 60 % of organic N in the SS was mineralized after 110 days. This N mineralization index was twice as high as that defined by Resolution 375/2006 of the National Environmental Council. The combined incorporation of SS and OS in the soil caused an immobilization of microbial N of 5.9 kg Mg-1 of OS (mean 3.5 kg Mg-1). The results of this study indicated that SS did not increase straw C mineralization, but the SS rate should be adjusted to compensate for the microbial N immobilization caused by straw.

雪被斑块对川西亚高山两个森林群落冬季土壤氮转化的影响

为了解气候变暖情景下雪况变化对高寒森林冬季土壤氮转化的影响，测定了川西亚高山冷杉（<em>Abies faxoniana</em>）+红桦（<em>Betula albo-sinensis</em>）混交林（MF）和冷杉次生林（SF）三类雪被斑块（浅雪被、中厚度雪被和厚雪被）内冬季土壤氮矿化特征。结果表明:经过一个冬季（2011-2012），两个森林群落土壤净氮氨化量都为负值，净氮硝化量都为正值，且净氮硝化量显著高于净氮氨化量；冬季土壤氮氨化、硝化、矿化和固持量都是中度雪被厚度最高，但各雪被斑块之间都未达到显著水平。各雪被斑块下，冷杉次生林土壤氮矿化参数都显著高于针阔混交林，但雪被斑块和林型交互作用对冬季土壤氮矿化无显著影响。这表明，该区冬季土壤氮矿化以硝化过程为主，硝化和氨化过程可能受不同微生物群落调控；短时期内，未来气候变化所导致的雪被减少对该区森林冬季土壤氮转化影响可能不明显。;A consensus in global ecology is that climate change is profoundly altering the process of terrestrial ecosystem. In particular, more and more attentions have been focused on the effects of snow pack on soil process in cold biomes recently, since climate change characterized by winter warming and extreme is changing the pattern of seasonal snow cover and freeze-thaw cycles in the high-latitude and high-altitute regions. As yet, the results concerning the effects of seasonal cover and freeze-thaw cycles on soil process remain uncertainty, and little information has been available on the effects of snow pack on soil nitrogen tansformation in the subalpine forest of western Sichuan, China. In order to deeply understand the process of soil nitrogen transformation in the subalpine forest of western Sichuan under climate change scenarios, therefore, wintertime net nitrogen mineralization, net nitrogen ammonification, net nitrogen nitrification and microbial nitrogen immobilization in soils under three depths of snowpacks (deep snowpack, middle snowpack and shallow snowpack) were measured in two subalpine forests of western Sichuan, which locates at the upper reaches of Yangtze River and the eastern Qinghai-Tibet Plateau. The top of soil columns sampled from<em> Abies faxoniana </em>+ <em>Betula albo-sinensis</em> mixed forest and secondary fir forest were closed, and then <em>in-situ</em> incubated in the forest floor under deep snowpack, middle snowpack and shallow snowpack of the corresponding forest from November 2011 to April 2012, respectively. The results indicated that wintertime soil net nitrogen ammonification ranged from -2.37mg/kg to -2.22 mg/kg under different snowpacks in the mixed forest, and from -7.73 mg/kg to -7.10 mg/kg under different snowpacks in the secondary fir forest. Wintertime soil net nitrogen nitrification varied from 71.98 mg/kg to 82.69 mg/kg under different snowpacks in the mixed forest, and from 101.56 mg/kg to 124.89 mg/kg under different snowpacks in the secondary fir forest. Net nitrogen nitrification was significantly (<em>P</em> < 0.05) higher than net nitrogen ammonification. The highest rates of soil nitrogen ammonification, nitrification, mineralization and microbial nitrogen immobilization were observed in the middle snow pack, but no significant (<em>P</em> > 0.05) differences were observed among snow packs. Meanwhile, all measured indexes in each snow pack were significantly (<em>P</em> < 0.05) higher in the secondary fir forest than those in the fir-birch mixed forest. However, the interaction of snow pack and forest type was not significant on wintertime soil nitrogen transformation. The results suggested that soil nitrogen nitrification be a major process of wintertime soil nitrogen mineralization, and the process of soil nitrogen nitrification and ammonification be regulated by different microbial communities in the subalpine forests of western Sichuan, China. The result also implies that the decline of snowpack caused by climate change, in the short-run, has little effects on soil nitrogen mineralization in our study area.

Litter Quantity and Nitrogen Immobilization Cause Oscillations in Productivity of Wild Rice (Zizania palustris L.) in Northern Minnesota

Microbial immobilization of nitrogen (N) in litter from one year’s production may cause oscillations in biomass production if it delays N availability the following year. We tested whether shoot and root litter and plant density affect biomass and seed production of populations of wild rice (Zizannia palustris L.) grown in 378 l stock tank mesocosms over four consecutive years. Half the tanks were thinned to a uniform seedling density whereas density in the remaining tanks was allowed to fluctuate ad libitum. Litter treatments included both shoot litter removal, leaving only root litter, and retaining shoot litter intact with root litter. A separate greenhouse fertilizer experiment tested whether N and/or phosphorus (P) limited productivity. Responses to N additions were much stronger than to P additions. Annual production and N availability in the tanks were correlated with each other and followed a concurrent cycle from 2004 to 2008. Furthermore, production in tanks with shoot + root litter did not fluctuate more than tanks with only root litter. Root litter immobilized more nitrogen and for a longer period than shoot litter. Neither litter immobilized P. Density did not affect mean seed weight, total seed production, or mean plant weight, but total seed production declined in years following productive years and was high only following years of low litter production. Root litter may therefore be primarily responsible for the delays in N availability that cause cycles in biomass and seed production. Consequently, both wild rice litter quantity and quality play central roles in production and population dynamics of wild rice stands.

Modeling soil respiration based on carbon, nitrogen, and root mass across diverse Great Lake forests

The variability in the net ecosystem exchange of carbon (NEE) is a major source of uncertainty in quantifying global carbon budget and atmospheric CO 2. Soil respiration, which is a large component of NEE, could be strongly influential to NEE variability. Vegetation type, landscape position, and site history can influence soil properties and therefore drive the microbial and root production of soil CO 2. This study measured soil respiration and soil chemical, biological and physical properties on various types of temperate forest stands in Northern Wisconsin (USA), which included ash elm, aspen, northern hardwood, red pine forest types, clear-cuts, and wetland edges. Soil respiration at each of the 19 locations was measured six times during 1 year from early June to mid-November. These data were combined with two additional data sets from the same landscape that represent two smaller spatial scales. Large spatial variation of soil respiration occurred within and among each forest type, which appeared to be from differences in soil moisture, root mass and the ratio of soil carbon to soil nitrogen (C:N). A soil climate driven model was developed that contained quadratic functions for root mass and the ratio of soil carbon to soil nitrogen. The data from the large range of forest types and site conditions indicated that the range of root mass and C:N on the landscape was also large, and that trends between C:N, root mass, and soil respiration were not linear as previously reported, but rather curvilinear. It should be noted this function appeared to level off and decline at C:N larger than 25, approximately the value where microbial nitrogen immobilization limits free soil nitrogen. Weak but significant relationships between soil water and soil C:N, and between soil C:N and root mass were observed indicating an interrelatedness of (1) topographically induced hydrologic patterns and soil chemistry, and (2) soil chemistry and root production. Future models of soil respiration should address multiple spatial and temporal factors as well as their co-dependence.

Ecosystem partitioning of 15N-glycine after long-term climate and nutrient manipulations, plant clipping and addition of labile carbon in a subarctic heath tundra

Low temperatures and high soil moisture restrict cycling of organic matter in arctic soils, but also substrate quality, i.e. labile carbon (C) availability, exerts control on microbial activity. Plant exudation of labile C may facilitate microbial growth and enhance microbial immobilization of nitrogen (N). Here, we studied 15N label incorporation into microbes, plants and soil N pools after both long-term (12 years) climate manipulation and nutrient addition, plant clipping and a pulse-addition of labile C to the soil, in order to gain information on interactions among soil N and C pools, microorganisms and plants. There were few effects of long-term warming and fertilization on soil and plant pools. However, fertilization increased soil and plant N pools and increased pool dilution of the added 15N label. In all treatments, microbes immobilized a major part of the added 15N shortly after label addition. However, plants exerted control on the soil inorganic N concentrations and recovery of total dissolved 15N (TD 15N), and likewise the microbes reduced these soil pools, but only when fed with labile C. Soil microbes in clipped plots were primarily C limited, and the findings of reduced N availability, both in the presence of plants and with the combined treatment of plant clipping and addition of sugar, suggest that the plant control of soil N pools was not solely due to plant uptake of soil N, but also partially caused by plants feeding labile C to the soil microbes, which enhanced their immobilization power. Hence, the cycling of N in subarctic heath tundra is strongly influenced by alternating release and immobilization by microorganisms, which on the other hand seems to be less affected by long-term warming than by addition or removal of sources of labile C.

Interactions of plant species mediated plant competition for inorganic nitrogen with soil microorganisms in an alpine meadow

Sources of competition for limited soil resources, such as nitrogen, include competitive interactions among different plant species and between plants and soil microbes. We hypothesized that plant interactions intensified plant competition for inorganic nitrogen with soil microorganisms. To test these competitive interactions, one dominant species (Kobresia humilis Serg) and one less abundant gramineous herb (Elymus nutans Griseb) in an alpine ecosystem were selected as target species to grow under interactions with their neighboring plants and without interaction treatments in field plots. 15N-labeled ammonium and nitrate were used to quantify their partition between plants and soil microorganisms for 48 h after tracer additions. Responses of K. humilis to interactions from their surrounding plants were negative, while those of E. nutans were positive. Species identity, inorganic nitrogen forms, and plant interactions significantly affected the total amount of nitrogen utilization by soil microorganisms and plants. Although plant interactions have negative effects on nitrogen uptake of K. humilis, there is an increase in microbial immobilization of nitrogen under presence of its neighbors. For E. nutans, facilitation from surrounding plants is in favor of their nitrogen uptake. Compared with K. humilis, competition for $$ ^{{15}} {\text{N}} - {\text{NO}}^{ - }_{3} $$ and $$ ^{{15}} {\text{N}} - {\text{NH}}^{ + }_{4} $$ was less intensive between E. nutans and microorganisms. $$ ^{{15}} {\text{N}} - {\text{NH}}^{ + }_{4} $$ recovery by soil microorganisms and plants were not more than or much lower than their utilization of $$ ^{{15}} {\text{N}} - {\text{NO}}^{ - }_{3} $$ under different interaction treatments. These results suggested that the partitioning of inorganic nitrogen between plants and soil microorganisms is mediated by plant–plant interactions and interactions between plants and soil microorganisms.

Evaluation of microbial immobilization of nitrogen during composting by a quantiative analysis of DNA extraction from the compost

Evaluation of microbial immobilization of nitrogen during composting by a quantiative analysis of DNA extraction from the compost

Direct and indirect effects of experimental warming on ecosystem carbon processes in a tallgrass prairie

This study was conducted to examine direct and indirect impacts of global warming on carbon processes in a tallgrass prairie in the U.S. Great Plains. Infrared radiators were used to simulate global warming, and clipping was used to mimic hay mowing. Experimental warming caused significant increases in green biomass in spring and autumn and total biomass in summer on most of the measuring dates. Green aboveground biomass showed positive linear correlations with soil temperature in spring and autumn whereas total aboveground biomass in summer was negatively correlated with soil temperature. Experimental warming also affected aboveground biomass indirectly by extending the length of growing season and changing soil nitrogen process. Elevated temperature tended to increase net nitrogen mineralization in the first year but decrease it in the second year, which could be attributable to stimulated plant growth and belowground carbon allocation and consequently enhanced microbial nitrogen immobilization. Warming‐induced changes in soil respiration were proportional to those of total aboveground biomass. Clipping significantly reduced aboveground biomass and increased root biomass, but had no effect on net nitrogen mineralization and annual mean soil respiration. The proportional changes in soil respiration to those of aboveground biomass indicate warming‐stimulated ecosystem carbon uptake could be weakened by increased carbon release through soil respiration.

Imobilização de nitrogênio em solo cultivado com milho em sucessão à aveia preta nos sistemas plantio direto e convencional

O presente trabalho teve como objetivos avaliar a quantidade de nitrogênio imobilizado e a sua remineralização, nos sistemas plantio direto (SPD) e convencional (SC), ao longo do ciclo do milho. Para tal, foram coletadas amostras da camada de 0-5cm de solo semeado com milho em sucessão à aveia preta, sob os sistemas convencional e plantio direto, no dia da semeadura do milho e 46, 62, 88 e 112 dias após. Amostras da parte aérea das plantas de milho foram coletadas nas mesmas épocas, com exceção da primeira, e avaliadas quanto à quantidade de nitrogênio acumulado. As amostras de solo foram avaliadas quanto à quantidade de nitrogênio mineral no solo, de nitrogênio e carbono potencialmente mineralizáveis, atividade de urease e de nitrogênio imobilizado na biomassa microbiana. A imobilização microbiana do nitrogênio foi maior no sistema plantio direto, levando a uma menor quantidade de nitrogênio mineral no solo e resultando em menor acúmulo de nitrogênio na parte aérea do milho ao final do seu ciclo neste sistema, em comparação com o convencional. Não foi observada remineralização do nitrogênio imobilizado, indicando que a biomassa microbiana atuou mais como agente da mineralização de nitrogênio orgânico do que como fonte de nitrogênio potencialmente mineralizável.

Mechanisms of plant species impacts on ecosystem nitrogen cycling

AbstractPlant species are hypothesized to impact ecosystem nitrogen cycling in two distinctly different ways. First, differences in nitrogen use efficiency can lead to positive feedbacks on the rate of nitrogen cycling. Alternatively, plant species can also control the inputs and losses of nitrogen from ecosystems.Our current understanding of litter decomposition shows that most nitrogen present within litter is not released during decomposition but incorporated into soil organic matter. This nitrogen retention is caused by an increase in the relative nitrogen content in decomposing litter and a much lower carbon‐to‐nitrogen ratio of soil organic matter. The long time lag between plant litter formation and the actual release of nitrogen from the litter results in a bottleneck, which prevents feedbacks of plant quality differences on nitrogen cycling. Instead, rates of gross nitrogen mineralization, which are often an order of magnitude higher than net mineralization, indicate that nitrogen cycling within ecosystems is dominated by a microbial nitrogen loop. Nitrogen is released from the soil organic matter and incorporated into microbial biomass. Upon their death, the nitrogen is again incorporated into the soil organic matter. However, this microbial nitrogen loop is driven by plant‐supplied carbon and provides a strong negative feedback through nitrogen cycling on plant productivity. Evidence supporting this hypothesis is strong for temperate grassland ecosystems. For other terrestrial ecosystems, such as forests, tropical and boreal regions, the data are much more limited. Thus, current evidence does not support the view that differences in the efficiency of plant nitrogen use lead to positive feedbacks.In contrast, soil microbes are the dominant factor structuring ecosystem nitrogen cycling. Soil microbes derive nitrogen from the decomposition of soil organic matter, but this microbial activity is driven by recent plant carbon inputs. Changes in plant carbon inputs, resulting from plant species shifts, lead to a negative feedback through microbial nitrogen immobilization. In contrast, there is abundant evidence that plant species impact nitrogen inputs and losses, such as: atmospheric deposition, fire‐induced losses, nitrogen leaching, and nitrogen fixation, which is driven by carbon supply from plants to nitrogen fixers. Additionally, plants can influence the activity and composition of soil microbial communities, which has the potential to lead to differences in nitrification, denitrification and trace nitrogen gas losses. Plant species also impact herbivore behaviour and thereby have the potential to lead to animal‐facilitated movement of nitrogen between ecosystems.Thus, current evidence supports the view that plant species can have large impacts on ecosystem nitrogen cycling. However, species impacts are not caused by differences in plant quantity and quality, but by plant species impacts on nitrogen inputs and losses.

Effect of microbial nitrogen immobilization during the growth period on the availability of nitrogen fertilizer for winter cereals

Pot and field experiments were conducted to determine microbial immobilization of N fertilizer during growth periods of winter wheat and winter barley. In a pot experiment with winter wheat, Ca(15NO3)2 was applied at tillering [Zadok's growth stage (GS) 25)], stem elongation (GS 31) and ear emergence (GS 49). Rates of 100 mg N pot–1, 200 mg N pot–1 or 300 mg N pot–1 were applied at each N application date. At crop maturity, 15N-labelled fertilizer N immobilization was highest at the highest N rate (3×300 mg N pot–1). For each N-rate treatment about 50% of the total immobilized fertilizer N was immobilized from the first N dressing, and 30% and 20% of the total 15N immobilized was derived from the second and third applications, respectively. In field trials with winter wheat (three sites) and winter barley (one site) N was applied at the same growth stages as for the pot trial. N was also applied to fallow plots, but only at GS 25. N which was not recovered (neither in crops nor in soil mineral N pools) was considered to represent net immobilized N. A clear effect of N rate (51–255 kg N ha–1) on net N immobilization was not found. The highest net N immobilization was found for the period between GS 25 (March) and GS 31 (late April) which amounted to 54–97% of the total net N immobilized at harvest (July/August). At GS 31, non-recovered N was found to be of similar magnitude for cropped and fallow plots, indicating that C from roots did not affect net N immobilization. Microbial biomass N (Nmic) was determined for cropped plots at GS 31. Although Nmic tended to be higher in fertilized than in unfertilized plots, fertilizer-induced increases in Nmic and net N immobilization were poorly correlated. It can be concluded that microbial immobilization of fertilizer N is particularly high after the first N application when crop growth and N uptake are low.