Amino acid 15N in long-term bare fallow soils:: influence of annual N fertilizer and manure applications

Long-term dynamics of amino acids (AAs), from a bare fallow soil experiment (established in 1928 at INRA-Versailles, France), were examined in unamended control (Con) plots and plots treated with ammonium sulphate (Amsul), ammonium nitrate (Amnit), sodium nitrate (Nanit) or with animal manure (Man). Topsoil (0-25 cm) from 1929, 1963 and 1997 was analysed for C, N and N-15 content and distribution of 18 amino acids recovered after acid hydrolysis with 6 M HCl. With time, soil N, C and AA content were reduced in Con, Amsul, Amnit and Nanit, but increased in Man. However, the absolute N loss was 3-11 times larger in Man than Nanit, Amsul, Amnit and Con, due to the much higher N annual inputs applied to Man. From 1929 to 1997 in Con, Amsul, Amnit and Nanit the whole soil and non-hydrolysable-N pool delta N-15 increased associated with the loss of N (indicative of Rayleigh (NN)-N-15-N-/14 fractionation). No delta N-15 change from 1929 to 1997 was found in the hydrolysable AA-N (HAN) pool. Fertilizer N inputs aided stabilization of soil AA-N, as AA half-life in the mineral N fertilizer treatments increased from 34 years in 1963 to 50 years in 1997. The delta N-15 values of alanine and leucine reflected both source input and (NN)-N-15-N-/14 fractionation effects in soils. The delta N-15 increase of ornithine (similar to 6 parts per thousand) was similar to the whole soil. The delta N-15 change of phenylalanine in Con (decrease of 7 parts per thousand) was related to its proportional loss since 1929, whereas for Amsul, Amnit, Nanit and Man it was associated with isotope effects caused by the fertilizer inputs. However, the soil delta N-15 value of most individual amino acids (IAAs) did not significantly change over nearly 70 years, even with mineral or organic N inputs. We conclude for these bare fallow systems that: (i) delta N-15 changes in the whole soil and non-hydrolysable AA pool were solely driven by microbial processes and not by the nature of fertilizer inputs, and (ii) without plant inputs, the delta N-15 of the HAN pool and (most) IAAs may reflect the influence of plant-soil interactions from the previous (arable cropping) rather than present (fallow) land use on these soil delta N-15 values.