Quantifying the supply of plant-available nitrogen from dairy effluents to grow crops

Johnstone P1, Norris M1, Houlbrooke D2, Dexter M2, Sharp J3, Selbie D2, Hedderley D4

1The New Zealand Institute for Plant & Food Research Limited, Havelock North, 4130

2AgResearch Limited, Hamilton, 3214

3The New Zealand Institute for Plant & Food Research Limited, Lincoln, 7608

4The New Zealand Institute for Plant & Food Research Limited, Palmerston North, 4442

Email: [email protected]

Abstract

The use of dairy effluent to grow forage and arable crops represents an opportunity to more sustainably reuse shed, feed pad and barn nutrients that are generated from intensive dairy systems. To do so in a profitable and low risk manner requires an understanding of the effect of effluent characteristics on nutrient supply patterns, including both the quantum of release and rate of release. Between 2014 and 2016 we have conducted several assays to investigate the nitrogen (N) supplying power of dairy effluents and link this to effluent characteristics measured at the time of application. This paper reports on Assay 1 where we quantified release patterns for five slurry and six solid dairy effluents collected from commercial farms in the Waikato region of New Zealand. These effluents were applied to a single, low N (0.36 % total N) soil at a target application rate of 100 kg N/ha and subsequently incubated in 500 ml units at 20°C and 90% of field capacity for 182 days. Units were leached a total of 15 times during the assay and the drainage water characterised for inorganic N levels. Estimates of N supply were calculated, corrected for background N supply from a non- effluent control, and relationships with a wide range of effluent characteristics assessed. The assay showed that the pattern and magnitude of N supply across slurry and solid effluent treatments varied considerably, consistent with the large variation in effluent characteristics. Strong positive correlations were found between the water-soluble N and carbon (C) effluent characteristics and the rate of N supply in the first month after effluent addition. There were few clear correlations between effluent characteristics and the rate of N supply during the later stages of the assay (112-182 days). At the end of the assay (182 days), final N supply for respective slurry and solid effluents ranged from 3.7 to 74.2 % and 1.5 to 34.3 % of total effluent N applied. Net N supply values which adjusted for inorganic N in the effluents at application (expressed as either a percentage of total N or organic N) were positive for seven of the eleven treatments (three slurries and four solids) indicating a net N mineralisation effect and negative for the remaining four (two slurries and two solids), indicating a net N immobilisation effect. Work is ongoing to identify the causes of the large variation in N supply.