Contribution of Winter Wheat and Barley Cultivars to Climate Change via Soil Respiration in Continental Croatia

Abstract

Agricultural greenhouse gas emissions can be reduced by the cultivation of cultivars with lower carbon footprint. Considering the hypothesis that there are differences in soil respiration, due to differences in physiological and morphological characteristics of wheat and barley, the aim of this study is an assessment of soil respiration rates and microclimate under different cover (bare soil, wheat, and barley) and cultivar (four barley and four wheat) types. Soil respiration was determined by in situ closed static-chamber method in continental Croatia, during the 2020/2021 season. The seasonal pattern of the soil respiration was similar for all cultivars, respiration was increasing with crop development stages until maturity, when it decreased until the harvest. Cover type did not have influence on soil microclimate but did have on soil respiration. Bare soil had significantly lower annual respiration rates, compared to the barley/wheat covers. Average annual respiration rates were similar between the barley and wheat covers, as well as between all studied barley cultivars. A significant difference between winter wheat cultivars have only been determined between the Renata (9.78 kg C-CO₂ ha⁻¹ day⁻¹) and El Nino (12.67 kg C-CO₂ ha⁻¹ day⁻¹) cultivars. However, the determination of the total carbon budget is needed, in order to determine the most suitable cultivar, in the light of climate change.

1. Introduction

Climate change represents one of the greatest problems the world is facing today. The impacts of climate change can already be seen across the globe. Croatia is already facing the impacts of climate change, as annual temperature trends are positive and significant for all parts of Croatia and statistically significant decreases in precipitation were found for some Croatian regions [1]. According to local and global climate changes scenarios for the near future, further climate changes and more frequent extreme weather conditions are expected [2]. A sector that will suffer the most from the impacts of climate change is agriculture. The agricultural sector also represents a source of greenhouse gasses. Therefore, agriculture has to both adapt to climate change and contribute to its mitigation.

In the global carbon cycle, carbon continuously moves between the soil and atmosphere. Plants, via photosynthesis, are removing atmospheric CO₂, incorporating carbon into their tissue and, after plant extinction, during the organic matter decomposition, carbon is released back into the atmosphere via plant roots and soil microorganism’s respiration, i.e., soil respiration. Soil respiration in agroecosystems represents one of the significant sources of soil CO₂ emissions to the atmosphere [3,4]. The key factors affecting soil respiration include soil physical properties (such as texture or porosity) [5,6,7], soil chemical properties (such as SOC content or inorganic phosphorus) [6,8], soil biological properties (such as soil microorganisms community) [6,7], land use types (such as annual and perennial croplands or grasslands) [6,9,10], vegetation types (such as forage or cereal crops) [11,12,13], climate patterns, i.e., soil microclimate elements (such as soil temperature and soil moisture) [12,14], as well as agrotechnical measures (such as tillage or fertilization) [3,6,15,16,17,18]. Soil respiration is influencing the global carbon cycle, regulating the global greenhouse effect and global climate [19]. Therefore, soil respiration has to be reduced, in order to conserve soil carbon. Soil organic carbon content has been severely reduced in the past, due to the soil degradation induced by climate change. Loss of soil organic carbon is closely associated with the loss of crop productivity [20] and can lead to increased food insecurity. Therefore, carbon has to be stored in the agricultural ecosystem. Carbon sequestration is considered to be one of the best ways to store carbon in the plant/soil pool [21]. An increase of soil organic carbon content in agricultural soils improves soil quality, soil resilience, and may also contribute to climate change mitigation [22].

There are lot of possibilities for mitigating climate change in the agricultural sector, through implementation of new technologies and sustainable agricultural practices. One of these technologies is agricultural biotechnology, a promising tool for the development of cultivars that can contribute to mitigation and adaptation to climate change [23]. In demanding times of higher food production, the cultivation of enhanced crop cultivars for increased food production was in the past accompanied by environmental problems [24]. Therefore, the development of cultivars with increased yield potential and lower environmental impact through modern breeding technologies and sustainable crop/soil agrotechnical measures were recognized as very important strategies [25]. Development of improved crop cultivars with higher yields, greater resistance to droughts or pest resistance, and greater carbon amount in plant residues or cultivars with lower soil respiration rates are desirable, as they can reduce agricultural greenhouse gas emissions by carbon sequestration in the plant/soil pool, as well as the reduction of cropland expansion and the application of fossil fuels, insecticides, and herbicides with a high carbon footprint [26,27]. To our knowledge, no studies in the literature can be found on the comparison of soil respiration rates under different barley and wheat cultivars. The existing studies are limited to the assessment of soil respiration under single cultivar or comparison of soil respiration rates under different land uses [6,11,12,13,15,16,18]. Differences in the soil–carbon dynamics are the result of the distinct physiological and morphological characteristics of each cultivar. Therefore, the aim of our research was to evaluate the sustainability of the two most cultivated arable crops in Croatia (beside maize), winter barley and winter wheat, and their four different cultivars, in light of climate change, via their impact on soil respiration and soil microclimate.

2. Materials and Methods

2.1. Experimental Site and Treatments

A study on soil carbon loss by soil respiration, under different cover and cultivar types, was conducted in 2020/2021 on experimental site of Agricultural Institute Osijek (Figure 1) near Osijek city (φ = 45°31′56.47′′ N, λ = 18°44′16.07′′ E; 90 m a.s.l). The experiment includes three different cover types (control—bare soil; winter barley—average of 4 different winter barley cultivars; winter wheat—average of 4 different winter wheat cultivars), and eight different cultivar types of wheat and barley, bred by the Agricultural Institute Osijek (AIO). The experimental treatments are:

• C—control, bare soil—black fallow
• BR—winter barley (Hordeum vulgare L.) Rex cultivar—medium late growing two-rowed cultivar with average yield of 10 t ha⁻¹, low habitus (87–92 cm), plant density 6,440,000 plants ha⁻¹
• BL—winter barley (Hordeum vulgare L.) Lord cultivar—medium late growing multi-rowed cultivar with average yield of 10 t ha⁻¹, medium high habitus (95 cm), plant density 4,720,000 plants ha⁻¹
• BB—winter barley (Hordeum vulgare L.) Barun cultivar—medium early growing two-rowed cultivar with average yield of 11 t ha⁻¹, low habitus (80 cm), plant density 8,980,000 plants ha⁻¹
• BP—winter barley (Hordeum vulgare L.) Panonac cultivar—medium late growing multi-rowed cultivar with average yield of 11 t ha⁻¹, medium high habitus (92 cm), plant density 6,710,000 plants ha⁻¹
• WS—winter wheat (Tritucum aestivum L.) Srpanjka cultivar—very early growing cultivar with average yield of 10 t ha⁻¹, very low habitus (64 cm), plant density 9,110,000 plants ha⁻¹
• WR—winter wheat (Tritucum aestivum L.) Renata cultivar—medium early growing cultivar with average yield of 11 t ha⁻¹, low habitus (65 cm), plant density 11,170,000 plants ha⁻¹
• WEN—winter wheat (Tritucum aestivum L.) El Nino cultivar—early growing cultivar ty with average yield of 11 t ha⁻¹, high habitus (73 cm), plant density 10,670,000 plants ha⁻¹
• WK—winter wheat (Tritucum aestivum L.) Kraljica cultivar—medium early growing cultivar with average yield of 11 t ha⁻¹, high habitus (75 cm), plant density 12,320,000 plants ha⁻¹

More on barley and wheat cultivars can be found at [28].

2.2. Soil Properties

In 2020, before the beginning of the research, soil sampling (0–30 cm) was conducted, in order to determine physical and chemical soil properties. Soil at the experimental site has silty clay texture, containing 2.33% sand, 56% silt, and 41.67% clay. Water holding capacity amounts 37.7%, air holding capacity 10.2%, soil porosity 47.8%, and bulk density 1.39 g cm⁻³. Soil pH_KCl amounts 7.24, soil contains 2.3% of humus, 0.11% of total nitrogen, 1.25% of total carbon, 0.06% of total sulphur, 17.9 mg of P₂O₅, and 15.5 mg of K₂O per 100 g of soil.

2.3. Climate Conditions

Climate of Osijek area is continental [29], with an average annual air temperature of 11.7 °C and average annual precipitation amount of 707 mm, in the period 1991–2018 [30]. Furthermore, evapotranspiration amounts 590 mm per year, soil water deficit occurs in period July–September in amount of 72 mm and water surplus in period December–March, in the amount of 116 mm. Analyses of climate conditions during studied period November-July, i.e., the 2020/2021 growing season and the calculation of agroclimatic indicators is made based on climate elements data (mean air temperature and precipitation amount) from Osijek-Čepin main meteorological station (φ = 45°30′9′′ N, λ = 18°33′41′′ E; 89 m a.s.l), of the Croatian Meteorological and Hydrological Service network. The analysis describes the annual course of monthly air temperatures and precipitation amounts, as well as the calculation of the soil–water balance according to the Thornthwaite method. More information can be found in [30].

2.4. Agrotechnical Measures

Culture on the field before the experiment establishment was soybean. Agrotechnical measures at the experimental field, i.e., tillage, fertilization, planting/harvesting dates, weed, and pest control are conducted, as presented in

Table 1. Agrotechnical measures in production of winter barley and wheat cultivars.

Date	Field Operation	Application	Application
		Winter Barley (Hordeum vulgare L.)	Winter Wheat (Triticum aestivum L.)
October 2020	Fertilization	Urea 46% (100 kg ha−1); NPK 7:20:30 (400 kg ha−1)
October 2020	Primary tillage	Up to 15 cm depth	Up to 25 cm depth
October 2020	Secondary tillage	Up to 5 cm depth	Up to 10 cm depth
October 2020	Seeding	Rex (200 kg ha−1); Lord (160 kg ha−1), Barun (220 kg ha−1) i Panonac (200 kg ha−1)	Srpanjka (290 kg ha−1); Renata (kg ha−1); El Nino (250 kg ha−1); Kraljica (270 kg ha−1)
November 2020	Rodenticide application	Arvalin
February 2021	Fertilization	KAN (100 kg ha−1)
March 2021	Fertilization	-	KAN (150 kg ha−1)
March 2021	Herbicide application	Trimur WG (15 g ha−1) + Fluxir (0.5 l ha−1)
April 2021	Fungicide application	Impact 25 SC (0.5 l ha−1) + Tebusha 25% EW (1 l ha−1)
July 2021	Harvest

.

2.5. Measurement of Soil CO₂ Concentrations and Climate Elements

Field measurements of soil CO₂ concentrations and climate elements were conducted once per month during vegetation growing season (November–July). The exception were two winter months (December–January) when measurement was not possible due to unfavorable weather conditions (frozen soil, snow cover). All measurements were conducted in three repetitions. Therefore, total number of measurements amount 189 (7 months’ × 9 treatments × 3 repetitions). Measurements of air temperature, air pressure and relative air humidity were conducted at the height of 1 m above soil surface with Testo 511 and 610 (2011). Soil temperature and soil–water content was determined at 10 cm depth with IMKO HD2 (2011). Soil CO₂ concentrations were measured by in situ closed static-chamber method with portable infrared detector of carbon dioxide (GasAlerMicro5 IR, 2011). The calculation of CO₂ efflux was conducted as:

F_CO2 = [M × P × V × (c₂ − c₁)]/[R × T × A × (t₂ − t₁)]

where:

F_CO2—soil CO₂ efflux (kg ha⁻¹ day⁻¹); M—molar mass of the CO₂ (kg mol⁻¹); P—air pressure (Pa); V—chamber volume (m³); c₂ − c₁—CO₂ concentration increase rate in the chamber during incubation period (μmol mol⁻¹); R—gas constant (J mol⁻¹ K⁻¹); T—air temperature (K); A—chamber surface (m²); t₂ − t₁—incubation period (day).

More on measurement of CO₂ concentrations and climate elements can be found in [31].

2.6. Statistical Analysis, Quality Management and Quality Control

All data were analyzed by statistical Software SAS [32]. Variability between treatments was evaluated with analysis of variance (ANOVA) and tested if necessary with Fisher’s LSD post-hoc test. Significance level was 5% in all statistical tests. Quality management (QM) system is in line with good laboratory practice and Internal and External proficiency testing quality control (QC) were included.

3. Results and Discussion

3.1. Climate Conditions

Climate conditions in the 2020/2021 growing season differed from recent period 1991–2018 (Figure 2). Average air temperature of the growing season 2020/2021 was 10.8 °C and was 0.9 °C lower compared to average of 1991–2018 growing season. Difference in precipitation amounts is also determined between studied growing season and multi-year average. 56 mm less precipitation occurred in the 2020/2021 compared to the period 1991–2018. Furthermore, in the recent period 1991–2018 evapotranspiration on average amounts 415 mm and soil–water deficit occurs in April. In 2020/2021 growing season evapotranspiration was lower by 38 mm and soil–water deficit occurs only in June in the amount of 98 mm (

Table 2. Soil–water balance, according to the Thornthwaite method in the 2020/2021 growing season.

1991–2018
	Nov.	Dec.	Jan.	Feb.	March	April	May	June	Season
p	57.0	52.7	44.6	42.6	43.9	51.2	71.5	80.7	706.7
t	6.5	1.3	0.8	2.2	6.8	12.3	17.3	20.7	11.7
SWR	10.8	57.4	100.0	100.0	80.0	0.0	0.0	0.0	348.2
AE	46.2	6.1	3.7	12.3	64.0	131.2	71.5	80.7	415.6
D	0.0	0.0	0.0	0.0	0.0	14.4	179.6	235.7	430
S	0.0	0.0	40.8	30.3	0.0	0.0	0.0	0.0	71.1
2020/2021
p	18.0	61.4	77.5	36.3	34.4	60.7	58.9	18.4	650.4
t	6.4	4.3	2.5	4.7	5.8	9.4	15.4	23.0	10.8
SWR	86.7	100.0	100.0	100.0	88.4	78.6	27.4	0.0	581.2
AE	31.3	22.4	17.6	33.8	46.0	70.5	110.1	45.8	377.5
D	0.0	0.0	0.0	0.0	0.0	0.0	0.0	98.3	98
S	0.0	25.7	59.9	2.5	0.0	0.0	0.0	0.0	88.1

p—precipitation amount (mm), t—air temperature (°C), SWR—soil water reserves (mm); AE—actual evapotranspiration (mm); D—water deficiency (mm), S—water surplus (mm).

).

3.2. Yearly Soil Respiration Rates and Soil Microclimate Influenced by Cover and Cultivar Types

Analysis of variance (

Table 3. Analysis of variance for yearly soil respiration and microclimate depending on the cover type.

Source	DF	Sum of Squares	Mean Square	F Value	Pr > F	R2	cv
C-CO2
Model	2	2204.23	1102.12	92.12	<0.0001	0.43	38.47
Error	249	2978.89	11.96
Corrected Total	251	5183.12
Soil temperature
Model	2	2.08	1.04	0.01	0.9897	0.0001	48.99
Error	249	25,118.81	100.88
Corrected Total	251	25,120.89
Soil moisture
Model	2	16.60	8.30	0.29	0.7492	0.0023	21.05
Error	249	7146.68	28.70
Corrected Total	251	7163.28

) showed that crop cover (F = 92.12, p < 0.0001) has significant impact on soil respiration but not on the soil microclimate, i.e., soil temperature and soil moisture (F = 0.01, p = 0.9897; F = 0.29, p = 0.7492, respectively).

Average annual soil respiration rate at bare soil (4.83 kg C-CO₂ ha⁻¹ day⁻¹) was significantly lower compared to winter barley (10.72 kg C-CO₂ ha⁻¹ day⁻¹) and winter wheat (11.42 kg C-CO₂ ha⁻¹ day⁻¹). Between winter barley and winter wheat not statistically different respiration rates were determined (

Table 4. Yearly soil respiration rates and soil microclimate influenced by cover type.

	Soil Respiration (kg C-CO2 ha−1 day−1) (LSD = 1.05)	Soil Moisture (%) (LSD = 1.63)	Soil Temperature (°C) (LSD = 3.05)
W. BARLEY	10.72 A	25.08 A	20.63 A
W. WHEAT	11.42 A	25.60 A	20.45 A
CONTROL	4.83 B	25.65 A	20.43 A

(Different capital letters within the columns denote a statistically significant difference between treatments according to the Fisher’s LSD test at a p < 0.05).

). The respiration rates under barely were on average 2.2 times higher and under wheat 2.4 timer higher compared to bare soil.

Higher respiration rates under barley/wheat cover occurred due to plant roots respiration and higher microorganism’s activity. Results in this study are in accordance with [33], who determined a significant influence of vegetation cover on soil respiration. The authors [33] determined 2–3 times higher soil respiration rates under fields with vegetation cover, compared to bare soil. Similar results were obtained by [34,35], who reported approximately 2 times greater respiration rate, and [36], who determined a 2.5 times greater respiration rate on fields with vegetation cover compared to bare soil. Similar respiration rates between barley and wheat cultures could be explained by plant’s physiological and morphological properties, i.e., plant height and plant density. Barley is on average higher (90 cm) and has lower plant density (6,710,000 plants ha⁻¹) compared to wheat that is lower (70 cm) and has higher plant density (10,820,000 plants ha⁻¹). Furthermore, it was observed that presence of crop cover does not significantly alter soil temperature and soil moisture. Soil temperature and soil moisture annually ranged respectively 20.43 °C–20.63 °C and 25.08–25.65% depending on cover type (Table 4). Furthermore, a significant impact of different winter barley and winter wheat cultivars on soil respiration (F = 7.48, p < 0.0001) was determined by the analysis of variance (

Table 5. Analysis of variance for yearly soil respiration and microclimate, depending on cultivar type.

Source	DF	Sum of Squares	Mean Square	F Value	Pr > F	R2	cv
C-CO2
Model	8	883.25	110.41	7.48	<0.0001	0.25	37.01
Error	180	2655.79	14751
Corrected Total	188	3539.04
Soil temperature
Model	8	2.34	0.29	0	1.0000	0.0001	49.27
Error	180	18,415.40	102.31
Corrected Total	188	18,417.74
Soil moisture
Model	8	134.54	16.82	0.60	0.7766	0.03	20.85
Error	180	5039.69	28.00
Corrected Total	188	5174.23

). The same analysis showed that cultivar type does not have a significant impact on soil temperature and soil moisture (F = 0, p = 1.0000; F = 0.60, p = 0.7766 respectively).

Average annual soil respiration rates between barley cultivars do not differ significantly. Annual soil respiration rates are in the range between 9.85 and 11.65 kg C-CO₂ ha⁻¹ day⁻¹ (Figure 3). A significant difference between wheat cultivars was only determined for the Renata (9.78 kg C-CO₂ ha⁻¹ day⁻¹) and El Nino (12.67 kg C-CO₂ ha⁻¹ day⁻¹) cultivars. El Nino has a 30% higher annual soil respiration rate, compared to Renata. This difference could occur due to difference in plant height and plant density. Renata cultivar is ~10% lower and has ~5% greater plant density, compared to the El Nino cultivar. El Nino also has significantly higher soil respiration rate, compared to two barley cultivars: Barun and Panonac. The average annual respiration rate under El Nino was 29% and 24% higher, compared to the ones of Barun and Panonac, respectively. Differences can be attributed to different plant properties of El Nino and Barun, i.e., Panonac cultivars. Barun and Panonac cultivars are, respectively, ~10% and ~30% higher and have, respectively, ~16 and ~38 % lower plant density compared to El Nino cultivar. Considering soil microclimate, it was observed that cultivar types do not significantly alter soil temperature or soil moisture. Soil temperature and soil moisture ranged respectively 20.32–20.68 °C and 24.28–26.97% depending on cultivar type (Figure 3).

Percentage of yearly carbon loss from soil pool determined at the beginning of the research by soil respiration differed between studied treatments by maximal of 5.5% (

Table 6. Total soil carbon loss by soil respiration under different barley and wheat cultivars and bare soil.

	Soil C Content (kg ha−1)	Soil Respiration (kg C-CO2 ha−1 day−1)	Daily Soil C Loss (%)	Yearly Soil C Loss (%)
REX	51 708	11.65	0.0225	8.22
LORD		11.14	0.0215	7.86
BARUN		9.85	0.0190	6.95
PANONAC		10.23	0.0198	7.22
SRPANJKA		11.75	0.0227	8.29
RENATA		9.78	0.0189	6.90
EL NINO		12.67	0.0245	8.94
PANONAC		11.5	0.0222	8.12
CONTROL		4.83	0.0093	3.41

). The lowest yearly soil total carbon loss is determined at bare soil where it amounted 3.41% annually. Comparing the treatments with vegetation cover, average yearly soil carbon loss under barley was 0.5% lower compared to the wheat. Considering barley cultivars, the lowest yearly soil carbon loss was determined for Barun (6.95%) and the highest one for Rex cultivar (8.22%). Considering wheat cultivars, the lowest yearly soil carbon loss is determined for Renata (6.90%) and the highest one for El Nino (8.94%) cultivar. The lowest respiration losses under Barun and Renata cultivar could be explained by the fact that both studied cultivars have the lowest height between studied barley, i.e., wheat cultivars.

Lower soil carbon losses under bare soil, compared to crop cover, were also determined in different agroecological conditions in central lowland Croatia [37]. These results are in accordance with the results obtained in this study. Average soil respiration rate under wheat, obtained by [36], in central lowland Croatia, is in accordance with the values obtained in this research. Ref. [13] determined the range of soil respiration under spring barley of 0.72–102.53 kg C-CO₂ ha⁻¹ day⁻¹ in Poland. The authors have also determined in crop rotation that soil respiration under lupine is higher compared to cereals. At two different locations in Italy, [12] determined the annual soil respiration of 8.97 and 7.43 t C ha^–1 yr^–1 under alfalfa and 4.67 and 5.22 t C ha^–1 yr^–1 under wheat. Ref. [38] have determined in one studied vegetation season 18% higher respiration rate under maize compared to maize/soybean while in the second studied vegetation season significant differences were not determined. Ref. [11] determined 45% and 51% greater soil respiration rate under winter wheat compared to corn and soybean, respectively. However, Ref. [39] have not determined significant differences in soil respiration rates between different crops in Sweden.

3.3. Monthly Soil Respiration Rates and Soil Microclimate Influenced by Cover and Cultivar Type

Analysis of variance (

Table 7. Analysis of variance for monthly soil respiration and microclimate, depending on the cover type.

Source	DF	Sum of Squares	Mean Square	F Value	Pr > F	R2	cv
C-CO2—barley (LSD = 2.18)
Model	6	747.32	124.55	17.28	<0.0001	0.57	25.04
Error	77	554.85	7.21
Corrected Total	83	1302.17
C-CO2—wheat (LSD = 2.61)
Model	6	632.21	105.37	10.23	<0.0001	0.44	28.10
Error	77	793.20	10.30
Corrected Total	83	1425.41
C-CO2—control (LSD = 1.30)
Model	6	85.91	14.32	11.71	<0.0001	0.67	24.30
Error	35	42.81	1.22
Corrected Total	41	128.71

) on the influence of cover type on monthly soil respiration rates showed that bare soil, barley, and wheat have a significant impact on monthly soil respiration rates (respectively, F = 17.28, p < 0.0001; F = 10.23; p < 0.0001; F = 11.71; p < 0.0001).

Depending on cover type, ranges of average monthly soil respiration rates are in the range of 2.74–16.55 kg ha⁻¹ day⁻¹ (Figure 4). Furthermore, soil temperature and soil moisture are, respectively, in the range of 10.06–38.23°C and 18.63–32.10 % (Figure 4). Pattern of monthly soil respiration showed to be similar under barley and wheat cover. Monthly soil respiration rates increased from November 2020 until June 2021 and, afterwards, decreased significantly before the harvest in July (Figure 4). The soil temperatures raised until March and then decreased significantly, compared to the winter months. Afterwards, the temperatures raised significantly until the harvest. Soil moisture under barley and wheat was decreasing during whole growing season, with the exceptions of May and July, when precipitation occurred few days before the measurements where conducted.

No significant differences were determined between studied cultivars, considering the influence of cultivar type on monthly soil temperatures. Considering time of measurement, significant difference in monthly soil temperatures is determined for all studied winter barley and wheat cultivars, which have similar monthly pattern (

Table 8. Monthly soil temperature (°C) rates, influenced by cultivar type and time of measurement.

	NOVEMBER (LSD = 1.79)	FEBRUARY (LSD = 1.64)	MARCH (LSD = 3.67)	APRIL (LSD = 2.86)	MAY (LSD = 1.61)	JUNE (LSD = 4.80)	JULY (LSD = 0.81)
REX (LSD = 3.09)	13.97 D a	13.40 D a	11.40 D a	13.30 D a	23.83 C a	31.57 B a	36.70 A d
LORD (LSD = 2.98)	13.87 D a	13.67 D a	11.40 D a	13.27 D a	24.53 C a	31.33 B a	36.67 A d
BARUN (LSD = 2.84)	13.83 D a	13.87 D a	11.40 D a	12.90 D a	24.90 C a	30.77 B a	36.77 A d
PANONAC (LSD = 2.82)	13.70 D a	14.07 D a	11.27 D a	12.57 D a	25.13 C a	30.60 B a	37.03 A cd
SRPANJKA (LSD = 2.92)	13.57 D a	14.40 D a	10.57 E a	11.97 DE a	24.93 C a	30.40 B a	37.97 A b
RENATA (LSD = 2.92)	13.50 D a	14.47 D a	10.17 E a	11.87 DE a	25.03 C a	30.27 B a	38.22 A ab
EL NINO (LSD = 3.09)	13.43 DE a	14.67 D a	9.83 F a	11.47 EF a	25.17 C a	29.93 B a	37.77 A bc
KRALJICA (LSD = 2.94)	13.30 DE a	14.97 D a	9.67 F a	11.27 EF a	25.13 C a	29.70 B a	38.87 A a
CONTROL (LSD = 1.77)	13.45 DE a	14.77 D a	10.25 F a	11.72 EF a	25.08 C a	29.47 B a	38.23 A ab

(Different letters denote a statistically significant difference between treatments according to the Fisher’s LSD test at a p < 0.05; capital letters-differences between months under same cultivar, lower case letters- differences between cultivars in same month).

). Soil temperatures, under barley cultivars, were similar in the period November–April, after which they started to increase significantly until the harvest (Table 8). Soil temperatures under wheat cultivars were similar, only in the winter months (November–February). In March, they have decreased significantly and afterwards temperatures were increasing significantly until the harvest (Table 8). Average monthly soil temperature ranges under Rex, Lord, Barun, and Panonac were, respectively, 11.40–36.70, 11.40–36.67, 11.40–36.77, and 11.27–37.03 °C (Table 8). Ranges of average monthly soil temperatures under Srpanjka, Renata, El Nino and Kraljica cultivars are, respectively, 10.57–37.97, 10.17–38.22, 9.83–37.77, and 9.67–38.87 °C (Table 8). Monthly pattern of soil temperature under bare soil is similar to the patter determined for barley and wheat cultivars with the temperature range of 10.25–38.23 °C.

Soil moisture varied significantly, considering the influence of the cultivar types and time of measurement (

Table 9. Monthly soil moisture rates (%), influenced by cultivar type and time of measurement.

	NOVEMBER (LSD = 5.43)	FEBRUARY (LSD = 6.84)	MARCH (LSD = 3.82)	APRIL (LSD = 4.10)	MAY (LSD = 2.44)	JUNE (LSD = 5.32)	JULY (LSD = 5.14)
REX (LSD = 5.03)	29.21 AB bc	32.20 A a	31.11 A a	22.26 CD ab	31.03 A a	18.42 D a	24.52 BC a
LORD (LSD = 4.49)	34.39 A ab	27.08 BC ab	23.93 BC c	23.24 C ab	28.02 B bc	17.21 D a	17.93 D c
BARUN (LSD = 5.91)	29.69 A bc	23.68 B b	23.65 B c	20.44 B b	29.81 A ab	20.79 B a	23.77 B ab
PANONAC (LSD = 4.10)	29.09 A bc	25.35 AB b	27.28 AB bc	23.93 B ab	27.49 AB bcd	18.10 C a	18.68 C bc
SRPANJKA (LSD = 5.14)	28.60 AB c	28.84 AB ab	27.40 AB abc	21.70 CD ab	30.91 A a	16.95 D a	24.66 BC a
RENATA (LSD = 4.74)	28.68 AB c	25.45 BC ab	25.93 BC bc	21.27 CD ab	31.90 A a	17.60 D a	23.07 C abc
EL NINO (LSD = 5.53)	31.24 A abc	24.87 B b	28.99 AB ab	25.25 B a	26.95 AB cd	18.83 C a	27.55 A ba
KRALJICA (LSD = 4.79)	35.16 A a	27.06 B ab	28.83 B ab	20.83 C b	28.41 B bc	21.77 C a	18.19 C c
CONTROL (LSD = 4.51)	32.10 A abc	26.57 B ab	25.29 B bc	23.88 BC ab	25.45 B d	19.89 C a	24.56 B a

(Different letters denote a statistically significant difference between treatments, according to the Fisher’s LSD test at a p < 0.05; capital letters: differences between months under same cultivar; lower case letters: differences between cultivars in same month).

). The seasonal pattern of the soil moisture was similar between the studied cultures. The soil moisture decreased with the time of measurement, with the exceptions of May and July, when the precipitation occurred a few days before the measurements were conducted. This resulted in increased soil moisture content. The average monthly soil moisture ranges, under Rex, Lord, Barun, and Panonac barley cultivars, were, respectively, 18.42–31.11, 17.21–34.39, 20.44–29.81, and 18.10–29.09 %. In four out of seven times of measurements, the highest soil moisture content, under barley cultivars, was determined for the Rex cultivar (Table 9). The ranges of the average monthly soil moisture under Srpanjka, Renata, El Nino, and Kraljica wheat cultivars were, respectively, 16.95–30.91, 17.60–31.90, 18.83–31.24, and 18.19–35.16 % (Table 9). Bare soil has a similar seasonal pattern of soil moisture, compared to barley and wheat cultivars, with the monthly soil moisture range of 19.89–32.10 %.

Different winter barley and wheat cultivars showed the same seasonal pattern of soil respiration rates (

Table 10. Monthly soil respiration rates (kg C-CO₂ ha⁻¹ day⁻¹), influenced by cultivar type and time of measurement.

CO2	NOVEMBER (LSD = 2.68)	FEBRUARY (LSD = 3.13)	MARCH (LSD = 3.57)	APRIL (LSD = 5.89)	MAY (LSD = 4.88)	JUNE (LSD = 8.04)	JULY (LSD = 2.16)
REX (LSD = 5.85)	6.07 C bcd	8.61 BC ab	10.55 BC a	12.99 AB a	13.75 AB a	18.02 A ab	11.57 BC a
LORD (LSD = 3.44)	7.59 D bc	7.83 D b	11.34 BC a	12.99 B a	13.75 AB a	16.55 A ab	7.96 CD bc
BARUN (LSD = 5.59)	6.07 BC bcd	7.83 BC b	9.77 ABC a	11.42 AB ab	13.75 A a	14.34 A ab	5.79 C d
PANONAC (LSD = 4.43)	7.59 C bc	8.61 BC ab	10.55 ABC a	10.63 ABC ab	12.26 AB a	14.34 A ab	7.59 C bcd
SRPANJKA (LSD = 3.90)	8.35 C b	9.40 BC ab	9.77 BC a	11.42 BC ab	12.26 B a	20.23 A a	10.85 BC a
RENATA (LSD = 3.45)	4.55 D d	8.61 BC ab	12.12 A a	12.99 A a	11.52 AB a	12.14 A bc	6.51 CD cd
EL NINO (LSD = 6.82)	11.38 AB a	10.96 B a	11.34 AB a	13.78 AB a	14.49 AB a	18.02 A ab	8.68 B b
KRALJICA (LSD = 6.17)	6.83 C bcd	7.83 BC b	11.34 ABC a	12.99 ABC a	13.75 AB a	15.81 A ab	11.93 ABC a
CONTROL (LSD = 1.30)	5.31 B cd	2.74 C c	3.13 C b	6.69 A b	5.57 AB b	5.33 B c	3.07 C e

(Different letters denote a statistically significant difference between treatments, according to the Fisher’s LSD test at a p < 0.05; capital letters: differences between months under same cultivar; lower case letters: differences between cultivars in same month).

). Average monthly soil respiration under Rex, Lord, Barun, and Panonac barley cultivars ranged, respectively, from 6.07–18.02, 7.59–16.55, 5.79–14.34, and 6.07–18.02 kg ha⁻¹ day⁻¹ (Table 10). Ranges of average monthly soil respiration rates under Srpanjka, Renata, El Nino, and Kraljica wheat cultivars were 8.35–20.23, 4.55–12.99, 8.68–18.02, and 6.83–15.81 kg ha⁻¹ day⁻¹, respectively (Table 10). Soil respiration rates increased with the cultivar development stages. During the phenophases of seedling growth, tillering, stem elongation, leaf and spike emergence, flowering, grain filling, and ripening the rates of soil respiration were the highest. In maturity phenophase, the rates of soil respiration decreased significantly under all studied cultivars with the exception of the Kraljica cultivar (Table 10). Soil respiration rates under bare soil was in the range of 2.74–6.69 kg ha⁻¹ day⁻¹. Under the bare soil peak respiration rate was determined in April while peak in respiration rates under barley and wheat cultivars was determined in June. Although significant differences in soil respiration have been determined between studied cultures, none of them showed to be prominent, considering the monthly soil respiration rates (Table 10).

High seasonal variability of respiration rates could be attributed to complex biotic and abiotic factors interactions. The results suggest that soil respiration depends on several soil, climate, and agronomic measures. The increase of soil temperatures contributes to higher respiration rates, as they stimulate plant growth and soil microbial activity that was previously dormant during the winter. Ref. [40] found that soil respiration increases with air and soil temperature increases. Over the spring, soil respiration could have been driven by autotrophic respiration, which is the result of root respiration and decomposition of root exudates, associated with the growth of cereals during the winter. Many authors have determined high seasonal variability and seasonal patterns, which have differed due to different pedological and climate conditions [16,34,41,42,43,44]. The lowest respiration rates during winter, gradually increasing from late spring and peaking in summer, were determined by [16] and were the same results [44] had obtained. Different seasonal pattern reported [41] who found higher respiration rates in the late winter and summer seasons and the lowest one during the autumn. The soil respiration was positively (R² = 0.0195) correlated with soil temperature and negatively (R² = 0.1011) correlated with soil moisture. This suggests that soil temperature plays more important role in soil respiration regulation than soil moisture. The negative correlation between soil respiration and soil moisture could be due to soil moisture distribution during growing season—higher in colder winter and spring months and lower in summer months. Positive correlation with soil temperature has also been reported by [44,45]. Furthermore, Ref. [13] determined, under rye monoculture in Poland, a positive correlation of soil respiration with soil and air temperature. Ref. [16] reported the range of correlation coefficients of 0.4–0.6 between the soil respiration and soil temperature in bamboo Forest in Subtropical China, while [46,47] found a negative correlation between soil respiration and temperature. Similar to our results, a negative influence of soil moisture on soil respiration has been reported by [48] and [49], while [16] reported no significant correlation between the monthly soil respiration rates and soil moisture.

4. Conclusions

In the research on soil respiration and microclimate conditions under different cover and cultivars types, it was determined that barley and wheat cover have a significant influence on soil respiration but not on soil microclimate, compared to bare soil. Soil respiration has similar seasonal pattern under all cultivars. Respiration increased with crop development stages, until the maturity stage, when it decreased significantly before the harvest. Differences in annual soil respiration rates, between the barley and wheat crop covers, were not determined. All studied barley cultivars have similar annual soil respiration rates. A significant difference between El Nino and Renata wheat cultivars was determined, where Renata showed to be the most suitable wheat cultivar, considering soil respiration. Thus, strategies to reduce greenhouse gas emissions in agriculture, such as the choice of crop cultivars that provide lower soil respiration rates, are important for the mitigation of climate change.