The effect of crop residues quality on N mineralization under drought stress in a calcareous soil

The incorporation of plant residues into soil is a recommended management practice for maintaining or increasing the level of soil organic matter. Although, its effect may be controlled by the soil type, climate and the initial quality of the plant residues. On the other hand, climate on a global scale is changing and the increase of drought stress is predicted in a large scale. As a result of these changes, all soil processes that are related to soil moisture and temperature, including mineralization of nutrients are being affected. This study was conducted to investigate the interactive effects of drought stress and plant residues quality on N mineralization in soils treated with different plant residues.
For this purpose, four types of plant residues including alfalfa, clover, corn and wheat shoots were collected. Plant residues-treated soils along with a control (without plant residue application) were incubated under three soil moisture levels 25, 35 and 55% of water holding capacity at 25৹C for 150 days. At the end of incubation period, net nitrification, net ammonification, net nitrogen mineralization and net production of soluble organic nitrogen were measured.
The results indicated that the maximum amount of net nitrification (282.33 mg NO3- kg-1), net ammonification (52.11 mg NH4 kg-1), net nitrogen mineralization (334.44 mg NO3-㑾 kg-1) and net changes of soluble organic nitrogen (141.43 mg N kg-1) were observed in the alfalfa-treated soils. The measured indices were significantly increased as the level of soil water during the incubation increased. Net nitrification (r=0.798***), net nitrogen mineralization (r=0.816***) and net changes of soluble organic nitrogen (r=0.803***) had a positive and significant correlation with nitrogen concentration of the plant residues. Negative significant correlations were observed between the measured N transformation indices and lignin content (r= -0.589***, r= -0.605*** and r= -0.612***) and ratios of C/N (r= -0.418***, r= -0.410*** and r= -0.424***) and LG/N (r= -0.650***, r= -0.655*** and r= -0.657***), respectively. Net ammonification did not significantly correlated with net nitrification (r=-0.007), net nitrogen mineralization (r=0.051) and net changes of soluble organic nitrogen (r=0.048). Under drought stress conditions, net nitrification (r=.998***) was more strongly correlated with net nitrogen mineralization than net ammonification (r=0.051).
In conclusion, the effect of drought stress on the N transformation indices were not similarly observed among the plant residue-treated soils. The soils treated with plant residues with greater N content (alfalfa and clover) showed to be more negatively affected by the drought stress compared to those treatments contained less initial N or greater C:N or lignin:N ratios (corn and wheat).