Assessment of soil chemical properties and the carbon sequestration in wheat-based production systems

Increasing soil carbon, both globally and on a farm level, has been considered as a fundamental strategy to reduce atmospheric carbon dioxide and increase soil productivity. Because agricultural ecosystems cover 11% of the earth's surface and wheat is one of the three most important grains in the world, any study on increasing soil carbon through land management and organic additive management can lead to a better understanding of our potential for soil improvement, ecosystem services, higher biomass yields, nutrient recycling, potentially increasing agricultural and ecological productivity. Crop rotation significantly affects the sequestration capacity of atmospheric carbon dioxide, and the change of cropping systems from fallowing to continuous cultivation can affect the rate of carbon sequestration in the soil through the introduction of more organic carbon. Also, managing the use of fertilizers, especially nitrogen, will not only increase crop yields, but also increase carbon sequestration as a secondary benefit to the land. Due to the alkalinity of most soils in arid and semi-arid regions, the addition of organic matter reduces soil acidity and improves plant growth conditions, increases the ability to absorb elements such as phosphorus and iron, moreover has a positive effect on increasing biomass production and carbon sequestration.

This study was conducted to take advantage of a set of desirable crop measures in diverse production systems with the aim of improving the chemical properties of soil and carbon sequestration with a focus on wheat during the two cropping years of 2018-19 and 2019-20 in a farm in Taybad plain. In this study, the factor of the cropping system in four levels (wild rocket-wheat, fallow-wheat, mung bean-wheat and corn-wheat) and the factor of nitrogen fertilizer levels (100, 50% and without nitrogen fertilizer application) were implemented in a randomized complete block design as factorial with three replications. Soil chemical properties such as acidity, organic carbon, total nitrogen, available phosphorus and iron were measured.

The highest amount of acidity was observed in corn-wheat treatment with 50% nitrogen fertilizer supply and the lowest amount of acidity was observed in wild rocket-wheat treatment with 100% nitrogen fertilizer supply. The study of treatments shows that wild rocket and mung bean showed the best results in increasing soil organic carbon and carbon sequestration. Corn less than were able to increase soil organic carbon, although with increasing nitrogen fertilizer supply in the corn-wheat cropping system, organic carbon improved compared to the control treatment. The results also showed that wild rocket-wheat treatment with 100% nitrogen fertilizer supply experienced the highest increase (56.7%) and corn-wheat treatment with 50% nitrogen fertilizer supply experienced the lowest (21.4%) increase in soil organic carbon. Soil nitrogen was also significantly affected by the increase of soil organic carbon mainly in two treatments of wild rocket-wheat (25.6%) and mung bean-wheat (17.9%) in conditions of 100% nitrogen fertilizer supply, while fallow-wheat and Wheat maize without nitrogen fertilizer showed the highest reduction in soil nitrogen content of 15.3% and 20.5%, respectively. Phosphorus and iron levels also increased in all treatments. The results of trait correlation also showed that reducing acidity in alkaline soils is the key to success in increasing plant access to phosphorus (r = -0.37 **) and iron (r = -0.33 **). It seems that the most important factor in the improving farming practices, which leads to an increase in organic carbon and consequently soil fertility, is the removal of fallow and continuous cultivation of agricultural lands.

The results showed that soil organic carbon increased in all treatments and led to increased carbon sequestration and improved soil chemical properties. The results regarding nitrogen also showed that non-use of nitrogen fertilizer in all cropping systems reduce or stop the increase of nitrogen and the need for optimal use of nitrogen fertilizer even in the case of using cover crops or legume as a nitrogen stabilizer. Reducing acidity in alkaline soils is the key to success in increasing plant access to phosphorus, although the capacity of soil organic carbon to retain phosphorus and iron in the soil should not be underestimated. Also, cropping systems affected by the type of crop rotation had different effects on soil properties. It seems that in addition to determining the quantity and quality of plant residues, plant rotation will have a different behavior on the concentration of soil elements depending on the amount of harvest of each nutrient.