THE EFFECT OF SEPARATING PIG SLURRY ON NITROGEN USE EFFICIENCY AND NITROGEN LOSS PATHWAYS FROM WINTER WHEAT ON CONTRASTING SOIL TYPES

THORMAN, R.E.1_{, MUNRO, D.G}1_{, BENNETT, G.}2_{, EDWARDS, D.J}1_{, KINGSTON, H.L}1_{, WILLIAMS, J.R.}1

1 _{ADAS Boxworth, Battlegate Road, Boxworth, Cambridge, CB23 4NH, UK;} 2 _{ADAS Gleadthorpe, Meden Vale, Mansfield, NG20 9PD, UK}

INTRODUCTION

Agriculture is fundamental in feeding a growing global population, but at the same time faces the challenge of minimising climate change and other environmental impacts. The use of new and modified manure management technologies has the potential to improve nitrogen (N) use efficiency (NUE) and minimise the loss of N to air (ammonia and nitrous oxide) and water (nitrate). Slurry separation on farms is a practical method to improve slurry management and has the potential to increase NUE by producing a high dry matter (DM), low readily available N, easily transported solid fraction, and a low DM, high readily available N liquid fraction (Fangueiro et al., 2015). The relatively low DM content of the liquid fraction compared to whole slurry can increase slurry

infiltration into the soil reducing ammonia (NH3) loss & hence indirect nitrous oxide (N2O) emissions, as well as

increasing grain yield and N uptake by the crop. However, the reduced NH3 loss conserves N in the soil, which may

result in an increase in direct N2O emissions and nitrate (NO3) leaching losses. The use of nitrification inhibitors

(NIs) has been shown to reduce direct N2O emissions, but NIs may also influence crop yields and N recovery, as

well as NH3 and NO3 losses with consequential effects on indirect N2O emissions. In the UK the use of slurry

separation is increasing, therefore there is a need to investigate the effect that slurry separation (& combined with NIs) has on N loss pathways and crop available N supply following application to contrasting soil types. MATERIAL AND METHODS

Field experiments were carried out on two commercial arable farms in England with contrasting soil types. Site 1 was located near Cambridge, eastern England (average annual rainfall 565 mm) on a clay soil (c.40% clay). Site 2 was located near Mansfield, central England (average annual rainfall 650 mm) on a sandy loam soil (c.12% clay). At both sites treatments were applied to replicated (x3) plots (5 x 15 m) on cereal stubble in autumn 2016 and to separate plots with a growing winter wheat crop in spring 2017. Whole pig slurry and separated pig slurry - liquid fraction was applied by trailing hose using a custom-made small plot applicator. Separated pig slurry - solid fraction was applied by hand. Application rates were based on manure N analysis to give a target N loading for all the

manures of 100-150 kg total N ha-1_{. In a separate treatment, the commercially available NI, dicyandiamide (DCD)}

was tested; prior to application, DCD (1% solution) was mixed with separated pig slurry - liquid fraction to give a

DCD application rate of 10 kg ha-1_{. The N supplied by the DCD was accounted for in the total N application value.}

Additionally, for comparison with the manure treatments, ammonium nitrate and urea fertiliser were also applied

at a rate of 150 kg N ha-1 _{to separate plots. A control treatment was included where no nitrogen was applied.}

Following N application, measurements of direct N2O were made over 6 months, using 5 static chambers (0.8 m2

total surface area) per plot and based on the method described by Chadwick et al., (2014). The sampling strategy was weighted with more intensive sampling in the first 6 weeks after application. Gas headspace samples were

analysed by gas chromatography. The N2O flux was calculated based on the linear increase in N2O concentration

inside the chamber. The assumed linear relationship was checked on each sampling occasion. Cumulative fluxes following land spreading were calculated using the trapezoidal rule. Nitrous oxide emission factors were calculated by subtracting the fluxes from the control plots and expressed as the percentage of total-N applied. Ammonia emissions were measured using a wind tunnel technique (Nicholson et al., 2017) for 1 week following applications of whole pig slurry and the separated fractions, and for 2 weeks following application of the manufactured N fertilisers. One wind tunnel per plot was set up immediately after application with each wind tunnel being moved on a daily basis. Nitrate leaching losses were measured following the autumn application at

site 2 only. Porous ceramic cups (5 per plot) were installed at a depth of 90 cm during the period of over-winter drainage (Webster et al., 1993). Samples were taken every 2 weeks or after 25 mm of rain, whichever was soonest.

Drainage volumes were estimated using IRRIGUIDE (Bailey and Spackman, 1986) and were combined with NO3

concentrations to quantify the amounts of NO3-N leached.

Soil temperature was logged continuously, along with measurements of air temperature and daily rainfall

recorded at a nearby meteorological station. On every N2O measurement occasion, representative soil samples

were taken (0-10 cm) from each plot for the determination of gravimetric moisture and soil mineral N (ammonium-N and nitrate-N) content.

The fertiliser N replacement value of the organic manure treatments was calculated by comparing yields and crop N offtakes from plots receiving applications of manufactured N fertiliser; ammonium nitrate was applied in spring

2017 to replicated (x3) plots (3 x 15 m), at 6 rates ranging from 0-300 kg N ha-1_.

Grain yields and total crop N offtake were measured from all plots (including the N response) at harvest in 2017. ANOVA was conducted to determine experiment and treatment differences.

RESULTS AND DISCUSSION

Results will be presented and discussed in terms of the potential that slurry separation has on improving winter wheat NUE and the effect that separation has on N loss pathways. We will examine the possible impact of using

a NI combined with separation on the mitigation of gaseous emissions and NO3 leaching losses, as well as any

benefits to crop production. Additionally, the replacement of manufactured N fertilisers with the organic manures tested in these experiments will be discussed in relation to possible agronomic and environmental damage or indeed benefits.

Acknowledgements: This experiment forms part of the UK-China Virtual Joint Centre for Closed-Loop Cycling of Nitrogen in Chinese Agriculture (N-CIRCLE), funded from the Newton Fund via The Biotechnology and Biological Sciences Research Council (BBSRC).

REFERENCES

Bailey R.J., Spackman E., 1996. A model for estimating soil moisture changes as an aid to irrigation scheduling and crop water-use studies: I. Operational details and description. Soil Use Manage., 12, 122-129.

Chadwick D.R., Cardenas L., Misselbrook T.H., Smith K.A., Rees R.M., Watson C.J., McGeough K.L., Williams J.R., Cloy J.M., Thorman R.E., Dhanoa M.S., 2014. Optimizing chamber methods for measuring nitrous oxide emissions from plot-based agricultural experiments. Eur. J. Soil Sci., 65, 295-307.

Fangueiro D., Surgy S., Fraga I., Cabral F., Coutinho J., 2015. Band application of treated cattle slurry as an alternative to slurry injection: implications for gaseous emissions, soil quality and plant growth. Agric. Ecosyst. Environ., 211, 102–111.

Nicholson F., Bhogal A., Cardenas L., Chadwick D., Misselbrook T., Rollett A., Taylor M., Thorman R., Williams J., 2017. Nitrogen losses to the environment following food-based digestate and compost applications to agricultural land. Environ. Pollut., 228, 204-516.

Webster C.P., Shepherd M.A., Goulding K.W.T., Lord E.I., 1993. Comparisons of methods for measuring the leaching of mineral nitrogen from arable land. J. Soil Sci., 44, 49-62.