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Soil nitrogen (N) fractions play an important role in contributing toward the availability pool of N in soil and thus enhancing the availability of N to the growing crops. Besides this, the distribution of these forms of N in soil also helps in understanding the conditions that control their availability to the crops. These fractions are relatively dynamic and respond more readily to the organic input like application of crop residues. Therefore, a long-term field experiment (2005–2012) was conducted at the ICAR-Central Research Institute for Dryland Agriculture, Hyderabad, India, for 8 years to assess the long-term effect of graded levels of surface crop residue application under minimum tillage on the quantum of N fractions in soil and their quantitative relationship with crop yield and N uptake under sorghum-cowpea cropping system in rainfed semi-arid tropical (SAT) Alfisol. The results of the study indicated that the graded levels of surface residue application under minimum tillage significantly influenced most of the N fractions studied, except nitrate-N, unidentified–N and fixed NH 4+ -N. Among the inorganic N fractions studied, the exchangeable NH 4+ -N was significantly influenced by the residue treatments and it varied from 20.4 mg kg −1  to 30.7 mg kg −1  whereas the nitrate N was not influenced by the residue application and varied from 14.4 mg kg −1  to 19.3 mg kg −1  across the treatments. Among the surface crop residue application treatments, the use of crop residue at 6 t ha −1  recorded significantly highest exchangeable NH 4+ -N (30.7 mg kg −1 ) as well as NO 3 -N (19.3 kg ha −1 ) followed by the treatment receiving sorghum residue at 4 t ha −1  (28.1 kg ha −1  and 17.8 kg ha −1 ). The total hydrolyzable N varied from 416.9 mg kg −1  to 545.7 mg kg −1  across the treatments. Among the total hydrolyzable N sub-fractions, the hydrolyzable NH 4+ -N ranged from 53.3 mg kg −1  to 71.7 mg kg −1 , hexosamine N from 13.0 mg kg −1  to 22.3 mg kg −1 , amino acid N ranged from 307.0 mg kg −1  to 384.3 mg kg −1 , and unidentified N from 43.7 mg kg −1  to 67.4 mg kg −1  across the treatments, and constituted 12.7%,, 3.8%, 72%, and 11.5% of the total hydrolyzable N, respectively, in the soil. When these N fractions were regressed with the crop response parameters such as sorghum grain yield (Y Sorghum yield ), cowpea grain yield (Y Cowpea yield ), sorghum N uptake, (Y Sorghum N uptake ), cowpea N uptake (Y Cowpea N uptake ), and available N in soil (Y Available N ), the coefficients of multiple determinations (R 2 ) were found to be 0.833, 0.877, 0.961, 0.958, and 0.976, respectively, and were found to be significant ( P  = .05). The magnitude of significant coefficients of multiple determination (R 2 ) indicated the importance of these nitrogen fractions in soil in influencing the crop yields of sorghum and cowpea, total N uptake by the crops, and nitrogen availability in the soil.

communications in soil science and plant analysis

Long Term Effect of Different Levels of Surface Crop Residue Application on Hydrolyzable and Non-Hydrolyzable Nitrogen Fractions in Sorghum ( Sorghum Bicolor (L.) Moench)  – Cowpea ( Vigna Unguiculata ) System in Rainfed Alfisols