Thursday, 13 July 2006 - 8:30 AM

Nutrient Use Efficiencies and Leaching in Organic and Conventional Cropping Systems in Sweden.

Lars F. Bergström and Holger Kirchmann. Dept of Soil Science, Swedish Univ of Agricultural Sciences, P.O. Box 7014, Uppsala, Sweden

In the past few years, organic farming has been proposed as a possible way of reducing N leaching from agricultural soils and improving the use efficiency of plant nutrients. This is, to a large extent, considered to be attributed to the fact that synthetic fertilizers are not allowed in such systems and the N inputs mainly originate in various types of organic manures. In this overview, results from a number of Swedish field studies are presented in which nutrient use efficiencies and leaching in organic and conventional systems are evaluated. Some studies were conducted in lysimeters and others in large tile-drained field plots. In two of the lysimeter experiments, leaching and crop uptake of N derived from either poultry or green manures were compared with fertilizer N, all labeled with 15-N. In another lysimeter experiment, the influence of increasing pig slurry applications on leaching and crop uptake of N and P by cereals was evaluated. In various plot experiments, comparisons between conventional and organic crop rotations were done. In the lysimeters on which poultry manure was applied, an average of 24% of N applied leached during three years, whereas only about 3% leached in ammonium nitrate fertilized lysimeters. The corresponding figure in lysimeters that received red clover manure was 6% of N applied. Crop uptake of N derived from the different N sources varied between 57 (ammonium nitrate) and 24% (red clover manure). When pig slurry was applied, the N leaching load increased with increasing slurry rates, to 139 kg total N ha-1 during the 3-yr period when 400 kg N ha-1 had been applied. In contrast, P leaching tended to decrease with increasing input of slurry, and it was lower in all treatments that received P with slurry or fertilizer than in unfertilized lysimeters. The use-efficiency of added N and P by crops was clearly best if inorganic N and P were used rather than slurry (i.e. 60 vs 35 % for N, and 38 vs <10 % for P). In organic farming plots on a sandy soil, N leaching represented 59 % of total N removal (leaching plus harvested N with crops) during a 6-yr period. In the conventional farming system in which only inorganic N fertilizers were used, N leaching was 33 % of total N removal. Adding cover crops in the conventional system to further reduce N leaching resulted in N leaching making up only 22 % of what was removed. What the results presented above clearly suggest is that the N-use-efficiency is less if organic manures are used rather than inorganic N fertilizers. The yields were always lower in the organic systems with no benefit for water quality.

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