Comparison of Nitrogen mineralization from15N-labeled organic amendments under flooded and upland conditions

Abstract

Knowledge of N availability from organic amendments is a key to improve N use efficiency and reduce environmental pressure from agriculture. Nitrogen mineralization from 15N-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 25 °C. 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 15N 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 15N abundance of total N and H1-N was higher than the 15N abundance of H2-N and non-hydrolyzable N. In the cattle dung treatment, inorganic 15N 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 15N was the highest at 42 d under flooded conditions and inorganic 15N 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 15N recovery in the soil between flooded and upland conditions at 63 d in the cattle dung treatment, while the 15N 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.