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In open-field vegetable production, high quantities of soil mineral nitrogen (Nmin) and N-rich crop residues often remain in the field at harvest. After the harvest of crops in autumn, this N can lead to considerable nitrate (NO3−) losses during the subsequent winter leaching period. In four field trials, different tillage depths (3–4, 10, 30 cm) and dates (early autumn, late autumn, early spring) were investigated to reduce N losses after growing spinach in the autumn. In a further treatment, the nitrification inhibitor 3,4-Dimethylpyrazole phosphate (DMPP) was directly applied to the crop residues. Potential N losses were calculated by a balance sheet approach based on Nmin concentration (0–90 cm), measured N mineralization and N uptake by catch crops. By postponing the tillage date from early to late autumn or spring, resprouting spinach stubbles acted as a catch crop, reducing N losses by up to 61 kg ha−1. However, if the spinach biomass collapsed, the N losses increased by up to 33 kg ha−1 even without tillage. The application of DMPP as well as the tillage depth were less effective. Overall, postponing tillage to spring seems to be the most promising approach for reducing N losses during the off-season. [ABSTRACT FROM AUTHOR]

Crop residues are an important component of N cycling in the agro-ecosystem. In this study, a loam soil was amended with cabbage residues and the N mineralization from soil organic matter and residues, as well as N uptake by Italian rye grass, were investigated. Four mineral fertilizer-residue combination treatments and two application times were tested in micro-plots. For uncropped micro-plots, application of residue resulted in higher soil mineral N content (43 mg N kg-1, 63 days post application) versus 30.5 mg N kg-1 for an unamended soil. Both bulked dry matter production and N uptake by the rye grass decreased in the order full mineral fertilizer > half mineral fertilizer + half residue > full residue applied 3 weeks before sowing > full residue applied at sowing, but the differences were more pronounced in the N uptake. Shoot N recovery from the applied residue (27.5-31.4%) was much smaller than that from the full fertilizer (70.2%), but was still considerable. Apparent net N mineralization from soil organic matter was significantly depressed in the treatments where mineral fertilizer was added. Cropping also reduced apparent net N mineralization, but this effect was reduced when residues were applied 3 weeks before sowing. Results showed that crop residues play an important role in crop production, in terms of obtaining comparable dry matter production to that of full mineral fertilizer application. Moreover, nitrogen supplied as residue is less liable for loss due to leaching during the growing season. The efficiency of crop uptake to residue-derived nitrogen could be increased by earlier application time. [ABSTRACT FROM AUTHOR]

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]

Abstract: In a laboratory study, organic biological wastes (OBW) were screened for their potential to manipulate the N release from vegetable crop residues in two phases: an immobilization and remineralization phase. During the first phase, celery leaves (Apium graveolens L.) were mixed with an immobilizer waste (straw, two green waste composts (GWC1 and GWC2), saw dust, paper sludge and tannic acid) in order to immobilize N released from crop residues. During the second phase, the treatments received a remineralizer waste (malting sludge, vinasses, molasses and dairy sludge) in order to stimulate remineralization of immobilized N. Straw showed the most pronounced N immobilization (on average 30.2mgNkg−1). N immobilization with tannic acid, saw dust and GWC2 was slower and less pronounced (on average 16.4, 15.9 and 8.0mgNkg−1, respectively). GWC1 and paper sludge immobilized almost no N. Only when GWC1 was mixed with vinasses, remineralization was observed (up to 55.4mgNkg−1) during a 30 days period. For all other remineralizers, positive priming effects were scarce and short-lived. Manipulating the N release of N-rich crop residues may be a suitable method to reduce the nitrate concentration in soil after incorporation of crop residues. Especially, easily decomposable waste materials (i.e. low lignin content) with a large C:N ratio seem to have a potential to immobilize N. However, stimulating remineralization of immobilized N seems not easy to accomplish. [Copyright &y& Elsevier]

We have studied the possibilities of manipulating N mineralization from high N vegetable crop residues by the addition of organic materials, with the aim of initially immobilizing the mineralized residue N with a view to stimulating remineralization at a later stage. Residues of leek (Allium porrum) were incubated with soil, alone and in combination with straw, two types of green waste compost (with contrasting C:N ratios) and tannic acid. Evolution of mineral N was monitored by destructive sampling. After 15 weeks, molasses was added to part of the samples in each treatment, and incubation continued for another 12 weeks. All materials added during the first incubation stage, except the low C:N compost, resulted in significant immobilization of the residue N. The immobilization with the high C:N compost (41.4 mg N kg−1 soil) was significantly larger than with tannic acid and straw (both immobilized about 26 mg N kg−1 soil). In the straw treatment, remineralization started in the first stage of incubation from day 50 onwards. The addition of molasses caused a strong and significant remineralization in the second stage (equivalent to 73% of the N initially immobilized) in the treatment with the high C:N ratio compost. In the case of tannic acid, there was no consistent effect on mineralization from addition of molasses. This was attributed to the fact that the immobilization observed was due to chemical rather than biological fixation of the residue N. A number of non-toxic organic wastes could be considered for use in mediating release of immobilized N from high N crop residue materials in an attempt to synchronize residue N availability with crop N demand. [Copyright &y& Elsevier]

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]

• After two decades, 60 croplands in Flanders were resampled to assess SOC evolution. • Two decades of aboveground residue incorporation did not increase SOC. • The humification coefficient of maize-C in MS was twice as high as in MG fields. • The stability of maize-C formed on the long-term under MS and MG fields was similar. The few available studies on maize biomass in soil of field experiments have shown that belowground maize biomass resides more than twice as long as aboveground residues. Our principal objective was to investigate the effect of maize crop residue incorporation on the long-term dynamics of soil organic carbon (SOC) for a selection of croplands with sandy to sandy loam texture in Flanders, Belgium. After a period of more than 20 years, we resampled 31 and 29 parcels that had either silage maize (MS, only belowground biomass input) or grain maize (MG, both above- and belowground biomass input) dominated crop rotations. Changes in maize-derived SOC (C maize) of MS and MG systems between 1993 and 2018 were evaluated by detecting SOC content and its δ 13C value. In addition, a 140-day incubation experiment (at 20.5 °C) was carried out to compare the stability of C maize and the related nitrogen (N) mineralization between the MG and MS fields. After two decades, δ 13C raised 2.4 and 2.6‰, while C maize increased 1.6 and 2.2 g kg−1 in MS and MG systems, respectively. The estimated average humification coefficient over this period of maize-C was 0.11 for the MS system, about twice as that of the MG system (0.06). This reconfirms findings of stronger contribution of belowground biomass versus aboveground residues to stabilized SOC pools over the medium to long term. By the end of the incubation, on average 10.4% and 11.6% of C maize had been decomposed in the MS and MG soils, respectively. These C maize mineralization rates were statistically equal for the two systems, which indicates that mainly belowground biomass remained in the soils. Also, net N mineralization was indifferent to the crop rotations (MS: 58.8 mg N kg−1 and MG: 52.6 mg N kg−1). To reconcile the two main observations in this study, we hypothesize that the lower preservation of shoot-C could be manifested on a shorter term. Detailed research is needed to determine the time scale of increased stabilization of root-C versus shoot-C under field conditions. [ABSTRACT FROM AUTHOR]

• No need to fertilize catch crops: they did well without fertilization. • Trade-off: more shoot and root C-input versus minor additional NO 3 − leaching. • Small N dose acceptable for catch crops sown under good conditions in August. • Immobilization of pig slurry N by cereal stubble critical to supplement N uptake. • Choice of catch crop depends on harvest date of previous and sowing date of next crop. Catch crops may strongly reduce nitrate leaching during autumn and winter in temperate climates. Authorities therefore aim to stimulate farmers to place catch crops in their rotations by allowing them to apply more nitrogen (N) to their fields. Furthermore, some regulations make it possible to fertilize catch crops after harvest of specific crops such as winter cereals, even if catch crops are sown during late summer. Some farmers claim that this practice is essential for a good development of the catch crop. However, it is not sure whether this summer fertilization to catch crops could result in any additional nitrate leaching, compared to nitrate leaching under non-fertilized catch crops. We therefore designed field experiments including four different catch crop species, each of them sown after harvest of winter cereals at two different dates, and each of them receiving two different doses of pig slurry or receiving no fertilizer at all. Experiments were installed on sites with different soil textures in northern Belgium. Based on the results of soil mineral N and N uptake, we assessed the risk of additional nitrate leaching caused by fertilization of catch crops. We found that the apparent uptake of N released from pig slurry was rather limited but sufficient to avoid additional nitrate leaching on sandy soils, mainly due to supplementary immobilization of the applied N occurring during mineralization of the cereal crop residues. It is critical to mention the limiting conditions that we found to be adequate from our results: catch crops were to be sown in good circumstances before September and could only receive the smallest dose of pig slurry, which was approximately 60 kg N ha−1. On silty soils, however, we found even under these conditions a few indications of small amounts of additional nitrate leaching. As fertilization did not seem mandatory to develop an effectively functioning catch crop and as there was no indication of reduced nitrate losses due to this practice, we would rather discourage fertilization of catch crops to exclude any environmental risk. However, fertilization significantly increased the catch crop yield, so the decision to fertilize or not may also be seen as a trade-off between minor amounts of additional nitrate leaching and enhanced biomass-related effects, such as the additional input of organic carbon into the soil. [ABSTRACT FROM AUTHOR]