The Use of ~(13)Cs and 15N Based Isotopic Techniques for Assessing Plant C and N Changes under Conservation Agriculture System

摘要

A long-term field experiment was conducted to investigate the effect of tillage and the addition of residues in a wheat-faba bean rotation. The soil was fertilized with a total of 150 kg nitrogen (N)/ha 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. Wheat delta carbon-13 (6~(13)Cs) values changed significantly with the addition of corn residues (R2 = 0.53-0.64) and with time. The wheat residues had the lowest 6~(13)Cs (-30.5%o) and the seeds the highest (-28.5%). Residue and till treatment (RT) had the highest wheat percent 15N values (1.5 and 1.9 percent, respectively). Residues and no-till (-) had the lower percent 15N values (1.1 and 1.1 percent, respectively). Residues increased mineralization by 50 percent as the quantity of 15N taken up by wheat increased with tillage. Tillage and residues treatments did not affect biological nitrogen fixation (BNF). Following wheat and faba bean, crops derived less N from fertilizer (10 to 2 percent). Residues and tillage increased significantly the percent N derived from fertilizer (Ndff). The wheat crop recovered 6.45 kg-N-ha~(-1) (4.3 percent), 13 kg-N-ha~(-1) (8.7 percent) and 11.1 kg-N-ha~(-1) (7.4 percent) for no residues no-till (NRNT), no-residues and till (NRT), residue and no till (RNT) and residue and till (RT) treatments, respectively. Faba bean recovered less N in all treatments (0.66-1.74 kg-N-ha~(-1) or 0.4 to 1.2 percent). The fourth wheat crop recovered between 1.5 (1 percent) and 5.63 kg-N-ha~(-1) (3.7 percent). Cumulative N recovery during the four growing seasons was 122 kg-N-ha~(-1) (81.5 percent) for RT and 116 kg-N-ha~(-1) (77.1 percent) for RNT. Less N was recovered in the NRNT treatments in the four cropping seasons: 104 kg-N-ha~(-1) (69.3 percent) and 106 kg-N-ha~(-1) (71 percent) respectively. Nitrogen not recovered by the crops amounted to 18.5 percent for RT, 22.9 percent for RNT, 31 percent for NRNT and 29percent for NRT. Most of the N not recovered was n the soil organic matter (SOM).