Remigiusz Łukowiak

Digestate, a by-product of methane fermentation in agricultural biogas plants, is a valuable source of nitrogen (D-N); however, the mode of action of this specific (mineral/organic) N carrier has not yet been elucidated. The influence of D-N on yield-forming action in winter oilseed rape (WOSR) was determined based on a three-year series of field experiments conducted between 2016 and 2018. The field experiments were arranged according to a two-factorial design. The first factor was the fertilization system (FS), based on two N carriers: (i) ammonium nitrate 34-0-0 (AN-FS) and (ii) digestate (D-FS). The second factor was the N dose, set as 0 (absolute N control), 60, 120, 180, or 240 kg ha⁻¹. The (i) mass, (ii) balance, and (iii) remobilization quantity of various nutrients were determined at two stages defining the seed-filling period (S-FP), namely, the onset of flowering and physiological maturity. These traits and developed indices were used to predict the seed yield (SEY), the number of seeds in the pods of lateral branches (SeSB) per m², and N mass at the onset of flowering (NAF) and at physiological maturity (NAH). The N source, averaged over N doses and years, did not significantly differentiate yield (2.68 vs. 2.78 t ha⁻¹ under the AN-FS and D-FS, respectively). The increases in seed yield under the D-FS in comparison to the AN-FS were −0.01, 0.05, 0.09, and 0.35 t ha⁻¹ at N doses of 60, 120, 180, and 240 kg N ha⁻¹, respectively. The lower yield efficiency for ammonium nitrate compared to the digestate was explained by the higher unit productivity of the N accumulated by the crop at the onset of flowering (NAF) under the latter treatment; in particular, this difference amounted to approximately 20% in favor of the digestate system (15.8 vs. 18.9 kg of seeds per kg of N). The WOSR production strategy based on the use of AN, which was found to saturate the rapeseed canopy with N at the onset of flowering, proved ineffective for yield formation. The higher yield-forming efficiency under the D-FS resulted from the higher efficiency of canopy N due to both the interaction between N and K (remobilization) and the net soil uptake of phosphorus (P) and micronutrients—namely, zinc (Zn) and copper (Cu)—during the S-FP. This study’s results demonstrate that digestate can be considered a significant input for sustainable winter rapeseed production.

Digestate, a by-product of methane fermentation in agricultural biogas plants, is a valuable source of nitrogen (D-N). The mode of action of this specific (mineral/organic) N carrier has not yet been elucidated. Strategies for D-N yield-forming action in winter oilseed rape (WOSR) were determined based on a three-year series of field experiments conducted between 2016 and 2018. The field experiment was arranged as a two-factorial design. The first factor was the fertilization system (FS), based on two N carriers: (i) ammonium nitrate, 34-0-0 (acronym AN-FS, and (ii) digestate (D-FS). The N doses were: 0 (absolute N control), 60, 120, 180, and 240 kg ha^-1. Nutrient: (i) mass, (ii) balance, and (iii) remobilization quantity were determined at two stages defining the seed-filling period (S-FP): the onset of flowering and physiological maturity of the rapeseed. These traits and developed indices were used to predict seed yield (SEY), the number of seeds in the pods of lateral branches (SeSB) per m², and N mass at the onset of flowering (NAF) and at physiological maturity (NAH). The type of N source, averaged over N doses and years did not differentiate yield (≈2.75 t ha^-1). The main reason was the lack of differences in the primary component of the yield structure, SeSB per m² (R² = 0.81. The reason for the lower yield efficiency of the ammonium nitrate (AN-FS) compared to the D-FS was the higher unit productivity of the N accumulated by the crop at the onset of flowering (NAF). This difference amounted to approximately 20% in favor of the digestate system (15.8 vs. 18.9 kg of seeds per kg of N). The WOSR production strategy based on the use of AN, which involved saturating the rapeseed canopy with N during the onset of flowering, proved ineffective for yield formation. The higher yield-forming efficiency of D-FS resulted from the higher efficiency of the canopy N resulted from both the interaction between N and K (remobilization) and net soil uptake of phosphorus (P), as well as micronutrients, including zinc (Zn) and copper (Cu) during S-FP. It is important to highlight the ameliorative effect of digestate regarding drought. Indeed, at the onset of flowering, increasing N rates in the digestate stabilized the Ca mass in the rapeseed canopy, whereas AN caused a progressive increase. Studies have shown that digestate can be considered a significant element of sustainable winter rapeseed production.

We hypothesized that the application of digestate (D) to winter oilseed rapeseed would have the same effect on seed production as nitrogen fertilizer (N_f). It impacts yield by altering the mass of readily available N in the vegetative and reproductive periods of plant growth. This allows for a good yield forecast. This hypothesis was assessed in field experiments with rapeseed carried out in 2015/2016, 2016/2017, and 2017/2018. The experiment included three N fertilization systems (FSs): AN, based on ammonium nitrate (AN); D, with digestate-based N; DAN, using ²/₃ of digestate + ¹/₃ of AN—and five N_f doses: 0, 80, 120, 160, and 240 kg N ha⁻¹. The net seed yield increase due to N application was 1.44 t ha⁻¹ in the AN system, 1.53 t ha⁻¹ in D, and 1.77 t ha⁻¹ in DAN. The optimal N rates were 160, 250, and 224 kg N ha⁻¹. The N economy of winter oilseed rapeseed was assessed in two periods: vegetative—before anthesis (from the rosette stage to the beginning of anthesis, BBCH 30–BBCH 60) and reproductive (from the beginning of anthesis to full maturity, BBCH 60–BBCH 89). The mass of available N at the beginning of anthesis increased by 54.3% (151 kg N ha⁻¹ to 233 N ha⁻¹) and doubled (151 kg N ha⁻¹ to 302 kg N ha⁻¹) compared to its value at the rosette stage, taking into account the mass of N in the rapeseed canopy and its total mass in the soil/rapeseed continuum. No differences in NUE were found for the tested N carriers. The net increase in N available resources resulting from the application of N fertilizer was 55.1, 104.9, 102.8, and 93.0 kg N ha⁻¹ for respective plots fertilized with 60, 120, 180, and 240 kg N ha⁻¹. Three N indices were measured at the beginning of rapeseed anthesis—N in crop biomass (NAF, r = 0.87 ***), N balance (Nb60, r = 0.87 ***), and N released from soil resources (Ngain60, r = 0.79 ***)—and showed potential for seed yield (SEY) prediction. The linear dependence of SEY on these indicators indicates that the potential of the rapeseed canopy to effectively accumulate N during the vegetative growth was too low. This limitation was fully confirmed by analogous N management indicators, but developed for rapeseed during the seed-filling period. The key indicator of SEY at harvest was the N mass in rapeseed biomass (NAH, r = 0.95 ***). N from digestate acted as a slow-release fertilizer, giving it an advantage over ammonium nitrate. In summary, digestate is an optimal N carrier under conditions of average rapeseed yield.

The research hypothesis of this study assumes that nitrogen (N) from digestate has an equivalent productive effect as mineral fertilizer. Therefore, the use of digestate as a N carrier by farmers significantly reduces the energy costs of oilseed rape production. This hypothesis was verified in field experiments with rapeseed conducted in the 2015/2016, 2016/2017, and 2017/2018 growing seasons. The experiment consisted of three N fertilization systems (FSs)—mineral ammonium nitrate (AN) (AN-FS), digestate-based (D-FS), and ²/₃ digestate + ¹/₃ AN (DAN-FS)—and five N_f doses: 0, 80, 120, 160, and 240 kg N ha⁻¹. Maximum seed yields were 3.26, 3.32 and 3.66 t ha⁻¹ and were obtained for optimal N doses of 170, 186 and 189 kgNn ha⁻¹, respectively. Similar trends were observed for the following yields: straw, crude oil, and cake. The values of the Nitrogen Fertilization Replacement Value for the above yield categories slightly exceeded 100%, and for the mixed variant, they were in the range of 108–112% in relation to the mixture with ammonium nitrate. The contribution of straw, oil, and oilcake in the total energy harvested accounted for 64%, 22%, and 14%, respectively. The most favorable energy efficiency ratio, E_e = 8.15 (seeds + straw), was achieved under the mixed N fertilization variant (DAN-FS; 8.3 and 15.6 m³ ha⁻¹, respectively) and N fertilizer doses of 40 and 80 kg ha⁻¹. For the highest nitrogen doses, the digestate significantly stabilized the yields and energy output of winter rapeseed production. Overall, based on the results of the field experiment and calculations conducted, it is recommended that winter rapeseed biomass should be used entirely for liquid fuel (oil) and solid fuel (oilcake and/or straw) production.

The effect of cyclic pig slurry (PS) application in long-term crop rotations with alfalfa is poorly recognized, particularly with regard to nitrogen use efficiency (NUE) in crops requiring relatively high nitrogen (N) inputs. A long-term field experiment was established in Prague-Ruzyně, Czechia, in 1955. The experiment evaluated the effects of eight fertilization combinations, involving PS application and various N, phosphorus (P) and potassium (K) rates (N₀P₀K₀; N₁P₁K₁; N₃P₂K₂; and N₄P₂K₂). The effect of fertilization was evaluated in a 9-year crop rotation, in which PS was applied only three times under root crops. Long-term different mineral fertilization treatments and the application of PS significantly affected the yield of the tested crops: winter wheat and sugar beet. The highest wheat yield (8.34 t ha⁻¹) was observed in the PS+N₃P₂K₂ treatment, while the highest beet yield (86.1 t ha⁻¹) was recorded in the PS+N₄P₂K₂ treatment. The differences compared with the absolute control (N₀P₀K₀) were 62.3% and 40.5%, respectively. However, statistically significant differences between treatments with different NPK rates were recorded only in plots without PS. With increasing NPK fertilizer rates, the uptake of macronutrients by plants also increased. The only exception was calcium in sugar beet in PS plots. The total N accumulation in plants was proportionally related to the total N input to the soil–plant system (N_in). For winter wheat, this trend was beneficial, as it resulted in higher protein yield, whereas in beet, the sugar yield did not increase significantly when N_in exceeded 250 kg N ha⁻¹. The obtained results indicate that, in the soil conditions of this experiment, N rates should be primarily balanced with appropriate rates of phosphorus.

The increase in biogas production has caused a simultaneous increase in the production of digestate, which is a valuable carrier of nutrients in crop plant production. Digestate-derived nitrogen ensures the optimal nutritional status of winter oilseed plants at critical stages of yield formation. This hypothesis was verified in field experiments with winter oilseed rape (WOSR) conducted in the 2015/2016, 2016/2017, and 2017/2018 growing seasons. The experiment consisted of three nitrogen fertilization systems (FSs)—mineral ammonium nitrate (AN) (AN-FS), digestate-based (D-FS), and ²/₃ digestate + ¹/₃ AN (DAN-FS)—and five N_f doses: 0, 80, 120, 160, and 240 kg N ha⁻¹. Plants fertilized with digestate had higher yields than those fertilized with AN. The highest seed yield (SY) was recorded in the DAN-FS, which was 0.56 t ha⁻¹ higher than that in the M-FS. The nitrogen fertilizer replacement value (NFRV), averaged over N doses, was 104% for the D-FS and reached 111% for the mixed DAN-FS system. The N content in WOSR leaves, which was within the range of 41–48 g kg⁻¹ DM at the rosette stage and within 34–44 g kg⁻¹ DM at the beginning of flowering, ensured optimal plant growth and seed yield. In WOSR plants fertilized with digestate, the nitrogen (N) content was significantly lower compared to that in plants fertilized with AN, but this difference did not have a negative impact on the seed yield (SY). The observed positive effect of the digestate on plant growth in the pre-flowering period of WOSR growth and on SY resulted from the impact of Mg, which effectively controlled Ca, especially in the third growing season (which was dry). Mg had a significant effect on the biomass of rosettes and on SY, but only when its content in leaves exceeded 2.0 g kg⁻¹ DM. It is necessary to emphasize the specific role of the digestate, which significantly reduced the Ca content in the indicator WOSR organs. Increased Ca content during the vegetative period of WOSR growth reduced leaf N and Zn contents, which ultimately led to a decrease in SY. Therefore, the rosette phase of WOSR growth should be considered a reliable diagnostic phase for both the correction of plants’ nutritional status and the prediction of SY. It can be concluded that the fertilization value of digestate-derived N was the same as that of ammonium nitrate. This means that the mineral fertilizer can be replaced by digestate in WOSR production.

The content of bioavailable forms of phosphorus (P) and potassium (K) in soil is essential for the proper functioning of agroecosystems. This study aimed to determine the effects of pig slurry (PS) and NPK mineral fertilizers on soil phosphorus (P) and potassium (K) fractions, the relationship between these fractions and basic soil agrochemical properties, and crop yield. The research material was collected from a long-term experiment established in 1955 in Prague-Ruzyně, Czechia. The effect of two constant factors was analyzed: manure application (control, PS) and different doses of NPK fertilizers (N₀P₀K₀, N₁P₁K₁, N₃P₂K₂, and N₄P₂K₂). A significant effect of fertilization on basic soil properties was demonstrated, including total soil carbon and nitrogen. PS and NPK fertilization also significantly affected the content of water-soluble and moderate labile fractions of P and K. These fractions were positively correlated with plant-available P and K (Mehlich 3). The best fertilization option, which resulted in the greatest increase in yield, was the use of PS and mineral fertilizers at the N₃P₂K₂ level. Increasing the nitrogen dose to the level of N₄ resulted in a decrease in the content of bioavailable forms of P and K in topsoil despite the application of PS.

Field zones at risk of low nitrogen use efficiency (NUE) can be identified by analyzing in-field spatial variability. This hypothesis was validated by analyzing soil mineral nitrogen (N_min) and several plant and soil N management indices. The research was conducted in Karmin (central Poland) during two growing seasons, with winter oilseed rape (2018/2019) and winter wheat (2019/2020). The study showed that the crop yield was positively related to N_min. However, this N trait did not explain all the observed differences in the spatial variation of crop yield and plant N accumulation. In addition, the soil N management indices were more spatially variable during the growing season than the plant N management indices. Particularly high variability was found for the indices characterizing the N surplus in the soil-plant system. The calculated N surplus (N_b = N fertilizer input − N seed output) ranged from −62.8 to 80.0 kg N ha⁻¹ (coefficient of variation, CV = 181.2%) in the rape field and from −123.5 to 8.2 kg N ha⁻¹ (CV = 60.2%) in the wheat field. The spatial distribution maps also confirm the high variability of the parameters characterizing the post-harvest N surplus, as well as the total N input (soil + fertilizer) to the field with rape. The results obtained indicate that a field N balance carried out in different field zones allows a more accurate identification of potential N losses from the soil-plant system.

Understanding the spatial–temporal variability of soil enzymatic activity and its relationship with nitrogen (N) resources in the soil and crop yield is crucial in rational management practices of mineral fertilization. The scarcity of comprehensive studies on geostatic analyses of agricultural soils and plant yields, which would take into account both temporal and spatial variability, was the reason for undertaking this research. The aim of this study was to determine the spatial and temporal variability of the activity of soil enzymes, such as acid (PAC) and alkaline (PAL) phosphatases, urease (URE) and protease (PROT), the content of N-NH₄ (ammonium ions), N-NO₃ (nitrate ions), phosphorus (P), pH, moisture, as well as crop yield on a conventionally managed farmland of 40 ha. During the two-year experiment, soil samples were collected from 37 measurement points. Wheat was the first tested crop, followed by oilseed rape. It was shown that all the tested soil parameters showed temporal and spatial variability, and a significant number of them were significantly higher in July. The creation of raster maps showing the distribution of the tested parameters allowed for the observation of the considerable activity of PAC, PAL, URE, and PROT, as well as a high application of N-NO₃ in the southern part of the field during the growth of the plants. The statistical analysis revealed a negative interaction between the N-NH₄ and N-NO₃ and the urease in the soil under the cultivation of plants. The pH and the percentage of moisture in the soil also had higher values in the south of the field. This pointed to the existence of separate production zones in the south-central part of the field, characterized by a higher yield of wheat and rape. On the basis of the conducted research, it was unequivocally stated that the values of enzymatic and chemical parameters of the soil were reflected in the size of the yield obtained, which allows conclusions to be drawn with respect to the rational management of N in the production process, laying the foundations for precision agriculture.

The Soil Fertility Clock (SFC) concept is based on the assumption that the critical content (range) of essential nutrients in the soil is adapted to the requirements of the most sensitive plant in the cropping sequence (CS). This provides a key way to effectively control the productivity of fertilizer nitrogen (N_f). The production goals of a farm are set for the maximum crop yield, which is defined by the environmental conditions of the production process. This target can be achieved, provided that the efficiency of N_f approaches 1.0. Nitrogen (in fact, nitrate) is the determining yield-forming factor, but only when it is balanced with the supply of other nutrients (nitrogen-supporting nutrients; N-SNs). The condition for achieving this level of N_f efficiency is the effectiveness of other production factors, including N-SNs, which should be set at ≤1.0. A key source of N-SNs for a plant is the soil zone occupied by the roots. N-SNs should be applied in order to restore their content in the topsoil to the level required by the most sensitive crop in a given CS. Other plants in the CS provide the timeframe for active controlling the distance of the N-SNs from their critical range.