Impacts of Using Mineral Fertilization Combined with Sewage Sludge in the Amendment of Luvisol on Oat Crop I. Influence on Yield and Mineral Nutrition

The present paper illustrates the results obtained on oat (Avena sativa L.), Someșan variety, concerning growth, grains and straw yields and also of macroelements accumulation (N, P, K, Ca, Mg) when various doses of sewage sludge were applied associated or not with mineral fertilization. Sewage sludge application produced significant increase of plant height starting with equivalent dose of 100 kg N/ha, meanwhile the highest height was observed in the case of plants fertilized with doses equivalent with 300 and 400 kg N/ha. With the increase of sewage sludge dose it has been observed the straw yield increase, this being statistically assured starting with sewage sludge equivalent dose of 100 kg N/ha. Sewage sludge fertilization, accompanied or not by mineral fertilizers, evidenced that nitrogen contents in oat grains increases with applied sewage dose, being significant from an equivalent dose of 200 kg N/ha. Low sewage doses equivalent with 200-300 kg K/ha evidenced for potassium concentration in oat grains a significant decrease in comparison with untreated variant, meanwhile mineral fertilization generated a significant increase of potassium levels in oat grains. Calcium contents in oat grains present a significant increase after sewage sludge fertilization and at a dose equivalent with 600 kg N/ha, the calcium levels (0.24%) increased 3 times in comparison with untreated variant (0.08%). Sewage sludge fertilization, associated or not with mineral fertilizers, did not produced significant changes of magnesium levels in oat grains, found concentrations being considered as normal values.


Introduction
Resulted from municipal wastewater treatment plants, sewage sludge is a nutrient rich organic material, used as fertilizer and soil conditioner. Considerable number of research papers has been accomplished worldwide on the effects of sewage sludge on soil and various crops. Hence, some studies indicated that successive application during seven years of 100 t/ha municipal sewage sludge for wheat and maize crops produced an increase with a percent of organic carbon from soil [1,2]. Other researches evidenced that sewage sludge application presented a positive influence on the content of total organic carbon and humus fractions [3]. Organic fertilization with compost obtained from municipal sludge with a dose of compost equivalent to 400 kg N/ha led to significant increases of mobile phosphorus levels in soil [4].
Mihalache et al [5] reported that sewage sludge application generate significant increase of soil organic carbon, total nitrogen, mobile forms of phosphorus and potassium, the highest effects being encountered for 240 t/ha sewage sludge dose.
There are many scientific reports that highlight the enhancement of crop productivity and crop quality parameters after sewage sludge application. Accordingly, application of 0, 40, 80, 120, 240 t/ha sewage sludge at rice crop evidenced that rate of 240 t/ha produced highest number of filled grains per panicle, dry weight of grains and weight of 1000 grains [6]. The efficiency of sewage sludge application on maize [7,8], sunflower [9,10], wheat [11], growth, yield, macroelements and metals accumulation was evidenced and reported. Also, positive effects on yield of dwarf bean [12] and growth of flax [13] have been reported at application of different doses of sewage sludge.
Fertilization with compost from municipal sludge associated with mineral fertilization led to better yields for various crops (oat, maize, soybean) in comparison with one type of fertilization [4]. Better crop parameters could be also achieved by using some promising materials, such as essential oils microcapsules [14].
Because of the presence of toxic heavy metals [15], organic pollutants [16][17][18] and pathogenic organisms [18,19], sewage sludge may present environmental risk. To eliminate the presence of pathogens in sewage sludge it is recommended to adopt some techniques as digestion and/or radiation [20]. A positive effect of sewage sludge (irradiated or not) was reported regarding dry matter yield and nitrogen uptake for wheat crop [20].
Having in view the acknowledged beneficial influence of sewage sludge application to different crops, the purpose of the present research was to investigate the sewage sludge effects on growth, yield responses and macronutrients (N, P, K, Ca, Mg) accumulation for oat (Avena sativa L.). The developed experimental model was a bifactorial one (7x2) and the studied factors were: A factorsewage sludge fertilization with 7 degrees representing equivalent doses varying between 0-600 kg/ha and B factormineral fertilization (NPK) with 2 degrees: b1 -N0P0K0 and b2 -N100P100K100.

Experimental design
In order to study the influence of sewage sludge application on yield and mineral nutrition, was used oat (Avena sativa L.), Someșan variety, as test plant cultivated in experimental pots. For experimental model which is a bifactorial one (7x2) (table 1), it were used 56 vegetation pots (20 L capacity), each treatment (totalizing 14) in four repetitions. For experiments was used soil collected from A0 horizon of luvisol meanwhile sewage sludge that was used in experiment was collected from the Wastewater Treatment Plant (WWTP) from Piteşti. sewage sludge -without mineral fertilization, N0P0K0 sewage sludge and mineral fertilizer N100P100K100 (fertilization 100 kg N/ha + 100 kg P/ha + 100 kg K/ha)

Chemical characterization of soil and sewage sludge
Soil and sewage sludge were fully characterized and the performed analyses are presented in Table  2.

Plant sampling and chemical analysis
Plant samples were processed and then analyzed according to methodology described in a previous paper [7], based on Singh and Agrawal researches [23]. Total nitrogen content was evaluated using Kjeldahl method (Gerhardt Vapodest automatic analyzer), meanwhile total phosphorus was assessed spectrophotometrically as molybdenum blue (Cecil 2041 UV/VIS spectrophotometer) and potassium using AES technique (Sherwood Scientific 420 flame photometer). Contents of calcium and magnesium were determined after filtering the digested samples through AAS technique (Thermo Scientific AA Spectrometer).
Sewage sludge chemical characteristics are depicted in table 3. It was analyzed 9 samples and according to the results, investigated sewage sludge contains high levels of organic matter (48.3%, as average) and presents a balanced macronutrient composition which recommends it as fertilizer.
Concerning metal composition of sewage sludge, it must be respected European Directive 86/278/CEE [25] and Order of the Minister of Agriculture, Forests, Waters and Environment no.344/2004 [26], metal content being a restrictive factor for sewage sludge application.
Excepting cadmium concentration which is 7.2 times higher than limit imposed by Order of the Minister of Agriculture, Forests, Waters and Environment no.344/2004 [26], found metal contents (as total form) in analyzed sewage sludge indicate that there are no restrictions concerning application it on agricultural soils and meet the requirements imposed by legislation [25,26].

Influence of sewage sludge application on height, grains yield and straw yield
The data regarding the influence of sewage sludge application associated or not with mineral fertilizers on height of oat plants Someșan variety, evidenced that sewage sludge application produce significant increase of plant height starting with equivalent dose of 100 kg N/ha ( Table 4). The highest height was observed in the case of plants fertilized with doses equivalent with 300 and 400 kg N/ha.
The mixed application of sewage sludge and mineral fertilizers resulted in no statistically assured increases of oat plant height, these being similar with those obtained after sewage sludge fertilization solely.
With respect to grains yield, the application of sewage sludge produced very significant increase of grains yield; the lowest value, as average (30g/pot) was encountered for sewage sludge without mineral fertilization variants, meanwhile the highest one, as average (95g/pot) was obtained for sewage dose equivalent with 500 kg N/ha and 600 kg N/ha (Table 4). Mineral fertilization led to significant oat grains yield in comparison with unfertilized variant.
With the increase of sewage sludge dose it has been observed the straw yield increase, the increase being statistically assured starting with sewage sludge equivalent dose of 100 kg N/ha. The lowest straw yield, as average (25g/pot) was obtained for sewage sludge unfertilized variants and the highest, as average (100g/pot) was obtained after application of sewage sludge dose equivalent with 600 kg N/ha (Table 4). Mineral fertilization increased significantly the straw yield.
Sewage sludge application associated with mineral fertilizers does not provide significant statistical increases of grains yield and straw yield but these parameters are slightly higher in comparison with the variants fertilized organic only.
Some researchers [27] evidenced that the highest oat yields were obtained when was applied a dose of sewage sludge equivalent with 400 kg N/ha. Also, sewage sludge treatments at oat crop at doses equivalent with 200 kg N/ha led to the same yields as those obtained after application of 100 kg N/ha as ammonium sulphate [27].
Mihalache et al. [5] evidenced that fertilization with sewage sludge favored yield increase, but the production decreased with the increasing the application rate of sewage sludge (at 120 t/ha for oat and 90 t/ha for sugar beet).

Influence of sewage sludge application on N, P, K contents in grains
Sewage sludge fertilization, accompanied or not by mineral fertilizers, evidenced that nitrogen contents in oat grains increases with applied sewage dose, being significant from an equivalent dose of 200 kg N/ha (table 5). Mineral fertilization led to statistic significant increases of nitrogen level in oat grains. Association of sewage sludge with mineral fertilizers did not led to statistic significant changes https://doi.org/10.37358/RC.20.4.8050 of nitrogen content, the nitrogen levels being slightly higher than those obtained after sewage sludge application only.
The obtained data, regarding sewage sludge fertilization, associated or not with mineral fertilizers, evidenced that no significant changes were recorded for phosphorus levels in oat grains (Table 5).
Concerning potassium content in oat grains after sewage sludge application, associated or not with mineral fertilizers, it was observed that at low doses equivalent with 200-300 kg N/ha, potassium concentration in oat grains present a significant decrease in comparison with untreated variant (table  5). Mineral fertilization generated a significant increase of potassium levels in oat grains.
Association and application of organic and mineral fertilizers did not produce significant potassium concentration, the obtained concentrations being similar with those obtained after sewage sludge application only.
Average concentrations for phosphorus and potassium in oat grains are in agreement with values reported for oat grains [28] and cereal grains [29,30]. In the case of applied sludge with and without mineral fertilization, the corresponding nitrogen doses correlate statistically very strong with the nitrogen content in grains.
The phosphorus and potassium content in grains analysis correlated with the nitrogen doses resulting from variants with sludge and mineral fertilization reveals statistically strong correlation coefficients (Figures 1, 2, 3).

Influence of sewage sludge application on Ca and Mg contents in grains
Calcium contents in oat grains present a significant increase after fertilization with sewage sludge, the values being statistically assured after application of a dose equivalent with 400 kg N/ha. At a dose equivalent with 600 kg N/ha, the calcium levels (0.24%) increased 3 times in comparison with untreated variant (0.08%) (table 6). Mineral fertilization, accompanied or not by organic fertilization did not led to significant changes of calcium in oat grains, the obtained values being similar with those obtained after organic fertilization.
Similar calcium levels were reported by Jakobsone et al. for oat grains [31,32]. Del Coco et al. found the same calcium levels in wheat grains [30]. Fertilization with sewage sludge, associated or not with mineral fertilizers, did not produced significant changes of magnesium levels in oat grains, found concentrations being considered as normal values and in agreement with those reported for cereal grains [29]. The content of calcium in oat grains correlates statistically very strong with nitrogen doses corresponding to the application of sludge in experimental variants with/without mineral fertilization ( Figure 4). The magnesium content in grains correlates statistically strong with nitrogen doses from variants with mineral fertilization and for those without mineral fertilization the coefficient reveals statistically a not-significant correlation ( Figure 5).

Conclusions
Based upon the findings of this study, the main conclusions are presented below. Starting with equivalent dose of 100 kg N/ha, sewage sludge application produce significant increase of plant height. The highest height was recorded in the case of doses equivalent with 300 and 400 kg N/ha.
Grains yields increase significant with sewage sludge application, the highest value, as average being obtained for sewage dose equivalent with 500 kg N/ha and 600 kg N/ha. Mineral fertilization led to significant oat grains yield in comparison with unfertilized variant. Also, straw yield increased with sewage sludge doses.
Sewage sludge application associated with mineral fertilizers does not provide significant statistical increases of grains yield and straw yield but these parameters are slightly higher in comparison with the variants fertilized organic only.
Nitrogen contents in oat grains increases with applied sewage sludge dose, being significant from an equivalent dose of 200 kg N/ha. No significant changes were recorded for phosphorus levels in oat grains in the case of sewage sludge application, associated or not with mineral fertilizers.
At low sewage sludge doses equivalent with 200-300 kg N/ha, potassium concentration in oat grains after sewage sludge application present a significant decrease in comparison with untreated variant.
Calcium contents in oat grains present a significant increase after fertilization with sewage sludge, the values being statistically assured after application of a dose equivalent with 400 kg N/ha.
Organic fertilization accompanied or not by mineral one, did not led to significant changes of calcium and magnesium in oat grains.