Evaluation of crop residues for mineralizable nitrogen in soils

A series of laboratory incubation experiments were conducted to evaluate various plant residues viz. soybean (Glycine max), lentil (Lens culinaris), pea (Pisum sativum), and wheat (Triticum aestivum) incorporation for potentially mineralizable N in three soils of Swat and Peshawar valleys. Soil samples at 0- 30 cm were collected from selected sites and amended with ground plant residues at 3% w/w in the absence or presence of urea N at 100 mg kg-1 soil. The samples were then incubated at 30 degree C acid taken out at 0, 1,2,4, 8, 12 and 16 wk for analysis of total mineral N. The mineral N content in the un- amended soil (control) increased gradually by 39 mg in Swat soil 1, 53 mg in Swat soil It and 21 mg kg-1 in Warsak soil during 16 wk of incubation indicating greater N mineralization potential in Swat than in Warsak soil. Wheat straw amendment always exhibited N immobilization even in the presence of urea N in all the three soils. All the four legume residues promoted N mineralization during 16 weeks of incubation. It was, however, observed that mungbean and pea residues exhibited immobilization during the first four week but later promoted mineralization. The lentil and soybean residues, on the other hand, promoted N mineralization from the very beginning and continued to do so till the end of 16 weeks in all the three soils. Urea amendment further promoted N mineralization in all the legume residue treatments. These results suggested that all the four legume residues tested had large potential for mineralizable N in soil, the amount and time of availability however varied with the type of residue. On the basis of these results, the lentil and soybean residues can be applied to soil just before sowing whereas those of mung and pea need to be applied at least 4 week before sowing. Wheat straw incorporation in soil cannot be recommended at this stage.