Nitrogen losses from two grassland soils with different fungal biomass

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 interest in 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. We performed two greenhouse experiments: in the main experiment the columns received either mineral fertilizer N or no N (control). We measured N leaching, N₂O emission and denitrification from the columns during 4 weeks, after which we analyzed fungal and bacterial biomass and soil N pools. In the additional ¹⁵N experiment we traced added N in leachates, soil, plants and microbial biomass. We found that in the main experiment, N₂O emission and denitrification were lower in the high fungal biomass soil, irrespective of the addition of fertilizer N. Higher ¹⁵N recovery in the high fungal biomass soil also indicated lower N losses through dentrification. In the main experiment, N leaching after fertilizer addition showed a 3-fold increase compared to the control in low fungal biomass soil (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. However, the ¹⁵N experiment did not confirm these results; N leaching was higher in high fungal biomass soil, even though this soil showed higher immobilization of ¹⁵N into microbial biomass. However, only 3% of total ¹⁵N was found in the microbial biomass 2 weeks after the mineral fertilization. Most of the recovered ¹⁵N was found in plants (approximately 25%) and soil organic matter (approximately 15%), and these amounts did not differ between the high and the low fungal biomass soil. 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 retention, but rather the result of low nitrogen availability. Both experiments confirmed that higher fungal biomass can be considered as an indicator of higher nitrogen retention in soils.