The Effects of Tillage and N Application Rate on Soil Quality in Corn-Canola-Corn Rotation

 Sustainable production in agriculture is closely related to the proper soil chemical, physical, and biological conditions which are considered as the main functions of soil organic matter. The amount of soil organic matter, especially soil organic carbon (SOC), in agricultural ecosystems depends on the tillage practices. Conventional tillage (CT) which generally uses moldboard plow, results in soil losses by intense erosion, a net loss of nutrients and organic carbon. Toward sustainable agriculture, decreasing plow intensity of CT through application of conservation tillage strategy in which tillage practices are omitted or limited to a considerable extent, has been reported as an essential alternative. The objective of this study was to evaluate the effect of tillage management practices and N application on soil chemical and physical properties as well as SOC in a corn-based rotation on a clay loam textured soil in semi-arid climate of Hashtgerd, Iran.

 A field experiment as the split plot design with three replications carried out in the research farm of agricultural research department of Atomic Energy Organization of Iran in two successive growing seasons during 2011-13. The tillage systems were (CT) conventional tillage (moldboard, rotary, and leveler) and; (MT) minimum tillage (disk) assigned as the main plot; and N rates of application, as the subplots, were 0, 50, 150, 250 kg ha-1. In CT treatment, moldboard plow to a depth of 25-30 cm was used as the primary tillage once in autumn and once in spring each year. As the secondary tillage, CT plots were rotavated to 10 cm depth in spring. MT treatment included two trips over the plots with disk harrow cutting to a soil depth of approximately 10 cm prior to sowing. Soil pH, bulk density (BD), total nitrogen (TN), soil organic carbon (SOC), exchangeable K and available P were then evaluated. Soil samples were collected in September 2013 after the end of three growing seasons from 0-30 cm depth at 5 locations per plot using a 3.5 cm diameter coring tube.

 The results showed that short-term (2 years) effect of tillage systems on soil pH, BD, TN, SOC and exchangeable K as well as available P, was not significant (p ≤ 0.05). However, N application rate significantly (p ≤ 0.05) changed soil TN, BD, exchangeable k and available P. Soil TN increased significantly (p ≤ 0.05) by increase in N application rate as the highest amount of TN was 1036 and 968 mg kg-1 in 250 and 150 kg ha-1, respectively. As the soil samples were taken after crops harvest, soil TN is illustrative of the residual soil N and high amount of TN implies the excessive N application. Soil BD decreased significantly in 250 kg N ha-1. Increasing the N application rate would decrease soil BD by increasing root growth. Exchangeable K and available P decreased significantly (p ≤ 0.05) by increasing N application rate. The lowest amount of soil exchangeable K, and available P was detected for N rate of 250 and 150 kg ha-1, 127 and 130 mg kg-1 for K, and 13.43 and 14.24 mg kg-1 for P, respectively. Increased N application promotes plant growth and improves nutrient uptakes such as K and P, consequently, the amount of soil exchangeable K and available P would decrease.

 Toward sustainable agriculture, conservation tillage seems to be an effective strategy to maintain crop yields as well as soil chemical, physical, and biological properties in the long-term. However, based on the results, tillage systems (CT and MT) had no significant effects on SOC as well as other investigated soil properties in the studied site in the short-term. However, N application rate increased soil TN and decreased BD, exchangeable K and available P. It seems that longer-term investigations are needed to evaluate the probable effects of different tillage systems on soil properties particularly SOC