Comparison of Nitrogen mineralization from 15N-labeled organic amendments under flooded and upland conditions

Knowledge of N availability from organic amendments is a key to improve N use efficiency and reduce environmental pressure from agriculture. Nitrogen mineralization from N-15-labeled cattle dung compost and rapeseed cake was investigated under flooded and upland (60% of water holding capacity) conditions in an incubation experiment for 63 d at 25degreesC. The relative abundance of N in the cattle dung compost by the simple step-wise acid hydrolysis method was in the following order: labile N (37% of total N, refluxing with 1 M HCl for 3 h, H1-N) > non-hydrolyzable N (32%) > recalcitrant N (18%, 3 M HCl for 3 h, H2-N). There was no significant difference in the N-15 abundance between total N and N in each fraction of the cattle dung compost. For the rapeseed cake, the H1-N accounted for 81% of total N and the N-15 abundance of total N and H1-N was higher than the N-15 abundance of H2-N and non-hydrolyzable N. In the cattle dung treatment, inorganic N-15 was the highest at 21 d of incubation and then decreased thereafter under flooded conditions, whereas it remained constant from 21 to 63 d under upland conditions. In the rapeseed cake treatment, inorganic N-15 was the highest at 42 d under flooded conditions and inorganic N-15 increased until 42 d and remained stable thereafter under upland conditions. The N mineralization rate from the cattle dung compost was slow both under flooded and upland conditions. More than half of N in the rapeseed cake was mineralized during the incubation period both under flooded and upland conditions. There was no significant difference in N-15 recovery in the soil between flooded and upland conditions at 63 d in the cattle dung treatment, while the N-15 recovery in the soil at 63 d was higher under upland than under flooded conditions in the rapeseed cake treatment. Although N mineralization from the rapeseed cake was greater under flooded conditions than upland conditions, there was no significant difference in N mineralization from the cattle dung compost between both conditions. Therefore, N mineralization from organic amendments is not always more rapid under flooded than upland conditions depending on the amendment type.