organic-N Pool Research Articles

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.

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.

Soil Organic Nitrogen Enrichment Following Soybean in an Iowa Corn–Soybean Rotation

Understanding soil organic N (ON) pool enrichment may help explain why rates of N fertilization required to attain maximum corn (Zea mays L.) yields are usually lower for corn following soybean [Glycine max (L.) Merr.] than for corn following corn. Our objectives were to quantify the ON pools within a 16‐ha Iowa field and to correlate those results with corn yield. Spring and fall measurements of ON content (0–15 cm soil) as amino acids (AAs), amino sugars (ASs), and NH4+ were made using samples collected between 1997 and 1999 from 10 soil map units. The chemical extraction method determined an average 87% of the total N content (n = 10 soils) as identified ON but gave reduced ON recovery from depression soils that experienced periods of water ponding. The total AA concentrations measured in May were positively correlated (r2 = 0.84, P < 0.01) with corn yield during a dry year (1997) and 7 out of 10 soils provided near maximum yields. A wetter 1999 boosted overall corn yields 6.6% but resulted in a poorer relationship between May AA concentrations and corn yield. Microbial N compounds measured (May 1997) as glucosamine, galactosamine, and ornithine were also positively correlated with corn yield (r2 = 0.84, ρ < 0.01; r2 = 0.94, P < 0.001; r2 = 0.93, P < 0.001, respectively). The ON concentration decreased during corn production from May to September 1997 an average of 367 kg N ha−1 but increased following soybean growth in 1998 by 320 kg N ha−1 The chemical extraction methodology identified soils that may not require the full amount of N fertilizer currently being applied, thus decreasing the potential for N loss to surface and ground water resources without decreasing opportunities to achieve optimum yield.

NITROGEN AND PHOSPHORUS LIMITATION OF BIOMASS GROWTH IN A TROPICAL SECONDARY FOREST

Understanding secondary successional processes in Amazonian terrestrial ecosystems is becoming increasingly important as continued deforestation expands the area that has become secondary forest, or at least has been through a recent phase of secondary forest growth. Most Amazonian soils are highly weathered and relatively nutrient poor, but the role of nutrients as a factor determining successional processes is unclear. Soils testing and chronosequence studies have yielded equivocal results regarding the possible role of nutrient limitation. The objective of this paper is to report the first two years' results of a nitrogen (N) and phosphorus (P) fertilization experiment in a 6‐yr‐old secondary forest growing on an abandoned cattle pasture on a clayey Oxisol. Growth of remnant grasses responded significantly to the N + P treatment, whereas tree biomass increased significantly following N‐only and N + P treatments. The plants took up about 10% of the 50 kg P/ha of the first year's application, and recovery in soil fractions could account for the rest. The trees took up about 20% of the 100 kg N/ha of the first year's application. No changes in soil inorganic N, soil microbial biomass N, or litter decomposition rates have been observed so far, but soil faunal abundances increased in fertilized plots relative to the control in the second year of the study. A pulse of nitric oxide and nitrous oxide emissions was measured in the N‐treated plots only shortly after the second year's application. Net N mineralization and net nitrification assays demonstrated strong immobilization potential, indicating that much of the N was probably retained in the large soil organic‐N pool. Although P availability is low in these soils and may partially limit biomass growth, the most striking result of this study so far is the significant response of tree growth to N fertilization. Repeated fire and other losses of N from degraded pastures may render tree growth N limited in some young Amazonian forests. Changes in species composition and monitoring of long‐term effects on biomass accumulation will be addressed as this experiment is continued.

Recovery of cover-crop-N in the soil-plant system in the Guinea savannah zone of Ghana

In order to understand the efficiency of residue-N use and to estimate the minimum input required to obtain a reasonable level of crop response, it is important to quantify the fate of the applied organic-N. The recovery of N from 15N-labelled Crotalaria juncea was followed in the soil and the succeeding maize crop. Apparent N recovery (ANR) by maize from unlabelled Crotalaria juncea, Crotalaria retusa, Calopogonium mucunoides, Mucuna pruriens and mineral fertilizer at three locations were also evaluated. The maize crop recovered 4.7% and 7.3% of the 15N-labelled C. juncea-N at 42 days after sowing (DAS) and at final harvest, respectively. The corresponding 15N recovery from the soil was 92.4% and 58.5%. The highest mean ANR of 57.4% was with mineral fertilizer, whereas the mean ANR of 14.3% from C. retusa was the lowest. A large pool substitution and added-N interaction effect was observed when comparing N recovery from the labelled and unlabelled C. juncea. The amount of residue-N accounted for by the isotope dilution method at 42 DAS was 97.1% and at final harvest 65.8%. The large residue-N recovery in the soil organic-N pool explains the residual effect usually observed with organic residue application.

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.

The influence of soil desiccation on plant production, nutrient uptake and plant nutrient availability in two French floodplain grasslands

The effects of summer soil desiccation on plant production and plant nutrient availability (determined by wet chemical extraction) in floodplain grasslands along the rivers Allier and Loire in France were investigated. Soil desiccation in these river floodplains is the result of human interference with the natural flooding regime of rivers, such as dam construction and gravel mining. Flooding periods along the Allier have a longer duration (maximum of 202 days as opposed to 38 days for the Loire). The main comparison was between floodplain grasslands along the two rivers. Additional comparisons were made between relatively high lying, wetter areas (‘ridges’) and low lying, drier areas (‘swales’) within both floodplains. Thus, areas with different soil moisture content were examined, independent of river influences. The availability of P was higher in the Allier floodplain than in the Loire floodplain, but it was similar between ridges and swales. It was concluded that P-availability was not related to soil wetness, but to river sedimentation. Plant production, plant nutrient uptake, and biologically mediated soil processes, such as N-mineralization and nitrification, were all higher on the wetter Allier floodplain and in the wetter swales. These higher process rates were noted where higher amounts of soil bound carbon and nutrients were found as well. Plant production, N-mineralization and nitrification were moisture limited at the dry ridge on the Loire floodplain, as moisture levels were below the wilting point here (pF>4.2). On the wetter parts of the floodplain, plant production was N-limited. This was concluded from low tissue N/P ratios (about 10) and a positive relation between plant production and N-mineralization. On the wetter parts, the rate of N-mineralization depended on the size of soil organic-N pools. The size of these pools was positively related to soil wetness, which can be interpreted as a positive effect of river flooding. Reduced flooding lowers the nutrient input to floodplains and contributes to the occurrence of soil desiccation in summer, which results in lower nutrient cycling and reduced accumulation of soil organic matter. These effects have negative consequences for important floodplain functions, such as nutrient retention and biomass production. © 1998 John Wiley & Sons, Ltd.

Sewage Sludge Proteins as Labile Carbon and Nitrogen Sources

The study of specific, organic sewage sludge constituents is necessary to augment our knowledge of C and N mineralization in sludge-amended soils. A laboratory incubation study of seven sewage sludges was initiated to test the hypothesis that sewage sludge proteins are labile C and N sources. Sewage sludge proteins were extracted with H2O, 10% (v/v) Triton X-100, and 1.0 M NaOH and determined by the Lowry assay. Sewage sludges were mixed with Bresser sandy loam soil (fine-loamy, mixed, mesic Aridic Argiustoll) at a rate of 10 g dry sludge kg−1 dry soil and incubated at 25 °C and 0.111 kg kg−1 soil water content for 12 wk to determine sludge C and N mineralization. Extractable sludge proteins were highly correlated to C mineralization (r2 = 0.94-0.96), but they were poorly correlated to N mineralization (r2 = 0.40-0.41). This supported the hypothesis that sludge proteins were a labile C source but not a labile N source. However, low molecular weight primary amines (assumed to be predominately protein degradation products) combined with the sludge C/N ratios were highly correlated to sludge N mineralization rates (r2 = 0.91). Nitrogen mineralization of sludge-amended soil followed either zero- or first-order kinetics. Kinetic models of the first-order systems showed that N mineralization was best described as the decomposition of two distinct organic-N pools. Sewage sludge proteins appear to be significant sources of labile C, and their degradation products apparently are critical N sources.

The effect of harvesting intensity on the fate of applied 15N-ammonium to the organic horizons of a coniferous forest in N. Wales

15N-ammonium sulphate equivalent to 0.5 kg N/ha was added as a tracer to lysimeters containing the organic horizons of an acid forest soil. The effect of logging debris (brash), vegetation and second rotationPicea sitchensis seedlings on the amount of the15N found in various soil, vegetation and leachate pools was followed over a period of 60 days. Transformation of15N-ammonium to nitrate occurred within 24 hours. Although total nitrate leachate losses were high, tracer-derived nitrate represented only 0.4%–4.2% of the applied15N-ammonium. The atom % excess of the KCI-extractable organic-N pool was initially lower than for the inorganic species but due to the large pool size, consistently represented 3–6% of the applied15N-ammonium. The similarity of the atom % excess of the ammonium and nitrate pools indicated an autotrophic nitrification pathway.

Nitrogen contribution by leucaena (Leucaena leucocephala) prunings to maize in an alley cropping system

The N uptake of maize was assessed on an Alfisol in a sole crop and in an alley cropping system in southwestern Nigeria. Although the application of prunings increased the maize N content in both sole and alley-cropped maize, the N contributed to the maize by the prunings was low, ranging between 4.4 and 23.8 kg ha−1. This was equivalent to 3.2% and 9.407% of the N released during decomposition of the prunings. Application of the prunings increased the grain yields of the sole maize by 38% and the maize yield in the alley-cropped plots by 104%, compared with yields in the corresponding plots where prunings were not applied. The results indicate that part of the N from the prunings was retained in the soil organic-N pool. Maize N, dry weights and grain yields were lowest in the alley-cropped plots where prunings were removed, probably because of competition between the maize and the hedgerow trees.

The nitrogen cycle in shallow water sediment systems of rice fields Part II: Fractionation and bio-availability of organic nitrogen compounds

The organic N-pool in soils provides an important part of the N metabolized by rice. As this pool is, however, very large compared to the yearly uptake, — which may involve a few percent only — direct evidence of its importance is difficult to establish. We tried therefore to distinguish different fractions with supposedly different availability. We did not succeed in developing a satisfactory fractionation scheme, but improved existing fractionation schemes by applying a sequential extraction using an alkaline extraction followed by an acid one; the residue of the latter was still an important portion of the total.