Nitrogen mineralization potentials of selected mediterranean soils

Abstract

To gain a better understanding of N availability in Mediterranean soils, 18 surface (0–20cm) soil samples were selected from soil groups of Syria for study of their N mineralization potentials. The chosen soils represented the predominant soil groups found in West Asia and North Africa, i.e. Inceptisols, Vertisols, Aridisols, Mollisols and Entisols. Soils were incubated in tubes at 25 ± 0.5°C after addition of N‐free nutrient solution according to the method used by Stanford and Smith (18). Mineralized N was recovered from tubes by leaching with nutrient solution at 2 week intervals up to 22 weeks of incubation; NH4 and NO3 nitrogen were analyzed in the leachates. The N mineralization potential (No) was estimated using three procedures: (i) application of the empirical linear equation suggested by Stanford and Smith (18) relating (No) to the slope of N mineralized (Nt) with the square root of time (t) i.e., No = 6.5 (δNt/δt1/2); (ii) the exponential equation Nt = No [(1‐exp (‐kt)], with calculation of No and k (the rate constant of mineralization) from best fit to experimental data using the BMDP non‐linear least square equation (NLLS); (iii) application of hyperbolic model equation (9) Nt = No.t/(t1/2 + t), where (t1/2) is the time required for half of No to mineralize. The NLLS equation was used to calculate t1/2 and No.

It was found that: i) estimates of No varied largely among soils, being highest for Mollisols and lowest for a sandy Entisol; ii) differences observed between No values estimated by methods (I) and (II) were not significant, validating the linear relationship between (No) and the slope of δNt/δt1/2 for Mediterranean soils and identifying it as a simple practical tool for estimation of (No); 3) higher estimates of (No) were obtained by the hyperbolic compared with the exponential models, though values obtained with the exponential and hyperbolic models were well correlated (with r2= 0.71); and 4) small differences in the rate of mineralization constant (k) (using the exponential model) were observed among various soils with an average of k = 0.0397 ± 0.0015/week and t1/2 equivalent to 18.5 ± 0.7 weeks.

It was concluded that in spite of large differences in (No) values among soils tested, rates of mineralization were similar. This suggests that the forms of organic N compounds contributing to the mineralizable forms of N in Mediterranean environments are similar among soils.