Nitrogen losses from two grassland soils with different fungal biomass.

de Vries, Franciska T. and van Groenigen, Jan Willem and Hoffland, Ellis and Bloem, Jaap (2011) Nitrogen losses from two grassland soils with different fungal biomass. Soil Biology and Biochemistry, 43 (5). pp. 997-1005. ISSN 0038-0717

Abstract

Nitrogen losses from agricultural grasslands cause eutrophication of ground- and surface water and contribute to global warming and atmospheric pollution. It is widely assumed that soils with a higher fungal biomass have lower N losses, but this relationship has never been experimentally confirmed. With the increased attention for soil-based ecosystem services and sustainable management of soils, such a relationship would be relevant for agricultural management. Here we present a first attempt to test this relationship experimentally . We used intact soil columns from two plots from a field experiment that had consistent differences in fungal biomass (68 ± 8 vs. 111 ± 9 μg C g-1) as a result of different fertilizer history (80 vs. 40 kg N ha-1 y-1 as farm yard manure), while other soil properties were very similar. In the greenhouse, the columns received either mineral fertilizer N or no N (control). We measured N leaching, N2O emissions and denitrification from the columns during 4 weeks, after which we analyzed fungal and bacterial biomass and soil N pools. We found that N2O emission and denitrification were lower in the high fungal biomass soil, irrespective of the addition of fertilizer N. After fertilizer addition, N leaching in low fungal biomass soil showed a 3-fold increase compared to the control (11.9 ± 1.0 and 3.9 ± 1.0 kg N ha-1, respectively), but did not increase in high fungal biomass soil (6.4 ± 0.9 after N addition vs. 4.5 ± 0.8 kg N ha-1 in the control). Thus, in the high fungal biomass soil more N was immobilized. An additional experiment with 15N–labelled mineral fertilizer, showed a 2-fold higher immobilization of 15N into microbial biomass in the high fungal biomass soil. However, only 3% of total 15N was found in the microbial biomass 2 weeks after the mineral fertilization. Most of the recovered 15N was in the plants (approximately 25%) or in the soil organic matter (approximately 15%). Our main experiment confirmed the assumption of lower N losses in a soil with higher fungal biomass. The additional 15N experiment showed that higher fungal biomass is probably not the direct cause of higher N immobilization, but rather the result of low nitrogen availability. Both experiments confirmed that higher fungal biomass can be considered as an indicator of higher nutrient retention in soils.