Characterization of the amino acid composition of soils under organic and conventional management after addition of different fertilizers

The classical nitrogen (N) cycling model has provided good understanding of inorganic N dynamics in agricultural soils, but largely ignores organic N available to plants. The ability of numerous crop plant species to take up and use amino acids underlines the importance of this N pool in agricultural systems; therefore, the soil free amino acids (FAA) pool was quantified in soils under organic (organic soil) and conventional (conventional soil) management after addition of different types of fertilizer. After application of the same amount of N as urea, alfalfa, rice straw, or compost in the organic soils and urea or alfalfa in the conventional soils, water-extractable amino acid composition and concentrations, and inorganic and microbial N were measured during a 56 day soil incubation. Alanine, glutamic acid, glycine, isoleucine, leucine, phenylalanine, serine, tryptophan, and valine were the most abundant soil FAA. Organic and conventional soils did not significantly differ in their soil FAA composition and concentrations. Urea significantly modified FAA composition, but only in organic soils, suggesting that urea disrupts microbial structure and/or metabolic pathways in organic soils. Alfalfa and compost did not alter FAA composition and concentrations, indicating that any pulses of amino acids from these materials are short lived. On the contrary, straw significantly increased FAA concentrations after 15 days, coinciding with an increase in microbial biomass N. FAA concentrations remain low and have a largely constant composition in both organic and conventional soils; however, the addition of some fertilizers can significantly alter FAA composition and concentrations, which may affect the importance of amino acid N in the total N budget of plants. These findings warrant further research into the mechanisms controlling soil FAA composition and concentration in agricultural soils.