Natural 15N and 13C abundance in Andisols influenced by long-term fertilization management in Japan

Two field experiments were conducted on Andisols in Japan to evaluate the changes in the natural N-15 and C-13 abundance in the soil profile and to determine whether the values of delta(15)N could be used as an indicator of fertilizer sources or fertilizer fate. The 6-year experiment conducted at the National Agricultural Research Center (NARC) consisted of the following treatments: application of swine compost (COMPOST), slow-release nitrogen fertilizer (SRNF), readily available nitrogen fertilizer (RANF), and absence of fertilization (CONTROL). Experimental plots located at the Nippon Agricultural Research Institute (NARI) received cattle compost at different rates for 12 years; a forest soil at this site was sampled for comparison. Swine compost application led to a considerable change in the delta(15)N distribution pattern in the soil profile, with the highest delta(15)N values recorded in the top 20 cut layers of the COMPOST plot, decreasing in the sequence of CONTROL greater than or equal to RANF > SRNF, mainly due to the relatively high delta(15)N value of swine compost and its subsequent decomposition. In contrast, SRNF application resulted in the lowest delta(15)N values in soil, indicating the presence of negligible nitrogen losses relative to input and low nitrogen cycling rates. Values of delta(15)N increased with compost application rates at NARI. In the leachate collected at 1-m depth, the delta(15)N values decreased in the sequence of COMPOST > RANF CONTROL > SRNF. The delta(13)C values in soil peaked in the 40-60 cm layers for all the fertilizers. The delta(13)C value was lowest in forest soil due to the presence of plant residues in soil organic matter. These results indicated that the delta(15)N values in the upper soil layers or leachate may enable to detect pollution sources of organic or inorganic nitrogen qualitatively in Andisols.