Comparison of laboratory methodologies to determine soil nitrogen mineralization from organic residues

Recycling organic waste for use as fertilizer requires prior knowledge of mineral nitrogen (N) availability for crops. Estimation of soil N release or potentially mineralizable N is an important tool for the design of fertilization strategies that aim to minimize the use of N fertilizer. The aerobic incubation method is considered a standard technique to measure soil potential to mineralize N. In this study, alternative methods of aerobic incubation were evaluated to help overcome its limitations (long time and equipment). In this regard, biological methods (anaerobic incubation at 7 and 14 days) and chemical extraction (hot KCl) procedures were examined. To determine potentially mineralizable N, a silty clay loam soil was fertilized with spent mushroom substrates and anaerobic digestates from different origins (C/N ratio of 4 to 38). Based on the results, chemical extraction emerges as a reliable alternative to the aerobic incubation method, particularly when the C/N ratio of the organic residues ranges from 12 to 15. Moreover, its implementation in routine soil laboratories is straightforward and faster, and it does not require any special equipment.

Effect of Urea Fertilizer on the Biochemical Characteristics of Soil

Urea-potash mixture was added to the manured soil at three different concentrations equivalent to 0.8, 1.6 and 2.4g f urea per 10Kg of soil. Nitrate and nitrite N concentration in the soil increased within 24h after addition of urea. The nitrate N content in soil without urea was 17 µg and in urea fertilized soils, it ranged from 39.9-47 µg/g of soil after 19h. . Increase in total mineralizable N was around 67- 160% in urea fertilized soils in comparison to the control. Percent conversion of urea to nitrate and nitrite N decreased at higher concentrations of the fertilizer. Addition of biochar to urea amended soil did not bring about significant change in the available N content. Decrease in total mineralizable N and accumalation of available P was observed over the period of 15 days. Addition of urea resulted in acidification of the soil. Acidification of the soil could be correlated with increase in acid phosphatase concentration. The soil amended with biochar exhibited significant buffering capacity in the region of pH 7.4-9. Int. J. Appl. Sci. Biotechnol. Vol 7(4): 414-420

Short-term nitrogen dynamics in a soil amended with anaerobic digestate

The co-product of anaerobic digestion, digestate, is nitrogen (N) rich; however, the forms and accessibility of this N by the crops have not been fully explored. This study aimed to determine the mineralization parameters of digestate N and to assess its availability for annual ryegrass (Lolium multiflorum Lam.). Four digestate rates of 0 (control), 38, 75, and 150 mg N kg−1 soil (equal to 0, 90, 180, and 360 kg total N ha−1) were applied to a silty clay loam soil in a completely randomized block design with four replications in a greenhouse study. A 100 d aerobic incubation experiment was also conducted with 0 and 150 mg digestate N kg−1 rates at 25 °C. Digestate feedstock included cattle manure (28%), hay (15%), and silage corn (Zea mays L.; 57%). Total plant biomass and N uptake increased linearly with digestate application rate with average apparent N recovery of 37%. Potentially mineralizable N (N0) and mineralizable N rate constant (k) were not significantly different in digestate and control treatments; however, a flush of digestate organic N (30 mg N kg−1) released right after mixing the digestate with soil. Evidences of N immobilization with digestate application were observed in greenhouse study. Majority of plant-available digestate N was in form of NH4+-N; therefore, NH4+-N can be used for estimation of available digestate N for crops. Results need to be validated for specific feedstock and soil properties under field conditions. Further research is needed to assess how long-term build-up of digestate organic N may impact the N availability for crops.

Soluble organic nitrogen in potentially mineralizable N assays: are we missing an important component?

We examined soluble organic nitrogen (SON) leached from long-term, sequentially leached, aerobic incubations. Leached SON, present in all depths (0–60 cm), ranged from 35% to 56% of total nitrogen (N). This unaccounted-for SON may have important implications in the estimation of plant available N and the potential for environmental N losses.

Nitrogen Mineralization Potential (No) in Three Kenyan Soils, Nitisols, Ferralsols and Luvisols

Nitrogen mineralization potential is important so as to prevent over-fertilization that could lead to groundwater contamination or under-fertilization that could lead to poor nutrient provision by crops leading to low yields. Three soil types were selected on the basis of groups, agro-ecological zone, organic matter content and land use. The soil samples were taken from the 0-15 and 15-30 cm depth. The samples were placed in incubation bags, water added to field capacity, sealed and incubated in laboratory at room temperature. The bags were opened at intervals of two weeks and soil sub-samples taken for analysis of mineral N for a period of 17 weeks. The calculated mineralizable N was 138.8 μg N and 116.4 μg N/g for Gituamba andosols, 46.0 μg N and 46.4 μg N/g for Kitale ferralsol and 260.1 μg N and 197.3 μg N/g soil for Katumani luvisols in the 0-15 and 15-30 cm depth, respectively. These calculated values compared well with the actual cumulative mineralizable N for Gituamba andosols at 127 μg N and 74.1 μg N/g, for Kitale ferralsols at 48.0 μg N and 64.1 μg N/g and for Katumani 80.6 μg N and 47.7 μg N/g soil in the 0-15 and 15-30 cm depth, respectively. The time taken for 50% of potentially mineralizable N to be mineralized (t½) ranged from 6.3 weeks for Katumani luvisols 15-30 cm to 30.1 weeks for Kitale ferralsols 0-15 cm soil depths. The soils with highest rate constant (k) had the least. For example, 15-30 cm depth of Katumani luvisols with of 6.3 weeks had the highest k of 0.112 week-1 compared with Kitale ferralsols 0-15 cm depth with t½ of 30.1 weeks and the lowest k of 0.023 week-1. The observed data indicates that 50% of N would be mineralized in all the soil types with the exception of Kitale ferralsols (0-15 cm depth) within the growing period of the crops which is approximately 20 weeks.

Depth distribution of mineralizable nitrogen pools in contrasting soils in a semi-arid climate

A better understanding of the depth distribution of soil mineralizable nitrogen (N) pools is important to improve prediction of net soil N mineralization. However, our understanding of the depth distribution of these N pools under the semi-arid conditions of western Canada is limited. This study examined the depth distribution of soil mineralizable N pools (kS, the rate constant of a nondepleting zero-order stable N pool, and NL, the size of a depleting first-order labile N pool) of six sites in western Canada chosen to vary with respect to soil zone, soil texture, and cropping system. The depth distribution of mineralizable N pools varied substantially among sites, indicating that this distribution needs to be considered in making predictions of net soil N mineralization. A single regression equation including soil total nitrogen (STN), Pool I (a labile mineralizable N pool determined through a 14-day aerobic incubation), and soil pH explained 67% of the variation in kS across sites and soil depths. In addition, 95% of the variation in NL was explained by a regression model with Pool I. Thus, although the depth distribution of soil mineralizable N pools can vary substantially among sites, the mineralizable N parameters can be adequately predicted across sites and soil depths from simple soil properties. Comparison with a study using surface soils under humid conditions in New Brunswick suggests that the relationship between NL and Pool I is applicable across a wide range of soils, climatic zones, and cropping systems, whereas the regression model to predict kS varied with climatic zone, perhaps reflecting different pedogenic processes stabilizing the organic matter in these climatic zones.