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 (NO) emission and nitrate (NO) leaching in soil. A mesocosm experiment was conducted to investigate the effect of P addition (0, 10 and 20 mg P kg) with and without plant presence (Phalaris aquatica, C3 grass) on NO emission, NO leaching and N recovery. Our results showed large variation in NO 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 NO emission rates, possibly by stimulating nitrifiers and/or denitrifiers in soil. Plant presence decreased NO emission at times when plants reduced water and NO in the soil, but increased NO emission at times when both water and NO 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 NO emission with plant presence. P addition increased N recovery in soil, but reduced it in leachates, suggesting increased N fixation in microbial biomass. Our results showed that both P addition and plant presence stimulated N loss as NO, but also increased N retention in the soil-plant system and thus reduced N loss through leaching.