Three-year Effects of Conservation Tillage and Cover Crop on Selected Soil Quality Indicators and Corn Yield Components

IntroductionConservation agriculture (CA), as a sustainable cultivation system, aims at efficient use of natural resources with least environmental impacts, while achieving food security through increasing yield and crop diversification. CA consists of three main principles: 1- reduction or elimination of mechanical soil disturbance; 2- maintaining a permanent cover of crop residues on soil; and 3- diversification of crops. However, the total area under CA in Iran is less than 5% of arable lands. In Hamedan province, CA is mostly implemented in rainfed farming. Therefore, there is a necessity to expand CA in irrigated areas. Nonetheless, a lack of sufficient technical and local knowledge about CA acts as a barrier for its expansion in irrigated lands. Despite the large body of research conducted on CA, there is no detailed information about the combined effects of cover crops and conservation tillage systems on soil functioning and corn productivity in semi-arid regions of Hamedan province. Therefore, our aim was to study three-year effects of conservation tillage practices (no tillage and minimum tillage) and cover crops (hairy vetch and grass pea) on ​​selected soil quality indicators and yield components of corn in a clay loam soil of a semi-arid region in Hamedan.  Materials and Methods Combined effects of various tillage practices and cover crops on selected soil quality indicators and corn productivity were examined in a three-year experiment conducted in the research field of Bu-Ali Sina University. A factorial experiment in the basis of randomized complete block design with 3 replications and 2 factors were carried out, in which three levels of tillage practices (no tillage (NT), minimum tillage (MT), and conventional tillage (CT)), and three levels of cover crops (hairy vetch (V), grass pea (L), and no cover crop) were the treatments. Surface soil samples (0-15 cm) were collected two weeks after corn harvesting in the third year of experiment. Total organic carbon (TOC), organic carbon stock (CS), active carbon (AC), carbon management index (CMI), basal respiration (BR), alkaline phosphatase activity (APA), bulk density (BD), mean weight diameter of water-stable aggregates (MWD), and available phosphorous (P) and potassium (K) were determined. Corn yield components (including number of kernel rows per corn, number of grains per corn row, ear cob weight, hundred weight of grains, ear weight, grain weights per ear, biological yield and grain yield) were measured.Results and DiscussionThe highest TOC (0.96%), CS (18.7 ton/ha), AC (398 mg/kg), CMI (74.8), BR (0.118 mgCO2/g.d) and MWD (1.82 mm) were observed in MT treatment. However, no significant difference was detected between MT and CT in terms of AC, CS and CMI. Moreover, the lowest TOC (0.74%) was measured in NT, which showed no significant difference with CT treatment (0.83%). Reduced destruction of soil structure coupled with the increased MWD, and increased inputs of crop residues through MT, resulted to the protection of organic matter against microbial decomposition. Soil structuring, represented by BD, was improved under conservation tillage treatments (NT and MT).Among cover crops, hairy vetch treatment demonstrated the highest TOC (1.0%), CS (19.5 ton/ha), AC (427 mg/kg), CMI (80.3) and MWD (1.73 mm). However, these indicators, except CMI, were not significantly different between the two cover crops. On the contrary, these indicators were lowest in the control (no cover crop). Moreover, AC and CMI were not significantly different between grass pea and the control. Carbon stock was increased by 54 and 40% in hairy vetch and grass pea treatments, respectively, relative to the control. In general, cover crop cultivation combined with conservation tillage practices introduced additional biomass to the soil which in turn improved soil organic matter over time and enhanced soil quality.The lowest amounts of biological yield (1663 g/m2), grain yield (507 g/m2), hundred weight of grains 11.0 g), ear weight (91.4 g), grain weights per ear (62.9 g), and number of kernel rows per corn (13) were measured in CT system. In contrast, the highest grain yield (637 g/m2), hundred grain weight (13.6 g), ear weight (108.4 g), and grain weights per ear (81.9 g) were measured in NT treatment. However, the biological yield showed no significant difference between NT and CT. Soil quality improvement in conservation tillage treatments explains the enhancement of certain yield components. Biological yield and number of grains per row demonstrated significant difference between cover crop treatments; the maximum of biological yield (2103 g/m2) and of number of grains per row (44) was measured in hairy vetch treatment. Moreover, the lowest of biologigal yield (1589 g/m2) was observed in the control (no cover crop) treatment. Conclusions All soil quality indicators, except available P, were improved under MT as compared with CT. Our three-year study revealed that among conservation tillage treatments, MT improved majority of soil quality indicators compared to NT. Therefore, minimum tillage practice seems to be more sustainable in this study area. Conservation tillage treatments (MT and NT) also enhanced corn grain yield, grain weights per ear and number of grain rows per ear compared to to the CT. Both cover crops improved most soil quality indicators. Moreover, both cover crops induced significant effect on biological yield, although hairy vetch was more effective than grass pea. As a whole, the integration of minimum tillage with hairy vetch cover crop is considered as a sustainable cropping system for the improvement of soil quality and corn yield in this area.