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1.

Purpose

Small but highly bioactive labile carbon (C) and nitrogen (N) pools are of great importance in controlling terrestrial C and N fluxes, whilst long-term C and N storage is determined by less labile but relatively large sizes of C and N pools. Little information is available about the effects of global warming and grazing on different forms of C and N pools in the Qinghai?CTibet Plateau of China. The aim of this study was to investigate the effects of warming and grazing on the sizes of different soil labile C and N pools and N transformation in this region.

Materials and methods

A free-air temperature enhancement system in a controlled warming?Cgrazing experiment had been implemented since May 2006. Infrared heaters were used to manipulate temperature, and a moderate grazing intensity was simulated by Tibetan sheep. After 3 years?? warming, soil samples were taken from the four treatment plots: no warming with no grazing; no warming with grazing; warming with no grazing; and warming with grazing. Concentrations of inorganic N in the 40?Ccm soil profiles were measured by a flow injection analyser. Microbial biomass C (MBC) and microbial biomass N (MBN) were measured by the fumigation?Cextraction method, and soluble organic C (SOC) and soluble organic N (SON) were determined by high-temperature catalytic oxidation. Total N (TN), C isotope composition (??13C) and N isotope composition (??15N) were determined using an isotope ratio mass spectrometer. Net N transformation under low temperature was studied in a laboratory incubation experiment.

Results and discussion

Warming and grazing treatments affected soil C and N pools differently, and these effects varied with soil depth. Warming significantly increased TN, MBC, MBN, and SON and decreased ??13C at the 10?C20 and 20?C30 cm soil depths, whilst grazing generally decreased SON at the 10?C20 and 20?C30 cm, and MBC at 20?C30 cm. At the 0?C10 cm depth, neither warming nor grazing alone affects these soil parameters significantly, indicating that there could be considerable perturbation on the soil surface. However, grazing alone increased NO 3 ? ?CN, total inorganic N, SOC and ??15N at the 0?C10 cm depth. Incubated at 4°C, warming (particularly with grazing) led to net immobilization of N, but no-warming treatments led to net N mineralization, whilst nitrification was strong across all these treatments. Correlations between MBC and SOC, and TN and MBN or SON were positive. However, SON was less well correlated with TN and MBN compared with the highly positive correlations between SOC and MBC.

Conclusions

It is clearly demonstrated that warming and grazing affected labile C and N pools significantly, but differently after 3 years?? treatments: Warming tended to enlarge labile C and N pools through increased litter inputs, whilst grazing tended to increase inorganic N pools, decrease SON and accelerate N cycling. Grazing might modify the mode that warming affected soil C and N pools through its strong impacts on microbial processes and N cycling. These results suggested that interactive effects of warming and grazing on C and N pools might have significant implications for the long-term C and N storage and productivity of alpine meadow ecosystem in the Qinghai?CTibet Plateau of China.  相似文献   

2.
Abstract

To understand the influence of basal application of N fertilizer on nitrification potential and N2O and NO emissions, four soil samples were collected from an upland Andisol field just before (sample 1) and 4 (sample 2), 36 (sample 3) and 72 (sample 4) days after the basal application of N fertilizer during the Chinese cabbage growing season from 12 September to 30 November 2005. The potentials of N2O production and nitrification of the soils were determined using a 15N tracer technique and the soils were incubated for 25 days at 25°C and 60% water-filled pore space (WFPS). The results revealed that as much as 84–97% N2O and almost all NO were produced by nitrification. The 15N2O emission peak occurred approximately 350 h after the beginning of incubation for samples 1 and 2, but just 48 h later in samples 3 and 4. Total 15N2O emission during the 25-day incubation of samples 3 and 4 ranged from 190 to 198 µg N kg?1 soil, which was significantly higher than the 99–108 µg N kg?1 soil recorded in samples 1 and 2. Basal application of N fertilizer did not immediately increase the nitrification potential and the ratio of N2O to N added, but did dramatically increase the nitrification potential and the ratio of N2O to N added as (15NH4)2SO4 36–72 days after the basal N fertilizer was added. In contrast, NO emission was negatively correlated with nitrification potential and total N2O emission. As a result, a trade-off relationship between total NO and N2O emissions was identified. The results indicated that there was a time-lagged induction of the change of N turnover in the soil, which was possibly caused by slow population growth of the nitrifiers and/or a slow shift in the microbial community in the soil.  相似文献   

3.
Forest soils contain a variable amount of organic N roughly repartitioned among particles of different size, microbial biomass and associated with mineral compounds. All pools are alimented by annual litter fall as main input of organic N to the forest floor. Litter N is further subject to mineralization/stabilization recognized as the crucial process for the turnover of litter N. Although it is well documented that different soil types have different soil N stocks, it is presently unknown how different soil types affect the turnover of recent litter N. Here, we compared the potential mineralization of the total soil organic N with that of recent litter-released N in three beech forests varying in their soil properties. Highly 15N-labelled beech litter was applied to stands located at Aubure, Ebrach, Collelongo, which differ in humus type, soil type and soil chemistry. After 4-5 years of litter decomposition, the upper 3 cm of the organo-mineral A horizon was sampled and the net N mineralization was measured over 112 days under controlled conditions. The origin of mineralized N (litter N versus soil organic N) was calculated using 15N labeling. In addition, soils were fractionated according to their particle size (>2000 μm, 200-2000 μm, 50-200 μm, <50 μm) and particulate organic matter (POM) was separated from the mineral fraction in size classes, except the <50 μm fraction. Between 41 and 69% of soil organic N was recovered as POM. Litter-released 15N was mainly to be found in the coarse POM fractions >200 μm. On a soil mass basis, N mineralization was two-fold higher at Aubure and Collelongo than at Ebrach, but, on a soil N basis, N mineralization was the lowest at Collelongo and the highest at Ebrach. On a soil N (or 15N) basis, mineralization of litter 15N was two to four-fold higher than mineralization of the average soil N. Furthermore, the δ15N of the mineral N produced was closer to that of POM than to that of the mineral-bound fraction (<50 μm). Highest rates of 15N mineralization happened in the soil with the lowest N content, and we found a negative relationship between accumulations of N in the upper A horizon and the mineralization of 15N from the litter. Our results show that mineral N is preferentially mineralized from POM in the upper organo-mineral soil irrespective of the soil chemistry and that the turnover rate of litter N is faster in soils with a low N content.  相似文献   

4.
Abstract

The rate and timing of manure application when used as nitrogen (N) fertilizer depend on N‐releasing capacity (mineralization) of manures. A soil incubation study was undertaken to establish relative potential rates of mineralization of three organic manures to estimate the value of manure as N fertilizer. Surface soil samples of 0–15 cm were collected and amended with cattle manure (CM), sheep manure (SM), and poultry manure (PM) at a rate equivalent to 200 mg N kg?1 soil. Soil without any amendment was used as a check (control). Nitrogen‐release potential of organic manures was determined by measuring changes in total mineral N [ammonium‐N+nitrate‐N (NH4 +–N+NO3 ?–N)], NH4 +–N, and accumulation of NO3 ?–N periodically over 120 days. Results indicated that the control soil (without any amendment) released a maximum of 33 mg N kg?1soil at day 90, a fourfold increase (significant) over initial concentration, indicating that soil had substantial potential for mineralization. Soil with CM, SM, and PM released a maximum of 50, 40, and 52 mg N kg?1 soil, respectively. Addition of organic manures (i.e., CM, SM, and PM) increased net N released by 42, 25, and 43% over the control (average). No significant differences were observed among manures. Net mineralization of organic N was observed for all manures, and the net rates varied between 0.01 and 0.74 mg N kg?1 soil day?1. Net N released, as percent of organic N added, was 9, 10, and 8% for CM, SM, and PM. Four phases of mineralization were observed; initial rapid release phase in 10–20 days followed by slow phase in 30–40 days, a maximum mineralization in 55–90 days, and finally a declined phase in 120 days. Accumulation of NO3 ?–N was 13.2, 10.6, and 14.6 mg kg?1 soil relative to 7.4 mg NO3 ?–N kg?1 in the control soil, indicating that manures accumulated NO3 ?–N almost double than the control. The proportion of total mineral N to NO3 ?–N revealed that a total of 44–61% of mineral N is converted into NO3 ?–N, indicating that nitrifiers were unable to completely oxidize the available NH4 +. The net rates of mineralization were highest during the initial 10–20 days, showing that application of manures 1–2 months before sowing generally practiced in the field may cause a substantial loss of mineralized N. The rates of mineralization and nitrification in the present study indicated that release of inorganic N from the organic pool of manures was very low; therefore, manures have a low N fertilizer effect in our conditions.  相似文献   

5.
Molecular nitrogen (N2) and nitrous oxide (N2O) generated by denitrification increase N losses in the soil–plant system. This study aimed to quantify N2 and N2O from potassium nitrate (K15NO3) applied to soils with different textures and moisture contents in the absence and presence of a source of carbon (C) using the 15N tracer method. In the three soils used (sandy texture (ST), sandy clay loam texture (SCLT), and clayey texture (CT)), three moisture contents were evaluated (40%, 60%, and 80% of the water holding capacity (WHC)) with (D+) and without (D?) dextrose added. The treatments received 100 mg N kg?1 (KNO3 with 23.24 atom% 15N). N2 emissions occurred in all of the treatments, but N2O emissions only occurred in the D+ treatment, showing increases with increasing moisture content. SCLT with 80% WHC in the D+ treatment exhibited the highest accumulated N emission (48.26 mg kg?1). The 15N balance suggested trapping of the gases in the soil.  相似文献   

6.
Abstract

Up to 50% of nitrogen (N) fertilizer can remain in soil after crop harvest in dryland farming. Understanding the fate of this residual fertilizer N in soil is important for evaluating its overall use efficiency and environmental effect. Nitrogen-15 (15N)-labeled urea (165 kg N ha?1) was applied to winter wheat (Triticum aestivum L.) growing in three different fertilized soils (no fertilizer, No-F; inorganic nitrogen, phosphorus and potassium fertilization, NPK; and manure plus inorganic NPK fertilization, MNPK) from a long-term trial (19 years) on the south of the Loess Plateau, China. The fate of residual fertilizer N in soils over summer fallow and the second winter wheat growing season was examined. The amount of the residual fertilizer N was highest in the No-F soil (116 kg ha?1), and next was NPK soil (60 kg ha?1), then the MNPK soil (43 kg ha?1) after the first winter wheat harvest. The residual fertilizer N in the No-F soil was mainly in mineral form (43% of the residual 15N), and for the NPK and MNPK soils, it was mainly in organic form. The loss rate of residual 15N in No-F soil over summer fallow was as high as 48%, and significantly (P < 0.05) higher than that in the NPK soil (22%) and MNPK soil (19%). The residual 15N use efficiency (RNUE) by the second winter wheat was 13% in the No-F soil, 6% in the NPK soil and 8% in the MNPK soil. These were equivalent to 9.0, 2.0 and 2.2% of applied 15N. The total 15N recovery (15N uptake by crops and residual in 0–100 cm soil layer) in the MNPK and NPK soils (84.5% and 86.6%, respectively) were both significantly higher than that in the No-F soil (59%) after two growing seasons. The 15N uptake by wheat in two growing seasons was higher in the MNPK soil than in NPK soil. Therefore, we conclude that a high proportion of the residual 15N was lost during the summer fallow under different land management in dryland farming, and that long-term combined application of manure with inorganic fertilizer could increase the fertilizer N uptake and decrease N loss.  相似文献   

7.
Sulfur (S) deficiency effects on nitrogen (N) and S fluxes during vegetative growth of Brassica napus was investigated by tracing 15N and 34S for 9 d of S-sufficient [1.5 mM sulfate (SO42-)] and S-deficient (0.05 mM SO42-) condition. A significant decrease in leaf osmotic potential and chlorophyll content was apparent after 9 d of S-deficiency. Sulfur uptake during 9 d was remarkably decreased by 94.3% by S-deficiency, whereas no significant change occurred for N uptake. The N and S deriving from uptake were mainly allocated to the leaves in control plants, but the S flow into leaves was largely restricted under S-deficient condition. The remobilization of stored N and S were mainly issued only from leaves in control plants, while from leaves and petiole in S-deficient ones. The remobilization of N and S mainly issued from leaves flows into the roots both in control and S-deficient plants.  相似文献   

8.
Abstract

To determine the relationships between microbial biomass nitrogen (N), nitrate–nitrogen leaching (NO3-N leaching) and N uptake by plants, a field experiment and a soil column experiment were conducted. In the field experiment, microbial biomass N, 0.5 mol L?1 K2SO4 extractable N (extractable N), NO3-N leaching and N uptake by corn were monitored in sawdust compost (SDC: 20 Mg ha?1 containing 158 kg N ha?1 of total N [approximately 50% is easily decomposable organic N]), chemical fertilizer (CF) and no fertilizer (NF) treatments from May 2000 to September 2002. In the soil column experiment, microbial biomass N, extractable N and NO3-N leaching were monitored in soil treated with SDC (20 Mg ha?1) + rice straw (RS) at five different application rates (0, 2.5, 5, 7.5 and 10 Mg ha?1 containing 0, 15, 29, 44 and 59 kg N ha?1) and in soil treated with CF in 2001. Nitrogen was applied as (NH4)2SO4 at rates of 220 kg N ha?1 for SDC and SDC + RS treatments and at a rate of 300 kg N ha?1 for the CF treatment in both experiments. In the field experiment, microbial biomass N in the SDC treatment increased to 147 kg N ha?1 at 7 days after treatment (DAT) and was maintained at 60–70 kg N ha?1 after 30 days. Conversely, microbial biomass N in the CF treatment did not increase significantly. Extractable N in the surface soil increased immediately after treatment, but was found at lower levels in the SDC treatment compared to the CF treatment until 7 DAT. A small amount of NO3-N leaching was observed until 21 DAT and increased markedly from 27 to 42 DAT in the SDC and CF treatments. Cumulative NO3-N leaching in the CF treatment was 146 kg N ha?1, which was equal to half of the applied N, but only 53 kg N ha?1 in the SDC treatment. In contrast, there was no significant difference between N uptake by corn in the SDC and CF treatments. In the soil column experiment, microbial biomass N in the SDC + RS treatment at 7 DAT increased with increased RS application. Conversely, extractable N at 7 DAT and cumulative NO3-N leaching until 42 DAT decreased with increased RS application. In both experiments, microbial biomass N was negatively correlated with extractable N at 7 DAT and cumulative NO3-N leaching until 42 DAT, and extractable N was positively correlated with cumulative NO3-N leaching. We concluded that microbial biomass N formation in the surface soil decreased extractable N and, consequently, contributed to decreasing NO3-N leaching without impacting negatively on N uptake by plants.  相似文献   

9.
An incubation experiment was carried out with maize (Zea mays L.) leaf straw to analyze the effects of mixing the residues with soil and N amendment on the decomposition process. In order to distinguish between soil effects and nitrogen effects for both the phyllospheric microorganisms already present on the surface of maize straw and soil microorganisms the N amendment was applied in two different placements: directly to the straw or to the soil. The experiment was performed in dynamic, automated microcosms for 22 days at 15 °C with 7 treatments: (1) untreated soil, (2) non-amended maize leaf straw without soil, (3) N amended maize leaf straw without soil, (4) soil mixed with maize leaf straw, (5) N amended soil, (6) N amended soil mixed with maize leaf straw, and (7) soil mixed with N amended maize leaf straw. 15NH415NO3 (5 at%) was added. Gas emissions (CO2, 13CO2 and N2O) were continuously recorded throughout the experiment. Microbial biomass C, biomass N, ergosterol, δ13C of soil organic C and of microbial biomass C as well as 15N in soil total N, mineral N and microbial biomass N were determined in soil samples at the end of the incubation. The CO2 evolution rate showed a lag-phase of two days in the non-amended maize leaf straw treatment without soil, which was completely eliminated when mineral N was added. The addition of N generally increased the CO2 evolution rate during the initial stages of maize leaf straw decomposition, but not the cumulative CO2 production. The presence of soil caused roughly a 50% increase in cumulative CO2 production within 22 days in the maize straw treatments due to a slower decrease of CO2 evolution after the initial activity peak. Since there are no limitations of water or N, we suggest that soil provides a microbial community ensuring an effective succession of straw decomposing microorganisms. In the treatments where maize and soil was mixed, 75% of microbial biomass C was derived from maize. We concluded that this high contribution of maize using microbiota indicates a strong influence of organisms of phyllospheric origin to the microbial community in the soil after plant residues enter the soil.  相似文献   

10.
Elevated CO2 may increase nutrient availability in the rhizosphere by stimulating N release from recalcitrant soil organic matter (SOM) pools through enhanced rhizodeposition. We aimed to elucidate how CO2-induced increases in rhizodeposition affect N release from recalcitrant SOM, and how wild versus cultivated genotypes of wheat mediated differential responses in soil N cycling under elevated CO2. To quantify root-derived soil carbon (C) input and release of N from stable SOM pools, plants were grown for 1 month in microcosms, exposed to 13C labeling at ambient (392 μmol mol−1) and elevated (792 μmol mol−1) CO2 concentrations, in soil containing 15N predominantly incorporated into recalcitrant SOM pools. Decomposition of stable soil C increased by 43%, root-derived soil C increased by 59%, and microbial-13C was enhanced by 50% under elevated compared to ambient CO2. Concurrently, plant 15N uptake increased (+7%) under elevated CO2 while 15N contents in the microbial biomass and mineral N pool decreased. Wild genotypes allocated more C to their roots, while cultivated genotypes allocated more C to their shoots under ambient and elevated CO2. This led to increased stable C decomposition, but not to increased N acquisition for the wild genotypes. Data suggest that increased rhizodeposition under elevated CO2 can stimulate mineralization of N from recalcitrant SOM pools and that contrasting C allocation patterns cannot fully explain plant mediated differential responses in soil N cycling to elevated CO2.  相似文献   

11.
Background, aim, and scope  Hoop pine (Araucaria cunninghamii) is a nitrogen (N) demanding indigenous Australia softwood species with plantations in Southeast Queensland, Australia. Soil fertility has declined with increasing rotations and comparison study of N cycling between hoop pine plantations, and adjacent native forest (NF) is required to develop effective forest management for enhancing sustainable forest production and promoting environmental benefits. Field in situ mineral 15N transformations in these two forest ecosystems have not been studied. Hence, the present study was to compare the differences in soil nutrients, N transformations, 15N fluxes, and fate between the hoop pine plantation and the adjacent native forest. Materials and methods  The study sites were in Yarraman State Forest (26°52′ S, 151°51′ E), Southeastern Queensland, Australia. The in situ core incubation method was used in the field experiments. Mineral N was determined using a LACHAT Quickchem Automated Ion Analyzer. 15N were performed using an isotope ratio mass spectrometer with a Eurovector elemental analyzer. All statistical tests were carried out by the SPSS 11.0 for Windows statistical software package. Results  Soil total C and N were significantly higher in the NF than in the 53-year-old hoop pine plantation. Concentrations of NO3 – were significantly higher in the NF soil than in the plantation soil. The plantation soil had significantly higher 15N and 13C natural abundances than the NF soil. The NF soil had significantly lower C/N ratios than the plantation soil. NO3 ––N was dominated in mineral N pools in both NF and plantation soils, accounting for 91.6% and 70.3% of the total mineral N pools, respectively. Rates of net nitrification and net N mineralization were, respectively, four and three times higher in the NF soil than in the plantation soil. The 15NO3 ––N and mineral 15N were significantly higher in the NF soil than in the plantation soil. Significant difference in 15NH4 +–N was found in the NF soil before and after the incubation. Discussion  The NF soil had significantly higher NO3 ––N, mineral N, total N and C but lower δ15N, δ13C, and C/N ratios than the plantation soil. Moreover, the rates of soil net N mineralization and nitrification were significantly higher, but ammonification rate was lower in the NF than in the plantation. The NF soil had many more dynamic N transformations than the plantation soil due to the combination of multiple species and layers and, thus, stimulation of microbial activity and alteration of C and N pool sizes in favor of the N transformations by soil microbes. The net rate of N and 15N transformation demonstrated differences in N dynamic related to the variation in tree species between the two ecosystems. Conclusions  The change of land use and trees species had significant impacts on soil nutrients and N cycling processes. The plantation had larger losses of N than the NF. The NO3 ––N and 15NO3 ––N dominated in the mineral N and 15N pools in both forest ecosystems. Recommendations and perspectives  Native forest soil had strong N dynamic compared with the plantation soil. Composition of multiple tree species with different ecological niches in the plantation could promote the soil ecosystem sustainability. The 15N isotope dilution technique in the field can be quite useful for studying in situ mineral 15N transformations and fate to further understand actual N dynamics in natural forest soils.  相似文献   

12.
Abstract

Nitrogen (N) concentrations and stable N isotope abundances (δ15N) of common reed (Phragmites australis) planted in a constructed wetland were measured periodically between July 2001 and May 2002 to examine their seasonal variations in relation to N uptake and N translocation within common reed. Nitrogen concentrations in P. australis shoots were higher in the growing stage (7.5 to 24.8 g N kg?1) than in the senescence stage (4.2 to 6.8 g N kg?1), indicating N translocation from shoots to rhizomes. Meanwhile, the corresponding δ15N values were higher in the senescence stage (+12.2 to +22.4‰) than in the growing stage (+5.1 to +11.3‰). Coupled with the negative correlation (R2=0.24, P<0.05, n=18) between N concentrations and δ15N values of shoots in the senescence stage, our results suggested that shoot N became enriched in 15N due to N isotopic fractionation (with an isotopic fractionation factor, αs/p, of 1.012) during N translocation to rhizomes. However, the positive correlation between N concentrations and δ15N values in the growing stage (R2=0.19, P<0.001, n=54) suggested that P. australis relies on N re‐translocated from rhizome in the early growing stage and on mineral N in the sediment during the active growing stage. Therefore, seasonal δ15N variations provide N‐isotopic evidence of N translocation within and N uptake from external N sources by common reed.  相似文献   

13.
Summary Surface additions of (15NH4)2SO4 were used to measure the immobilization and subsequent movement of exogenous N added to two litter types of contrasting quality (Cornus florida and Quercus prinus). Litterbaskets were used to measure the litter mass loss and N dynamics and to follow the movement of the 15N label through litter, F layer, and soil pools. Half of the litterbaskets of each species were treated with naphthalene to reduce microarthropod densities. The faster decomposing C. florida litter maintained a higher excess atom % 15N, and a greater relative concentration of the labeled input (g 15N g–1) than did Q. prinus litter. In both litter types the excess atom % 15N, relative concentration (g 15N g–1), and absolute amount of label recovered in the litter declined over time. This occurred during a period of net accumulation of total litter N, implying simultaneous release of the initial input and immobilization of N from other sources. The concentration of 15N in the soil increased over time, while the F layer apparently acted as an intermediary in the transfer of 15N from litter to soil. Naphthalene effectively reduced microarthropod numbers in all horizons of the litterbaskets and significantly reduced the decay rates of Q. prinus, but not C. florida litter. Naphthalene did not appear to affect total N dynamics in the litter. However, with all horizons taken together, the naphthalene-treated litterbaskets retained more total 15N than the control litterbaskets. Naphthalene also changed the vertical distribution of 15N within litterbaskets, so that the litter retained less of the 15N-labeled input and the F layer and soil horizons retained more of the labeled input than in control litterbaskets. Our major conclusions are: (1) the N pool of decomposing litter is dynamic, with simultaneous N release and immobilization activating N turnover even during the net accumulation phase; (2) litter quality is an important determinant of immobilization and retention of exogenous N inputs and, therefore, turnover of the litter N pool; and (3) microarthropod activity can significantly affect the incorporation and retention of exogenous N inputs in decomposing litter, although these changes are apparently not reflected in net N accumulation or release during the 1st year of decomposition. However, the naphthalene may have affected microbially mediated N dynamics and this possibility needs to be considered in interpreting the results.  相似文献   

14.
A new 15N tracing model was developed to analyse nitrogen (N) transformations in old grassland soil. There was a need to develop a new model because existing models such as FLUAZ were not able to simulate the observed N dynamics. The new features of the model are: (a) simulation of heterotrophic nitrification, (b) simulation of dissimilatory nitrate (NO3−) reduction to ammonium (NH4+) (DNRA), (c) release of adsorbed or stored fertiliser N into the available mineral N pools and (d) immobilisation of NH4+ and NO3− into two separate organic N pools with different re-mineralisation characteristics. The tracing model contains six N pools and nine simultaneous N transformations either at zero- or first-order kinetics. The model is set up in the modelling software ModelMaker which contains non-linear optimisation routines based on the Marquardt-Levenberg algorithm. The model is able to simulate data obtained from triple labelling studies where either the NH4+, the NO3− or both pools were labelled with 15N. The flexible modelling environment allows the user to develop the model further.  相似文献   

15.

Purpose

Carbon (C) flux is largely controlled by the highly bio-reactive labile C (LC) pool, while long-term C storage is determined by the recalcitrant C (RC) pool. Soil nitrogen (N) availability may considerably affect changes of these pools. The aim of this study was to investigate the effects of N treatments on soil LC and RC pools.

Materials and methods

A field experiment was conducted in a city lawn soil for 600 days with three N treatments, i.e., the control (0 kg N ha?1 year?1), low-N (100 kg N ha?1 year?1), and high-N (200 kg N ha?1 year?1) treatments. As the N source, NH4NO3 solution was added to soil surface monthly. Measurements of LC, RC, and other soil biochemical properties, including pH, soil respiration rates, microbial biomass, and enzymes activities, were taken during the experiment period.

Results and discussion

The low-N and high-N treatments increased 6.3 and 13% of the LC pool, respectively, which was caused by decreased microbial biomass and soil respiration rates under the N treatments. By contrary, the low-N and high-N treatments decreased 5.9 and 12% of the RC pool, respectively. The N addition treatments enhanced phenol oxidase activities. The enhanced oxidase activities decreased new RC input and the increased dissolved organic C stimulated RC pool decomposition. The LC and RC pools were highly influenced by the N treatments, whereas effect of the N treatments on soil organic C was not significant. The N addition treatments also caused soil acidification and reduced bacterial biomass proportion in the soil microbial composition.

Conclusions

The N addition increased the LC pool but decreased the RC pool in the soil. These changes should greatly impact soil long-term C storage.  相似文献   

16.
ABSTRACT

There are many nitrogen (N) pools in soil, so their availability and different status can give information about bulk soil response to N deposition. However, the different size of N pools in forest soils and the relationship between them have not been well studied under N deposition when considering the role of litter. Here soil in an N-deposition experiment carried out for 5 years in a broad-leaved forest was used as an object to study the response of N pools to N deposition by stepwise extraction using water or solutions containing 0.5 M K2SO4, 2.5 M H2SO4 (LPI), or 13 M H2SO4 (LPII), and calculation of recalcitrant (RC) N pool. Under N control (CT), soil with the presence of litter had a higher N of 23.8–106.8% in the first four pools, but lower of 80.6% in recalcitrant N pool compared with soil with the absence of litter. In the absence of litter, N addition increased soil N in labile pool but decreased N in the RC pool compared to CT and these impacts were greater at high added N (HN) than low-added N (LN) rates. However, in the presence of litter, LN increased the amount of N in the K2SO4- extracted pool and HN reduced that in the water extracted pool. Additionally, LN and HN increased TN in the RC pool and HN increased the total soluble N (TSN) in the LPI and LPII pool. N changes in the water extraction pool were attributed to inorganic N, whereas they were NH4 + and soluble organic N (SON) in the K2SO4-extracted, LPI, and LPII pools. In the presence of litter, HN increased the SON concentration in the K2SO4, LPI, and LPII extractions; thus, SON may be a potentially important N form for N availability. These results suggested that N additions improve the accumulation of N in RC pool with the presence of litter. The different effects of N additions on soil N pool or N form in each pool depend on litter present or not.  相似文献   

17.
Abstract

The short‐term fate and retention of ammonium (NH4)‐15nitrogen (N) applied to two types of forest soils in east Tennessee was investigated. Four ridgetop forests, predominantly oak (Quercus spp.), were studied. Five applications of NH4‐15N tracer were made to the forest floor at 2‐ to 4‐week intervals over a 14‐week period in 2004. Nitrogen‐15 recovery in the forest floor, fine roots (<2 mm), and the mineral soil (0–20 cm) was calculated at 6, 21, and 42 weeks after the last application. Most of the 15N was retained in the forest floor and the mineral soil, with only small amounts (≤2%) found in roots from both soil layers. Recovery of NH4‐15N was greater in Inceptisols, which had a wider carbon (C)‐to‐N ratio than Ultisols. For both soil types, higher NH4‐15N recoveries and long retention times (half‐lives>100 weeks) indicated the forest floor is an effective filter for atmospheric N inputs.  相似文献   

18.
Relative control of soil moisture [30, 60, and 80 percent water-holding capacity (WHC)] on nitrous oxide (N2O) emissions from Fargo-Ryan soil, treated with urea at 0, 150, and 250 kg N ha?1 with and without nitrapyrin [2-chloro-(6-trichloromethyl) pyridine] (NP), was measured under laboratory condition for 140 days. Soil N2O emissions significantly increased with increasing nitrogen (N) rates and WHC levels. Urea applied at 250 kg N ha?1 produced the greatest cumulative N2O emissions and averaged 560, 3919, and 15894 µg kg?1 at 30, 60, and 80 percent WHC, respectively. At WHC ≤ 60 percent, addition of NP to urea significantly reduced N2O losses by 2.6- to 4.8-fold. Additions of NP to urea reduced N2O emission at rates similar to the control (0 N) until 48 days for 30 percent WHC and 35 days for 60 and 80 percent WHC. These results can help devise urea-N fertilizer management strategies in reducing N2O emissions from silty-clay soils.  相似文献   

19.
Optimal fertilizer nitrogen (N) rates result in economic yield levels and reduced pollution. A soil test for determining optimal fertilizer N rates for wheat has not been developed for Quebec, Canada, or many other parts of the world. Therefore, the objectives were to determine: 1) the relationship among soil nitrate (NO? 3)- N, soil ammonium (NH + 4)- N and N fertilizer on wheat yields; and 2) the soil sampling times and depths most highly correlated with yield response to soil NO? 3-N and NH + 4-N. In a three year research work, wet and dried soil samples of 0- to 30- and 30- to 60-cm depths from 20 wheat fields that received four rates of N fertilizer at seeding and postseeding (plants 15 cm tall) were analyzed for NH + 4-N and NO? 3 -N using a quick-test (N-Trak) and a standard laboratory method. Wheat yield response to N fertilizer was limited, but strong to soil NO? 3-N.  相似文献   

20.
Abstract

A soil test for mineralizable soil N had been calibrated for winter wheat in the Willamette Valley of western Oregon. Seventy‐eight percent of the variation in spring N uptake by unfertilized wheat was explained by N mineralized from mid‐winter soil samples incubated anaerobically for 7 days at 40°C. Mineralizable N (Nmin) ranged from 10 to 30 mg N kg?1 and was used to predict N fertilizer needs. Recommended rates of N were correlated (R2=0.87) with maximum economic rates of N fertilizer. Subsequent farmer adoption of no‐till sowing and a high frequency of soil tests>30 mg N kg?1 prompted reevaluation of the soil test. Four N fertilizer rates [0, 56, G, and G+56 kg N ha?1] were compared in 12 m×150 m farmer‐managed plots. Grower's N rates (G) ranged from 90 to 180 kg N ha?1 and were based on Nmin and NH4‐N plus NO3‐N soil tests. Averaged across ten no‐till and five conventionally tilled sites, grain yield and crop N uptake were maximized at the recommended rate of N. Results demonstrate that N fertilizer needs for winter wheat can be predicted over a wide range of mineralizable soil N (10 to 75 mg N kg?1) and that the same soil test calibration can be used for conventionally sown and direct‐seeded winter wheat.  相似文献   

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