The use of 13C and 15N based isotopic techniques for assessing soil C and N changes under conservation agriculture

A four years experiment was conducted to investigate the effect of tillage and addition of crop residues in wheat-faba bean rotation. The soil was fertilized with a total of 150 kg nitrogen (N) ha(-1) enriched with 9.96 percent nitrogen-15 (N-15) atom excess, in four applications. The first crop was corn, a C-4 plant cropped under till (T) and no-till (NT) conditions. Percent N derived from fertilizer (Ndff) by corn was 37.12-48.62. The leaves had the lowest delta carbon-13 (delta C-13) values of -12.7 and the seeds the highest (-11.8). Soil delta C-13 was affected by addition of C-4 plant residues. Soil under residue and till treatment (RT) had the highest percent N-15 values. Residues and no-till (RNT) had the lower percent N-15 values. At the llend of the corn crop soil percent N-15 was 0.211, 0.26, and 0.253 in the three soil depths. Residues and tillage increased significantly the Ndff of wheat: from 6.43 to 6.46 kg N ha(-1) for no residues no-till (NRNT) and no-residues and till (NRT) and from 11.1 to 13 kg N ha(-1) in RNT and RT treatments. In wheat nitrogen derived from residues (Ndfr) was 4.68 and 1.53 kg N ha(-1) in grain and residues, respectively. Residues and tillage affected significantly soil C, N, N-15, and delta C-13 from seeding to two months after and have no effect at harvest. The interaction was always significant. After four years the N-15 fertilizer contributed only with 1.5-2.85 kg N ha(-1) in NRNT and NRT, respectively, and 3.3-5.63 kg N ha(-1) in RNT and RT, respectively. Cumulated N recovered during the three growing seasons following corn was 8.59-11.07 kg N ha(-1) for NRNT and NRT; 20.24-15.6 kg N ha(-1) for RT and RNT, respectively. Residues increased N mineralization by 50 percent and the quantity of N-15 available to plants increased with tillage. (C) 2014 Elsevier B.V. All rights reserved.