Impacts of long-term phosphorus and nitrogen fertiliser application on soil biology: a New Zealand perspective

Soil biology encompasses a vast diversity of organisms contributing to soil processes and functions that underpin the delivery of soil ecosystem services. We review the response of soil biology to long-term phosphorus (P) and nitrogen (N) fertiliser application, focusing on trials conducted at Winchmore and Ballantrae Research Stations under sheep grazing. Application of P and N fertiliser increases soil fertility and pasture production and can have impacts on soil biology. Higher earthworm abundance was associated with increasing P, but not N fertiliser application. Microbial biomass or respiration tended to increase with P application but decrease with N application. Both P and N fertiliser decreased fungal biomass. The few microbial functional groups studied also appear responsive to fertilisation. A decline in soil pH may have driven some of these changes and could have been mitigated with lime. Other factors, such as plant composition, may also be important to the soil biology but have received limited attention in these studies. There are currently few indicators that can be used on-farm to assess soil biology. We discuss these as well as measures that can be used in a research context to inform the impacts of fertiliser use on soil biology and its functioning.