Saturday, 15 July 2006

Fungal/Bacterial Ratios in Grasslands with Contrasting Nitrogen Management.

Franciska T. De Vries1, Ellis Hoffland1, Jaap Bloem2, Nick Van Eekeren3, and Lijbert Brussaard1. (1) Wageningen Univ and Research Centre, Dept Soil Quality, P.O. Box 8005, 6700 EC, Wageningen, Netherlands, (2) Wageningen Univ and Research Centre, Alterra, P.O. Box 47, 6700 AA, Wageningen, Netherlands, (3) Louis Bolk Institute, Dept Organic Agriculture, Hoofdstraat 24, 3972 LA, Driebergen, Netherlands

It is frequently hypothesized that high fungal/bacterial ratios are indicative for more sustainable agricultural systems. Increased F/B ratios have been reported in extensively managed grasslands. To determine the shifts in fungal/bacterial biomass ratio as influenced by grassland management and to find relations with nitrogen leaching potential, we sampled a two year old field experiment at an organic experimental farm in the eastern part of The Netherlands. The effect of crop (grass and grass-clover), N application rate (0, 40, 80, 120 kg N/ha) and manure type (no manure, farm yard manure and slurry) on the F/B ratio was tested, as well as relations with soil and crop characteristics, nitrate leaching and partial N balance. Biomass of fungi and bacteria was calculated after direct counting using epifluorescence microscopy. Fungal and bacterial biomass and the F/B ratio were higher in grass than in grass-clover. The F/B ratio decreased with increasing N application rate and multiple regression analysis revealed a negative relationship with pH. Bacterial activity (measured as incorporation of [3H]thymidine and [14C]leucine into bacterial DNA and proteins) showed the exact opposite: an increase with N application rate and pH. Leaching increased with N application rate and was higher in grass-clover than in grass. Partial N balance was more positive at a higher N application rate and showed an inverse relationship with fungal biomass and F/B ratio. From these results we conclude that the fungal/bacterial biomass ratio quickly responded to changes in management. Grasslands with higher N input showed lower F/B ratios. Grass-clover had a smaller fungal biomass and higher N leaching than grass. In general, a higher fungal biomass indicated a lower nitrogen leaching and a more negative partial N balance (or smaller N surplus), but more observations are needed to confirm the relationship between the F/B ratio and sustainability.

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