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.