No abstract

During a complex investigation programme on the evaluation of the long‐term ecological impact of cropping Miscanthus, the substrate composition of residues in 4–8‐year‐old stands of Miscanthus × giganteus and the contribution of Miscanthus residues to soil organic matter (SOM) were investigated. Only about 50 % of the above‐ground biomass produced yearly was suitable for harvest because of pre‐harvest losses and harvesting residues. The potential supply to SOM was therefore 3.1 t ha−1 carbon annually accumulated by the litter, and 9.1 t ha−1 carbon accumulated by rhizomes and roots in the long term. These organic residues mineralized differently in laboratory experiments at generally high rates of decomposition. Analysis of soluble portions of the organic substance of these biomass components by hydrolysis with hot water, 2 % HCl and 80 % H2SO4 confirmed the results. Setting the SOM effect of an equivalent farmyard‐manure (FYM) supply to 100 %, Miscanthus residues reached 60 % (stubble, rhizomes), 80–90 % (pre‐harvest losses) and 100 % (roots). Established Miscanthus stands are able to produce about 8.2 t ha−1 organic substance, which is comparable with FYM in terms of SOM impact. This kind of calculation showed higher values for Miscanthus than for the agricultural crops investigated to date. An SOM increase of about 0.5 % on sandy soils and 0.2 % on silt soil was determined after 6–8 years of cropping Miscanthus × giganteus.

Short‐rotation forestry (SRF) on arable soils has high potentials for biomass production and leads to long‐term no‐tillage management. In the present study, the vertical distributions of soil chemical and microbial properties after 15 y of SRF with willows and poplar (Salix and Populus spp.) in 3‐ and 6‐year rotations on an arable soil were measured and compared to a pertinent tilled arable site. Two transects at different positions in the relief (upper and lower slope; transect 1 and 2) were investigated. Short‐rotation forestry caused significant changes in the vertical distribution of all investigated soil properties (organic and microbial C, total and microbial N, soil enzyme activities), however, the dimension and location (horizons) of significant effects varied. The rotation periods affected the vertical distribution of the soil properties within the SRF significantly. In transect 1, SRF had higher organic‐C concentrations in the subsoil (Bv horizon), whereas in transect 2, the organic‐C concentrations were increased predominantly in the topsoil (Ah horizon). Sufficient plant supply of P and K in combination with decreased concentrations of these elements in the subsoil under SRF pointed to an effective nutrient mobilization and transfer from the deeper soil horizons even in the long term. In transect 1, the microbial‐C concentrations were higher in the B and C horizons and in transect 2 in the A horizons under SRF than under arable use. The activities of β‐glucosidases and acid phosphatases in the soil were predominantly lower under SRF than under arable use in the topsoil and subsoil. We conclude, that long‐term SRF on arable sites can contribute to increased C sequestration and changes in the vertical distribution of soil microbial biomass and soil enzyme activities in the topsoil and also in the subsoil.

The effects of stand establishment and nitrogen fertilizer on yield and yield stability of winter oil‐seed rape (Brassica napus L.) were investigated in two field experiments in north eastern Germany (Mecklenburg‐Vorpommern). During six years of testing from 1984/85 to 1989/90 three sowing dates (10 Aug, 20 Aug, 5 Sept) as combined with three plant densities (60, 100, 140 plants/m2 in autumn) were compared. Although seed yield did not respond to the main effects of these treatments, the two‐way classification resulted in significant differences from the average yield. August sowing combined with lower plant densities caused comparably high yields, while delayed sowing was somewhat effective only with higher plant density. Highest yield stability was achieved at early sowing when combined with lower plant densities. From 1986/87 to 1988/89 nitrogen top‐dressings were applied during early March, at start of stem extension and shortly before start of flowering at five locations. 200 kg N/ha in one dose resulted in lowest yield performance and enhanced instability. Contrastingly, the 240 kg N/ha treatment advanced yield stability regardless of split regimes. Nevertheless, split application to 100 kg N/ha + 50 kg N/ha + 50 kg N/ha also guaranteed that high yield performance and optimum yield stability were reached simultaneously and, that the total nitrogen input could be limited to 200 kg N/ha.