The impacts of inorganic nitrogen application on mineralization of C-labelled maize and glucose, and on priming effect in saline alkaline soil

Organic matter dynamics and nutrient availability in saline alkaline soil of the former lake Texcoco will determine the success of a planned reforestation program. Uniformly labelled ¹⁴C-maize (MAI-treatment) and glucose (GLU-treatment) with or without 200 mg  kg⁻¹ soil (MAI-N treatment and GLU-N treatment, respectively) were added to soils with electrolytic conductivity (EC) 56 dS m⁻¹ (soil A) and 12 dS m⁻¹ (soil B) to investigate the importance of N availability on decomposition of organic material. Production of CO₂ and and inorganic N dynamics were monitored. The amount of ¹⁴C-glucose mineralized increased 1.8-times in the soil A, but had no effect in the soil B when 200 mg  kg⁻¹ soil was added, while the amount of ¹⁴C-maize mineralized increased 1.7 and 1.3-times when 200  kg⁻¹ soil was added in the soils A and B, respectively. Application of increased priming effect 3.7-times in the MAI-treatment of the soil A and 3.4-times in the GLU-treatment, while in the soil B the increase of priming effect was 4.1-times in the MAI-treatment and 3.7-times in the GLU-treatment. Of the 200 mg  kg⁻¹ added to both soils less than 10 mg NH₃-N kg⁻¹ was volatilized within one day, while 22 and 44 mg  kg⁻¹ soil was fixed on the soil matrix in the soil A and the soil B, respectively. Therefore more than 100 mg −N kg⁻¹ was immobilized into the microbial biomass within the first day. Concentration of nitrite increased sharply in all the treatments of soil A at the onset of the incubation followed by a decrease. A similar pattern was observed in the GLU-N and MAI-N treatments of the soil B, but not in the other treatments. A decrease in concentration of was observed in both soils followed by an increase in the MAI-N and GLU-N treatments of the soil B. It was found that application of had a stimulating effect on the decomposition of maize and glucose, and on the priming effect, while assimilatory reduction of resulted in an increase of in the soil A, and nitrification in the soil B.     

Ammonium production during microbial nitrate removal in soil microcosms from a developing marsh estuary

Rapid development and urbanization in the South Carolina (SC) coastal plain may introduce significant nutrients to adjacent tidal creeks and salt marsh estuaries, and threaten estuarine water quality. Microbial denitrification in estuarine soils plays an important role in removing excessive nitrate in coastal waters. Relative contributions of denitrification and ammonium production during nitrate reduction via dissimilatory nitrate reduction to ammonium (DNRA) and soil mineralization determine whether N is lost from the system or retained as ammonium . The objectives of this study were to compare background, short-term and long-term potential denitrification ( and glucose added) rates, and production during microbial conversion in a developing marsh estuary, SC (USA). Denitrification rates were measured using the acetylene block technique in an undeveloped fresh water site (T1W), an undeveloped Spartina marsh (Grave's Dock, GD), and a Spartina marsh at a golf course resort (Chechessee marsh, C3M). Background denitrification with no added was primarily controlled by concentration in soils and surface water. Adding glucose did not enhance either short-term or long-term potential denitrification rates in GD marsh soils. production during microbial removal was significant at both marsh sites, and N-mass balance based on N₂O and production suggested a significant contribution of from sources other than DNRA. DNRA was estimated to account for approximately 16.3% and 0% of total added removal in GD surface (0-10 cm) and subsurface (30-40 cm) soils, and 1.9 and 23.2% in C3M surface and subsurface soils. Excessive generation from processes other than DNRA may be attributed to stimulated mineralization, and this stimulation was estimated to enhance soil ammonification by 0.5∼4 times compared to background generation with no added. Our results suggest that although the marsh soils displayed high potential of removal via denitrification, the produced via a combination of DNRA and enhanced mineralization may allow to accumulate and be transported to coastal waters.     

Nitrate uptake by Eriophorum vaginatum controls N2O production in a restored peatland

The clear dependence of N₂O production through denitrification on available nitrate in soil has been shown in many studies. Since N availability similarly limits the growth of plants, the resource competition with vegetation limits the activity of denitrifying microbes and may consequently moderate the N₂O emissions from peatlands. We used uptake by Eriophorum vaginatum L. as a vegetation competition factor for microbes. The species was selected for the experiment because it has high nutrient use efficiency in low-nutrient conditions and high nutrient uptake efficiency in luxuriant nutrient conditions. We measured gaseous N flux as N₂O (end product of denitrifier activity) in a restored peatland in central Finland with acetylene inhibition technique over a growing season from sample plots with varying addition levels and E. vaginatum cover. The resource competition effects were analysed with a model that used exponential decay dependence of N₂O flux on the leaf area of E. vaginatum, and saturating response of N₂O flux to addition level. The model explained the variation in N₂O fluxes well (R²=0.86). The model simulation showed that the increasing nutrient uptake of E. vaginatum decreased the N₂O fluxes exponentially. Simultaneously, denitrification appeared to saturate even in conditions with high availability of and low level of competition by vegetation. Thus, E. vaginatum is an effective competitor for in sedge-dominated peatlands that controls the availability of for denitrification, and consequently moderates the N₂O emissions from peatlands.     

Nitrite production by Nitrosomonas europaea and Nitrosospira sp. AV in soils at different solution concentrations of ammonium

Although it remains unclear why NH₃-oxidizing bacteria (AOB) of the genus Nitrosospira dominate soil environments, and why Nitrosomonas spp. are less common, virtually no studies have compared their behavior in soil. In this study, the NH₃ oxidation rates of Nitrosomonas europaea (ATCC 19718) and Nitrosospira sp. AV were compared in three differently textured soils containing a range of extractable contents (2-11 μg soil). Soils were adjusted to pH 7.0-7.4 with CaCO₃ and sterilized with γ-radiation. Cell suspensions of each bacterium were inoculated into the soils to bring them to two-third of water-holding capacity and cell densities ∼2.5×10⁶ g⁻¹ soil. In virtually all cases, rates of production for both N. europaea and Nitrosospira sp. AV were linear over 48 h, and represented between 13 and 75%, respectively, of the maximum rates achieved in soil-free bacterial suspensions. Soil solution concentrations that supported these rates ranged between 0.2 and 1.5 mM. Addition of 21-36 μg soil raised soil solution levels to 1.8-2.5 mM and stimulated production to a greater extent in N. europaea (3.3-6.6-fold) than in Nitrosospira sp. AV (1-2.1-fold). Maximum rates of production were obtained by raising soil solution levels to 3-4 mM with a supplement of ∼80-90 μg soil. K_s values in soil for Nitrosospira sp. AV and N. europaea were estimated as 0.14 and 1.9 mM , respectively, and estimates of V_max were about 3.5-times higher for N. europaea (0.007 pmol h⁻¹ cell⁻¹) than for Nitrosospira sp. AV (0.002 pmol h⁻¹ cell⁻¹). The cell density of N. europaea increased in sterile Steiwer soil independent of supplemental . In the case of treatments receiving supplemental , growth yields of N. europaea calculated from either produced or consumed were similar to those reported in literature (3.5×10⁶-6×10⁶ cells μmol⁻¹). A higher growth yield was measured in the case of zero added (2.7×10⁷ cells μmol⁻¹), indicating that use of organic carbon compounds might have occurred and resulted in some energy sparing. Our results suggest that Nitrosospira spp. with a K_s similar to Nitrosospira sp. AV may have an advantage for survival in soil environments where soil solution levels are less than 1 mM. However, it is apparent that AOB like N. europaea are poised to take advantages of modest increases in extractable that raise soil solution levels to about 2.0-2.5 mM.     

Forest floor gross and net nitrogen mineralization in three forest types in Quebec, Canada

The effect of high nitrogen (N) depositions on forest ecosystems is an important concern in North America and may lead to N saturation of forest ecosystems and contribute to soils and surface water acidification. In this study, nitrogen dynamics in the FH layers of a sugar maple (SM), a balsam fir (BF) and a black spruce (BS) forest was characterized using a short term ¹⁵N isotopic pool dilutions approach and mid-term FH material incubation both in situ and in the laboratory. The short term dilutions approach indicated that the mean residence times of and in the FH material of the three sites were low (<1 d). The amount of inorganic nitrogen () recycled annually within the exchangeable forest floor reservoir was between one and two orders of magnitude larger than the annual atmospheric N deposition found at each of the sites. The BS site was clearly distinct than the two other forest types in that net N mineralization was negligible, even in absence of root uptake, suggesting that soil microorganisms were severely N limited. While net nitrification was not observed within the FH material of the BF site, did accumulate in the FH of the SM despite a low pH of 3.72 presumably because of heterotrophic nitrification or as a result of acid-tolerant autotrophic nitrification. The difference in N dynamics between the sites were most probably caused by dominant tree species. Transformation rates of inorganic N were higher in SM, followed by BF and BS stands. Given that the potential to mineralize inorganic N matches with a superimposed N atmospheric deposition gradient in Québec, the sugar maple forest is more likely to be affected by N saturation than coniferous forests.     

Oxygen exchange with water alters the oxygen isotopic signature of nitrate in soil ecosystems

Combined oxygen (O) and nitrogen (N) stable isotope analyses are commonly used in the source determination of nitrate . The source and fate of are studied based on distinct O and N isotopic signatures (δ¹⁸O and δ¹⁵N) of various sources and isotopic effects during transformation processes, which differ between sources like fertilizer, atmospheric deposition, and microbial production (nitrification). Isotopic fractionation during production and consumption of further affects the δ¹⁸O and δ¹⁵N signal. Regarding the δ¹⁸O in particular, biochemical O exchange between O from and H₂O is implicitly assumed not to affect the δ¹⁸O signature of . This study aimed to test this assumption in soil-based systems. In a short (24 h) incubation experiment, soils were treated with artificially ¹⁸O and ¹⁵N enriched . Production of from nitrification during the incubation would affect both the ¹⁸O and the ¹⁵N enrichment. Oxygen exchange could therefore be studied by examining the change in ¹⁸O relative to the ¹⁵N. In two out of the three soils, we found that the imposed ¹⁸O enrichment of the declined relatively more than the imposed enrichment. This implies that O exchange indeed affected the O isotopic signature of , which has important implications for source determination studies. We suggest that O exchange between and H₂O should be taken into consideration when interpreting the O isotopic signature to study the origin and fate of in ecosystems.     

Ammonium oxidation coupled to dissimilatory reduction of iron under anaerobic conditions in wetland soils

In exploring the dynamics of iron and nitrogen cycling in sediments from riparian forests we have observed a redox reaction that has not been previously described. During incubations of soil slurries under strictly anaerobic conditions, we repeatedly measured an unexpected production of both nitrite () and ferrous iron [Fe(II)]. Using this indirect evidence we hypothesize that, under anaerobic conditions, there is a biological process that uses ferric iron [Fe(III)] as an electron acceptor while oxidizing ammonium () to for energy production. This oxidation under iron reducing anaerobic conditions is thermodynamically feasible and is potentially a critical component of the N cycle in saturated sediments.     

Nitrogen dynamics in co-composted drilling wastes: Effects of compost quality and N fertilization

A better understanding of N availability in co-composted drilling wastes is required to evaluate the potential use of the composts as growth media. We investigated N dynamics in co-composted drilling wastes by examining the changes in the concentrations and partition of applied ¹⁵N in various soil N pools (, , dissolved organic N, microbial biomass N, and non-extractable N) in a 4-month greenhouse incubation experiment using 1-, 2-, 3-, and 4-year-old (referred to below as 1Y, 2Y, 3Y, and 4Y, respectively) composts, representing substrates with different quality. Regardless of compost age, after 4 months of incubation extractable N concentrations decreased (P<0.05), in contrast with the increasing pattern of the non-extractable N, indicating stabilization of the extractable N into the recalcitrant soil organic fraction. Fertilizer N application increased (P<0.05) extractable N concentrations. In the younger composts, a major part of the applied ¹⁵N was recovered in the non-extractable N fraction (44.0% for 1Y and 38.5% for 2Y) with little recovered as mineral N. On the other hand, a considerable percentage of the applied ¹⁵N (21.8% for 3Y and 18.8% for 4Y) was found in the pool in the older composts with relatively high mineral N but low organic C contents. This study shows that the dynamics of biologically available N and fate of applied N in the composts depend on compost quality such as mineral N and organic C contents, and compost C:N ratio. To use the co-composted drilling waste as growth media, different N management strategies need to be established for those composts with differed substrate quality.     

Mineralization of forest litter nutrients by heat and combustion

Temperature dependant mineralization dynamics during fire of litter species characteristic of the New Jersey pine barrens was determined. Senescent leaf material of pitch pine (Pinus rigida), white oak (Quercus alba) and black huckleberry (Gaylusssacia baccata) were collected at the time of abscission; sorted, ground and oven-dried at 70 °C. Replicate samples were then heated for 2 h at: 70, 100, 200, 300, 400, and 550 °C. Mass loss and total nitrogen and total phosphorus concentration of the heated material were determined. Additional samples of the residual material were extracted with deionized water, and the filtrate was assayed for the anions: , , ; and cations: , K⁺, Mg⁺⁺, and Ca⁺⁺.By heating leaf litter over a range of temperatures, to simulate the heterogeneous nature of forest litter burning, we identified patterns of nutrient mineralization characteristic of specific temperatures, some of which were common to all three litter species and others unique to individual species. In general, it appears that black huckleberry leaf litter was the most nutrient rich and the most labile. In huckleberry litter, there was a large reserve of soluble nitrogen, sulfur, phosphate, calcium and magnesium that became available upon heating to 200 °C. Pitch pine litter was the most nutrient poor, and the rates of nutrient mineralization were also generally the lowest of the three species studied. White oak litter nutrient concentration and rates of mineralization along the temperature gradient were intermediate. For all three litter species examined organic and inorganic nitrogen losses due to volatilization were >99% upon heating to 550 °C, and soluble magnesium concentrations declined significantly at temperatures of 300 °C, despite having a volatilization temperature greater than 1100 °C. Under the temperature range employed, heating of leaf litter resulted in little volatilization loss of phosphorus; however, the amount of soluble phosphate phosphorus was much lower in all three litter types at temperatures of 300 °C and above. With increasing temperatures, inorganic phosphate ions presumably became bound to cations in the ash, forming insoluble metal phosphates. The dramatic increase of the ratio of total phosphorus to soluble inorganic phosphate at higher temperatures, the loss of soluble magnesium above 300 °C, and the near complete loss of nitrogen at 550 °C suggests that after intense fires availability of these minerals may be dramatically reduced.     

Collembola effects on plant mass and nitrogen acquisition by ash seedlings (Fraxinus pennsylvanica)

We studied the effects of varied collembolan numbers on three compensatory mechanisms of nutrient uptake: fine root mass, endomycorrhizal development, and physiological uptake capacity. We grew ash (Fraxinus pennsylvanica) with or without the arbuscular mycorrhizal fungus Glomusintraradices, with 0, 10 or 50 initial Collembola (Folsomia candida). After 83 d root and uptake rates, endomycorrhizal development, and plant biomass were determined. Plant mass increased with Collembola number. Collembola interacted with mycorrhizae in their effects on N uptake and leaf N. Collembola in the absence of mycorrhizal roots were associated with lower N uptake and leaf N at 10 than at 0 or 50 initial Collembola. In contrast, Collembola in the presence of mycorrhizal roots were associated with the highest rate of N uptake and leaf N at 10 versus 0 or 50 initial Collembola. Hence as initial Collembola number increased, the relative importance of root system traits that determined N uptake changed from root physiological uptake capacity, presence of mycorrhizal roots, to fine root biomass.     

Impact of harvesting and logging slash on nitrogen and carbon dynamics in soils from upland spruce forests in northeastern Ontario

The potential impact of timber harvesting in the boreal forest on aquatic ecosystem water quality and productivity depends in part on the production of nutrients within the soil of the harvested catchment. Nitrogen supplied by organic matter decomposition is of particular interest because of the important role that N plays in biotic processes in surface waters, and in forest nutrition in general. Logging slash quality and input to the forest floor has the potential to influence N availability after harvest on clearcut sites. Net production of organic and inorganic-N and microbial biomass C and N concentrations were determined during a 90-day laboratory incubation at constant temperature and moisture. Incubated soils included F horizon and shallow mineral soil horizons (0-5 cm) from unharvested and full-tree harvested (2 and 12 growing seasons since harvest) boreal forest sites at the Esker Lakes Research Area (ELRA), in northeastern Ontario, Canada. In an ancillary experiment, black spruce foliage was added to unharvested forest floor material after 30 days during a 90-day laboratory incubation to simulate the influence of logging slash from full-tree harvesting on C and N dynamics. Twelve-year old clearcut F horizon material released on average 75 and 5 times more -N and 3 and 2 times as much inorganic-N than soil collected from unharvested and 2-year-old clearcuts, respectively. This increase in -N accumulation during the incubation was accompanied by decreases in both exchangeable -N and microbial biomass C and N levels. Net daily changes in microbial biomass N were significantly related to organic and inorganic-N accumulation or loss within the F horizon. Mineral soil release of inorganic-N was lower than release from the forest floor. Nitrate-nitrogen accumulation was lower, and -N accumulation was higher in mineral soil from unharvested sites when compared to 12-year-old clearcuts. Calculated harvest response ratios indicated that incubated mineral soil from the 12-year-old clearcut sites released significantly greater amounts of -N than 2-year-old clearcuts. Incorporation of black spruce needles into F horizon material reduced the production of organic and inorganic-N and increased microbial biomass N. Laboratory incubations of F horizon and shallow mineral soil from 12-year-old clearcuts suggested that these boreal soils have the capacity for increased inorganic-N production compared to uncut stands several years after harvesting. This has the potential to increase N availability to growing boreal forest plantations and increase N leaching due to greater -N levels in the forest soil.     

Functional stability of the nitrate-reducing community in grassland soils towards high nitrate supply

To study the effects of short-term fluctuation of nitrate concentrations on the nitrate-reducing community, repacked soil cores were amended with 0, 100 and soil and incubated for 3, 7 and 14 days, respectively. The nitrate reductase activity was determined in a laboratory-based enzyme assay. In parallel, the community structure of nitrate-reducing microorganisms was characterised by RFLP-PCR using the functional gene narG, which encodes the catalytic site of the membrane-bound nitrate reductase. The community structure remained constant over the experimental period indicating that this functional community is characterised by a high resistance towards fluctuating nitrate concentrations. Decreases in nitrate concentration as well as increase in pH values indicated a very active nitrate-reducing community under nitrate addition. Surprisingly, inhibition of nitrite reductase by 2,4-dinitrophenol, which is a precondition for the measurement of nitrate reductase activity, could not be achieved in the treatment despite increased concentrations of the inhibitor. However, comparison of the nitrate reductase in the control and the treatment showed a significant increase in the latter at day 3. No further differences were observed at days 7 and 14, which suggests a high resilience of the nitrate reductase activity.     

The role of plant residues in pH change of acid soils differing in initial pH

Reports on the effect of plant residues on soil pH have been contradictory. The conflicting accounts have been suggested to result from differences in compositions and types of plant residues and characteristics of soils. This incubation study examined the effect of plant residues differing in concentrations of N (3-49 g kg⁻¹) and of alkalinity (excess cations) (220-1560 mmol kg⁻¹) on pH change of three soils differing in initial pH (3.9-5.1 in 0.01 M CaCl₂). The addition of plant residues at a rate of 15 g kg⁻¹ soil weight increased the pH of all soils by up to 3.4 units and the pH reached the maximum at day 42 after incubation for Wodjil (initial pH 3.87) and Bodallin (pH 4.54) soils and day 14 for Lancelin soil (pH 5.1). The amount of pH buffering was decreased by residue addition in Wodjil soil, increased in Bodallin soil and remained unchanged in Lancelin soil, which closely related to changes of soil pH. Residue addition increased concentration and the increase in concentration generally correlated positively with the concentration of residue N. The concentration increased with time, reached the peak at Days 42-105 for Wodjil soil, Days 14-105 for Bodallin soil and Days 14-42 for Lancelin soil, and then decreased only in Lancelin soil. The concentration of was kept minimal in Wodjil and Bodallin soils. In Lancelin soil, concentrations increased with incubation time from days 14-28. Irrespective of plant residue and incubation time, the amounts of alkalinity produced due to residue addition correlated highly with the sum of the alkalinity added as plant residues (excess cations) and those resulting from mineralization of residue N, with the slope of regression lines decreasing with increase of the initial soil pH. Direct shaking of soil with the residues at the same rate of alkalinity (excess cations) under sterile conditions increased the pH of the Wodjil soil but decreased it in the Lancelin soil. It is suggested that the decarboxylation of organic anions (as indicated by excess cations) of added plant residues and ammonification of the residue N causes soil pH increase whereas nitrification of mineralised residue nitrogen causes soil pH decrease, and that the association/dissociation of organic compounds also plays a role in soil pH change, depending initial pH of the soil. The overall effect on soil pH after addition of plant residues would therefore depend on the extent of each of these processes under given conditions.     

Prolonged elevated atmospheric CO2 does not affect decomposition of plant material

Prolonged elevated atmospheric CO₂ might alter decomposition. In a 90-day incubation study, we determined the long-term (9 years) impact of elevated CO₂ on N mineralization of Lolium perenne and Trifolium repens plant material grown at ambient and elevated CO₂ and low- and high-¹⁵N fertilizer additions. No significant differences were observed in recovery rates between any of the treatments, except an N addition effect was observed for L. perenne (0.4 versus 0.5% day⁻¹ in high versus low N). The results suggest that elevated CO₂ did not change plant N mineralization in any of the soils, because of a surplus of available N in the fertilized and leguminous systems, and because of insignificant plant responses to elevated CO₂ in the low soil N availability systems.     

Constraining the conditions conducive to dissimilatory nitrate reduction to ammonium in temperate arable soils

Here we offer the first assessment of conditions conducive to dissimilatory nitrate reduction to ammonium (DNRA) in temperate arable soils, through an examination of the potential for this process to occur in a range of soils of contrasting characteristics. NH₄¹⁵NO₃ (6.2 g N m⁻², 25 atom % excess ¹⁵N) was applied, and recovery of ¹⁵N in the pool taken as indicative of occurrence of DNRA. Up to 5% of applied ¹⁵N was recovered in the pool 2 d after addition of N, glucose (44.6 g C m⁻²) and l-cysteine (7.7 g m⁻², 0.9 g N m⁻², 2.3 g C m⁻²). concentrations were positively correlated with soil pH, ratio, bulk density, sand content and concentration, but negatively correlated with soil C and organic N content. Our results demonstrate the potential for DNRA to contribute to N cycling in temperate arable soils, but its detection and significance is likely to depend on the provision of a low molecular weight C source.     

A hot-spot approach applied to nitrification in tropical acid soils

Several authors have reported that nitrification in acid soils may be restricted to microsites having a more favorable pH. The aim of this study was to propose a conceptual model of the functioning of nitrification in hot-spots, and to test it with the experimental data obtained in laboratory conditions using twelve tropical unamended and -amended soils with a wide range of pH (from 4.2 to 6.9). Nitrification was also measured in two selected soils where the pH was adjusted from 3.5 to 6.2. The model characterizes the relationship between the nitrification rates in unamended and -amended soils as a function of pH. It is based upon the assumption that nitrification of the coming from N mineralization occurs in the hot-spot (RN_h), and the nitrification of the added occurs in the hot-spot and also in its adjacent surrounding region (RN_s). The experimental design was chosen to be able to estimate both nitrification rates. Soil acidity limited nitrification more in -amended soils than in unamended ones. From our approach, this is due to less favorable conditions for nitrification in the region surrounding the hot-spot. The effect of self-induced acidity on nitrification was not noticeable neither in unamended nor in -amended treatments. The model described well three observations made in the experiments: (i) the minimum pH for nitrification to occur was lower for RN_h (pH<4.2) than for RN_s (pH<4.7), (ii) the RN_h/RN_s ratio increased with the decrease of pH (from 1.5 at pH 6 to 8.5 at pH 4), and (iii) for a given pH, the RN_h/RN_s ratio increased with the decrease of the initial pH of the soil. Among the soil parameters determined in this study (i.e. exchangeable Al, EDTA-extractable Cu and Zn, total C and N), only pH was related to nitrification. However, for a given pH, nitrification varied 3-fold among soils, depending upon their initial pH. This suggests that soil pH as determined on bulk soil is not suitable to predict nitrification in each individual soil, because it is not representative of the acidity level within the hot-spot.     

Evaluation of nitrate leaching from mine tailings amended with biosolids under Mediterranean type climate conditions

Mine tailings are difficult to revegetate due to the lack of organic matter, severe nutrient limitations, and potential metal toxicity. Biosolids has been shown to be favorable for improving properties of mine tailings. The rates of biosolids required to reclaim mine tailings (up to ) may produce conditions where significant amounts of nitrates can leach into groundwater. Leaching column experiments were conducted to determine the influences of biosolids placement and plant cover on nitrate leaching from biosolids-amended mine tailings. PVC columns packed with 7.7 kg of tailings were treated with 168 g of biosolids (approximately 270 kg mineral ). Biosolids were either placed on the surface or mixed with the tailings and half of the columns were seeded with perennial ryegrass (Lolium perenne, L.). Columns were drip irrigated at a rate of 758 mm of water y^-1. This rate was twice the average precipitation for Central Chile. All leachates were collected weekly for up to 21 weeks and analyzed for nitrate, pH, electrical conductivity, and chemical oxygen demand. The electrical conductivity and nitrate concentration of percolates decreased with time, while the pH remained constant. In some cases the percolate had nitrate concentrations greater than the maximum amount allowed for human consumption (10 mg ). Vegetation cover and mixing the biosolids with tailings reduced NO₃-N concentrations in the percolate.     

Prolonged compost curing reduces suppression of Sclerotium rolfsii

Composts have long been recognized to facilitate biological control of soil borne plant pathogens. Composts can introduce biocontrol agents into growth media and serve as a food base for their establishment and activity. Mature biosolids compost (a blend of sewage sludge and yard waste) was found to be suppressive to germination of the sclerotia of S. rolfsii on compost plates and also suppresses the disease development in bean plants (Phaseolus vulgaris L.). Microscopic observations revealed that sclerotia placed on suppressive compost were attacked by mycoparasites. However, prolonged curing of compost negated this phenomenon. This research was aimed to study the changes in chemical and biological properties occurring during prolonged curing and their relation to compost suppressiveness. Correlations were found between the decrease and subsequent loss of suppression of sclerotia germination and the decrease in basal respiration, dissolved organic carbon (DOC) and concentrations, and the increase in concentration and specific UV absorbance. A shift of both bacterial and Ascomycetes populations as a consequence of curing was observed. Interactions between micro-organisms and their chemical environment are discussed.     

Nutrient spatial variability in biogenic structures of Nasutitermes (Termitinae; Isoptera) in a gallery forest of the Colombian ‘Llanos’

By definition ‘ecosystem engineers’ are those organisms capable to modify physically the environment by producing ‘biogenic’ structures (BS). Large macroinvertebrates like termites, earthworms and ants produce BS with distinguishable physico-chemical properties. We measured total C_org, and contents in the BS produced by two species of Neotropical termites (subfamily Nasutermitinae) in a gallery forest (GF) of the Eastern Plains of Colombia. We sampled from the top of the BS to the edge at proportional distances, i.e. 20-100% for the largest BS in the soil surface and 50-100% for the smallest arboricole BS. Control soil was sampled 1 m apart from the BS. Values of total C_org were high in the BS produced by Nasutitermes sp1 (epigeic mound), while a high N mineralization process was observed in the same BS and in the Nasutitermes sp2 arboreal nest. The role of these two ecosystem engineers in nutrient cycling is discussed.