Labile organic-N Research Articles

Seasonal soil-plant nitrogen dynamics of a cordgrass salt marsh in response to coastal embankments in Eastern China

The effects of coastal embankments on nitrogen (N) cycling in the Spartina alterniflora salt marsh have been extensively reported. However, it remains unclear effects of the embankment on the sizes of diverse N subpools in the plant-soil subsystems year-round. This study examined seasonal changes in various N subpools of plant subsystems; soil subsystems [e.g., soil organic N (SON), recalcitrant organic N (RON), labile organic N (LON), dissolved organic N (DON), ammonium N(NH+4 N), and nitrate N(NO3-N)]; N mineralization [e.g., soil net ammonification (RA) and nitrification (RN) rate]; and immobilization [e.g., microbial biomass N (MBN)] in embanked and adjacent S. alterniflora natural salt marshes on the coast of Eastern China. The embankment significantly reduced the litter N storage by 62.7–71.8% over the four seasons and decreased the root N storage by 53.0% during winter. The SON, LON, RON, and N H +4 N concentrations declined significantly by 43.0–60.2%, 35.8–64.8%, 44.9–59.0%, and 20.8–42.2%, respectively, over the four seasons following the embankment construction. Furthermore, the embankment dramatically reduced the DON concentrations by 21.9% in spring, 14.6% in summer, and 10.4% in winter, while notably diminishing the NO3 N concentrations by 33.4% in autumn and 44.9% in winter, and the RA and RN in spring and summer. However, the embankment clearly increased the MBN concentrations during summer and autumn, the NO3 N concentrations in spring, and the RA and RN in winter at different levels. Due to the decreased soil N inputs from plants, the embankment decreased the organic and inorganic N subpools every season to varying degrees, except for the NO3 N concentration in spring. We suggest that the decreased soil salinity following embankment establishment might increase the uptake of ions by microbes, while stimulating the production of MBN. Ultimately, the NO3 N and DON were two vital N sources for S. alterniflora, and plants absorbed N from the soil to promote their biomass, as well as N concentration and storage. This study is conducive toward understanding the mechanisms behind the effects of coastal embankments on the N transfer among various N subpools in the plant and soil systems.

Gross nitrogen transformation rates differ in reconstructed oil-sand soils from natural boreal-forest soils as revealed using a 15N tracing method

The Athabasca Oil Sands deposit is one of the largest single oil deposits in the world. Following surface mining, companies are required to restore soil-like profiles that can support the previous land capabilities. Re-establishment of nutrient cycles in reconstructed soils is one of the most critical factors in ensuring long-term sustainability of these reconstructed boreal landscapes. We compared soils from nine reconstructed oil-sand sites with those from three natural boreal-forest sites of similar age since wildfire disturbance. We (1) measured soil total N, NH4+, and NO3− content; (2) quantified gross N transformation rates in reconstructed and natural soils using a 15N tracing method; (3) compared gross rates of N transformations in reconstructed soils under different vegetation types to compared N-cycling processes in these soils. The 15N tracing approach highlighted key distinctions in N-cycling processes in reconstructed and natural soils. In reconstructed soils, NH4+ was mainly cycled through the recalcitrant organic-N pool, whereas in natural soils, NH4+ was produced from the recalcitrant organic-N pool but predominantly consumed in the labile organic-N pool. The mineralization of NH4+ from the labile organic-N pool was also higher in natural soils compared to reconstructed soils, suggesting greater prominence of microbial N-cycling activity in the natural soils compared to the reconstructed soils. Gross nitrification rates were similar in natural and reconstructed soils, but net nitrification rates were higher and apparently of heterotrophic origin in reconstructed soils. The higher net nitrification rates in reconstructed soils indicate a surplus of N relative to microbial requirements in reconstructed soils. N-transformation rates were similar in reconstructed soils under the three types of vegetation.

Cattle manure and straw have contrasting effects on organic nitrogen mineralization pathways in a subtropical paddy soil

Organic materials are widely recommended for the maintenance and/or accumulation of organic carbon and total nitrogen in agricultural soils. However, the relative effectiveness of different organic materials on gross N transformation and inorganic-N supply is not known. Here, a 15N tracing incubation study was conducted to investigate the rates of gross N mineralization, nitrification, and microbial immobilization of and in a paddy soil managed using different organic materials. Soil samples were collected from a rice field that has been under long-term study (30 years) and is receiving four different fertilizer treatments: no fertilizer (CK), chemical fertilizer (NPK), chemical fertilizer plus cattle manure (NPKM), and chemical fertilizer plus straw (NPKS). The samples were incubated with 15NH4NO3 or NH415NO3, and the results were calculated based on a 15N tracing model. The analysis showed that mineralization of labile and recalcitrant organic-N pools was significantly stimulated by NPKM and NPKS treatments, respectively, but not by NPK alone. Heterotrophic nitrification was negligible in all treatments. Therefore, the enhanced inorganic-N supply rates (total mineralization + heterotrophic nitrification) due to NPKM and NPKS application could be attributed to the increasing mineralization of the labile organic-N pool and recalcitrant organic-N pool, respectively. The rate of autotrophic nitrification rate was significantly increased by NPK and NPKM application, but not by NPKS application. However, both consumption rates (immobilization of and dissimilatory reduction to ) were low and unaffected by fertilizer application. Our results suggest that NPKM stimulated the processes of mineralization of the labile organic-N pool and autotrophic nitrification to increase accumulation in soil. In contrast, application of NPKS increased the rate of mineralization of recalcitrant organic-N pool but did not affect the autotrophic nitrification rate.

The effect of dicyandiamide addition to cattle slurry on soil gross nitrogen transformations at a grassland site in Northern Ireland

SUMMARYMany studies have shown the efficacy of the nitrification inhibitor dicyandiamide (DCD) in reducing nitrous oxide (N2O) emissions and nitrate (NO3−) leaching. However, there is no information on the effect of DCD on gross soil N transformations under field conditions, which is key information if it is to be used as a mitigation strategy to reduce N losses. The current field study was conducted to determine the effect of DCD on ten gross nitrogen (N) transformations in soil following cattle slurry (CS) application to grassland in Northern Ireland on three occasions (June 2010, October 2010 and March 2011).Ammonium (NH4+) oxidation (ONH4) was the dominant process in total NO3−production (ONH4+ONrec(oxidation of recalcitrant organic N to NO3−)) following CS application, accounting for 0·894–0·949. Dicyandiamide inhibited total NO3−production from CS by 0·781, 0·696 and 0·807 in June 2010, October 2010 and March 2011, respectively. The lower inhibition level in October 2010 was thought to be due to the higher rainfall and soil moisture content in that month compared to the other application times. As DCD strongly inhibited NH4+oxidation following CS application, it also decreased the rate of total NO3−consumption, since less NO3−was formed. The rates of mineralization from recalcitrant organic-N (MNrec) were higher than from labile organic-N (MNlab) on all occasions. The DCD significantly increased total mineralization (MNrec+MNlab) following CS application in June 2010 and March 2011, but had no significant effect in October 2010. In contrast, the rate of immobilization of labile organic-N (INH4_Nlab) was higher than from recalcitrant organic-N (INH4_Nrec) on all occasions, accounting for 0·878–0·976 of total NH4+immobilization from CS. The DCD significantly increased total immobilization (INH4_Nrec+INH4_Nlab) when CS was applied in June 2010, but had no significant effect at other times of the year.Dicyandiamide was shown to be a highly effective inhibitor of ammonium oxidation at this grassland site. Although there was evidence that it increased both NH4+mineralization and immobilization following CS application, its effect on these processes was inconsistent. Further work is required to understand the reason for these inconsistent effects: future improvements in15N tracer models may help.

Spatial pattern and variability in soil N and P availability under the influence of two dominant species in a pine forest

The presence of a legume in a nitrogen (N)- limited forest ecosystem may not only create "islands of N fertility" but also affect the phosphorus (P) availability. The main objective of this study was to compare the effect of a pine (Pinus canariensis) and a leguminous (Adenocarpus viscosus) species on the spatial pattern and variability of different labile organic-N (microbial biomass-N (MB-N) and dis- solved organic-N (DON)), as well as inorganic-N (IN) and -P fractions (NH4-N, NO3-N, and PO4-P), in a forest soil of the Canary Islands (Spain). Assuming some litter quantity and quality differences between these two species, we expected to find higher soil labile organic-N concentrations under isolated indi- viduals of P. canariensis than under isolated individ- uals of A. viscosus. We also expected to find higher concentrations and spatial dependence (percentage of total variance explained by spatial autocorrelation) of NO3-N beneath A. viscosus than beneath P. canar- iensis canopies, and higher spatial scaling of soil variables under the influence of P. canariensis canopies than under the presence of A. viscosus individuals. Moreover, we tested whether the soil variables measured under isolated individuals of both species showed a different spatial variability than the same soil variables measured under overlapping pine canopies inside a pine forest. To test these hypotheses, soil samples under isolated mature individuals of each species were collected in the winter and summer, whereas under a pine forest canopy, the sampling was performed only in the winter. The winter MB-N and DON concentrations were significantly higher be- neath the pine individual, whereas the winter NO3-N, NO3-N-to-IN ratio, and PO4-P were significantly higher under the leguminous individual; these differ- ences were not observed in the summer samples. We found higher spatial ranges under the pine than under the legume canopy in the winter sampling, and the spatial dependence of NO3-N was twice as high beneath the legumes as under the pines at both sampling dates. The soil spatial variability was higher (up to 17 times higher) under isolated individuals than inside the pine forest. The results of this study suggest that both the morphological and physiological char- acteristics of P. canariensis and A. viscosus, as well as the spatial pattern of P. canariensis, may influence the spatial pattern and variability of soil resources.

Effects of repeated fertilizer and cattle slurry applications over 38 years on N dynamics in a temperate grassland soil

The effects of repeated synthetic fertilizer or cattle slurry applications at annual rates of 50, 100 or 200 m 3 ha −1 yr −1 over a 38 year period were investigated with respect to herbage yield, N uptake and gross soil N dynamics at a permanent grassland site. While synthetic fertilizer had a sustained and constant effect on herbage yield and N uptake, increasing cattle slurry application rates increased the herbage yield and N uptake linearly over the entire observation period. Cattle slurry applications, two and four times the recommended rate (50 m 3 ha −1 yr −1, 170 kg N ha −1), increased N uptake by 46 and 78%, respectively after 38 years. To explain the long-term effect, a 15N tracing study was carried out to identify the potential change in N dynamics under the various treatments. The analysis model evaluated process-specific rates, such as mineralization, from two organic-N pools, as well as nitrification from NH 4 + and organic-N oxidation. Total mineralization was similar in all treatments. However, while in an unfertilized control treatment more than 90% of NH 4 + production was related to mineralization of recalcitrant organic-N, a shift occurred toward a predominance of mineralization from labile organic-N in the cattle slurry treatments and this proportion increased with the increase in slurry application rate. Furthermore, the oxidation of recalcitrant organic-N shifted from a predominant NH 4 + production in the control treatment, toward a predominant NO 3 − production (heterotrophic nitrification) in the cattle slurry treatments. The concomitant increase in heterotrophic nitrification and NH 4 + oxidation with increasing cattle slurry application rate was mainly responsible for the increase in net NO 3 − production rate. Thus the increase in N uptake and herbage yield on the cattle slurry treatments could be related to NO 3 − rather than NH 4 + production. The 15N tracing study was successful in revealing process-specific changes in the N cycle in relationship to long-term repeated amendments.

Nitrogen cycling in forest soils across climate gradients in Eastern China

A 15N tracing study was carried out to investigate the potential gross nitrogen (N) dynamics in thirteen forest soils in Eastern China ranging from temperate to tropical zones (five coniferous forests, six deciduous broad-leaf forests, one temperate mixed forest, one evergreen broad-leaf forests ecosystems), and to identify the major controlling factors on N cycling in these forest ecosystems. The soil pH ranged from 4.3 to 7.9 and soil organic carbon (SOC) ranged from 6.6 g kg−1 to 83.0 g kg−1. The potential gross N transformation rates were quantified by 15N tracing studies where either the ammonium or nitrate pools were 15N labeled in parallel treatments. Gross mineralization rates ranged from 0.915 μg N g−1 soil day−1 to 2.718 μg N g−1 soil day−1 in the studied forest soils. The average contribution of labile organic-N (MNlab) to total gross mineralization (MNrec+MNlab) was 86% (58% to 99%), indicating that turnover of labile organic N plays a dominant role in the studied forest ecosystems. The gross mineralization rates in coniferous forest soils were significantly lower (ranging between 0.915 and 1.228 μg N g−1 soil day−1) compared to broad-leaf forest soils (ranging from 1.621 to 2.718 μg N g−1 soil day−1) (p < 0.01). Thus, the dominant vegetation may play an important role in regulating soil N mineralization. Nitrate production (nitrification) occurred via two pathways, oxidation of NH4+ and organic N the forest soils. Correlations with soil pH indicated that this is a key factor controlling the oxidation of NH4+ and organic N in theses forest ecosystems. NH4+ oxidation decreased with a decline in pH while organic N oxidation increased. The climatic conditions (e.g. moisture status) at the various sites governed the NO3−-N consumption processes (dissimilatory NO3− reduction to NH4+ (DNRA) or immobilization of NO3−). Total NO3− consumption and the proportion of total NO3− consumption to total NO3− production decreased with an increase in the drought index of ecosystems, showing that strong interactions appear to exist between climatic condition (e.g. the drought index), N mineralization and the rate of DNRA. Interactions between vegetation, climatic conditions govern internal N cycling in these forests soils.

Effect of elevated CO 2 on soil N dynamics in a temperate grassland soil

The response of terrestrial ecosystems to elevated atmospheric CO 2 is related to the availability of other nutrients and in particular to nitrogen (N). Here we present results on soil N transformation dynamics from a N-limited temperate grassland that had been under Free Air CO 2 Enrichment (FACE) for six years. A 15N labelling laboratory study (i.e. in absence of plant N uptake) was carried out to identify the effect of elevated CO 2 on gross soil N transformations. The simultaneous gross N transformation rates in the soil were analyzed with a 15N tracing model which considered mineralization of two soil organic matter (SOM) pools, included nitrification from NH 4 + and from organic-N to NO 3 − and analysed the rate of dissimilatory NO 3 − reduction to NH 4 + (DNRA). Results indicate that the mineralization of labile organic-N became more important under elevated CO 2. At the same time the gross rate of NH 4 + immobilization increased by 20%, while NH 4 + oxidation to NO 3 − was reduced by 25% under elevated CO 2. The NO 3 − dynamics under elevated CO 2 were characterized by a 52% increase in NO 3 − immobilization and a 141% increase in the DNRA rate, while NO 3 − production via heterotrophic nitrification was reduced to almost zero. The increased turnover of the NH 4 + pool, combined with the increased DNRA rate provided an indication that the available N in the grassland soil may gradually shift towards NH 4 + under elevated CO 2. The advantage of such a shift is that NH 4 + is less prone to N losses, which may increase the N retention and N use efficiency in the grassland ecosystem under elevated CO 2.

Soil microbial activity and biomass is stimulated by leguminous cover crops

AbstractThe leguminous cover cropsAtylosia scarabaeoides(L.) Benth.,Centrosema pubescensBenth., andPueraria phaseoloides(Roxb.) Benth., were grown in the interspaces of a 19 y–old coconut plantation and incorporated into the soil at the end of the monsoon season every year. At the end of the 12th year, soils from different depths were collected and analyzed for various microbial indices and their interrelationships. The objectives were to assess the effects of long‐term cover cropping on microbial biomass and microbial‐community structure successively down the soil profile.In general, total N (TN), organic C (OC), inorganic N, extractable P, and the levels of biological substratesviz., dissolved organic C (DOC) and N (DON), labile organic N (LON), and light‐fraction organic matter (LFOM) C and N decreased with depth at all the sites. Among sites, the cover‐cropped (CC) sites possessed significantly greater levels of TN, OC, DOC, DON, and LON compared to the control. Consequently, microbial biomass C (MBC), N (MBN), and P (MBP), CO2evolution, and ATP levels, in general, decreased with depth at all sites and were also significantly higher in the CC sites. Among the ratios of various microbial indices, the ratio of MBC to OC and metabolic quotient (qCO2) declined with depth. Higher MBC‐to‐OC ratios and largeqCO2levels in the surface soils could be ascribed to greater levels of readily degradable C content and indicated short turnover times of the microbial biomass. In contrast, the ratios of MBC to MBN and MBC to MBP increased with depth due to low N/P availability and relatively higher C availability in the subsoils. Cover cropping tended to enhance the ratios of MBC to OC, MBC to MBN, MBC to MBP, and ergosterol to MBC and decreased the ATP‐to‐MBC ratio at all depths. The relatively lower ATP‐to‐MBC ratios in the CC site, especially in the subsoil indicated microbial‐community structure possibly dominated by fungi. By converting the ergosterol content to fungal biomass, it was observed that fungi constituted 52%–63% of total biomass C at the CC site, but only 33%–40% of total biomass C at the control site. Overall, the study indicated that leguminous cover crops likeP. phaseoloidesorA. scarabaeoidessignificantly enhanced the levels of OC, N and microbial activity in the soils, even down to 50 cm soil depth.

Short-term wildfire effects on the spatial pattern and scale of labile organic-N and inorganic-N and P pools

The spatial heterogeneity of essential plant resources plays a crucial role in the structure, composition and productivity of many terrestrial ecosystems. Fires may affect both the availability and spatial pattern of soil nutrients. However, little is known about the effect of fire on the spatial pattern of soil resources. We hypothesized that shortly after a wildfire, the spatial patterns of soil mineral-N, organic labile-N (microbial biomass-N and dissolved organic-N) and extractable-P pools would become more clumped because of ash accumulation and post-fire deposition of litter around individual adult trees. To test this hypothesis, we used plots within a Pinus canariensis forest (with both Pinus canariensis and Adenocarpus viscosus present) and sampled them one month before and one month after a wildfire. Using geostatistical analyses, we examined the spatial patterns of soil mineral-N (NH 4-N and NO 3-N), dissolved organic-N (DON), microbial biomass-N (MB-N) and soil extractable-P (PO 4-P). Burned plots of P. canariensis and A. viscosus both had values that were significantly greater than the unburned plots for all variables, except for DON in both cases, and the N:P ratio in the case of A. viscosus, which showed significantly lower values. Except for DON, we observed an increased spatial dependence and range after a fire for all studied variables in the P. canariensis plots (large individuals). However, in plots with A. viscosus (smaller individuals), we only found differences before and after the fire for the PO 4-P and DON spatial patterns. Our results confirm the changes in the spatial structure of soil variables with fire, and suggest that, on a short-term basis, the physical structure of the plant community may determine the new spatial structure after fire, with a more clumped distribution around large surviving trees and shrubs. The spatial patch size of limiting resources has important consequences for the success of restoration of forest communities on burned areas.

Microbial and biochemical soil quality indicators and their potential for differentiating areas under contrasting agricultural management regimes

The aim of this study was to examine interrelationships between functional biochemical and microbial indicators of soil quality, and their suitability to differentiate areas under contrasting agricultural management regimes. The study included five 0.8ha areas on a sandy-loam soil which had received contrasting fertility and cropping regimes over a 5 year period. These were organically managed vegetable, vegetable–cereal and arable rotations, an organically managed grass clover ley, and a conventional cereal rotation. The organic areas had been converted from conventional cereal production 5 years prior to the start of the study. All of the biochemical analyses, including light fraction organic matter (LFOM) C and N, labile organic N (LON), dissolved organic N and water-soluble carbohydrates showed significant differences between the areas, although the nature of the relationships between the areas varied between the different parameters, and were not related to differences in total soil organic matter content. The clearest differences were seen in LFOM C and N and LON, which were higher in the organic arable area relative to the other areas. In the case of the biological parameters, there were differences between the areas for biomass-N, ATP, chitin content, and the ratios of ATP: biomass and basal respiration: biomass. For these parameters, the precise relationships between the areas varied. However, relative to the conventionally managed area, areas under organic management generally had lower biomass-N and higher ATP contents. Arbuscular mycorrhizal fungus colonization potential was extremely low in the conventional area relative to the organic areas. Further, metabolic diversity and microbial community level physiological profiles, determined by analysis of microbial community metabolism using Biolog GN plates and the activities of eight key nutrient cycling enzymes, grouped the organic areas together, but separated them from the conventional area. We conclude that microbial parameters are more effective and consistent indicators of management induced changes to soil quality than biochemical parameters, and that a variety of biochemical and microbial analyses should be used when considering the impact of management on soil quality.

Uranium Power Corp: catalytic process licence agreement with Earth Energy

The Athabasca Oil Sands deposit is one of the largest single oil deposits in the world. Following surface mining, companies are required to restore soil-like profiles that can support the previous land capabilities. Re-establishment of nutrient cycles in reconstructed soils is one of the most critical factors in ensuring long-term sustainability of these reconstructed boreal landscapes. We compared soils from nine reconstructed oil-sand sites with those from three natural boreal-forest sites of similar age since wildfire disturbance. We (1) measured soil total N, NH4+, and NO3− content; (2) quantified gross N transformation rates in reconstructed and natural soils using a 15N tracing method; (3) compared gross rates of N transformations in reconstructed soils under different vegetation types to compared N-cycling processes in these soils. The 15N tracing approach highlighted key distinctions in N-cycling processes in reconstructed and natural soils. In reconstructed soils, NH4+ was mainly cycled through the recalcitrant organic-N pool, whereas in natural soils, NH4+ was produced from the recalcitrant organic-N pool but predominantly consumed in the labile organic-N pool. The mineralization of NH4+ from the labile organic-N pool was also higher in natural soils compared to reconstructed soils, suggesting greater prominence of microbial N-cycling activity in the natural soils compared to the reconstructed soils. Gross nitrification rates were similar in natural and reconstructed soils, but net nitrification rates were higher and apparently of heterotrophic origin in reconstructed soils. The higher net nitrification rates in reconstructed soils indicate a surplus of N relative to microbial requirements in reconstructed soils. N-transformation rates were similar in reconstructed soils under the three types of vegetation.

Nitrogen mineralization in relation to site history and soil properties for a range of Australian forest soils

Rates of N mineralization were measured in 27 forest soils encompassing a wide range of forest types and management treatments in south-east Australia. Undisturbed soil columns were incubated at 20°C for 68 days at near field-capacity water content, and N mineralization was measured in 5-cm depth increments to 30 cm. The soils represented three primary profile forms: gradational, uniform and duplex. They were sampled beneath mature native Eucalyptus sp. forest and from plantations of Pinus radiata of varying age (<1 to 37 years). Several sites had been fertilized, irrigated, or intercropped with lupins. The soils ranged greatly in total soil N concentrations, C:N ratios, total P, and sand, silt, and clay contents. Net N mineralization for individual soil profiles (0–30 cm depth) varied from 2.0 to 66.6 kg ha-1 over 68 days, with soils from individual depths mineralizing from <0 (immobilization) to 19.3 kg ha-1 per 5 cm soil depth. Only 0.1–3.1% of the total N present at 0–30 cm in depth was mineralized during the incubation, and both the amount and the percentage of total N mineralized decreased with increasing soil depth. N fertilization, addition of slash residues, or intercropping with lupins in the years prior to sampling increased N mineralization. Several years of irrigation of a sandy soil reduced levels of total N and C, and lowered rates of N mineralization. Considuring all soil depths, the simple linear correlations between soil parameters (C, N, P, C:N, C:P, N:P, coarse sand, fine sand, silt, clay) and N mineralization rates were generally low (r<0.53), but these improved for total N (r=0.82) and organic C (r=0.79) when the soils were grouped into primary profile forms. Prediction of field N-mineralization rates was complicated by the poor correlations between soil properties and N mineralization, and temporal changes in the pools of labile organic-N substrates in the field.