Study site. A field experiment was conducted in two growing seasons (2018-2019) at the research farm of Agriculture and Natural Resources Faculty, University of Mohaghegh Ardabili, located in Babolan village, Ardebil, Iran. Ardebil is located between latitude 38˚15́N and longitude 48˚15́E and 1350 m above sea level. The climate in the study area is semi-arid and cold (very cold winters, and mild summers) according to the Koppen classification. The climatic conditions of the area during the experiment are indicated in Table 1. The soil type was loamy with 35, 42, and 23 (%) sand, silt, and clay, respectively. In the last year, the field was fallow with the dominance of annual broadleaf weeds such as common Amaranth (Amaranthus retroflexus L.) and goosefoot (Chenopodium album). To determine the initial soil chemical properties (PH, soil organic matter, Zn2+, K+, P, total N, lime, and base saturation), soil samples were taken at depths of 0-20 cm soil with several replications in the field and are shown in Table 2. Soil pH was measured in a 0.01mol L−1 CaCl2 suspension. The chromic acid digestion method was used to determine soil organic matter. Available P was determined by spectrophotometer and Zn2+, K+ were extracted with a flame atomic absorption spectrophotometry. Total N was measured by the Kjeldahl method.
Field establishment and treatment application
The experimental design was a factorial based on a randomized complete block design with three replicates in each growing season. Experimental treatments have consisted of faba bean densities (25, 35, 40, and 80 plants per square meter) and different levels of nitrogen (N) fertilizer in forage corn (0, 100, 200, and 300 kg ha−1). Al-Suhaibani expressed that the optimum plant density of faba bean to obtain high productivity can range 10 to 100 plant per m2 and in order to, agronomic management of weeds, faba bean density was considered higher in this experiment. The size of each plot was 3 m × 4 m which separated with a 1m path between replicates and to avoid mixing effects of fertilizers, a 1 m path between all plots was considered. Faba bean (Vicia faba L. cv. Shadan/G-Faba-133) (LineF6/Latt338/08) was used in this experiment. This cultivar was introduced by Seed and Plant Improvement Institute of Iran in 2016. 1. 5. 5. 6 code is a registry code for this cultivar in Seed and Plant Certification and Registration Institute. Seeds were sown on 3th March after soaking in water and inoculated with Rhizobium leguminosarum var viciae. To improve nodulation of faba bean plant, 20 kg ha−1 of N fertilizer was added to the soil as starter fertilizer. It was also the first time the faba bean was planted in the area. Weeds controlling were done manually at the emergency to vegetative stage. The faba bean was harvested manually at the physiological maturity stage. After harvesting, the entire plant residues were left on the soil surface as mulch. Thereafter, maize seeds were planted on faba bean residue rows, as the second planting, with the population of 80000 plants ha−1 without plowed of soil. Early maturing maize hybrid KSC 201 (a cross between K1263/17 × S61 inbred lines) was sown on 23th and 25th June 2018 and 2019, respectively. This cultivar is developed by Seed and Plant Improvement Institute of Iran in 2015. 1. 4. 1. 57 code is a registry code for this cultivar in Seed and Plant Certification and Registration Institute in Iran. The maize seedlings were emerged 7 days after sowing, on average, in each growing season. Due to the soil calcareous conditions and making iron (Fe) unavailable for plant uptake and to the prevention of severe iron deficiency in the plant, iron foliar application (Fe-EDTA) was done in three stages and with ten days intervals in the plant. N fertilizer was applied in the form of urea at the V5 stage of maize development by superficial broadcasting (in three stages). Due to the cool weather, forage corn was harvested at the kernel milk stage and 100 days after sowing. The germination test was over 98% for both maize and we have prepared seeds by official permission with Seed and Plant Improvement Institute (SPII), Karaj, Iran. Seeds were hand-planted and irrigation was done properly and uniformly to grow successful crops for both crops. During the growing season of both plants, aphids appeared that were naturally controlled by ladybirds.
Sampling and Analyses
At the maturity stage of the faba bean, samples were taken from a quadrat of 0.5m X 0.5m for each plot. Agronomic data were collected such as the number of pods, and seeds (per plant), yield and total dry biomass (tons ha−1). The seeds and pods were removed from the plants in each plot and were counted. Seed yield (per hectare) was determined by harvesting seeds of plants from per plot and dried in an oven at 65°C for 48h and weighted by digital scale (0.01g). For total dry biomass, all parts of the plant at maturity from the above-ground were harvested and dried oven 65°C for 48h and weighted by digital scale (0.01g). Finally, all data was converted to the hectare.
To calculate the amount of Nitrogen mineralization of faba bean residues was used mesh bag technique. Mesh bags of polyamide nylon were made with a size of 20 × 10 cm. Mesh bags (each sample was contained 100gr of Faba bean root) were buried in the 20 cm deep of soil and were out of the soil after the second week and weekly.
Soil samples were collected of 0–15cm soil depth and from three random spots in each plot before harvesting of faba bean. The soil composite sample was dried under shade and passed through a 2 mm sieve for soil physical and chemical analyses (water infiltration rate, organic carbon, organic matter, EC, bulk density, soil moisture content, soil moisture stock, and total porosity). The soil infiltration rate was determined by a double ring infiltrometer method, based on measuring the height of water infiltrated into the soil (three replicate per experimental unit between the two central plants). The soil organic carbon was obtained by Walkely and Black procedure, Organic matter (OM) was computed from multiplying organic carbon by 1.724 (“Van Berminelen factor”). Soil electrical conductivity was measured in a 1:25 soil/water solution using EC meters (model: Mi 180 Bench Meter). Bulk density was determined by the core method. The total porosity (TP%) was calculated by TP = [1-(BD/2.65)] × 100. Soil moisture content was determined by the gravimetric method and with the following formula (the soil sample was weighted, then dried in the oven 105°C for 72h and weighed again);
Soil moisture stock was calculated by the following calculation:
Three plants of corn were selected randomly and hand-cut above the soil surface in each plot thereafter were weighed to determine fresh forage yield. All above parts of the plant were dried in an oven at 70°C for 72h and the weighted and dry matter production of these samples was calculated (data converted to hectare). The plant samples were ground with a mill and passed through a 2 mm sieve. Sieved samples were packed in small paper bags and sent to Central Laboratory of Kharazmi University (Tehran, Iran) for total nitrogen analysis (by Kjeldahl method) and N accumulation value (kg ha−1) was estimated with the following formula:
Data gotten from this research were analyzed for differences in variance (ANOVA) using the software of SAS (version 9.4) to determine the main plot and interactions treatment effects. Significant differences between the treatment means were tested with Least Significant Difference (LSD) value at 5% level of probability.