Effect of nitrogen fertilizer on nitrogen pools and soil communities under grazed pastures

Nitrogen (N) fertilizer increases pasture production in New Zealand in a near linear manner and affects pasture composition, soil below-ground communities and N losses. We monitored N fertilized plots established in long-term low-fertility pasture over different time periods to compare changes in N availability on below-ground soil communities (particularly ammonia-oxidizing archaea [AOA] and ammonia-oxidizing bacteria [AOB] as they appear to be sensitive to change). Although the most significant effects were seen in the 30 gN (300 kg/ha) treatment, there were indications that even 5 gN had effects on archaea. AOA gene copies dominated in control and 5 gN treatments but decreased in the 10 gN treatment. The ratio of AOA to AOB in 10 gN was lower and more similar to 30 gN suggesting that the AOA/AOB ratio may be a sensitive indicator of change in N status. In the 30 gN treatment, both the soil C/N ratio and the fungal phospholipid fatty acids were reduced (consistent with changes in DNA profiles) and microbes were suppressed. The number of AOB gene copies significantly increased in this treatment and corresponded to a switch from ammonium-N to nitrate-N as the dominant inorganic form of N in the 56-day incubation. This was consistent with increased soil ammonium-N and nitrate-N concentrations, leading to increased nitrate-N leaching that occurred at a threshold of between 10 gN and 30 gN, and suggesting that, with 30 gN, nitrification and nitrate leaching are influenced more by AOB than archaea. Generally there was no significant change in mesofauna, microfauna or bacterial DNA profiles with the treatments. In a high-fertility pasture, DNA profiles for bacterial, fungal and archaeal groups clustered away from low-fertility pasture suggesting that changes in soil communities, with increased soil fertility, take more time to be fully expressed.