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Article

Soil Mineral Nitrogen and Mobile Organic Carbon as Affected by Winter Wheat Strip Tillage and Forage Legume Intercropping

by
Viktorija Gecaite
1,
Jurgita Ceseviciene
2,* and
Ausra Arlauskiene
1
1
Joniškėlis Experimental Station, Institute of Agriculture, Lithuanian Research Centre for Agriculture and Forestry, Pasvalys Distr., 39301 Joniškėlis, Lithuania
2
Chemical Research Laboratory, Institute of Agriculture, Lithuanian Research Centre for Agriculture and Forestry, Instituto av. 1, Kėdainiai Distr., 58344 Akademija, Lithuania
*
Author to whom correspondence should be addressed.
Agriculture 2024, 14(9), 1490; https://doi.org/10.3390/agriculture14091490 (registering DOI)
Submission received: 2 July 2024 / Revised: 23 August 2024 / Accepted: 27 August 2024 / Published: 1 September 2024
(This article belongs to the Section Agricultural Soils)
Browse Figures
Versions Notes

Abstract

:
Diversifying crop rotations by incorporating legumes is recommended to enhance the resilience of agricultural systems against environmental stresses and optimize nitrogen utilization. Nonetheless, ploughing forage legumes or grass-legumes poses a significant risk of nitrate leaching. The study aimed to assess the impact of strip tillage intercropping management on soil mineral nitrogen, water-extractable organic carbon, mobile humic substances content, and winter wheat (Triticum aestivum L.) grain yield compared to forage legume and winter wheat monocropping with conventional tillage. In the intercropping systems, the following bicrops were used: black medick (Medicago lupulina L.) with winter wheat, white clover (Trifolium repens L.) with winter wheat, and Egyptian clover (Trifolium alexandrinum L.) with winter wheat. Research was conducted in two experiments. The results indicated that after implementing strip tillage and winter wheat intercropping, the soil mineral nitrogen content was similar to or lower than that observed in conventional tillage and winter wheat sowing after forage legumes. Winter wheat grain yield in intercrops decreased compared to the legumes monocultures that were ploughed before winter wheat sowing. The highest amount of water- extractable organic carbon was in intercropping growing white clover and winter wheat bicrops or in all fields (except Egyptian clover and winter wheat bicrops) after applying strip tillage. During the research period, the quantities of mobile humic substances and mobile humic acids exhibited similar changes. Their content increased substantially in fields with white clover and Egyptian clover, regardless of whether the legumes were ploughed or grown with winter wheat.

1. Introduction

Soils are the largest reservoir of terrestrial organic carbon (C), containing approximately three times more C than the atmosphere [1]. Soil organic matter (SOM), constituting the largest geological C pool on the earth’s surface [2], is critical to ecosystem sustainability and plays a vital role in maintaining soil fertility, structure, and vitality [3]. In addition, increasing soil organic carbon (SOC) sequestration can mitigate climate change [4,5,6].
A relatively stable part of SOM are humic substances (HSs), constituting substantial amounts (80–90%) of humus. They arise from the decomposition and transformation of plant, animal, and microbial residues. HSs mainly consist of three types—humic acids (HAs), which are soluble but precipitate at pH levels below 2, fulvic acids (FAs), which remain soluble across all pH values, and humin, which is an insoluble residue [2,7,8]. Among all HSs, HAs represent the largest and most important, dynamic, and versatile fraction because of their complex structure and numerous functional groups that interact with other soil components. HAs contributes more available C, which used as a source of energy for soil microbes, improves nutrient supply to plants, and mitigates both biotic and abiotic stresses by regulating soil properties [7,9,10]. The water-extractable organic carbon (WEOC) accounts for only a small portion of SOC; nevertheless, it is considered the most mobile, reactive, and relatively easily decomposable organic carbon fraction. Consequently, WEOC can influence various physical, chemical, and biological processes in both aquatic and terrestrial soil environments [11].
Farming practices affect SOM by altering the input of carbon from crop residues or organic fertilizers and by indirectly affecting SOC turnover through soil disturbance. A combination of practices such as high use of organic inputs, permanent soil cover, and reduced tillage can increase soil carbon sequestration. Tillage is one of the used tools that influences biological C sequestration and impacts greenhouse gas production. Globally, transitioning from conventional tillage to no-tillage has been effective in protecting soils under cropping, improving soil quality by slowing the decline of SOM, and increasing the resilience of cropping systems [12]. Intensive tillage (i.e., plough-based tillage) can destroy soil aggregates [13] and other essential ecosystem services, such as crop biodiversity, and lead to a decline in C and HSs storage in many agricultural soils [1,2,14,15,16]. WEOC responds more rapidly to different tillage or residual management practices than SOC; therefore, it is a potentially more sensitive indicator of agricultural management-induced changes [17,18].
Transition from arable farming to direct seeding for some time can also cause some direct seeding problems [19], and clay loam soils are sensitive to these problems. In contrast, strip tillage combines two soil zones with different functions and properties [20], creating favourable conditions for seed germination and plant growth in the sowing area while the uncultivated inter-row serves to restore soil fertility [20]. By combining cover crops with strip seeding, the negative impact of direct seeding on the soil and the plant can be reduced. Cong et al. [21] indicate that soil C sequestration potential of strip intercropping depends on management practises aimed at conserving organic matter in the soil.
Wheat, grown on approximately 217 million hectares worldwide, is the most extensively cultivated crop and provides around 20% of the world’s food calories and protein [22]. Population growth necessitates additional wheat production to meet food needs. At the same time, due the global climate change processes, there is a need for improvement of technologies, using nutrient use efficiency, sustainable intensification, and climate change mitigation.
Recently, the practice of intensifying and diversifying crop rotations by replacing fallow periods with leguminous crops has emerged as a new approach, making a comeback in an innovative form to boost wheat yields and enhance overall soil health [23]. Diversification of crop systems with leguminous crops improves nitrogen (N) use efficiency [24,25] by providing additional N through the mineralization of legume residues. Legumes can be grown in various ways, such as main crops, catch crops, or intercropped. In arable farming, they are important as supporting crops and can provide many other ecosystem services [19]. According to Domnariu [26], even less is known about the specific effects of legume N on SOC when transitioning to a more diverse cropping system.
The results of earlier research show that leguminous crops can gradually enhance SOC stocks, fraction-C concentrations, macroaggregates stability, soil moisture levels, and total porosity, while also leading to a consistent increase in the yield of following crops [27,28,29]. When combined with reduced tillage, these benefits could be increased [30]. Biological N fixation by legumes is considered a key factor [31]. As a result, retaining legume residues in the soil can sequester more C compared to cereal residues. However, some studies have suggested that the increased soil N levels caused by legume plant residue decomposition may promote microbial activity and extracellular enzymes production via the priming effect, potentially leading to increased local C and N losses [32]. It is clear that the impact of leguminous crops is crucial for C accumulation in the soil. Hu et al. [33] found that after intercropping, the SOC, water-extractable organic carbon (WEOC), and readily oxidized organic carbon content significantly increased in the soil. This is also influenced by the granulometric composition of the soil. As the size of soil aggregates decrease, SOC content and its stability increase. Therefore, when determining general patterns and drawing conclusions, researchers must consider specific local site pedoclimatic conditions.
Incorporating legumes into crop rotations is recommended as a strategy to improve the resilience of the cropping systems against environmental stresses and optimize N resource use. However, ploughing forage legumes or grass-legumes can significantly increase the risk of nitrate leaching, especially in sandy soils [34]. Implementing cover crops and the adoption of reduced tillage are vital conservation practices for boosting SOC and N stocks as well as increasing soil microbial activity [35]. So, greater attention should be focused on the interaction between N and SOC sequestration when combining strip tillage with legume intercropping. We hypothesize that winter wheat (Triticum aestivum L.) and forage legume strip intercropping technologies could optimize crop residue mineralization and increase SOC. The object of the research was legume–winter wheat bicrops. This study aimed to assess the impact of strip tillage and intercropping management on soil mineral nitrogen levels, water-extractable organic carbon, content of mobile humic substances, and the yield of winter wheat grain.

2. Material and Methods

2.1. Site and Soil

Field experiments were conducted in 2018–2020 at Joniškėlis Experimental Station of the Institute of Agriculture, Lithuanian Research Centre for Agriculture and Forestry (LAMMC), situated in the northern part of Central Lithuania’s plain (56°04′ N, 24°16′ E). The soil at the experimental site was classified as an Endocalcari-Endohypogleyic Cambisol (Siltic, Drainic), the texture of which is clay loam on silty clay with deeper-lying sandy loam. The tests were conducted on the soil with the following agrochemical properties of the plough layer (0–25 cm): pHKCL—(6.1), available phosphorus—146 mg P2O5 kg−1, available potassium—276 mg K2O kg−1, humus—2.54%, and total nitrogen—0.14%.

2.2. Design and Details

Two analogous field experiments were conducted during 2018–2019 (Experiment I) and 2019–2020 (Experiment II). In the first year (2018 and 2019), the primary crop was spring oat (Avena sativa L.), variety ‘Migla DS’ (O), which was undersown with black medick (Medicago lupulina L.) variety ‘Arka DS’ (O+BM), white clover (Trifolium repens L.) variety ‘Nemuniai’ (O+WC), and Egyptian clover (Trifolium alexandrinum L.) variety ‘Cleopatra’ (O+EC). Oat and forage legumes (BM, WC, and EC) were also tested in monocultures. For the subsequent years (2019 and 2020), winter wheat (Triticum aestivum L.), variety ‘Gaja DS’, was grown in bicropping with forage legumes (BM+WW, WC+WW, EC+WW) or as a monocrop (WW). Eight different management strategies for winter wheat, incorporating two soil tillage methods—conventional deep inversion tillage (CTS) and strip tillage (STS)—were evaluated (Table 1).
The treatments consisted of pure stands of winter wheat grown after three different forage legume monocrops using conventional tillage and sowing (CTS): O–WW(CTS), BM–WW(CTS), WC–WW(CTS), and EC–WW(CTS). Additionally, wheat was grown in forage legume–winter wheat bicrops using strip tillage (STS): O–WW(STS), O+BM–BM+WW(STS), O+WC–WC+WW(STS), and O+EC–EC+WW(STS). The oats were sown at a density of 450 seeds m−2, while the legume species were sown at 50 seeds m−2 for monocrops and bicrops. Winter wheat was drill-seeded at a density of 450 seeds m−2 under conventional tillage and at 380 seeds m−2 under strip-tillage. The mass of EC froze in winter, while BM and WC were left to grow. The control treatments consisted of winter wheat cultivated in a cereal rotation after oats as a sole crop: O–WW(CTS). Oat straw was used as fertilizer in all experimental plots where oats were grown. The plants were grown according to organic farming standards. The seeds of forage legumes were not inoculated, oats and winter wheat were not fertilized, and no plant protection products were used. The experiment plots were arranged as a complete one-factor randomized block design with four replicates. Each individual plot was 6 × 20 m.

2.3. Sampling, Preparation, and Analyses

The winter wheat grain yield of every separate experimental plot was fully harvested and weighed (with a small-plot combine harvester) when most crops reached the stage BBCH 87. The grain yields were converted to kg ha−1 at 14% moisture basis.
To determine soil mineral nitrogen content (SMN = N-NO3 + N-NH4), soil samples were collected in spring before winter wheat growth resumed, 25 March 2019 and 2020, at two depths (at 0–30 cm and 30–60 cm). Five soil cores were randomly taken from each plot then crushed and stored in a deep freezer (−18 °C) until the N-NO3 and N-NH4 analyses were conducted. Soil N-NO3 concentrations were measured using the potentiometric method with a 1% extract of KAI(SO4)2·12H2O (1:2.5, w/v) [36]. This method involves measuring the electrical potential (voltage) generated by nitrate ions in the solution, which correlates with their concentration. Soil N-NH4 concentrations were measured using spectrophotometry with a UV/Vis Cary 50 (Varian Inc., Palo Alto, CA, USA). This technique measures the intensity of light absorbed by the ammonium ions in the 1 M KCl extract (1:2.5, w/v) solution at a wavelength of 655 nm, which is specific for ammonium determination [37].
Soil samples for agrochemical characterization were taken from the plough layer (0–25 cm) and collected three times: in spring, before oats and forage legumes were sown; in autumn, after winter wheat sowing; and after wheat harvest. Each composite soil sample was air-dried, crushed, and visible roots and plant residues were manually removed, then the soil was passed through a 0.25 mm sieve and stored at room conditions until the water-extractable organic carbon (WEOC) and mobile humic fractions analyses. WEOC was quantified in a deionized water extract (1:5, w/v) using infrared (IR) detection following UV-catalysed persulphate oxidation, with analysis conducted on an ions chromatograph (SKALAR, Netherlands). The procedure adhered to SKALAR’s recommended methodology, using C8H5KO4 as the calibration standard [38].
Mobile humic substances (HSs) were extracted using 0.1 M NaOH [39,40]. A soil suspension with the NaOH solution (v/w, 1:10) was intermittently agitated at ambient temperature for 24 h. Subsequently, 10 mL of a saturated Na2SO4 solution was introduced, and the extract was separated by centrifugation at 3800 rpm for 10 min using a centrifuge (Universal 32, Hettich, Germany). The solution containing MHSs was evaporated to obtain a dry mass (DM) and then quantified spectrophotometrically at 590 nm, using glucose as a standard [41]. To determine the content of mobile humic acids (HAs) in the soil samples, a portion of the extract was acidified to a pH of 1.3–1.5 with 1 M H2SO4 and heated at 68–70 °C to precipitate the HAs. The precipitated HAs were filtered and washed with a 0.01 M H2SO4 solution to remove remaining mobile fulvic acids. The HAs were then dissolved in 0.1 M NaOH, evaporated, and quantified spectrophotometrically. All concentrations of soil compounds were reported on a dry matter (DM) basis. The chemical analyses of the soil were performed at the Chemical Research Laboratory of the Institute of Agriculture, LAMMC.

2.4. Statistical Analysis

Winter wheat grain yield and soil data were analysed with two/three-way analysis of variance (ANOVA). Significant differences among factors and their interactions were assessed with an F-test at p < 0.05 and p < 0.01 levels. Post hoc comparisons were carried out using Tukey’s test at p < 0.05, where means sharing the same letter are not significantly different. The standard error of the mean (SE) was used to indicate error values.
Statistical analyses were conducted using Statistica software, version 7.1 (StatSoft Inc., Tulsa, OK, USA).

2.5. Meteorological Conditions

Weather data were sourced from a meteorological station situated 1.0 km from the experimental site (Figure 1a,b). In the first half of the 2018 growing season, rainfall was close to the Standard Climate Norm (SCN). In April and May, there was no lack of heat and sunshine, which led to good plant development in the early growth stages. The low rainfall in June slowed down plant growth. Dry weather persisted in July and August, with lower rainfall compared to the SCN.
Autumn 2018 was dry, especially in September (only 13.3 mm of precipitation). The low rainfall limited soil processes and the migration of mineral N to deeper layers. December was warm, but rainfall was low (17.5 mm). Following this, 2019 was slightly wetter and the seasonal distribution of rainfall was much more even than in 2018. April was dry, and the dry period ended only at the end of May. This may have had a negative impact on the release of N from plant residues and soil and on winter wheat nutrition. In the third ten-day period of May, 42 mm of precipitation fell, which accelerated the growth of plant biomass. June was unusually warm, with an average daily temperature 6 °C above the SCN. However, the limited rainfall did not lead to a very intense growth of plant biomass. July was the wettest month, with 109.3 mm of rainfall, 36.5 mm more than the SCN.
In August and September (2019), there was sufficient rainfall and warmth for plant development. The autumn of 2019 was much warmer than that of 2018. The months of December–February 2019–2020 were characterised by positive average monthly daily temperatures, which is not common in Lithuania. Spring 2020 was warm and early, but rainfall was low. April was dry, with only 14.9 mm of precipitation, 18.8 mm below the SCN. However, the dry period was not prolonged, with May rainfall close to the SCN. June and July were characterised by an excess of rainfall and warmth, which led to an intensive growth of cereal and forage legume biomass. The 2020 growing season experienced an uneven rainfall distribution, featuring inadequate precipitation during the first half and excessive rainfall in the latter half.

3. Results

Soil mineral nitrogen and carbon were analysed using three-way analysis of variance (ANOVA), considering the factors of year/experiment (2018–2019 and 2019–2020), soil tillage methods (CTS and STS), and legumes and their cultivation strategies (BM–WW(CTS); WC–WW(CTS); EC–WW(CTS); BM+WW(STS); WC+WW(STS); EC+WW(STS)) (Table 2). The studied indicators were most influenced by the year and their interaction with other factors.

3.1. Soil Mineral Nitrogen

Tillage method (p < 0.01) and its interaction with year (p < 0.01) had a significant effect on the SMN content. On average, in 2019, SMN was significantly lower (33.6% on average) with winter wheat grown using strip tillage and sowing compared to conventional tillage and sowing methods (Figure 2a). In 2020, due to meteorological conditions, the differences in SMN were not significant. At the resumption of winter wheat vegetation, substantially more (30.4%) SMN was detected in winter wheat field (WC–WW(CTS)) after WC was ploughed compared to the preceding crop of oats (O–WW(CTS)) (Figure 2b). Other pre-sown legumes (BM, EC) tended to increase (5.9–12.5%) the SMN content. Strip tillage and sowing of winter wheat into forage legumes (except EC) resulted in a 5.7–9.1% increase in SMN content compared to sowing into oat stubble.

3.2. Productivity of Winter Wheat

Statistical analysis showed that winter wheat grain yield was significantly affected by the year (p < 0.01) conditions. On average, the grain yield in 2020 was significantly higher (51%) compared to 2019. The interaction between tillage and cropping also contributed to a significant increase in grain yields (p < 0.01). Table 3 shows the average data for both trials. A comparison of grain yields of winter wheat (oats as a preceding crop) sown in ploughed soil and sown using the strip tillage method showed an average yield reduction of 7%, but this was not significant. The cultivation of forage legumes further accentuated these differences. The winter wheat grain yield increase with forage legume ploughing was between 47 and 58% compared to the control of the oat pre-crop. The highest yield increase was due to ploughing WC (1854 kg ha−1), but it was not significantly different from ploughing other forage legumes.
On average, strip tillage reduced wheat yield by 30% compared to wheat under conventional tillage and sowing. Grain yield decreased from 105 to 348 kg ha−1 (except for EC+WW(STS)). The negative effect of strip tillage and sowing was mitigated by BM (BM+WW(STS)). Here, the grain yield increase was 322 kg ha−1 compared to O-WW(STS). This yield increase was 10% and 5% higher compared to the oat pre-crop with conventional or strip tillage and WW sowing, respectively. Winter wheat yields were lowest when grown in a binary crop with WC+WW(STS), which competed with the winter wheat between the rows. The results show that winter wheat grown in a binary crop with forage legumes has significantly lower grain yield than winter wheat grown after grasses under ploughing and conventional sowing.

3.3. Forms of Soil Organic Carbon

In the year of the experimental set-up, water-extractable organic carbon (WEOC) and mobile humic substances (HSs) were higher in the soil of experiment II and mobile humic acids (HAs) were higher in the soil of experiment I. The WEOC content of the soil depended on the interaction of three factors: year, tillage/sowing method, and crop (p < 0.05) (Figure 3).
During the study period, WEOC changed little (Experiment I) or decreased (Experiment II). In 2018, after sowing winter wheat, the highest WEOC was found in the soil of the binary crop WC+WW(STS) and the lowest in the soil of BM (irrespective of tillage and sowing method). Experiment II showed the highest WEOC in EC+WW(STS) soil and the lowest in BM+WW(STS) soil. These mobile organic carbon compounds decreased after harvesting winter wheat and depended significantly on the year (p < 0,01). There were no differences between the treatments. The studies show that strip tillage and sowing, as well as legume crops, tend to increase WEOC content.
In both study periods, the amount of mobile humic substances (HSs) was significantly increased by year and plant interactions (p < 0.01). On average, HSs increased over the study period. In Experiment I, after sowing winter wheat, the highest HSs was found in the soil of the variants EC–WW(CTS), WC+WW(STS), and EC+WW(STS) (Table 4). Irrespective of tillage methods, legume WC and EC significantly increased the HSs content compared to the preceding crop of oats. Annual EC demonstrated the earliest aboveground mass formation [42].
Experiment II showed that forage legumes BM and WC had a positive effect on the accumulation of these substances. The lowest levels of HSs were found after oats and EC. Tillage and sowing method had no significant effect. These differences were maintained after harvesting winter wheat. In Experiment I, the HSs content was significantly higher in WC and EC plots compared to the control, irrespective of tillage and sowing method. Experiment II showed that the positive effect of BM–WW(CTS) and WC–WW(CTS) remained. In the strip tillage and sowing plots, the highest HSs content was found in the WC+WW(STS) binary crop. It can be concluded that the performance of forage legumes (except WC) is influenced by their growing conditions.
After sowing winter wheat, the content of humified organic carbon compounds—HAs—depended on the interactions between year and tillage (p < 0.05) and year and crop (p < 0.05). On average, the highest levels of HAs were found in the plots of Experiment II under strip tillage and sowing (Table 5).
In Experiment I, annual clover (EC) had the greatest positive effect. It can be argued that the roots and residues of annual plants restructure and prepare to replenish the soil with organic matter, unlike perennial grasses. According to the average data from Experiment II, BM and WC increased the mobile humic acid content by 14.4 and 15.3%, respectively, compared to the oat pre-crop, irrespective of tillage method. The effect of EC was not consistent. After cereal harvesting, the year had the greatest influence on soil HAs (p < 0.01). The data from Experiment I showed that the positive influence of EC remained, with a significant increase in HAs compared to the oat pre-crop. Strip tillage and sowing tended only to increase the HAs content. In Experiment II, the data were less consistent and contradictory. Compared to the data before Experiment II, the HAs content consistently increased. Strip tillage and WW sowing tended to decrease HAs, while the oat pre-crop tended to increase them. Ploughed WC also showed positive results.

3.4. Correlations

This section provides a correlation analysis between the obtained soil characteristics and WW grain yield under different tillage and sowing systems, demonstrating how parameters can interact at different growth stages (Table 6). In both CTS and STS systems, the correlation between HSs and HAs after WW sowing and after harvest is consistently strong (0.53, 0.79 for CTS and 0.90, 0.73 for STS), indicating a close relationship between these two soil characteristics. WEOC shows a stronger correlation with itself across growth stages in CTS (0.71) compared to STS (0.11), suggesting that WEOC is more stable in CTS. Winter wheat grain yield has particularly strong positive correlations with SMN in STS (0.79), underscoring the importance of SMN for yield outcomes, while in CTS, yield has a moderate positive relationship with soil-mobile HSs (0.66).
The overall relationships among variables suggest that conventional tillage may lead to more complex interactions among soil characteristics, whereas strip tillage shows more straightforward, though generally weaker, correlations. These insights help to understand the dynamics of soil quality and crop yield under different tillage practices, highlighting areas for potential improvement in agricultural management.

4. Discussion

Wheat grain yield: In the forage legume–winter wheat strip bicropping system, release of N from legumes was weak (2019) or similar (2020) and did not meet the nitrogen requirements of wheat. Forage legume mass incorporated during ploughing decomposed rapidly and increased the amount of mineral N in the soil, as noted by other researchers [12]. Effective combinations of crop management practices and soil health play crucial roles in regulating the production potential of various crops within conservation agriculture systems [43,44,45]. According to Verma et al. [46], conventional tillage without residue resulted in lower yields compared to zero-tillage with residues and conventional tillage with residues. Research has shown that excessive tillage operations combined with no-residue covering expose the soil to increased water runoff, nutrient losses, reduced microbial diversity, heightened weed competition, and moisture loss due to heat on the bare soil surface [45,47,48] According to Plaza-Bonilla et al. [49], the use of cover crops did not have a significant negative impact on yield. Pittelkow et al. [50] indicate that direct seeding reduced plant yield by 12% without fertilizer and by 4% with the addition of N fertilizer (80–120 kg N ha−1). The decrease in yield depended on the type of plants, hydrothermal conditions, the method of managing plant residues, and N fertilizer rates. After 3 to 10 years, the yield is said to be equivalent to that obtained with conventional tillage [50]. Our research also demonstrated that legumes, tillage practices, and the year influenced winter wheat yield. Consistent with Ernst et al.’s [51] study, the limitations of no-tillage technology have been shown to reduce winter wheat yields. Therefore, it is essential to enhance yields under conservation tillage to achieve the combined benefits of carbon sequestration and crop production [32]. In the short term, the negative effect of strip tillage and sowing can be mitigated by selecting legumes (such as EC in our experiments). We assume that yield increase could be due to the frozen aboveground mass and the nutrients released from it. Several studies have shown that incorporating legumes into a cropping system can increase overall yield by increasing soil organic carbon [26,52].
Soil organic carbon: Soil water-extractable organic carbon (WEOC), as the most active component of SOC, plays a key role in migration and transformation of SOC [17,18]. WEOC represents a collection of dissolved carbon compounds and is a crucial indicator of the labile organic carbon pool in soil [53]. The concentration of WEOC is primarily influenced by soil temperature, water content, and NH4+-N levels [54]. Intercropping increased soil dissolved organic carbon (WEOC) by 2.6–14.5% and accumulation by 8.0–21.1% [54]. This is in line with our results, where legume crops, as well strip tillage and sowing, tended to increase WEOC content.
In soil, the largest proportion of C is humic substances, mainly composed of humin, humic acid, and fulvic acid [2,7,8]. As a reactive part of soil HSs, HAs plays an important role in maintaining soil fertility and nutrient supply [7,9,10]. Several studies demonstrated that humic matter, a humified component of soil organic matter (SOM), consists of plant residues that have undergone transformation processes in the soil and lost their cellular structure [55,56]. Kelleher and Simpson [57] described HSs as an operationally defined fraction of SOM, representing the largest pool of unfavourable organic carbon in the environment. Previous studies have indicated that SOC is more active within large aggregates, and the SOM can be stabilized or protected from decomposition to form stable microaggregates [58,59]. Intercrops have also been found to increase SOM. Total root biomass in intercrops was on average 23% higher than the average root biomass of common plants [21]. Research shows that reduced tillage and no-till can improve soil organic carbon (SOC) sequestration compared to conventional tillage [3].
Higher lability, carbon pools, and the carbon management index are directly correlated with increased carbon fractions in rotations that include legumes and organic nutrition management [43,60,61]. The integration of legumes with organic manures in the rice–wheat cropping system also enhanced both the carbon pool and the carbon management index [62]. In just two years, the addition of residues under zero-tillage and conventional-tillage increased C inputs by 3 Mg ha−1 yr−1 compared to conventional-tillage without residue incorporation [46]. Retaining crop residues under zero-tillage conditions promotes the formation of larger macro-aggregates, and the presence of micro-aggregates within these macro-aggregates serving as a protective barrier for SOC, shielding it from microbial degradation [63].
According to Datta et al. [64], for the last nine years, a considerable amount of crop residues has been recycled in conservation agriculture-based managements (zero-tillage wheat and mung bean with partial residue retention to zero-tillage maize followed by zero-tillage wheat and mung bean). It is known that organic materials (fertilizers, composts, etc.) containing a high FAs content indicates a low degree of maturity and humification, making them unsuitable for plant nutrition; conversely, an increased HAs content and HAs/FAs ratio reflect higher humification [8]. The roots and residues of annual plants restructure and prepare to replenish the soil with organic matter (HSs and HAs), likely under a shorter time, unlike perennial grasses. Therefore, annual clover (EC) in our experiment had the greatest positive effect on HSs accumulation in spring (one-year conditions), but the white clover (WC) had the greatest effect after winter wheat harvest (both years). Jat et al. [65] found that the addition of large amount of C into the soil, primarily from crop residues, eventually decomposes into stable fractions such as HAs. The higher content of HAs in soils under conventional tillage might be attributed to the C from roots and rhizodeposition of rice and wheat crops accumulated over the nine years of the experiment, as evidenced by the grain yields during this period and the avoidance of crop residue burning [64]. Liaudanskiene et al. [66] reported that minimum tillage practices significantly increased the contents of HAs, which were strongly bound to calcium through cation bridging at the soil surface layer. Similarly, Wolschick et al. [8] observed higher amounts of HAs in soil under 27 years of conservation tillage. The results of the present study support the hypothesis of Jat et al. [65,67] that higher carbon input through crop residues, combined with zero tillage and the inclusion of legumes, promotes the conversion of crop residue carbon into SOC. This process leads to the formation of more condensed humic acid under conservation agriculture scenarios, as evidenced by increased SOC and soil aggregation.
Cong et al.’s [21] study suggested that increased biological N fixation and/or reduced gaseous N losses contributed to the increases in soil N within intercrop rotations involving legumes.

5. Conclusions

In clay loam soils, strip sowing of winter wheat into forage legumes resulted in a significantly lower soil mineral nitrogen content (on average, 17.3%) compared to conventional sowing after forage legumes. When applying strip tillage and winter wheat sowing, the grain yield decreased by an average of 29.8% compared to ploughing. The grain yield of winter wheat grown in a binary crop with forage legumes was not significantly lower than that of winter wheat grown after oats under ploughing and conventional sowing. The content of mobile humic substances and humic acids in the soil was influenced by the incorporation of forage legumes (white and Egyptian clover with oat straw) and their interaction with the varying conditions over the years. However, these soil components were not significantly affected by the tillage practices employed.
Future research should focus on the management of forage legumes and cereal binary crops.

Author Contributions

Conceptualization, A.A.; methodology, A.A.; software, A.A., V.G. and J.C.; validation, A.A., V.G. and J.C.; formal analysis, A.A., V.G. and J.C.; investigation, A.A. and V.G.; resources, A.A., V.G. and J.C.; data curation, A.A. and V.G.; writing—original draft preparation, A.A. and V.G.; writing—review and editing, A.A., V.G. and J.C.; visualization, V.G.; supervision, A.A.; project administration, A.A.; funding acquisition, A.A., V.G. and J.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the long-term program ‘Biopotential and quality of plants for multifunctional use’ implemented by the Lithuanian Research Centre for Agriculture and Forestry.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Our institution does not have a data collection database.

Acknowledgments

We acknowledge the technical personnel and other contributors for support in fieldworks and laboratory analyses.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agriculture 14 01490 g001
Figure 1. Monthly average temperature and precipitation at the experimental sites for the periods 2018–2019 (a) and 2019–2020 (b). Note. SCN—standard climate norm.
Figure 1. Monthly average temperature and precipitation at the experimental sites for the periods 2018–2019 (a) and 2019–2020 (b). Note. SCN—standard climate norm.
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Figure 2. Variation in soil mineral nitrogen (SMN) at depths of 0–60 cm based on soil tillage (a) and cultivated plants (b) after regeneration of winter wheat vegetation, 2019 and 2020 average. Note: CTS—conventional tillage with WW sowing, STS—strip tillage with WW undersowing, O—oat, WW—winter wheat, BM—black medick, WC—white clover, EC—Egyptian clover, BM+WW—black medick and winter wheat bicrop, WC+WW—white clover and winter wheat bicrop, EC+WW—Egyptian clover and winter wheat bicrop. Means that share the same letters are not significantly different at p ≤ 0.05, error bars represent SE.
Figure 2. Variation in soil mineral nitrogen (SMN) at depths of 0–60 cm based on soil tillage (a) and cultivated plants (b) after regeneration of winter wheat vegetation, 2019 and 2020 average. Note: CTS—conventional tillage with WW sowing, STS—strip tillage with WW undersowing, O—oat, WW—winter wheat, BM—black medick, WC—white clover, EC—Egyptian clover, BM+WW—black medick and winter wheat bicrop, WC+WW—white clover and winter wheat bicrop, EC+WW—Egyptian clover and winter wheat bicrop. Means that share the same letters are not significantly different at p ≤ 0.05, error bars represent SE.
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Figure 3. Variation in soil water-extractable organic carbon (WEOC) at a depth of 0–25 cm throughout the winter wheat growing season. Note: O—oat, BM—black medick, WC—white clover, EC—Egyptian clover, O+BM—oat undersown with black medick, O+WC—oat undersown with white clover, O+EC—oat undersown with Egyptian clover, WW(CTS)—conventional tillage and WW sowing, WW(STS)—strip tillage and WW undersowing BM+WW(STS)—black medick and winter wheat bicrop, WC+WW (STS)—white clover and winter wheat bicrop, EC+WW (STS)—Egyptian clover and winter wheat bicrop. Means followed by the same letters within columns across separate experiments are not significantly different at p ≤ 0.05; error bars represent SE.
Figure 3. Variation in soil water-extractable organic carbon (WEOC) at a depth of 0–25 cm throughout the winter wheat growing season. Note: O—oat, BM—black medick, WC—white clover, EC—Egyptian clover, O+BM—oat undersown with black medick, O+WC—oat undersown with white clover, O+EC—oat undersown with Egyptian clover, WW(CTS)—conventional tillage and WW sowing, WW(STS)—strip tillage and WW undersowing BM+WW(STS)—black medick and winter wheat bicrop, WC+WW (STS)—white clover and winter wheat bicrop, EC+WW (STS)—Egyptian clover and winter wheat bicrop. Means followed by the same letters within columns across separate experiments are not significantly different at p ≤ 0.05; error bars represent SE.
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Table 1. Crops, tillage practices, and sowing methods used in the rotation sequence for the 2018–2019 and 2019–2020 periods.
Table 1. Crops, tillage practices, and sowing methods used in the rotation sequence for the 2018–2019 and 2019–2020 periods.
Rotation Sequences
Main Vegetation Period 2018 (Exp. I) or 2019 (Exp. II)Autumn
2018 (Exp. I) or 2019 (Exp. II)		Main Vegetation Period
2019 (Exp. I) or 2020 (Exp. II)		Abbreviation
Legumes and Their Cultivation StrategySoil Tillage and SowingCrops
Oats, O (Control)Conventional deep inversion tillage at depth 23–25 cm (CTS)Winter wheat WW (CTS)O–WW(CTS)
Black medick, BMBM–WW(CTS)
White clover, WCWC–WW(CTS)
Egyptian clover, ECEC–WW(CTS)
Oats, OStrip tillage and sowing in oats or forage legumes (STS)Winter wheat WW (STS)O–WW(STS)
Oats and undersown black medick, O+BMBlack medick and winter wheat bicrop BM+WW (STS)O+BM–BM+WW(STS)
Oats and undersown white clover, O+WCWhite clover and winter wheat bicrop WC+WW (STS)O+WC–WC+WW(STS)
Oats and undersown Egyptian clover, O+WCEgyptian clover and winter wheat bicrop EC+WW (STS)O+EC–EC+WW(STS)
Note: O—oat, BM—black medick, WC—white clover, EC—Egyptian clover, WW—winter wheat, O+BM—oat undersown with black medick, O+WC—oat undersown with white clover, O+EC—oat undersown with Egyptian clover, CTS—conventional tillage with WW sowing, STS—strip tillage with WW undersowing WW(CTS)—conventional tillage and WW sowing, WW(STS)—strip tillage and WW undersowing, BM+WW(STS)—black medick and winter wheat bicrop, WC+WW (STS)—white clover and winter wheat bicrop, EC+WW (STS)—Egyptian clover and winter wheat bicrop.
Table 2. Analysis of variance for the effects of year (experiments), soil tillage, and strategy of growing leguminous on soil mineral N, winter wheat grain yield, and organic carbon quality during wheat growth (The table includes sources of variation and significance levels for the F-test of each factor and their interactions).
Table 2. Analysis of variance for the effects of year (experiments), soil tillage, and strategy of growing leguminous on soil mineral N, winter wheat grain yield, and organic carbon quality during wheat growth (The table includes sources of variation and significance levels for the F-test of each factor and their interactions).
EffectsSMNGrain YieldWEOCHSsHAs
After WW SowingAfter WW HarvestAfter WW SowingAfter WW HarvestAfter WW SowingAfter WW Harvest
Year (Y)0.2153.9 **16.7 **7.0 *57.3 **23.7 **4.32 **24.3 **
Soil tillage (ST)13.3 **0.00.20.21.70.38.57 **0.0
Legumes and their cultivation strategy (LCS)2.614.2 **1.90.25.5 **7.7 **1.951.5
Y × ST16.5 **0.00.00.20.02.15.67 *1.6
Y × LCS1.71.30.20.69.0 **7.7 **4.1 *2.7
ST × LCS0.811.4 **1.20.30.20.10.750.7
Y × ST × LCS0.92.23.2 *0.71.00.90.610.5
Note: SMN—soil mineral nitrogen; WEOC—water-extractable organic carbon; HSs—humic substances; HAs—humic acids; WW—winter wheat. Significance at: **—p < 0.05, *—p < 0.01 level.
Table 3. Grain yield of winter wheat grown in monocrops and bicrops, 2019 and 2020 average.
Table 3. Grain yield of winter wheat grown in monocrops and bicrops, 2019 and 2020 average.
CropsGrain Yield
kg ha−1		Change
%kg ha−1
Conventional tillage and sowing of WW
O–WW (CTS), control3175 ± 163.4 ab1000
BM–WW (CTS)4676 ± 401.3 cd1471501 cd
WC–WW (CTS)5029 ± 395.2 d1581854 d
EC–WW (CTS)4693 ± 346.8 cd1481519 cd
Mean 4393 ± 326.7 B1000
Strip tillage and sowing of WW
O–WW(STS)2944 ± 345.4 a93−231 a
O+BM–BM+WW (STS)3070 ± 309.5 ab97−105 ab
O+WC–WC+WW (STS)2827 ± 294.5 a89−348 a
O+EC–EC+WW (STS)3497 ± 407.6 b110322 b
Mean3084 ± 339.3 A701309
Mean of years 20192978 ± 263.1 A1000
20204499 ± 275.2 B1511521
Note: O—oat, BM—black medick, WC—white clover, EC—Egyptian clover, O+BM—oat undersown with black medick, O+WC—oat undersown with white clover, O+EC—oat undersown with Egyptian clover, WW(CTS)—conventional tillage and WW sowing, WW(STS)—strip tillage and WW undersowing, BM+WW(STS)—black medick and winter wheat bicrop, WC+WW (STS)—white clover and winter wheat bicrop, EC+WW (STS)—Egyptian clover and winter wheat bicrop. Means (±SE) that share the same letters with same formatting are not significantly different at p ≤ 0.05.
Table 4. Content of soil-mobile humic substances (HSs) at a depth of 0–25 cm during the winter wheat growing season.
Table 4. Content of soil-mobile humic substances (HSs) at a depth of 0–25 cm during the winter wheat growing season.
Crops Mobile Humic Substances %
After WW SowingAfter WW Harvest
Exp IExp IIExp IExp II
Before experiments0.249 ± 0.0160.276 ± 0.020
O–WW (CTS) 0.243 ± 0.002 a0.280 ± 0.009 ab0.288 ± 0.008 a0.356 ± 0.007 abc
BM–WW (CTS) 0.227 ± 0.013 a0.326 ± 0.003 d0.308 ± 0.018 ab0.386 ± 0.005 bc
WC–WW (CTS) 0.265 ± 0.006 abc0.317 ± 0.006 bcd0.347 ± 0.005 cde0.388 ± 0.005 bc
EC–WW (CTS) 0.289 ± 0.008 bc0.279 ± 0.009 a0.343 ± 0.009 cde0.337 ± 0.011 a
O–WW (STS) 0.243 ± 0.002 a0.297 ± 0.005 abcd0.293 ± 0.012 a0.341 ± 0.009 a
O+BM–BM+WW (STS) 0.237 ± 0.009 a0.316 ± 0.003 bcd0.319 ± 0.011 abc0.354 ± 0.007 abc
O+WC–WC+WW (STS) 0.284 ± 0.012 bc0.316 ± 0.008 bcd0.340 ± 0.008 bcde0.397 ± 0.009 c
O+EC–EC+WW (STS) 0.290 ± 0.015 c0.305 ± 0.015 abcd0.361 ± 0.005 e0.319 ± 0.017 a
Mean:
Year ×
Tillage/sowing 		CTS 0.256 ± 0.009 A0.300 ± 0.008 B0.322 ± 0.010 A0.367 ± 0.008 B
STS 0.264 ± 0.010 A0.308 ± 0.006 B0.328 ± 0.009 A0.353 ± 0.011 B
Year × Crop O 0.243 ±0.002 A0.289 ± 0.008 B0.291 ± 0.010 A0.349 ± 0.008 B
BM 0.232 ± 0.011 A0.321 ± 0.004 B0.314 ± 0.014 A0.370 ± 0.009 B
WC 0.275 ± 0.010 B0.317 ± 0.007 B0.344 ± 0.006 B0.393 ± 0.007 B
EC 0.290 ± 0.011 B0.292 ± 0.013 B0.352 ± 0.008 B0.328 ± 0.013 B
Experiment 0.2600.3050.3250.360
Note: O—oat, BM—black medick, WC—white clover, EC—Egyptian clover, O+BM—oat undersown with black medick, O+WC—oat undersown with white clover, O+EC—oat undersown with Egyptian clover, WW(CTS)—conventional tillage and WW sowing, WW(STS)—strip tillage and WW undersowing, BM+WW(STS)—black medick and winter wheat bicrop, WC+WW (STS)—white clover and winter wheat bicrop, EC+WW (STS)—Egyptian clover and winter wheat bicrop; means (±SE) that share the same lowercase letters within columns or the same uppercase letters at interactions in different growing seasons (not in italics—at Year × Tillage/sowing interaction; in italics at Year × Crop interaction) are not significantly different at p ≤ 0.05.
Table 5. Content of soil-mobile humic acids (HAs) at a depth of 0–25 cm during the winter wheat growing season.
Table 5. Content of soil-mobile humic acids (HAs) at a depth of 0–25 cm during the winter wheat growing season.
Crops Mobile Humic Acids %
After WW SowingAfter WW Harvest
Exp IExp IIExp IExp II
Before experiments0.121 ± 0.0010.091 ± 0.022
O–WW (CTS) 0.099 ± 0.002 abc0.087 ± 0.011 a0.108 ± 0.008 a0.178 ± 0.009 ab
BM–WW (CTS) 0.087 ± 0.019 a0.125 ± 0.016 abcd0.111 ± 0.018 a0.161 ± 0.032 ab
WC–WW (CTS) 0.109± 0.007 abc0.112 ± 0.024 abcd0.131 ± 0.005 ab0.213 ± 0.018 b
EC–WW (CTS) 0.125 ± 0.006 bc0.089 ± 0.020 ab0.142 ± 0.005 bcd0.151 ± 0.011 ab
O–WW (STS) 0.099 ± 0.002 abc0.134 ± 0.007 cd0.108 ± 0.015 a0.174 ± 0.016 ab
O+BM–BM+WW (STS) 0.087 ± 0.011 a0.129 ± 0.017 bcd0.125 ± 0.005 ab0.165 ± 0.019 ab
O+WC–WC+WW (STS) 0.119 ± 0.011 bc0.144 ± 0.015 d0.141 ± 0.012 bcd0.165 ± 0.031 ab
O+EC–EC+WW (STS) 0.129 ± 0.013 c0.130 ± 0.017 bcd0.163 ± 0.011 d0.158 ± 0.016 ab
Mean:
Year ×
Tillage/sowing 		CTS 0.105 ± 0.006 A0.103 ± 0,009 A0.123 ± 0.006 A0.176 ± 0.011 B
STS 0.108 ± 0.007 A0.134 ± 0.006 B0.134 ± 0.008 A0.165 ± 0.009 B
Year × Crop O 0.099 ± 0.002 A0.111 ± 0.012 A0.108 ± 0.008 A0.176 ± 0.009 B
BM 0.087 ± 0.010 A0.127 ±0.010 B0.118 ± 0.009 A0.163 ± 0.016 B
WC 0.114 ± 0.006 A0.128 ± 0.014 B0.136 ± 0.006 A0.189 ± 0.020 B
EC 0.127 ± 0.007 B0.110 ± 0.015 A0.152 ± 0.007 B0.155 ± 0.009 B
Experiment 0.1070.1190.1290.171
Note: O—oat, BM—black medick, WC—white clover, EC—Egyptian clover, O+BM—oat undersown with black medick, O+WC—oat undersown with white clover, O+EC—oat undersown with Egyptian clover, WW(CTS)—conventional tillage and WW sowing, WW(STS)—strip tillage and WW undersowing, BM+WW(STS)—black medick and winter wheat bicrop, WC+WW (STS)—white clover and winter wheat bicrop, EC+WW (STS)—Egyptian clover and winter wheat bicrop; means (±SE) that share the same lowercase letters within columns or the same uppercase letters at interactions in different growing seasons (not in italics—at Year × Tillage/sowing interaction; in italics at Year × Crop interaction) are not significantly different at p ≤ 0.05.
Table 6. Correlation analyses between the obtained soil characteristics using different tillage and sowing systems (n = 24).
Table 6. Correlation analyses between the obtained soil characteristics using different tillage and sowing systems (n = 24).
VariableWEOCHSsHAsWEOCHSsHAsSMNGrain Yield
CTS After WW SowingAfter WW Harvest
After WW sowingWEOC1.000
HSs−0.0131.000
HAs0.3090.5251.000
After WW harvestWEOC0.710−0.2270.0821.000
HSs0.0240.7140.274−0.2261.000
HAs−0.2000.573−0.058−0.3930.7851.000
SMN0.188−0.241−0.0090.2100.142−0.0441.000
Grain yield−0.3310.6620.074−0.1960.6390.538−0.1061.000
STS
After WW sowingWEOC1.000
HSs0.0011.000
HAs0.0460.9001.000
After WW harvestWEOC0.1130.0120.0121.000
HSs−0.1080.5270.4460.3301.000
HAs0.0280.5550.4390.2900.7281.000
SMN−0.3510.213−0.016−0.3120.3040.3611.000
Grain yield−0.3400.3640.153−0.4350.1400.3930.7921.000
Note: WEOC—water-extractable organic carbon, HAs—mobile humic acids, HSs—mobile humic substances; SMN—soil mineral nitrogen, CTS—conventional tillage and sowing, STS—strip tillage and sowing, WW—winter wheat. Correlations in bold are significant at p < 0.05.
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Gecaite, V.; Ceseviciene, J.; Arlauskiene, A. Soil Mineral Nitrogen and Mobile Organic Carbon as Affected by Winter Wheat Strip Tillage and Forage Legume Intercropping. Agriculture 2024, 14, 1490. https://doi.org/10.3390/agriculture14091490

AMA Style

Gecaite V, Ceseviciene J, Arlauskiene A. Soil Mineral Nitrogen and Mobile Organic Carbon as Affected by Winter Wheat Strip Tillage and Forage Legume Intercropping. Agriculture. 2024; 14(9):1490. https://doi.org/10.3390/agriculture14091490

Chicago/Turabian Style

Gecaite, Viktorija, Jurgita Ceseviciene, and Ausra Arlauskiene. 2024. "Soil Mineral Nitrogen and Mobile Organic Carbon as Affected by Winter Wheat Strip Tillage and Forage Legume Intercropping" Agriculture 14, no. 9: 1490. https://doi.org/10.3390/agriculture14091490

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