Effects of Size and Placement Depth of Plant Residues on Organic Carbon and Nitrogen Dynamics

Prediction of mineralization rate of organic carbon and nitrogen amounts of plant residues is important due to plant nutrient management, carbon dioxide production and environmental issues. Plant residues characteristics such as total nitrogen content (N), carbon: nitrogen (C/N), lignin content and particle size, Soil characteristics (texture, structure, pH and the microbial population) and Climate (temperature and moisture) are the most important factors affecting plant residues decomposition. Decomposition process of plant residues is influenced by substrate quality, decomposer community and environmental factors. Within a given climatic region, litter chemistry is the main determinant of litter decomposition. Litter decay and nutrient release are controlled by the litter quality, including the nitrogen (N) concentration of the litter, the carbon to nitrogen (C/N) ratio, as well as other chemical properties. To investigate the effects of size and placement depth of plant residues on organic carbon and nitrogen dynamics, a split – split plot layout based on a randomized complete block design and three replications was conducted using litter bag method. The factors were depths of incubation periods of plant residues (1, 2, 3 and 4 months), placement of plant residues (5, 15, 30 and 45 cm) and sizes of plant residues (0.2 - 0.5, 1 - 2 and 5 - 10 cm) which were located in main, sub and sub-sub plots respectively. At the end of the incubation period, the litter bags were pulled out of the pots; after the weights of the remaining plant residues in the bags were measured, the residue organic carbon was measured via the dry combustion method at 450°C for 5 h and the total nitrogen via the kjeldahl method. We analyzed the collected data during desert-lab studies by SAS/STAT software release 9.1. Statistical differences among size and placement depth of plant residues and time duration were determined using a generalized linear model (Proc GLM), P ≤ 0.05 and LSMEANS, which allows mean comparisons even when data points are missing. Results of data variance decomposition indicated that size and placement depth of plant residues had a significant effect on carbon and nitrogen loss at the probable level of 1℅ .The highest organic carbon and nitrogen loss were measured after four month of incubation and when the size and the depth of placement of plant residues were 0.2 - 0.5 and 30 cm, respectively. The lowest organic carbon and nitrogen loss were also obtained after the first month of incubation and when the size and the depth of placement of plant residues were 5 - 10 and 5 cm, respectively. After four months of incubation 49.73 and 54.07% of organic carbon and 34.48 and 39.78 of organic nitrogen of plant residues mineralized when the depths of placement of plant residues were 5 and 30cm respectively. Aridity, soil hilling and availability to nutrients are determining factors of the carbon cycle in the decomposition process. Conceptually and analytically advanced models from diverse studies suggest three factors affecting decomposition in arid ecosystems: quality and quantity of the organic matter under decomposition, the physical environment (including temperature, precipitation and soil type) and the nature and entity of the decomposing organs in the soil. From the results it was concluded that when the soil moisture level is a limiting factor for plant residue mineralization, increasing the depth of placement of plant residues enhances the rate of mineralization of organic carbon by providing sufficient moisture for plant residues decomposition. The results also showed that reducing plant residues particle size with increases the surface area and plant residues contact with the soil, enhances the rate of decomposition of plant residues.