Joost Salomez

Modeling Soil Moisture Effects on Net Nitrogen Mineralization in Loamy Wetland Soils

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°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.

Within-field variability of mineral nitrogen in grassland

Abstract The within-field variability of soil mineral nitrogen (Nmin) in a grazed grassland of 8000 m2 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, before 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 Nmin 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 Nmin, the number of samples needed to estimate the average Nmin in a field could be calculated for different probabilities and various degrees of precision. From the spatial distribution of the Nmin concentrations and the restrictions imposed by the new European decree, adapted fertilizer strategies can be proposed at least for places where systematically higher Nmin concentrations can be expected.

NITRATE EXTRACTION FROM FRESH PLANT MATERIAL BY MEANS OF A METHANOL:WATER EXTRACTION SOLUTION

Nitrate determination from fresh plant material is common in routine analysis, but comparisons between extraction solutions, detection techniques or combinations of both show that there is a wide range in performance in methods determining plant nitrate. In this paper, the performance of methanol:water (1:1, V:V) as an extraction solution was compared against water and KCl as extractants. Measurement results of methanol:water extracts were consistently higher than the water or KCl results. Even compared to a recommended nitrate determination method, using dried material, significantly higher results were obtained with methanol:water as an extraction solution. This has consequences on the interpretation of internal quality of produce, like some vegetables, where maximum allowable nitrate concentrations are put forward in various countries. Furthermore, the results of the methanol:water extracts of two common determination techniques, i.e., segmented flow analysis and HPLC detection technique, were highly comparable.

A soil temperature/short-wave radiation growth model for butterhead lettuce under protected cultivation in Flanders

ABSTRACT The objective of this study was to provide Flemish greenhouse farmers an accurate growth model for butterhead lettuce, based on two environmental parameters, i.e. soil temperature and short-wave radiation. During two consecutive years, a total of 27 growth experiments were followed up, whereby head fresh weight (at a 14 d interval), soil temperature at 10 cm depth (on a half-hour basis), and short-wave radiation (14 d summation) were measured. Separate Gompertz functions, with either radiation or soil temperature as input variable, accurately modelled growth; but via a combined approach, an almost perfect fit (R2 = 0.91) between measured and simulated head fresh weight was obtained. This modelling approach provides lettuce growers a tool for the quantitative estimation of crop weight in relation to changes in soil temperature and short-wave radiation. Weather forecast (radiation) and managerial decisions (soil temperature), now serve as the input data of a scientifically based lettuce growth model.

Effect of organic fertilizers on nitrate accumulation in vegetables and mineral nitrogen in tropical soils of Morogoro, Tanzania

Nitrogen (N) nutrition is a key factor for vegetable growth and yield. However, different rates of nitrogen fertilization may trigger different responses to vegetables. A survey was conducted to investigate the effect of soil fertility management practices on nitrate concentration in vegetables. The survey results were used to plan experiments on the effect of chicken and cattle manures on nitrate levels in Chinese cabbage (Brassica rapa) and amaranthus (Amaranthus cruentus) grown in Tanzania and the patterns of mineral nitrogen in soils under open field conditions. Chicken or cattle manure at 200, 300 kg N ha(-1) and 170 250 kg N ha(-1) for Chinese cabbage and amaranthus respectively, and control were compared in a randomized complete block design. We observed a higher nitrate content in fertilized Chinese cabbage at day 30 than at day 44 after sowing, ranging from 3243 to 4993 mg kg(-1) fresh matter regardless of the N source and rates. Only application of manures at high levels (250 kg N ha(-1)) induced significantly (p < 0.05) higher nitrate contents in amaranthus at day 28 after sowing, although there was a clear indication of nitrate accumulation even at 170 kg N ha(-1) application. Soil NH(4)(+)-N + NO(3) -N in both Chinese cabbage and amaranthus plots were increased with increasing N application rates and differences between control and amended soils were significant (p < 0.01). There was a positive relationship between NO(3)(-) concentration in vegetables and NO(3) -N in the rooted top soil layer (0-15 cm). However, higher NH(4)(+) concentrations depressed NO(3) build up in crops and a significant negative relationship between soil (NH(4)(+)-N)/(NO(3)(-)-N) ratio and crop NO(3)(-) content was found. It is concluded that low manure application rates result in similar yields to high rates but reduces nitrate accumulation in vegetables and excess mineral nitrogen in soils.

A Future for Vegetable Cultivation Within the Flemish Manure Decree

Abstract Most vegetables, despite N fertilization recommendations, are heavily fertilized, leaving high NO3 −‐N residues in the soil profile at harvest. This contrasts with the Flemish Manure Decree stating that the NO3 −‐N residue in the soil profile (0–90 cm) must remain below 90 kg ha−1 during a fixed curfew (i.e., October 1st until November 15th). This study was conducted to examine the feasibility of this prevailing standard for properly fertilized open‐air vegetables (cauliflower, leek, lettuce, etc.). During 3 consecutive yr (i.e., 2000–2002), a scientifically sound N fertilization recommendation, based on an Nmin measurement in the soil profile, was given for 136 experimental fields. Yield and product quality, as well as the NO3 −‐N residue, were determined for each plot, and results were compared with a control plot. The results showed that during the curfew, only 44.9% of the experimental plots remained below 90 kg NO3 −‐N ha−1 as demanded by the Flemish Manure Decree. For the years 2000 and 2001, even at the lowest NO3 −‐N‐residues, no significant loss of yield and quality was observed for the harvested vegetables. However, results of the year 2002 showed distinct losses for both yield and quality, especially for cauliflower and leek. The results further indicated that for the vegetables mentioned above, which have a limited rooting depth and root development, the excessive NO3 −‐N rates are partly linked with a high organic matter content in the soil and/or a labile organic matter pool, giving rise to large uncertainties in N fertilization recommendations. Thus, it appears to be rather difficult to meet the needs of the market for fresh vegetables, especially during late summer and early autumn, and meet the requirements of the manure decree. Furthermore, lowering the NO3 −‐N residue in the soil after harvest will require specific measures and will result in an increase of the production cost for Flemish vegetables.

Nitrogen Mineralization of Two Manures as Influenced by Contrasting Application Methods under Laboratory Conditions

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 (NO3 −) N and ammonium (NH4 +) 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 NO3 −-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.

Sampling strategy for mineral nitrogen in a greenhouse soil

Introduction Greenhouse crops typically have a high net economic added value, whereby suboptimal cultivation conditions have repercussions on the economic return. Especially mineral nitrogen (Nmin), being the most important macro-nutrient, is required in a sufficient amount and uniform spatial distribution, as crop responses to variability in mineral nitrogen exert their effect on both yield and quality (Roberson, 2000). As the natural Nsupplying process, i.e. mineralization from soil organic matter, seldom provides enough nitrogen for economic crop production, fertilizers are necessary to make up the deficiency. Quantitative studies on the availability of soil N to plants (N-fertilizer advices) require best estimates of mean mineral N concentrations and a measure of the reliability of those estimates. Therefore, bulking is practiced to minimize the variability that exists over short distances between soil cores collected in a sample area (Wollenhaupt et al., 1997). Although it was stated already 40 years ago that a mere concern with the quantity of nitrogen is not sufficient for optimum plant production (Harmsen and Kolenbrander, 1965), the uniform management of N across fields with variable soil and plant N relations remains common practice. As greenhouse soils are expected to have a low spatial variability because only mineral fertilizer are applied and, on the contrary to e.g. grazed pastures and arable land, neither excreta nor organic amendments introduce supplementary variability (Bogaert et al., 2000) only a limited number of investigations have been performed concerning the spatial variability and its effect on sampling strategy for mineral nitrogen in greenhouse soils. The aim of this investigation was to calculate the number of soil samples necessary to obtain an acceptable mean value for this nutrient constituent. Classical statistical analysis of the data was performed to accomplish this objective.