مجله آب و خاک (May 2024)
The Effect of Cultivation Systems on Stability Index and Organic Carbon of Aggregates in Soils with Long-Term Sugarcane Cultivation History
Abstract
IntroductionSoil aggregate stability is a crucial indicator for evaluating soil structure, quality, and health. This index affects the physical and hydrological functions of the soil, which, in turn, depend on plant primary production and the capacity of organic carbon decomposition. Soil organic carbon plays a positive role in the formation and stability of soil aggregates. Soil organic carbon (SOC) causes a rapid decrease in water penetration into soil aggregates by creating a water-repellent coating around them and increases their stability against instant wetting stress. Land use and management, including cultivation systems and tillage methods, have an important impact on the stability and size distribution of soil aggregates. Mechanized sugarcane cultivation has a long history in Khuzestan province, particularly in Haft Tepe sugarcane cultivation and industry. Haft Tepe Agriculture is the first sugar production unit in Iran. Despite the increase in the use of chemical fertilizers, the yield of sugarcane crops has been decreasing due to the destruction of the physical properties of the soil. The study aimed to investigate the effects of different sugarcane cultivation systems on soil physicochemical-biological properties and soil stability indices in parts of Khuzestan province. Materials and MethodsSoils were sampled from the surface of five farms in the Haft Tepe sugarcane cultivation complex located in the northwest of Khuzestan province. The farms included single-row, new planting cultivation (S-P); single-row, third ratoon cultivation (S-R3); double rows, new planting cultivation (D-P); double rows, first ratoon cultivation (D-R1); and uncultivated land (barren) that had been left unused for a long time. Soil organic carbon content, active carbon content, basal respiration, induced respiration, water-stable aggregates, and aggregate organic carbon fractions were measured in the sampled soil. Mean weight diameter (MWD) and geometric mean diameter (GMD) of soil aggregates were also calculated. Results and DiscussionThe study found that the organic C content was highest in the double-rows+new planting (D-P) method and lowest in uncultivated land (0.95% and 0.12%, respectively). The increase in plant density, biomass, and plant residue addition in D-P cultivation has led to an improvement in SOC content. The higher SOC content in cultivated lands compared to uncultivated land indicates the positive effect of many years of cultivation and irrigation. Among the different cultivations, double-row new planting (D-P) cultivation had significantly higher active carbon. In D-R1 cultivation, returning plant residues to the soil increased the SOC (0.59%) and active carbon content. The burning of sugarcane plant residues during harvesting and land preparation for new sugarcane cultivation in S-P fields appears to have led to a decrease in active carbon. Basal respiration and induced respiration values were significantly higher in single-row, third ratoon (S-R3) and double-row, new planting (D-P) cultivations, respectively. In S-R3 cultivation, the older plants and increased root biomass provided more rhizospheric organic C for microorganisms, resulting in higher microbial activity and respiration. Microorganisms transform and decompose soil organic matter, which is a source of energy for their metabolic processes. Therefore, there is a close relationship between organic matter and soil microorganisms. Lower basal respiration in newly planted lands may be due to the process of land preparation for cultivation. Additionally, single-row new-planted farms had a clayey texture, which could reduce soil respiration. In general, the recycling of organic matter and microbial activity is lower in fine-textured soils compared to coarse-textured soils. The highest MWD and GMD were found in single-row, third ratoon (S-R3) and single-row, new planting (D-P) cultivations. The uncultivated land had the lowest MWD and GMD, indicating unstable soil structure due to low SOC content. The lower MWD observed in S-P cultivation could be related to tillage and hilling up operations. S-R3 cultivation had more plant residues compared to other cultures. Higher plant ages and increased root biomass and rhizodeposits led to an increase in soil aggregate formation and stability. Soil tillage, which reduces soil organic carbon, can decrease the stability of soil aggregates and structure. The S-P and D-P cultivations had the highest value of coarse aggregates (larger than 2 mm) and fine aggregates (0.53-2 mm). The highest amount of medium aggregates were observed in S-P, D-P, and D-R1 cultivations. Agricultural operations can break large soil aggregates into smaller ones, while low SOC content and burning of sugarcane residues can reduce the formation of large aggregates. The study found statistically significant differences in the OC content of aggregates among the different cultivations. The highest content of aggregates OC was found in coarse aggregates (0.25-2.0 mm) of D-R1, D-P, and S-P cultivations. ConclusionThis study investigates the impact of mechanized and long-term sugarcane cultivation on the physical and biological properties of soil. Overall, the water stable aggregates and MWD were found to be unsuitable in some of the studied fields due to the low amount of SOC. This is primarily caused by the annual burning of sugarcane residue. Therefore, returning plant residues after harvesting is suggested as a significant solution to improve problems related to compaction, soil instability, and their harmful consequences.
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