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 150kgnitrogen (N)ha−1 enriched with 9.96 percent nitrogen-15 (15N) atom excess, in four applications. The first crop was corn, a C4 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 (δ13C) values of −12.7 and the seeds the highest (−11.8). Soil δ13C was affected by addition of C4 plant residues. Soil under residue and till treatment (RT) had the highest percent 15N values. Residues and no-till (RNT) had the lower percent 15N values. At the end of the corn crop soil percent 15N 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 6.46kgNha−1 for no residues no-till (NRNT) and no-residues and till (NRT) and from 11.1 to 13kgNha−1 in RNT and RT treatments. In wheat nitrogen derived from residues (Ndfr) was 4.68 and 1.53kgNha−1 in grain and residues, respectively. Residues and tillage affected significantly soil C, N, 15N, and δ13C from seeding to two months after and have no effect at harvest. The interaction was always significant. After four years the 15N fertilizer contributed only with 1.5–2.85kgNha−1 in NRNT and NRT, respectively, and 3.3–5.63kgNha−1 in RNT and RT, respectively. Cumulated N recovered during the three growing seasons following corn was 8.59–11.07kgNha−1 for NRNT and NRT; 20.24–15.6kgNha−1 for RT and RNT, respectively. Residues increased N mineralization by 50 percent and the quantity of 15N available to plants increased with tillage.