Paola Gioacchini

Influence of urease and nitrification inhibitors on N losses from soils fertilized with urea

Abstract. The aim of this study was to evaluate how the N losses through volatilization and leaching from soils fertilized with urea can be affected by the application of a urease inhibitor or a urease plus a nitrification inhibitor. The experiment was carried out using lysimeters with 15N-labelled urea and N-(n-butyl) thiophosphoric triamide (NBPT) as urease inhibitor and dicyandiamide (DCD) as nitrification inhibitor, comparing three different treatments: urea alone (U), urea + NBPT (UN) and urea + NBPT + DCD (UND). Both volatilization and leaching were significantly different in the soils used, according to their physico-chemical characteristics. However, the pattern of the loss was similar: the volatilization was significantly reduced by NBPT (UN), but the presence of DCD (UND) significantly increased the loss, with respect to UN. Considering leaching, the highest amount of NO3– was lost with UND, the lowest with U. The greatest amount of N lost by leaching was soil-derived N produced by the N mineralization-immobilization turnover. We suggest that, by maintaining the NH4+ in the soils, the inhibitors, in particular DCD, caused a priming effect with a subsequent increase in the rate of soil organic matter mineralization and an extra release of soil organic N. The priming effect was real in the sandy loam (SL) soil where a net N release was observed, whereas in the clay loam (CL) soil the effect of the inhibitors was less pronounced and an apparent priming effect was observed; however, a real priming effect also cannot be excluded in this soil.

Effects of the urease inhibitor N-(n-butyl)phosphorothioic triamide in low concentrations on ammonia volatilization and evolution of mineral nitrogen

Laboratory incubation experiments were conducted to study the influence of increasing concentrations of N-(n-butyl)phosphorothioic triamide (NBPT) on NH3 volatilization and rate of urea hydrolysis and evolution of mineral N in Ozzano, Rimini and Carpi soils with different physicochemical characteristics. Low concentrations of NBPT reduced NH3 losses due to volatilization after urea fertilization and the effectiveness of the inhibitor was related to the soil characteristics (e.g. high concentrations of organic C and sand). After 15 days of incubation, no significant reductions of losses were found for any of the NBPT concentrations employed in Rimini soil. The application of NBPT led to a considerable reduction of the formation of nitrite. This process was completely annulled with the highest dose of NBPT (0.5% w/wurea) in the Carpi soil after 15 days. In Rimini soil, however, the use of NBPT was less effective in influencing nitrite formation. The use of NBPT favoured accumulation of nitrate proportional to the NBPT concentration employed while it had no influence on the NHinf4sup+fixation by 2:1 layer silicates. The data obtained support previous evidence that NBPT is effective in reducing the problems encountered in using urea as fertilizer. However, environmental conditions and soil physicochemical characteristics may have an important influence on the effectiveness of NBPT.

Dynamics of Mineral Nitrogen in Soils Treated with Slow‐Release Fertilizers

Abstract In this study, five different slow‐release fertilizers were incubated in two soils, and their nitrogen (N) dynamics was followed for 4 months. Four of the fertilizers used were organic: two were hydrolyzed fur, one was made by hydrolyzed fur, farmyard manure, horn, and hoofs (Endurance), and one contained synthetic organic N [isobutylidendiurea (IBDU)]. The fifth, on the contrary, was a mineral fertilizer (Entec 26) containing both nitrate (NO3 −) and ammonium (NH4 +) and a nitrification inhibitor (3,4 DMPP). The two soils used were a sandy loam (SL) and a clay loam (CL). The fertilizer‐N release was conditioned by both the fertilizers and the soil characteristics. The highest accumulation of mineral N as NO3 − was observed in the SL soil for all the treatments. The lowest net N release was observed in both soils with End, which was found to be the most resistant to microbial attack and degradation. The two fur‐based fertilizers released similar amounts of N, which was higher in the SL soil than in the CL soil. The highest accumulation of fertilizer‐derived N in the CL soil resulted from IBDU; however, the net accumulation of N in the SL soil with this fertilizer was even higher than the amount of N initially added. This fact was attributed to an increase in mineralization of the soils native organic N. A real positive “priming effect” was also observed in the SL soil with E26. On the contrary, E26 caused a low level of N accumulation in the CL soil.

Fate of 15N derived from soil decomposition of abscised leaves and pruning wood from apple (Malus domestica) trees

Abstract The fate of nitrogen (N) derived from soil incorporating 15N-labeled apple (Malus domestica) leaves and wood from pruning (hereafter referred to as “pruning wood”) was studied in an 8-month pot experiment. The net mineralization of N was measured as 15N recovery in perennial ryegrass (Lolium perenne) that was allowed to grow in soils amended with residues < 2 mm in size (litter : soil ratio, w/w, 1:250 for leaves and 1:330 for wood). The immobilization of native soil N as a consequence of residue addition was measured by comparing the amount of total N taken up by ryegrass in residue-amended soil and in control soil. Net immobilization of soil N occurred during the first 2 months after litter addition and was especially high in the soil amended with leaf litter. During the period of soil N immobilization, the amount of soil microbial N was high in the soils treated with both types of residues, while that of mineral N was markedly reduced only in the leaf-litter-amended soil. Net N uptake from the control soil almost stopped after 3 months of plant growth, while ryegrass in the litter-amended soil continued to take up N, indicating a likely release of previously immobilized N. Net mineralization of the 15N from apple residues was slow during the first 2 months after their incorporation and then increased. In total, 6% (leaves) and 12% (wood) of the N added via residues underwent mineralization, while 67% (leaves) and 85% (wood) were found in the extractable soil N pool (humic and fulvic acids and non-humified fractions). The data indicated that, even if N was incorporated into the soil, apple leaves and pruning wood did not mineralize significant amounts of N in the short term. The evidence suggested that during the decomposition of both types of apple residues the N originally present was incorporated into the stable soil N pool.

Seasonal changes in microbial nitrogen in an old broadleaf forest and in a neighbouring young plantation

Soil microorganisms are actively involved in many processes of the soil N cycle and are strong competitors with plants for soil N. Therefore, microbial dynamics are important factors in controlling forest productivity. Nevertheless, they are poorly studied especially in relation to forest age, which can produce strong effects on the microbial community by affecting the forest floor environment. In the present study, seasonal variations of soil microbial N (Nmic) were monitored in an old floodplain hardwood forest (270 years) and in a young hardwood plantation (19 years) in two soil horizons (0–15 and 15–30 cm). Although the differences according to time of sampling and soil horizon were statistically significant, Nmic was significantly higher in old than in young forest, especially for the deeper soil layer. However, the highest percentage of total N (Ntot) immobilised in microbial biomass was found in the surface soil layer of the young plantation. Soil organic C (Corg) explained 23% of the spatial–temporal variation of Nmic over all sampling periods in the old forest, whereas the linear combination of Ntot, total extractable soil N (Ntotex) and the C/N ratio explained 59% of variation in Nmic when considering only the growing season. In contrast, Corg and Ntotex explained 59% of variation in Nmic in the young stand when considering all sampling periods and 75% when the analysis was limited to growing season. Soil moisture did not show any significant correlation with Nmic in either site. The sensitivity of Nmic to variation in Corg and Ntot seems to be affected by forest age, being higher in young than in old forest. Finally our results indicate that during the growing season, when the Ntotex availability is low, the dynamics of Nmic and Ntotex are temporally interdependent, suggesting the existence of a reciprocal control whose mechanisms deserve to be elucidated.

Isotopic–geochemical study of nitrogen and carbon in peat from the Tunguska Cosmic Body explosion site

Abstract Isotopic–geochemical investigations were carried out on peat samples from the 1908 Tunguska Cosmic Body (TCB) explosion area. We analyzed two peat columns from the Northern peat bog, sampled in 1998, and from the Raketka peat bog, sampled during the 1999 Italian expedition, both located near the epicenter of the TCB explosion area. At the depth of the “catastrophic” layer, formed in 1908, and deeper, one can observe shifts in the isotopic composition of nitrogen (up to Δ 15 N = +7.2‰) and carbon (up to Δ 13 C = +2‰) and also an increase in the nitrogen concentration compared to those in the normal, upper layers, unaffected by the Tunguska event. One possible explanation for these effects could be the presence of nitrogen and carbon from TCB material and from acid rains, following the TCB explosion, in the “catastrophic” and “precatastrophic” layers of peat. We found that the highest quantity of isotopically heavy nitrogen fell near the explosion epicenter and along the TCB trajectory. It is calculated that 200,000 tons of nitrogen fell over the area of devastated forest, i.e., only about 30% of the value calculated by Rasmussen et al. (1984) . This discrepancy is probably caused by part of the nitrogen having dispersed in the Earth’s atmosphere. The isotopic effects observed in the peat agree with the results of previous investigations (Kolesnikov et al., 1998a, 1998b, 1999; Rasmussen et al., 1999) and also with the increased content of iridium and other platinoids found in the corresponding peat layers of other columns (Hou et al., 1998, 2000) . These data favor the hypothesis of a cosmochemical origin of the isotopic effects.

Dynamics of nitrogen uptake and partitioning in early and late fruit ripening peach (Prunus persica) tree genotypes under a mediterranean climate

The dynamics of N uptake and N partitioning in peach (Prunus persica, Batsch) trees of a very early (cv. Flordastar) and a very late (cv. Tudia) fruit ripening varieties grown under a mediterranean climate was assessed during one season. Labelled N was applied to two-year old potted trees which were destructively harvested at regular intervals during the vegetative and reproductive cycle. Tree phenology as well as vegetative and reproductive growth of the two genotypes strongly differed: bud burst started in late January in Flordastar and late March in Tudia. Leaf senescence in Flordastar was almost complete by mid October, while Tudia still retained a significant fraction of leaves at the December harvest. Fruit yield averaged 4.0 and 6.9 kg tree−1 (fresh weight) in cv. Flordastar and Tudia, respectively, and fruit size was within commercial standards for the two genotypes. After growth resumption, shoot and fruit growth mainly relied on N remobilised from reserves, which accounted for 72–80% of total N in new growth. Nitrogen uptake by both genotypes was relatively low in the first month after bud burst, then was more rapid until the end of the season. Total labelled N uptake did not differ between the two genotypes and accounted on average for 65–70% of total N supplied. The kinetics of labelled N uptake were similar in the two varieties despite the great difference in the timing of their fruit ripening. Leaves were the main sink for N during much of the experimental period. The fruits, when present, also used a significant fraction of the absorbed N, which was almost constant until fruit ripening in Flordastar. Nitrogen partitioning to leaves declined progressively after summer, when a greater fraction of the absorbed N was recovered in the twigs, the trunk, the fine roots and especially in the coarse roots. The data provide evidence for guiding the kinetics of N supply to peach orchards under a Mediterranean climate.

Nitrogen dynamics and microbial response in soil amended with either olive pulp or its by-products after biogas production

Olive pulp (OP), the residual material of a two-phase olive oil extraction system, and effluents from hydrogen (EH2) and methane (ECH4) production, have been evaluated as soil amendments particularly for their impact on soil mineral nitrogen (N) dynamics, gross N mineralization, and soil microbial biomass N (Nmic). Both N transformation and microbial growth were mainly influenced by the amount and quality of added organic carbon (C). Both OP and EH2, which contain more carbohydrates and lipids than polyphenolic compounds, stimulated NO3− immobilization during the early incubation period and increased Nmic, saprophytic fungi, and N mineralization. On the contrary, soil amended with ECH4, which is characterized by the lowest C content but the highest content of polyphenolic compounds, behaved as the control; neither NO3− immobilization nor microbial growth were observed and gross N mineralization was stimulated only at the beginning of the incubation period. Bacterial plate count was significantly correlated with direct bacterial count and fungal count was correlated with Nmic. Therefore, it is suggested that both bacterial and fungal plate counts may be used as indicators of the overall bacterial and fungal populations inhabiting soil, respectively. The knowledge of the impact of these materials on soil N dynamics is crucial for their correct use in agriculture because it has been shown that NO3− availability can be strongly influenced by the addition of different amounts and quality of organic amendment.

Activity and stability of urease-hydroxyapatite and urease-hydroxyapatite-humic acid complexes

In agricultural calcareous soils, hydroxyapatite (APA) may well represent an important support for urease immobilisation and could be present in both mineral and organo-mineral complexes. In this paper we studied the formation of APA-urease-humic acid (HA) complexes after the addition of urease either before or after HAs. We then proceeded to evaluate the role of HAs on the activity and stability of the complexes as compared to the APA-urease complexes and free urease. When increasing amounts of HAs were added after urease, they did not affect the activity of the enzymes that had already adsorbed onto the complexes. On the contrary, adding the same amount of HA before the enzyme caused a significant reduction in the amount of enzyme adsorbed. However, when urease adsorption onto the APA-HA complexes was carried out in the presence of NaCl, the enzyme activity of the complexes increased sharply to 86% of the initial activity. The immobilisation of the enzyme on the support increased urease stability against pronase treatment as well as directly in soil over time. The inhibition of urease activity by Cu2+ was reduced by urease immobilisation. However, the presence of HA did not alter the stabilisation capability of APA when alone.

Isotopic discrimination during litter decomposition and δ13C and δ15N soil profiles in a young artificial stand and in an old floodplain forest

In the present study, rates of litter decomposition and microbial biomass nitrogen were monitored over an 8-month period in a young broadleaf plantation (18 y) and in an old floodplain forest. Moreover, δ13C and δ15N temporal variations within soil profiles were evaluated at both sites. Rates of litter decomposition were higher in spring and autumn than in summer, in both forests. At the end of the observation period the percentage of original litter remaining was not statistically different between the young and the old forest and accounted for 60–70% of the original amount. Microbial biomass nitrogen in the remaining litter and the percentage of litter mass lost during decomposition were positively correlated. The difference in litter quality affected the decomposition rate and also the changes in carbon isotopic composition during the decomposition process. In contrast, 15N isotopic signatures showed a similar trend in the litter of the two forests irrespective of the litter quality. Although δ13Csoil and δ15Nsoil showed considerable temporal variation they increased with depth in the soils of both sites but their seasonal changes did not reflect those of the decomposing litter. Within the same soil horizon, both δ13C and δ15N showed similar seasonal trends in the soils of the two forests, suggesting the involvement of environmental factors acting at regional level, such as soil temperature and rainfall variations, in regulating seasonal δ13C and δ15N soil variations. # Revised version of a paper presented at the 1st Joint European Stable Isotope Users Group Meeting (JESIUM), August, 30 to September, 3, 2004, Vienna, Austria.