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Recommendations for nitrogen (N) fertilization are based on the analysis of the soil mineral N (SMN) content of one composite sample that is assumed to represent the average concentration, but that almost by definition fails to properly represent the inherent spatial variability of the field. Improvement in N management can thus be achieved by high-resolution measurements of the soil N status. However, soil sampling, transportation, sample preparation and laboratory analysis are laborious, costly and time-consuming. Additionally, errors associated with all these steps (particularly sampling and sample preparation) are notorious sources of inaccuracy. In situ and on-line measurement modes could overcome these drawbacks. This paper presents a state-of-the-art review of proximal soil sensing technologies for in situ and on-line measurements of total nitrogen (TN) and mineral N in soils. Visible and near-infrared spectroscopy (vis-NIRS) and mid-infrared spectroscopy (MIRS) have been successfully used in in situ and on-line measurement platforms for determining TN. The review also focuses on data fusion (DF) approaches not only to improve the prediction performance of TN by vis-NIRS and MIRS but also to delineate management zones for site-specific N management. Electrochemical sensors such as ion-selective electrodes (ISE) or ion-selective field-effect transistors (ISFET) were found to be the most suitable to measure SMN. However, it was found that both sensor technologies are difficult to use for on-line measurements because they require the preparation of a soil solution and sufficient time delay is needed to reach a stable plateau. These limitations restrict the implementation of electrochemical sensing tools for site-specific N management, although they are the best methods to measure SMN, a needed parameter as a direct measure to determine N recommendations. Research on proximal soil sensing (PSS) for N measurement in soils should focus on the development of robust and accurate in situ and on-line sensors for SMN, whereas improvement in the performance of vis-NIRS and MIRS for TN prediction by using advanced modelling and DF techniques is recommended. This development is essential to promote the implementation of variable rate N fertilization (VRNF). The review suggested the fusion of on-line collected soil data with crop data, present and historical yield, topography and weather conditions as the optimal solution for map-based and sensor-based VRNF scenarios. Furthermore, a new approach for VRNF, designated as the map-sensor-based approach, is proposed. In this approach, management zones (MZ) maps for the soil is developed in advance at the beginning of the cropping season, and this information is combined with current crop growth status [e.g., normalized differential vegetation index (NDVI)], measured with a proximal crop sensor to calculate the recommendation and implement VRNF in real-time. [ABSTRACT FROM AUTHOR]

In a forest on sandy, metal polluted soil, we examined effects of six tree species on litter decomposition rates and accompanied changes in metal (Cd, Zn) and nutrient (base cations, N, C) amounts. Decomposition dynamics were studied by means of a litterbag experiment lasting for 30 months. The decomposition peak occurred within the first year for all tree species, except for aspen. During litter decomposition, high metal litter types released part of their accumulated metals, whereas low metal litter types were characterized by a metal enrichment. Base cations, N and C were released from all litter types. Metal release from contaminated litter might involve risks for metal dispersion towards the soil. On the other hand, metal enrichment of uncontaminated litter may be ecologically relevant as it can be easily transported or serve as food source. [Copyright &y& Elsevier]

The effect of straw and vinasses on the N mineralization–immobilization turnover of celery residues was investigated in a field experiment in Flanders (Belgium). The field was laid out in raised beds on a loamy sand soil. At the start of the experiment, 15N-labelled celery residues (4t dry matter (DM)ha−1) were mixed with straw (12tDMha−1) in order to immobilize the released celery-N followed by an incorporation of vinasses (4tDMha−1) after 200 days aiming to remineralize the immobilized N. Total N, mineral N and their 15N enrichments as well as microbial biomass N were determined at regular time intervals. During the first 62 days after the incorporation, straw immobilized the celery derived 15N in the microbial biomass and reduced the total celery-15N losses from the top 25cm by 38%. However, after 62 days, the N immobilization ceased due to low temperatures (<10°C) in the raised beds, and was followed by natural remineralization of immobilized celery-15N (without addition of vinasses) at a moment when the risk of nitrate leaching was still high. Hence, straw was not able to reduce the celery-N losses during the complete winter period. The addition of vinasses in spring caused no real positive priming effect, although it did increase the amount of remineralized celery-15N (7.6% of celery derived 15N) compared to the straw treatment without vinasses (1.7% of celery derived 15N) probably due to an apparent added nitrogen interaction caused by displacement reactions with the soil microbial biomass. In conclusion, in raised beds, it is not possible to reduce nitrate leaching and to achieve a synchronization between the N release from crop residues and N demand by the following crop by using straw and vinasses to manipulate the N release from crop residues. [Copyright &y& Elsevier]

Following mineralization of N-rich crop residues, large amounts of mineral N can be released into the soil. Manipulating N mineralization of crop residues may be an option to reduce NO3- in soil. The potential to manipulate the N release from vegetable crop residues by using organic wastes was tested under field conditions. At the start of the experiment, cauliflower (Brassica oleracea L. var. botrytis L.) residues (~73 Mg fresh matter ha 1) together with an immobilizer waste (~5 Mg C ha-1 of straw, green waste compost, sawdust, or paper sludge) were incorporated into a silt loam soil. After 154 d, a remineralizing waste (~1 Mg C ha-1 of vinasse or dairy sludge) was incorporated. During the field experiment, the mineral N content in the soil was measured at regular time intervals, and net N release, NO3- leaching, and denitrification were simulated using a N mineralization-immobilization model coupled to a N3- leaching model. Straw, green waste compost, and sawdust were able to immobilize between 54 and 68% of the N released by the cauliflower residues and reduced N3- leaching by 56 to 68%. Paper sludge released an extra amount of N due to its low C/N ratio. No consistent remineralization of N could be found in any of the treatments, probably due to an unsuitable composition of the remineralizer wastes. Manipulating N release of N-rich crop residues by using organic wastes may be a suitable method to reduce N3- leaching; however, stimulating remineralization of immobilized N by the start of the following spring may not be easy to achieve. [ABSTRACT FROM AUTHOR]

• Vis-NIRS was evaluated for predicting soil nitrogen mineralisation rate (SNMR). • Vis-NIR spectra predicted SNMR with moderate to high accuracy (0.44 ≤ R2 ≥ 0.90). • The highest model accuracy was obtained for sandy-loam texture group. • Spectral bands of 2150–2260 & 2470–2480 nm contributed most significantly in SNMR prediction. Soil nitrogen mineralization rate (SNMR) influences crop N uptake and nitrate leaching leading to environmental pollution. This study aims at (i) examining whether visible and near-infrared reflectance spectroscopy (vis-NIRS) can predict SNMR and (ii) investigating if incubation periods and ancillary soil attributes can improve the prediction accuracy. Total 133 soil samples collected from seven fields were incubated under aerobic conditions for 60 days with seven batches of sub-samples. Mineral N was measured at regular time intervals and soil samples were scanned using a vis-NIRS sensor (Tec5 Technology, Germany) parallelly. SNMR was determined by fitting a zero-order kinetic to the net mineralized N as a function of the incubation time. Soil total nitrogen (TN), total carbon (TC) and electrical conductivity (EC) were determined once. Partial least squares regression (PLSR) models were calibrated individually for each field both for vis-NIR spectra and its combinations with TN, TC and EC. Six out of seven batches of sub-samples were used for calibrating PLSR when remaining one batch was used for model validation, and it rotated across all seven batches. Vis-NIRS alone predicted SNMR with moderate accuracy in five of seven fields (coefficient of determination, 0.53 ≤ R2 ≥ 0.66, ratio of prediction to deviation, 1.51 ≤ RPD ≥ 1.76), while models were poor in two fields (R2 = 0.23–0.26, RPD = 1.18 – 1.20). Inclusion of soil TC, TN and/or EC was expected to improve accuracy, but improvements varied across fields (R2 = 0.23–0.79, RPD = 1.18 – 2.26). Similarly, the incubation period increased vis-NIRS prediction accuracy, but frequently occurred among 2nd to 6th batches (R2 = 0.35–0.82, RPD = 1.28 – 2.44). Even incorporating secondary properties and increasing incubation duration hardly improved predictions, improvement can be compromised since it is not significant mostly and often underperformed or remained unchanged. Considering the time and effort required to incubate and analyze soil properties, this study suggests using a vis-NIRS sensor to estimate SNMR in fresh soil conditions i.e., without incubation and incorporation of secondary properties. [ABSTRACT FROM AUTHOR]

Several ecosystem processes such as nitrogen mineralization are mainly controlled by complex multitrophic interactions in which nematodes play major roles. Despite the abundance and diversity of these nematodes, studies on their contribution to N mineralization have been limited to a few selected species and often in completely sterilized media. Such simplified experiments are unrealistic and usually give inaccurate results as part of the interaction is missed. Therefore the contributions of nematodes to N mineralization need to be quantified more accurately in realistic conditions where native microbes, nematodes and plants interact. To do this, we set up a microcosm incubation experiment in which the whole nematode communities were extracted from fresh soil and reinoculated into the soil defaunated by 5 kGy dose gamma irradiation that leaves the microbial community largely intact. Three treatments namely control, +Nem (with nematodes) and −Nem (without nematodes), with or without plants were incubated for 86 days. In bare microcosms, +Nem cores increased total mineral nitrogen with 32% compared to −Nem. However, no significant (p > 0.05) differences were observed on total mineral nitrogen and plant N uptake between +Nem and −Nem treatments in planted microcosms. Nematode abundances and community structures were significantly changed over time and differed in bare and planted microcosms. Bacterivores dominated in bare, while herbivores dominated in planted soils throughout the incubation period. This could explain the contrasting effects of nematodes in N mineralization in bare and planted microcosms. Optimization of gamma irradiation technique is required to further reduce its side effects on nutrient flush and non-targeted organisms. [ABSTRACT FROM AUTHOR]

Abstract: To evaluate the effect of tillage intensity on the N mineralization pattern of winter wheat residues, sugar beet residues, Italian ryegrass and maize residues undisturbed soil samples were taken from six sites under different tillage management. Site NTK had been managed for 10 years under reduced tillage (RT), whereby the last 4 years the crops were sown using direct seeding (NT). Site RTCSE had been managed for 20 years under reduced tillage (RT) and site RTH for 3 years. For each site under RT a nearby site under conventional tillage (CT) was selected (CTK, CTCSE and CTH). On site NTK and site RTCSE a significantly higher amount of SOC in the 0–10cm was accumulated compared to the respective CT sites. Between site RTH and site CTH no such significant difference was found. However, the content of microbial biomass C (MB-C) and the β-glucosidase and urease activities were higher on all RT sites compared to the respective CT sites. This indicates that these microbiological and biochemical parameters seem to be very sensitive for alterations in management intensity. After 98 days, more N was immobilized under NTK than under CTK by adding winter wheat residues (expressed as kgha−1 and as % of total added N). This higher immobilization potential can be explained by a higher microbial activity and a change in microbial population. Under RTCSE and RTH net N immobilization of the winter wheat residues was found, but the pattern was less pronounced than for NTK. However, when expressed as % of total N added, N immobilization of winter wheat residues was higher under CT than under RT, which indicates that high C:N residues when incorporated, decompose more slowly under RT than under CT. Similar results were found comparing the N mineralization pattern of maize residues under RTH and CTH. The residues of sugar beet and Italian ryegrass at site CTH released N more rapidly and to a higher extent, 74.1% and 66.2%, respectively (expressed as % of total N added) than under RTH at the end of the incubation. The slower mineralization of N rich crop residues under RT compared to CT means that there is less potential risk for nitrate leaching to occur, which may result in a higher N efficiency in RT compared to CT. [Copyright &y& Elsevier]

Abstract: Crop rotations in Western Europe contain crops that seem not suitable for reduced tillage (RT) agriculture because they often include beets and potatoes, resulting in a high disturbance of the soil at the formation of the ridges and at harvest. Therefore, the short- and long-term effects of RT on the stratification and stock of total nitrogen (TN) in the 0–40cm depth layer and the nitrogen (N) mineralization in the upper 15cm depth layer of silt loam soils was evaluated. For doing so, 10 fields at seven locations representing the important types of RT systems in Belgium, applied for a different number of years, and eight fields under conventional tillage (CT) with comparable soil type and crop rotation were selected. Despite the presence of root and tuber crops in these rotations, the stratification of the percentage of TN and of the carbon to nitrogen ratio was more pronounced under RT than CT fields. The TN stock in the RT fields worked with a cultivator or soil loosener was comparable to CT fields, even after 20 years. No trend could be found in the change in TN stock of reduced tillage by direct drilling compared to CT. The N mineralization rate in undisturbed soil cores under controlled conditions in the laboratory was on average 0.20±0.08mgNkg−1 dry soil day−1 for RT fields compared to an average of 0.13±0.05mgNkg−1 dry soil day−1 for CT fields. This increase in N mineralization rate was correlated with a higher microbial biomass nitrogen content. [Copyright &y& Elsevier]

Biobased fertilizers recovered from animal manure are sustainable substitutes for synthetic mineral nitrogen (N) fertilizers, showing high potential to minimize environmental pollution while maintaining nutrient supply. This study investigated the response of maize (Zea mays L.) and soil microbes to the application of biobased fertilizers, i.e., pig manure (PM), the liquid fraction of digestate (LFD), and evaporator concentrate (EVA) in replacement of calcium ammonium nitrate (CAN) in a 56-day pot experiment. Apart from maize plant growth and soil chemical properties, the abundance of N-cycling-related genes was determined by destructive sampling on days 8, 16, and 56 after fertilization. The detected gene copies of bacterial and archaeal amoA in the soil were significantly increased by N fertilization. Relatively high NH 4 +-N concentrations (37.5–62.5 mg kg−1 soil dry weight) applied in this experiment may have promoted gaseous N losses via nitrifier nitrification and nitrifier denitrification shortly (8–16 days) after fertilization. Consequently, net N loss was observed in all the fertilized treatments, however, biobased fertilizers resulted in lower N loss as compared to CAN. The presence of maize plants also reduced the N loss, probably driven by the continuous NO 3 −-N uptake which reduced the N source for denitrification. Overall, the application of PM and LFD revealed no significant difference with CAN regarding either plant growth or soil biochemical properties. Whereas the EVA application resulted in lower biomass and nutrient uptake in the young maize plant compared to other treatments, probably attributed to salt stress due to the imbalanced ratio of N and Na in this product. • Ammonium-rich biobased fertilizers increased the gene abundance related to nitrification and denitrification. • Replacing synthetic fertilizers with biobased fertilizers resulted in a lower short-term N loss. • Pig manure and liquid fraction of digestate showed similar effect on maize growth as calcium ammonium nitrate. • Manure-derived evaporator concentrate may contain surplus Na and pose a risk of salt stress to young plant roots. [ABSTRACT FROM AUTHOR]

Both herbivorous and bacterivorous microfauna have been shown to influence root development, soil nitrogen (N) mineralization, and plant productivity. However, our knowledge of these effects is limited as multitrophic interactions remain largely unexplored. We investigated whether and how herbivorous nematodes (Pratylenchus zeae) and bacterivorous nematodes (Poikilolaimus oxycercus), alone and in combination, affect plant biomass (Lolium multiflorum) through root traits and/or soil N mineralization. Bacterivorous nematodes increased, whereas herbivorous nematodes decreased, plant productivity. We found that root trait coordination in response to soil microfauna was consistent with the concept of root economics space. The negative interaction between herbivorous and bacterivorous nematodes on plant productivity at high herbivorous nematode infestation could be explained by reduced N mineralization and variation in the root nitrogen concentration-root tissue density (RNC-RTD) axis aligned with increased herbivory intensity. This study revealed that herbivorous and bacterivorous nematodes moderated each other's effect on plant productivity via root trait coordination and N mineralization, and suggests, for the first time, the value of the root economics space concept for interpreting phenotypic root plasticity and functioning in response to local biotic factors. • We investigated whether and how microfauna interactively affect plant productivity. • Herbivorous and bacterivorous nematodes had a negative interaction on plant biomass. • The negative interaction was due to variation in N mineralization and root traits. • Root traits influenced by microfauna were in line with the root economics space. [ABSTRACT FROM AUTHOR]

Nitrogen (N) depositions in Europe are decreasing, but this could not explain faster than expected improvement of N saturation indicators in temperate forests. Alongside there were local signs of initial recovery from acidification during the past three decades and enhanced leaching of dissolved organic carbon and nitrogen (DOC, DON). These two global change processes both affect total dissolved nitrogen (TDN) levels and often occur simultaneously, hence complicating mechanistic explanations for changing European forest N status. We aimed to test the hypothesis that forest N status in northwest Europe has started to improve. If this hypothesis is confirmed, we wanted to investigate to what extent such improvement is due to reduced N deposition. We evaluated the evolution of multiple N saturation indicators in five ICP Forests Level II plots in northern Belgium, using long-term soil solution and foliage datasets. The DON:TDN ratio (molar) in soil solution increased overall in the O horizon (mean 0.279–0.463, slope 0.023–0.037 yr −1 ) and in the mineral soil (mean 0.134–0.78, slope 0.007–0.051 yr −1 ) between 2005 and 2014. The DOC:NO 3 − ratio (molar) in soil solution increased in three plots in the O horizon (mean 6.84–22.15, slope 0.58–1.92 yr −1 ) and in four plots in the mineral soil (mean 2.07–25.32, slope −0.06–5.76 yr −1 ) between 2002 and 2014. The ratio of N and phosphorus (P) concentrations in foliage (mg g −1 ) and the ratio of base cations (Bc = Ca + K + Mg) and N concentrations in foliage (molar) remained unaltered between 1999 and 2013. Changes in the soil solution chemical composition thus confirmed an improvement in forest N status, despite sustained high NO 3 − concentrations, but biotic recovery appeared to be lagging behind. This demonstrates that insight in forest recovery from N saturation requires a multiple indicator approach, and further monitoring of tree nutritional status alongside soil processes is needed to monitor the evolution of European forest N status. [ABSTRACT FROM AUTHOR]

Sustainable agriculture requires the careful optimization of the use of organic amendments to improve soil fertility while minimizing any harmful environmental effects. To understand the events that occur in soil after the addition of different organic amendments, we evaluated the nitrogen (N) mineralization dynamics in soil after adding organic amendments, and evaluated changes in the microbial population. The four organic amendments were fresh dairy cattle manure, fresh white clover, vegetable, fruit, and yard waste compost, and poplar tree compost. The N mineralization potential of each organic amendment was determined by analyzing total mineral nitrogen during a 97-day laboratory incubation experiment. Soils amended with clover released 240 μg N g −1 soil during the 97-day incubation, more than twice as much as that released from soils amended with manure or composts (76–100 μg N g −1 soil). At the end of the incubation, the net N mineralization in clover-amended soils was 54%, more than five times higher than that in soils amended with composts or manure (4%–9%). Nitrogen was mineralized faster in clover-amended soil (1.056 μg N g −1 soil day −1 ) than in soil amended with composts (0.361–0.417 μg N g −1 soil day −1 ). The microbial biomass carbon content was higher in clover-amended soil than in the soils amended with manure or composts. We monitored changes in the microbial population in amended soils by a phospholipid fatty acid (PLFA) analysis. On day 97, there were higher concentrations of total PLFAs in soils with organic amendments (e.g., 14.41 nmol g −1 in clover-amended soil) than in control soil without amendments (9.84 nmol g −1 ). Bacteria (Gram-positive and Gram-negative), actinomycetes, and fungi were more abundant in clover-amended soils than soils amended with manure or composts. The N mineralization potential varied among the four organic amendments. Therefore, the timing of application and the type of organic amendment should be matched to the nutrient needs of the crop. [ABSTRACT FROM AUTHOR]

Abstract: Soil fauna, particularly nematodes, are considered to strongly contribute to nitrogen mineralisation through grazing on microflora during decomposition. Demonstration of this effect has mostly relied on calculation-based soil food web analyses or experiments involving simplified and artificially constructed food webs. We carried out an incubation experiment in which defaunated soil cores were reinoculated with entire soil nematode populations extracted from bulk soil and during which nitrogen mineralisation was measured. Both undisturbed and disturbed cores were prepared to investigate whether a representative pore structure influences the effect of entire free-living nematode populations on nitrogen mineralisation. Cores were subjected to a 5 kGy gamma irradiation dose sufficient to eliminate all soil fauna while leaving the microbial biomass largely intact. Half of the irradiated cores were reinoculated with nematodes extracted from a corresponding volume of bulk soil and incubated for 82 days. The microbial biomass was not strongly affected by gamma irradiation or nematode addition but declined strongly in all treatments during incubation. Reinoculation of nematodes was successful in establishing populations of a similar size and composition as in the control samples. Net nitrogen mineralisation from indigenous soil organic matter was observed in all treatments throughout the incubation, but was always more pronounced in irradiated cores. Total mineral nitrogen concentrations did not differ significantly between simply irradiated and irradiated then reinoculated cores. However by the end of the incubation period nematode addition resulted in 87% and 23% more NO3 −–N g−1 dry soil in undisturbed and disturbed cores respectively, while NH4 +–N g−1 dry soil decreased by 50% in both core types. We found no convincing evidence for a contribution of free-living nematodes on total nitrogen mineralisation, but the activity of nitrifying organisms was clearly stimulated by nematode grazing. [Copyright &y& Elsevier]