Differences of Energy Balance and GHG Emissions in Legume Intercropped Maize Agroecosystem as a Result of Climate Change

1. Introduction

Agriculture exerts the most significant influence on the environment among all human activities and is closely tied to changes in land use, energy consumption and greenhouse gas (GHG) emissions Agriculture creates about 22% Lithuania’s and up to 32% of global CO₂ emissions [1,2,3].

The most important factor in controlling GHG emissions are reduced use of organic fertilizers [4], tillage intensity and fertilization rates [5]. However conversely, in some no-till practices, there were found the significant increase in GHG emissions by 7.1% compared to conventional tillage [6]. Multi-cropping is also an effective example of the use of resources. This provides prerequisites for more efficient fuel and energy consumption, as well as a relatively lower CO₂ equivalent. Multi-cropping also initiates a rise of soil fertility, phytosanitary conditions, protection against weeds, diseases, and pests. Successful increase in yield is also achieved with the help of multi-cropping [7,8], especially in the case of climate change, which frequently amplifies crop and yield losses [9].

It is important to choose the appropriate plants because the functions and benefits are performed differently. Clover, maize, faba bean, and alfalfa could serve as effective companions in multi-crops, given that maize has a high potential for biomass production [10]. Alfalfa and clovers provide distinctive advantages as a perennial crop, contributing to the development of organic matter for enhanced structure, stability, and water retention capacity. Incorporating clovers and alfalfa into a farm's rotation can enhance the yield of other crops and potentially decrease the requirement for chemical inputs. It is notably recognized for its positive alfalfa impact on maize, which utilizes the nitrogen fixed in alfalfa's roots [11]. Faba beans are great plants for intercropping and ecological service [12]. However, multi-crops cultivation has still not been widely studied [13].

There is a lack of a standardized approach for determining greenhouse gas (GHG) emissions in crop production systems, and there remains a necessity to enhance the sustainability of agricultural technologies [14]. Estimating the total amount of carbon inputs in agrotechnologies poses challenges. It is beneficial to convert different inputs into equivalent carbon dioxide emissions for agricultural.

The main aim of this study was to evaluate the fuels, working time, inputs of energy for main materials and biomass energy outputs for production of differently biodiverse single and binary maize crops in humid and arid vegetative conditions. We hypothesize that use of leguminous intercrops in maize cultivation will balance fuel and energy use, also stabilize the GHG emission.

2. Results and Discussion

2.1. Energy Inputs

Energy consumption is a necessary factor in agriculture [15], because agrotechnologies use plenty different powerful machines for seeding, soil tillage, harvesting and crop care [16,17] and other scientists found that the largest part of energy inputs was used in fertilizing chemical fertilizers. The lowest energy input was human labor [18].

In our experiments, energy consumption for human labor increased about 14% when growing intercrops compared to the control treatment (Table 1). Fuel consumption also increased up to 14% in all intercropped cultivations. With the use of agricultural machinery, the same trends emerged as in the previously mentioned analyzed indicators.

The lowest total energy consumption was calculated by the agrotechnology without intercrops (CO).

2.2. Crop Productivity and Energy Indices

Energy is an important driving force of development, but it is particularly important in the agricultural sector, as agriculture is not only a consumer of energy, but also a producer [15], because maize and hemp biomass are the beneficial resource for biofuel production [19,21].

In our study, the highest yield of dried biomass was received in single maize crops without companions (Table 2). The difference in biomass yields between vegetation conditions showed that maize makes excellent use of even lower rainfall amounts, and excess rainfall is harmful. However, in arid years, intercrops are more affected and cope poorly with soil conservation tasks (data are not presented).

Diesel fuel consumption, energy input, energy output, energy efficiency ratio and net energy of the various mechanized technological operations for tillage, sowing, fertilizing, and harvesting are presented in Table 3. In agrotechnologies, energy output is the energy value of the harvest. Therefore, the largest energy output, the energy consumption ratio and the net energy are calculated in control plots without intercrops. In the absence of moisture, intercrops germinate poorly and develop slowly, so various methods of sowing, fertilizing, selecting plant species or varieties, as suggested by some authors [22,23], bring little benefit.

2.3. Environmental Impact

A large proportion of anthropogenic emissions come from industrial processes, but agriculture is considered one of the most polluting sectors in the world. Agriculture is a major source of greenhouse gases (GHG) [24,25]. Considering that climate change is caused by the increasing emission of greenhouse gases due to anthropogenic effects [26], sustainable farming ensures lower emissions to the environment and the entire food chain [27,28].

The GHG emissions for the agrotechnological inputs were recalculated into a CO_2eq system using the conversion equivalents (Table 4).

Total GHG emissions were the highest in intercropped maize cultivations, when assessing fuel (from 13.9% to 31.0%), agricultural machinery (from 13.7% to 21.4%), and sowing work (from 0.9% to 9.6%) compared to the controls K1 and K2, because no agricultural machinery, fuel and labor hours were used for intercrop sowing. In our experiment, GHG emissions varied from 803.8 to 897.8 kg CO_2eq ha⁻¹ and were similar for all technologies. Juarez-Hernandez et al. [29] found that the total GHG emissions in tested maize agrotechnologies ranged from 152.9 kg CO_2eq ha⁻¹ to 3475.8 kg CO_2eq ha⁻¹.

3. Materials and Methods

3.1. Experimental Site

Stationary field experiments on maize intercropping were performed in 2009-2011 and the newest one was started in 2022 at the Experimental station of Vytautas Magnus University, Agriculture Academy. Experimental soil was sandy loam (sand 57.4%, clay 14.9%) Planosol (Endohypogleyic-Eutric, Ple-gln-w [30]. The pH of the soil is 7.1—7.5, content of P₂O₅- 130—249, K₂O - 74—121, MgO - 560—791 mg kg^-1 and N_tot — 1.14—1.30 g kg^-1. Data from 2010 (humid vegetative season) and 2023 (arid vegetative season) were analyzed.

Lithuania is a country with surplus precipitation balance, but in nova days, 300–400 mm precipitation rates distribute not even with several drought periods during vegetative period (Table 5). Meteorological data of 2010 show that the weather during the maize growing season was warm and more humid than usual. The precipitation rate ranged 557 mm. Warm and humid summers are good for maize, but the rainfall was too much. Water often stagnated in the rows, soil aeration and nutritional conditions worsened. In 2023 vegetative season, average air temperatures were similar as long-term average or higher, but vegetative period was arid with 250 mm of precipitations only.

Arid conditions negatively influenced the germination, development, and productivity of legume intercrops, sowed after maize sprouting.

3.2. Treatments and Agronomic Practice

In our study, following treatments were evaluated:

1. Inter-row mellowing (CO);

2. Intercropping and mulching with Crimson/red clover (RD);

3. Intercropping and mulching with Persian clover (PC);

4. Intercropping and mulching with alfalfa (AA).

The field experiment was carried out in 4 replicates. Crops were grown as continued cultivation.

In the fall, before the experiment was set up, the soil was ploughed with a Kverneland semi-screw plough. In the spring, when the soil reached physical maturity, it was cultivated with a compound cultivator KLG - 4.3—4 cm deep. On the same day, mineral fertilizers NPK 5:15:29 were distributed. The fertilizer rate was 300 kg ha^-1. After fertilizing, up to 3 days, the maize was sown with a Kverneland Accord Optima pneumo-mechanical seeding machine in 45 cm wide rows with a distance between seeds of 21 cm. After the maize germinated, the inter-rows were loosened and inter-row Fabaceae crops were inter-sown with a hand seeder for greenhouses, which sowed 4—6 rows. Maize and intercrops were sown according to the intended sowing rates (Table 6).

The inter-rows of maize were mellowed, intercrops and weeds were cut and mulched 2—3 times during the maize growing season until the maize reaches a height of 50—70 cm.

The intercrops were cut with a hand brush cutter ‘Stihl’ FS-550. Intercrops were started to be cut when they reach a height of 20—25 cm. The green mass of intercrops and weeds was distributed in the inter-rows of maize. The inter-rows of Control 1 were mellowed manually.

Pesticides were not used in agrotechnologies. The biomass was harvested at the end of the maize vegetative period in October (after the grain has reached the beginning of hard maturity) manually. In the experiment, the energy of manual work was transformed into machine work (Table 7).

In the calculations of the energy and environmental assessment of agrotechnologies, we used the normative data of the agricultural machinery of the Lithuanian Institute of Economy and Rural Development [31,32]. We used a field area of 2–10 ha for the calculations. The power of tractors varied from 45 to 102 kW, biomass harvester - 250 kW. The data of tractor-operated drills are presented in calculations when up to 200 kg ha⁻¹ seeds were sown.

For cutting the inter-rows, we chose the closest available mounted rotary mower. In the case of a plot up to 10 ha in area, biomass yield up to 12 t ha^-1, and a swath width of 3 m, a 6-furrow biomass harvester was chosen. We modelled that the CO plots, where no intercrops were grown, would have a lower harvester load than in intercropped maize.

In Table 8, we presented the main technical indicators of the technological processes, including machine power, working width, output rate and working time costs, and diesel fuel costs. The highest fuel consumption is determined for harvesting operations. Under higher load conditions, fuel consumption will reach as much as 27.6 L ha^-1.

3.3. Methodology

Samples for maize and intercrops biomass productivity evaluation were taken in at least 5 spots per each experimental plot and for each species of crop. Biomass was dried at a temperature of 105 °C to dry form. The results of dried biomasses are presented in this study.

By selecting the energy equivalents of technological operations (Table 9), it is possible to evaluate the energy efficiency of different agrotechnologies. The seed rates used for the calculations, indicated in Table 2, were fertilized at the rate of N₁₅P₄₅K₈₇ kg ha^-1 in all experimental plots.

It is convenient to evaluate agrotechnologies according to the relative emissions of greenhouse gases. The equivalent of CO₂ gas (CO_2eq) is used for this (Table 10).

A computer program ANOVA from the statistical software SELEKCIJA (vers. 5.00, author dr. Pavelas Tarakanovas, Lithuanian Institute of Agriculture, Akademija, Kedainiu distr., Lithuania) was used for the data analysis. LSD test was performed. Letters refer to significant differences between treatments at p ≤ 0.05.

4. Concussions

The highest energy inputs were calculated in technologies with intercropped clovers and alfalfa or about 30% higher than in single maize crop. Maize crops produced higher biomass yields without competing with intercrops regardless of whether it was arid or humid.

Humid vegetative conditions were more favorable for the development of leguminous intercrops than arid conditions. Under wet conditions, the intercrops biomass was about 4 times higher than under dry conditions. Drought caused clover and alfalfa intercrops biomass losses by 23-40% compared with single maize cultivations. Lowe harvests reduced the energy output by up to 38%, energy efficiency ratio - by up to 2.5 times and net energy - by up to 42%. On the contrary, wet conditions were less suitable for maize and when growing with intercrops they produced about 3-5 t ha^-1 less dried biomass, and when growing sole - about 6 t ha^-1 less biomass than in dry conditions. Due to the higher yield of maize biomass in arid season, better energy indicators of the crop were obtained in arid conditions than in humid. The difference between the vegetation conditions was about 122-123 MJ ha^-1 in all treatments, except for the maize crop with alfalfa, where the difference was 62 MJ ha^-1.

According to the CO₂ equivalent, all tested technologies were environmentally friendly and were similar - differed by about 10%.

We expected that, although intercrops would compete with maize and reduce its productivity, it would produce abundant biomass to compensate for the losses. Due to climate change, the increased number of waterlogging and drought periods make us think about the improvement of intercrops sowing technologies. Sowing intercrops at the same time with maize could solve the problem of its germination, but there would be a problem of abundant weeds. Therefore, it is necessary to study in more detail what will be the effect of the spread weeds on the intercropped maize agroecosystem and its energy and GHG balance.