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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]

Renewed interest in temporal soil organic carbon (SOC) stock changes has stressed the importance of reliable methods for quantitative assessment of organic compound (OC) content. Particularly with the establishment of modern dry-combustion analyzers, which are replacing the traditional wet-oxidation methods, the need for correct relationships between both is of crucial importance for comparison of past and current SOC data in long-term SOC stock change studies. Dry combustion with a Variomax CNS-analyzer was the standard to evaluate three other methods for Belgian agricultural soils. Excellent linear relationships were found with the Walkey and Black method and the Springer and Klee method, whereas a Shimadzu TOC-analyzer slightly underestimated the OC content. Precision of the investigated methods was comparable and tended to be dependent on the sample size used for measurement. The OC oxidation efficiency of the most widely applied method of Walkey and Black for the soils in this study was very close to the generally accepted 75%. Mass loss on ignition at 800°C could be very well related to the soil OC content and the clay content. The traditional factor of 1.724 used to convert OC measurements to organic matter percentages is not valid for the investigated soils, which demonstrates that rather regional-specific factors (in this study 1.911) should be determined and adopted. [ABSTRACT FROM AUTHOR]

Abstract Evaluations of soil organic carbon (SOC) stocks are often based on assigning a carbon density to each one of a number of ecosystems or soil classes considered, using data from soil profiles within these categories. A better approach, in which the use of classification methods by which extrapolation of SOC data to larger areas is avoided, can only be used if enough data are available at a sufficiently small scale. Over 190 000 SOC measurements (0–24 cm) have been made in the Flemish cropland (the Northern part of Belgium) in the 1989–2000 period. These SOC data were grouped into 3-year periods and as means plus standard deviation per (part of) community (polygons). This large dataset was used to calculate SOC stocks and their evolution with time, without data extrapolation. Using a detailed soil map, larger spatial groups of polygons were created based on soil texture and spatial location. Linear regression analysis showed that in the entire study area, SOC stocks had decreased or at best had remained stable. In total, a yearly decrease of 354 kton OC yr-1 was calculated, which corresponds with a net CO2 emission of 1238 kton CO2 yr-1 . Specific regions with a high carbon sequestration potential were identified, based on SOC losses during the 1989–2000 period and the mean 1999 SOC content, compared to the average SOC content of soils in Flanders with a similar soil texture. When restoring the SOC stocks to their 1990 level, we estimated the carbon sequestration potential of the Flemish cropland soils to be some 300 kton CO2 yr-1 at best, which corresponds to a 40-year restoration period. In conclusion, we can say that in regions where agricultural production is very intense, carbon sequestration in the cropland may make only a very modest contribution to a country's effort to reduce greenhouse gas emissions. [ABSTRACT FROM AUTHOR]

• Variable rate outperformed uniform treatments in profitability and nitrogen applied • Variable rate by adding more nitrogen to the least fertile zones is the best approach • Ranking of management zones into fertility classes needs not only soil but yield data • Variable rate reduces environmental impact, rises fertilizer use efficiency and yield Nitrogen (N) is a main parameter affecting crop growth and yield. However, when variable rate N fertilization (VRNF) is adopted, a main question arises of whether to add more on the highly fertile zone or on the less fertile zone. This study aimed to compare two map-based VRNF treatments, e.g., variable-rate 1 (VRNF1) with the application of recommended rate (RR) of N fertilizer plus 40-50% to the high fertility zones, RR to the medium-fertility zones and RR minus 40-50% to the least fertile zones, and VRNF2 following the opposite scenario to that of VRNF1. Recommendations for both VRNF treatments were derived from the fusion of soil data and crop data, e.g., normalized difference vegetation index (NDVI) and historical yield and were compared with traditional uniform rate (UR) application. Online soil measurement was carried out in two fields in Belgium, using a multi-sensor platform that collects high-resolution data on key soil properties using a fiber type visible and near-infrared (vis-NIR) spectrophotometer. The vis-NIR sensor was calibrated using a partial least squares regression (PLSR) to predict pH, organic carbon (OC), moisture content (MC), calcium (Ca), sodium (Na), magnesium (Mg), phosphorous (P) and potassium (K). The predicted soil properties were fused with NDVI and historical yield data to delineate management zone (MZ) maps with different fertility classes. A cost-benefit analysis was carried out using the cost of fertilizer applied as input cost and the price of barley and wheat in 2019. Results showed that the VRNF2 treatment was more profitable compared to both VRNF1 (11.55 and 19.52 EUR ha-1) and UR (16.40 and 148.78 EUR ha-1). The VRNF2 treatment maintained or increased crop yield by up to 10.4% and reduced the amount of N fertilizer applied by up to 19.4%, compared to the traditional UR. Because of the lower amount of N fertilizer used compared to the other two treatments, the VRNF2 would reduce the negative environmental impact by reducing the amount of N fertilizer applied. Therefore, we recommend the VRNF2 based-on data fusion of soil and crop parameters as the best approach to manage N fertilization site specifically. [ABSTRACT FROM AUTHOR]

Diffuse groundwater contamination by intense fertilizer use is a widespread problem in most of the agricultural regions in West-Europe and in many other countries worldwide. As a result elevated nitrate concentrations in groundwater are found, and as groundwater is the source of baseflow into rivers and streams, also surface waters show increased nitrate levels. Flanders in the north of Belgium is no exception and despite restrictions on the fertilizer use and remediation practices initiated in the mid-1990s, and still ongoing, in many stream monitoring points, peak concentrations are measured above the 50 mg/l criterium. In many monitoring stations there is apparently a long delay between the start of the restrictions and effect on water quality. Often this delay is related to high concentrations in the groundwater inflow and long residence and travel times of groundwater feeding the streams. To better understand this behavior and estimate response times a tool was developed that incorporates the main water flows in a typical catchment and considers the effects of varying groundwater ages. This tool is conceived as a parsimonious lumped parameter model, simple and easy enough to be used by many people but still capturing the main mechanisms end processes. In this paper the tool is explained and it is applied on two testcases in Flanders to show its performance. • Presenting a model for simulating nitrate concentrations in surface waters (streams, rivers) at catchment scale due to inflow of nitrate rich groundwater. • Consists of a root zone model for nitrate leaching on a parcel scale and a saturated zone model at the catchment scale. • Uses fractioning of the groundwater flow in different components. • Conceived as a lumped parameters model with a minimum of parameters • Results are presented for 2 catchments, but the model has been tested on 10 sites in different hydrogeological settings [ABSTRACT FROM AUTHOR]

• 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]

Abstract: Acidification of forest soils in Europe and North America has been an important concern over the last decades. The last area-covering survey of forest soil acidification in Flanders (North Belgium) goes back to 1985 [Ronse A, De Temmerman L, Guns M, De Borger R. Evolution of acidity, organic matter content, and CEC in uncultivated soils of North Belgium during the past 25 years. Soil Sci; 146, (1988), 453–460] and highlighted a significant acidification of the upper layer (0.3–0.4 m) of forested podzols during the period 1950–1985. The present study aimed to assess (1) to what extent further acidification of forested podzols occurred during the period 1985–2000 at different depths and (2) whether the average annual acidification rate accelerated or slowed down between 1985 and 2000 compared to the period 1950–1985. Average soil pH–KCl values of podzols in northern Belgium dropped during the period 1985–2000. This decline extends to a depth of about 50 cm but was most pronounced and significant in the A horizon. In the A0, A1 and A2 horizons, average pH dropped with 0.2, 0.3 and 0.1 units, and in the Bir and C horizons with 0.1 units. No change in average pH value occurred in the Bh horizon. Average annual acidification rate of the A1 horizon was significantly higher in the period 1985–2000 than in the period 1950–1985. Changes in pH occurred in the entire soil profile during the period 1950/67–1985 likely because sulphate was the major form of acid deposition before 1985. After 1985, acid sulphur deposition decreased with more than 50% in North Belgium. In contrast, ammonium deposition almost doubled between 1950 and 1980, which may explain why soil acidification between 1985 and 2000 has been restricted to the upper soil horizons. [Copyright &y& Elsevier]

• 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]

• Site rather than variety had a strong impact on root biomass and traits. • Biochemical quality of roots was co-determined by both site and variety. • Belowground biomass was not necessarily accordant with aboveground yield. • Prediction of root biomass based on a fixed R/S ratio may cause misestimation. Knowledge about the response of root biomass and root system architecture (RSA) to soil conditions would increase efficiency in selecting high performance crop varieties and predicting the impact of rotations on long-term soil organic matter (SOM) stock. We evaluated the effect of variety and site on maize (Zea mays L.) root growth. Two-dimensional (2D) and 3D techniques were used to quantify changes of RSA. Commercial maize varieties were grown in three sites with typical soil types for maize cultivation in Belgium. We observed that at Merelbeke, above- and belowground biomass and root-to-shoot ratios (R/S) were larger compared to those at Ravels. Both factors interactively influenced an index of biological stability (ISB) based on biochemical quality of the roots. There were also more fine roots at Merelbeke than at Bassevelde and Ravels. Together with a lower ISB value, this suggests that site had an inherent effect on root biochemical quality. We conclude that a strong impact of site exists on both above- and belowground biomass; however, they are not necessarily correlated. With a fixed R/S, predictions of root biomass were misestimated in one half of the cases. This complex response of root phenotyping to varying soil conditions should not be overlooked. [ABSTRACT FROM AUTHOR]