Nitrogen mineralization of leguminous crops in soils

Organic-N production by legumes is a key benefit of growing cover crops and green manures. A soil sample was mixed with legume residue commonly used as green manure in Kenya at a rate of 500 mg N (kg soil)(-1). Silica sand equal to the weight of the soils was added and mixed thoroughly. The mixture was packed in a leaching tube and leached with 100mL of 5 mM of CaCl2 center dot 2H(2)O and incubated at 30 degrees C. The leaching was repeated every 2 weeks for a total of 16 weeks and analyzed for N as NH4+, NO3-, and NO2-. Five legume residues and five different soils were used in this study. Nitrogen mineralization of the legume residues conformed to an exponential model. Application of a two-components exponential model showed two phases of N mineralization. The relationship between the organic N remaining after each incubation period and time of incubation was controlled by two first-order reactions. The initial fast rate (k(1)) changed to a slow rate (k(2)) at incubation times ranging from 2 to 8 weeks, depending on the legume residue and the soil used. The percentage of N in each phase varied among the legume residue and soils. Linear regression analyses showed that net cumulative amounts of N mineralized from individual legume residues was significantly correlated with the total polyphenols and polyphenol-to-N ratios for two soils. Nitrogen mineralization of dolichos and field bean was significantly and negatively correlated with clay and sand, respectively; of field bean and alfalfa was significantly correlated with C-mic; and of dolichos significantly but negatively correlated with the total N and organic N in soils. Linear regression analysis of the pooled data showed that net cumulative amounts of N mineralized and percentage N mineralized were significantly correlated with C : N ratios of the residues (r = 0.44 and 0.48 at p < 0.05, respectively), and that net cumulative N mineralized was significantly correlated with (lignin + polyphenols)-to-N ratios (r = 0.41 at p < 0.05) and with lignin contents (r = 0.61 at p < 0.001).