Net Nitrogen Mineralization as Affected by Residue Quality and Soil Moisture

Introduction Soil organic matter is one of the main sources of carbon, nitrogen, phosphorus and sulfur and the agronomic value of organic materials depends on their nitrogen release. Nitrogen dynamics varies considerably depending on soil properties (e.g. soil texture and moisture content), residue location (incorporation or surface placement residues) and intrinsic characteristics of residues, especially carbon to nitrogen ratio. The presence of carbonaceous compounds easily accessible by microorganisms increases organic nitrogen mineralization whereas more recalcitrant organic residues with large amounts of lignin reduce nitrogen release. Nitrogen content of residue which is rich in N releases and accumulates in soil during decomposition. Considerable portion of nitrogen content of non-leguminous residues harvested at green stage, with C/N ratio lower than 25, might be also released when the residues are incorporated into the soil. Material and Methods In order to study the nitrogen mineralization patterns of residues with different qualities and soil moisture contents, an experiment was conducted at Faculty of Agriculture, Ferdowsi university of Mashhad as slit-plot in time arrangement based on a completely randomized design with three replications. Five mature plant residues including wheat (Triticum aestivum L.), oilseed rape (Brassica napus L.), maize (Zea mays L.), soybean (Glycine max L.) and cotton (Gossypium hirsutum L.) were used. Un-amended soil was considered as control. Soil moisture consisted of three levels of 30, 60 and 100 percentage of field capacity. Litterbag method was used and sampling was conducted in 10, 20, 50, 90, 140, 190, 240, 290, 340 and 390 days after incubation (25°C and darkness) to measure mineral nitrogen. Net cumulative N mineralized was calculated as the difference between mineral nitrogen in each sampling and at day 0 and net N mineralization rate was defined as mineralized nitrogen divided by incubation period. Data analysis was performed using Minitab 16. Means were compared by Duncansʼ test at a significance level of 0.05. Results and Discussion Results indicated that soil mineral nitrogen was almost the same in all three levels of soil moisture in early day of the experiment and increased during incubation period. Mineral nitrogen was significantly affected by residue quality and soil moisture content. Soil moisture as a key factor in residue decomposition and microbial biomass activity affected nitrogen mineralization as the highest (59.9 mg.kg-1) and lowest (26.9 mg.kg-1) mineral nitrogen was found in soil moisture content of 100 and 30% FC, respectively. Net cumulative N mineralized was increased in un-amended control soil during the incubation period and reached to 61 mg.kg-1 in day 390 but a different trend was observed in amended soils. In these treatments mineral nitrogen changes had two distinct phases: the first phase included mineral nitrogen immobilization and the intensity and duration of this phase was related to residue type and especially their initial nitrogen content. The second phase lasted to the end of the incubation period, included nitrogen mineralization. Soil amendment with plant residue led to soil nitrogen immobilization. The highest immobilization was observed in soils containing wheat (-7 mg.kg-1) and cotton (-5.2 mg.kg-1) residues (containing high carbon to nitrogen ratio). No net N mineralization was found in amended soils. The highest net N mineralization rate was found in control followed by soils amended with soybean residues (0.14 mg.kg-1.d-1) and the lowest in soils amended with wheat and cotton. Conclusion Results of the present study indicated that the net N mineralization rate and soil mineral nitrogen was significantly affected by residue quality and residues with higher nitrogen content led to nitrogen immobilization. Soil moisture also played an important role in nitrogen mineralization as higher mineral nitrogen was found in soils with higher moisture content