Combining fertilizer and organic inputs to synchronize N supply in alternative cropping systems in California

One of the principal aims of alternative cropping systems is to minimize excessive loss of N while maximizing N use efficiency and meeting crop N requirements. Many such cropping systems substitute intensive application of synthetic fertilizer with organic inputs, such as N-2-fixing legumes. The effectiveness of legume residues as a N source for subsequent crops depends heavily on temporal N release from the residue during the growing season. A field experiment with N-15-labeled fertilizer and N-15-labeled vetch residue was conducted to determine the temporal pattern of N release from both sources in conventional and alternative cropping systems in California. The experiment was conducted within conventional (fertilizer), low-input (fertilizer and organic N), and organic (organic N only) cropping systems established 9 year previously. Availability of N from the labeled inputs was determined based on uptake by maize (Zea mays L.). Uptake of total N and N-15 by maize in each cropping system was monitored at 10 day intervals from 50 to 90 days after seeding for determination of uptake rates. Uptake of N from fertilizer in the conventional system was greater than uptake of N from vetch in the low-input and organic systems. Uptake of N from vetch was delayed, but with a sustained availability later in the season. Uptake rates of N from fertilizer peaked at 4.3 kg N ha(-1) per day between 70 and 80 days while those from vetch residue reached a maximum of 0.6 kg N ha(-1) per day during the same time period. Grain and N yield at harvest did not differ between cropping systems despite different temporal and quantitative availability of N from organic and inorganic N inputs. This demonstrates that optimum yields can be achieved under management which uses alternative sources of N and can successfully match N availability with crop uptake. (C) 2002 Elsevier Science B.V. All rights reserved.