Effect of continuous compost application on humus composition and nitrogen fertility of soils in a field subjected to double cropping

We investigated the effect of continuous compost application on humus composition and N fertility of soils in a field subjected to double cropping (paddy rice and barley) for 25 years. Soil samples were collected from three different plots: (a) No-NF, fertilizer containing P and K but no N; (b) F, fertilizer containing N, P, and K; and (c) F+C, fertilizer plus compost. The amounts of total humus, extracted humus, and humic and fulvic acids increased in the order No-NF < F << F+C. The amounts of humic and fulvic acids were 2.7 and 1.7 times larger in the F+C plot than in the F plot, respectively. The degree of humification of the humic acids decreased in the order No-NF < F < F+C. The absorption curves and C-13-NMR spectra (TOSS method) of the humic acids indicated the presence of lignin-like structure, and its degree was the strongest in the F+C plot. The C-13-NMR spectra showed distinct differences in the distribution of carbon species between humic and fulvic acids. In humic acids, the content of aromatic-C, ranging from 37 to 44%, was the highest among carbon species. In fulvic acids, the content of O-alkyl-C, ranging from 45 to 51%, was the highest. The amounts of phosphate buffer-extractable N (PEON) and total N (TN) increased in the order No-NF < F < F+C. The amounts of PEON and TN were 1.2 and 1.7 times larger in the F+C plot than in the F plot, respectively. Present and previous findings indicated that continuous compost application could improve various properties of soils in a field subjected to long-term double cropping.