Influence of roots and mycorrhiza on the internal nitrogen cycle in an organic forest soil ­revealed by a 15N tracing experiment

Abstract:

The cycle of nitrogen in soil is complex, consisting of many simultaneous occurring transformation processes. So far, microorganisms have been thought to govern N cycling in soil. Nevertheless, plant roots and their associated mycorrhizal symbionts may exert control on N turnover for example by input of labile C to soil. However, studies investigating the effect of roots on gross N turnover rates are scarce. We conducted a ¹⁵N tracer study under field conditions to reveal the effect of plants on soil N cycle. The experiment includes three treatments: (a) control, (b) excluding roots and (c) excluding roots + mycorrhiza. On the study site, exclusion of roots + mycorrhiza has previously been shown to increase N₂O emissions which indicate that plants affect internal N cycling. ¹⁵NH₄NO₃ and NH₄¹⁵NO₃ were given to the soil and traced for a period of 10 days. Gross N turnover rates were determined applying a numerical ¹⁵N tracing model. Results on N turnover rates showed that roots and their fungal symbionts increased N cycling probably by input of labile C to soil which may results in an activation of the microbial biomass. While gross N mineralization increased by 270 and 313 % compared to the treatment excluding roots + mycorrhiza, NH₄⁺ immobilization increased by 402 and 489 %. Differences in ammonium and nitrate immobilization further indicated that ammonium was the preferred N source for roots and microorganisms. While ammonium availability decreased with trenching (0.59 compared to -0.47 and -0.96 µg N g^-1 d^-1), the opposite was true for nitrate (0.50 compared to 2.08 and 2.18 µg N g^-1 d^-1), explaining the increased N₂O emissions which were likely caused by denitrification. Further, plants increased dissimilarity nitrate reduction to ammonium (DNRA) and affected autotrophic nitrification probably by the release of nitrification inhibitors and by influencing ammonium availability. We conclude that plants and their mycorrhizal symbionts actively control N cycling in soil. By mechanism such as increased microbial N assimilation, increased DNRA or inhibition of nitrification plants may increase N availability and prevent losses of nitrate