Effect of Residue Nitrogen Concentration and Time Duration on Carbon Mineralization Rate of Alfalfa Residues in Regions with Different Climatic Conditions

Introduction Various factors like climatic conditions, vegetation, soil properties, topography, time, plant residue quality and crop management strategies affect the decomposition rate of organic carbon (OC) and its residence time in soil. Plant residue management concerns nutrients recycling, carbon recycling in ecosystems and the increasing CO2 concentration in the atmosphere. Plant residue decomposition is a fundamental process in recycling of organic matter and elements in most ecosystems. Soil management, particularly plant residue management, changes soil organic matter both qualitatively and quantitatively. Soil respiration and carbon loss are affected by soil temperature, soil moisture, air temperature, solar radiation and precipitation. In natural agro-ecosystems, residue contains different concentrations of nitrogen. It is important to understand the rate and processes involved in plant residue decomposition, as these residues continue to be added to the soil under different weather conditions, especially in arid and semi-arid climates.
Material and methods Organic carbon mineralization of alfalfa residue with different nitrogen concentrations was assessed in different climatic conditions using split-plot experiments over time and the effects of climate was determined using composite analysis. The climatic conditions were classified as warm-arid (Jiroft), temperate arid (Narab) and cold semi-arid (Sardouiyeh) using cluster analysis and the nitrogen (N) concentrations of alfalfa residue were low, medium and high. The alfalfa residue incubated for four different time periods (2, 4, 6 and 8 months). The dynamics of organic carbon in different regions measured using litter bags (20×10 cm) containing 20 g alfalfa residue of 2-10 mm length which were placed on the soil surface.
Results and discussion The results of this study showed that in a warm-arid (Jiroft), carbon loss and the carbon decomposition rate constant were low in a cold semi-arid (Sardouiyeh). The most suitable temperatures occurred from April to October in arid and semiarid climates and soil moisture is probably the key contributor to the rate of decomposition. The highest carbon loss in alfalfa in the cold, semiarid climate for a period of 8 months was 32.64%. The highest carbon decomposition rate constant was observed in the first 2 months of the incubation time. These results indicate that higher nitrogen residue resulted in greater decomposition of plant residue and lower carbon remaining in all tested climates. The higher nitrogen content of plant residue potentially increases the concentration of nitrogen in crop residue and may increase the decomposition rate.
The strong relation between decomposition and climate has led to the belief that favorable climatic conditions can increase the decomposition rate on a global scale and positively decrease and distribute greenhouse gases in the atmosphere. In arid and semi-arid ecosystems, it is difficult to assess the decomposition rate based on climatic data; it seems to be related to temperature and available humidity. Furthermore, Austin & Vivanco (2006) reported that, in semi-arid climates, the litter decomposition rate decreased by 60 % when solar radiation was attenuated; they concluded that photodegradation exerts dominant control over litter decomposition in a dry ecosystem.
Conclusions The results showed that, precipitation of the study area and soil moisture played a key role in the plant residue decomposition rate. In the cold semi-arid climate which moisture was available for decomposition of plant residues for a longer period of time, OC loss and decomposition rate constant were higher than those obtained for warm-arid and temperate-arid climatic conditions. It may be concluded that crop fertilization, which increases P and N concentrations of plant residue, increases decomposition rate of plant residue but decrease its mean residence time in soils.