Phosphorus availability and plants alter soil nitrogen retention and loss

Availability of phosphorus (P) can directly and/or indirectly affect nitrogen (N) retention and loss from soil by stimulating microbial and plant root activities. However, it is not clear how P availability and plant presence interact on nitrous oxide (N₂O) emission and nitrate (NO₃^-) leaching in soil. A mesocosm experiment was conducted to investigate the effect of P addition (0, 10 and 20 mg P kg^-1) with and without plant presence (Phalaris aquatica, C3 grass) on N₂O emission, NO₃^- leaching and ¹⁵N recovery. Our results showed large variation in N₂O emission with significant increases after leaching events. We observed that initially low but later (after 53 days of sowing) high levels of P addition increased N₂O emission rates, possibly by stimulating nitrifiers and/or denitrifiers in soil. Plant presence decreased N₂O emission at times when plants reduced water and NO₃^- in the soil, but increased N₂O emission at times when both water and NO₃^- in the soil were abundant, and where plants may have stimulated denitrification through supply of labile organic C. Furthermore, an increase in net N mineralization, possibly due to increased decomposition stimulated by root derived C, may also have contributed to the higher cumulative N₂O emission with plant presence. P addition increased ¹⁵N recovery in soil, but reduced it in leachates, suggesting increased ¹⁵N fixation in microbial biomass. Our results showed that both P addition and plant presence stimulated N loss as N₂O, but also increased N retention in the soil-plant system and thus reduced N loss through leaching.