Nutrient dynamics in wetland ecosystems are largely controlled by soil moisture content. Therefore, the influence of soil moisture content on N mineralization should be explicitly taken into account in hydro-ecological models. The aim of this research was to establish relationships between N mineralization and soil moisture content in loamy to silty textured soils of floodplain wetlands in central Belgium. Large undisturbed soil cores were taken, incubated for 3 months under various moisture contents, and zero order and first order N mineralization rates were calculated. We used the percentage water-filled pore space (WFPS) as an expression of soil moisture because it is a better index for aeration dependent biological processes than volumetric moisture content or water retention. The relationship between the N mineralization rate and %WFPS was described by a Gaussian model. The optimum WFPS for N mineralization ranged between 57% and 78%, with a mean of 65% +/- 6% WFPS. Expected annual net N mineralization rates at field temperature (9.7 degrees C) and at optimal moisture content varied between 30 and 186 kg N ha(-1) (0-15 cm depth) year(-1), with a mean of 110 +/- 42 kg N ha(-1) (0-15 cm) year(-1). The mean N turnover rate amounted to 2.3 +/- 1.1 g N 100 g(-1) N year(-1). Multiple linear regressions between N mineralization and general soil parameters showed that soil structure has an overriding impact on N mineralization in wetland ecosystems
The within-field variability of soil mineral nitrogen (N-min) in a grazed grassland of 8000 m(2) was examined. NO3--N concentrations were characterized by a high spatial variability. This can be explained by the uneven deposition of animal excreta. All NH4+-N as well as NO3--N values were lognormally distributed, be fore and after the grazing season. At the end of the grazing season the largest part of the variability of NO3--N was found for NO3--N concentrations measured within a distance of a few metres. A high variability for NO3--N over very short distances was also indicated by a large nugget variance. During the grazing season, observed mean N-mn, values increased from 22 to 132 kg N ha(-1). Regions with clearly higher NO3--N concentrations could be identified. These zones matched with the drinking place and the entrance of the pasture, places which were more frequently visited than others. High residual N levels in autumn led to relatively high losses of N, mostly by leaching, during the subsequent drainage period. Knowing the variability of N-min, the number of samples needed to estimate the average N-min in a field could be calculated for different probabilities and various degrees of precision. From the spatial distribution of the N-min concentrations and the restrictions imposed by the new European decree, adapted fertilizer strategies can be proposed at least for places where systematically higher N-min concentrations can be expected
The effect of soil fumigation on N mineralization and nitrification needs to be better quantified to optimize N fertilizer advice and predict NO(-)(3) concentrations in crops and NO(-)(3) leaching risks. Seven soils representing a range in soil texture and organic matter contents were fumigated with Cyanamid DD 95 (a mixture of 1,3-dichloropropane and 1,3-dichloropropene). After removal of the fumigant, the fumigated soils and unfumigated controls were incubated for 20 wk and N mineralization and nitrification were monitored by destructive sampling. The average short-term N mineralization rates (k(s)) were significantly larger in the fumigated than in the unfumigated soils (P = 0.025), but the differences in k(s) between fumigated and unfumigated soils could not be related to soil properties. The average long-term N mineralization rates (k(l)) were slightly larger in the fumigated soils but the difference with the unfumigated soils was not significant. Again, the differences in k(l) values could not be related to soil properties. Nitrification was inhibited completely for at least 3 wk in all soils, and an effect on nitrification could be observed up to 17 wk in one soil. An S-shaped function was fitted to the nitrification data corrected for N mineralization, and both the rate constant (gamma) and the time at which maximum nitrification was reached (t(max)) were strongly correlated to soil pH. However, since no correlations were found between the effect of fumigation on N mineralization and soil properties, taking into account the effects of fumigation in fertilizer advice and in the prediction of NO(-)(3) leaching risks will need further research.
The decomposition and the associated nitrogen (N) dynamics of organic N sources are affected by their contact with soil. While several authors have examined the effect of surface application or incorporation of crop residues on their decomposition rate, less information is available about the relationship between the placement of animal manure and their N mineralization rate. This study investigated the influence of chicken manure and cattle manure placement on soil N mineralization. The manures were incorporated or surface applied at 175 mg N kg −1 , and N release was periodically determined over 56 days by measuring inorganic N [nitrate (NO 3 − ) N and ammonium (NH 4 + ) N] in a 2 M potassium chloride (KCl) extract at a ratio of 1:10 (w/v). Results indicated that the control soil released a maximum of 64 mg N kg −1 soil at day 21, a sixfold increase over the initial concentration, which indicates its substantial mineralization potential. Manure treatments showed an initial increase in net NO 3 − -N content at the start of the experiments (until day 7) before an extended period of immobilization, which ended at day 21 of the incubation. A small but positive net N mineralization of all manures was observed from 28 days of incubation. At each sampling time, the mean mineral N released from the control was significantly less (P < 0.01) than surface-applied chicken manure, incorporated chicken manure, and surface-applied cattle manure. Treatments exceptions were at days 21 and 28 where N immobilization was at its peak. In contrast, incorporation of cattle manure showed a different N-release pattern, whereby the mineral N amount was only significantly greater than the control soil at days 42 and 56 with 84 and 108 mg N kg −1 soil respectively. Incorporation of chicken manure and cattle manure did not favor nitrification as much as surface application and cattle manure caused a much greater immobilization when incorporated than when surface applied.
Leafy vegetables are the major source of dietary nitrate intake of humans. Nitrates can have many detrimental effects on human health besides some beneficial effects. A further reduction in nitrate concentration thus can represent added value for vegetable products rich in carotenoids, vitamins C and E, selenium, dietary fiber, plant sterols, and so on. Though nitrate accumulation is governed by a plenitude of factors (i.e., genetic setup, environmental conditions and nutrients), the factor most readily controllable is nitrogen nutrition. This study examined the dependence of butterhead lettuce crops' nitrate concentration on soil mineral nitrogen (N) content. It was shown that the effect of the soil's mineral N content at harvest was strongly associated with the nitrate concentration of lettuce at a low to intermediate mineral N content (,100 kg N ha 21 ). This demonstrates the importance of N-fertilization practices. Results of 24 comparative experiments, based on two different N-fertilization recommendation procedures, did show that an N application difference at the start of the growing period has an effect on the nitrate concentration at harvest. A lower N application at either recommendation resulted in 71% of the experiments (17/24) having a lower leaf nitrate concentration. The head weight was negatively affected in only 2 of these 17 experiments. Minimizing the N input and hence lowering the soil N content at harvest points to the possibility of further reducing the nitrate concentration level in greenhouse lettuce, while having no significant negative effect on economic yield.
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