Nitrogen Mineralization in Semiarid Soils of Morocco: Rate Constant Variation with Depth

Development of drought-resistant crops through breeding for extensive root systems emphasizes the importance of subsoil N mineralization for crop production in arid regions. This study was conducted to determine the influence of soil depth on N-mineralization rate, total N, and amino acid N contents for eight representative soils of the subarid zone of Morocco. Nitrogen-mineralization ability within the profiles was evaluated through long-term soil incubation/leaching experiments. Total N and amino acid N were measured by the Kjeldahl method and acid hydrolysis, respectively. A hydrolysable-N-production rate was also evaluated, through successive acid hydrolysis of samples from two soil profiles. The changes with depth for the three variables studied were described by two equations. One is a power law of the type Y = AD⁻ᴮ, where Y is one of the variables, A and B are two positive constants, and D is depth. The second is an exponential equation, Y = A exp(−BD). Parameter B had values of 1.3 and 0.04 for N-mineralization rate, 0.7 and 0.021 for amino acid N, and 0.40 and 0.012 for total N for the power-law and exponential models, respectively. Significant differences were found between the rates of change with depth (B value) of the three quantities studied using the second equation. The hydrolysable-N-production rate decreased with depth in the profiles, indicating a reduction of the chemical degradability of N compounds in deeper horizons. These results suggest that the decrease of N mineralization with depth may be attributed to a recondensation of amino acids in complex biochemical compounds more resistant to biodegradation. Joint contribution of the Dep. of Soil Science, Inst. of Agronomy and Veterinary Science Hassan II and Agro/MBLA, Faculty of Agronomic Sciences, Catholic Univ. of Louvain.