Rapid intrinsic rates of amino acid biodegradation in soils are unaffected by agricultural management strategy

Amino acids represent one of the largest inputs of dissolved organic nitrogen to soil and consequently they constitute a major component of the organic N cycle. The effect of agricultural management on the rate of amino acid turnover in soil, however, remains largely unknown. The aim of this study was to evaluate in long-term field experiments the effect of fertilizer addition (N, P and K), grazing, pH manipulation (lime addition), vegetation cover and shifts (grassland versus arable) and drainage on the mineralization of 14C-labelled amino acids in agricultural topsoils. Our results showed that the intrinsic rate of amino acid mineralization was rapid for all management regimes, irrespective of the tested soil type. The average (±SEM) half-life of the amino acids in all soils (n=155) was calculated to be 2.3±0.5 h. The relative amount of amino acid-C partitioned into respiration (25% of total C) versus biomass production (75% of total C) was also unaffected by management strategy. The rate of amino acid mineralization was shown to be slightly sensitive to soil pH, peaking at around pH ( CaCl 2 ) 5.0 with an approximate twofold reduction at the pH extremes (pH 3.8 and 6.4). We conclude that management regime has little effect on the intrinsic rate of amino acid mineralization in agricultural soils. We propose therefore that total microbial activity rather than microbial diversity or community structure is likely to be the key determinant governing amino acid turnover in agricultural soils.