Optimal soil phosphorus reduces greenhouse gas emissions

Environment research in Johnstown Castle investigated the effect of long term P fertilisation on emissions of the potent greenhouse gas nitrous oxide in a number of laboratory experiments by Karl Richards and Rosie O’ Neill

For decades farmers have been applying phosphorus  (P) fertiliser and manure to soils to increase grass yields by ensuring that this key nutrient is adequate to meet the requirements of grass. Two long term trials were established in 1995 at Johnstown Castle where 4 rates of P fertiliser (0, 15, 30 and 45 Kg P ha/year) were applied to identify the optimal soil Ps levels for grass growth. Previous research on the site identified that these fertiliser treatments were impacting the soil microbial community but little was known about the effect on soil microbial processes such as denitrification. Denitrification is the process that reduces nitrate from fertiliser or manure to the atmospheric gas - di-nitrogen gas but can also frequently result in the emission of the potent greenhouse gas nitrous oxide.

Left: Long-term P trial at Johnstown Castel and  Right: Nitrous oxide reduces with increasing P fertilisation

The effect of long term P fertilisation on emissions of the potent greenhouse gas nitrous oxide was investigated in a number of laboratory experiments. Soils from the long term experiments were sampled and incubated under optimal conditions for denitrification to occur. These recently published experiments found that nitrous oxide emissions were significantly related to P fertilisation and that emissions decreased with increasing P fertilisation. The lowest nitrous oxide emissions were observed from the 45 kg P/ha/year treatment which is the recommended rate for index 3 soils. Index 3 is the recommended soil P level for most soils in Ireland and our recently published research indicates that optimal P for grass growth also compliments reductions in greenhouse gas emissions.

The initial experiment was a controlled laboratory experiment and a follow up field experiment (currently being prepared for publication) was also conducted, whose findings have reinforced those of the laboratory incubations. This research highlights the importance of the soil microbial ecology research in Johnstown Castle and long term experimentation.  With such a thorough knowledge of a soil system like this, land management can be tailored to provide exactly that which the soil requires and can be honed to simultaneously stimulate a microbial community most likely to achieve maximum productivity and minimum greenhouse gas emissions. Research such as this can be carried out across a range of soil types with a view to produce a database of management techniques most appropriate for the soil system in question.

Greenhouse gas mitigation must go hand in hand with sustainable production to meet the growing food demand. Finding solutions that satisfies both of these criteria lies in expanding our knowledge of ecosystem processes and reducing that which we do in obsolescence. For example, in-depth knowledge of a soil system allows for nutrient-additions of precisely what the system requires and avoids any fertiliser being wasted as a result of being spread in excess. Management techniques such as this have the potential to reduce costs, reduce emissions and maintain productivity, all which complement the future goals of sustainable practices and the endeavour to create a greener economy.