MANAGING PHOSPHORUS LOSSES Rich. McDowell, AgResearch
BIOAVAILABILITY: N-, CO-, OR P-LIMITATION Co-Limited Co-Limited 14% 19% N-Limited 13% P-Limited 73% N-Limited 17% P-Limited 64%
Where does it come from? Rain Dung Fertilizer & feed Infiltration excess Overland flow Erosion of particulate P Release of dissolved P Effective depth of interaction Deposition Percolation; > P retention Infiltration & saturation excess overland flow Preferential flow via macropores; < sorption Drainage Ground water Subsurface flow Variable source area
LOSSES FROM DIFFERENT SYSTEMS 12 10 Catchment losses (1970-present) Phosphorus loss (kg P ha -1 yr -1 ) 8 4 3 2 1 Wide range due to: climate soil type topography management 0 Native Sheep Mixed Deer Dairy n =7 n = 8 n = 9 n = 6 n = 22 Land use
SOIL OLSEN P Yield
FERTILISER Phosphorus concentration (mg/l)
FERTILISER Soil Dung Fertiliser 2004 2005 Runoff 7 days after application Soil Dung Fertiliser
GRAZING
DIRECT DEPOSITION Enriched source of phosphorus, nitrogen and faecal bacteria
FENCING The impact of fencing-off and planting? 2004 2009 after fencing Guideline
EFFLUENT Rotating twin gun travelling irrigators - soil water deficits low in spring & autumn - poor control of application depth & application uniformity - high rate of application (potential to be > infiltration) Effluent applied, mm 40 35 30 25 20 15 10 5 0 16 12 8 4 0 4 8 12 16 Distance from centre of irrigator
EFFLUENT Direct contamination of surface water via: - mole-tile drainage - surface runoff - other (e.g. infrastructure failure) 450 mm Macropores Trench soil 600 mm Mole drain Gravel backfill Intercepting pipe drain
FLOOD IRRIGAITON OUTWASH Olsen P mg/kg Photo courtesy of Bob Wilcock, NIWA
TRACKS/LANES Used each day Concentrated source of faecal deposits
TRACKS/LANES E Coli Location of collectors Raw effluent NH4-N Close to Shed Bridge crossing TP 1 10 100 1000 10000 100000 1000000 % of water quality guidelines
WINTERING BLOCK P N loss per ha N/P loss on a farm scale 6 4 2 Restricting grazing to 3-4 hr can decrease losses by 30%
FENCE-LINE PACING Associated with stress Denudes pasture + compacted soil = increased runoff mg/l or log 10 E. coli /100 ml ANZECC limits
SELF FEEDING Low cost and labour, but high losses NOx-N Total P Self feeding silgae pit Bottom Middle Top DRP SS E. coli 0 500 1000 1500 2000 2500 11000 Percent of water quality guideline
MITIGATION STRATEGIES NEED TO BE FULLY ASSESSED Strategy Effectiveness (%) Cost ($/kg P conserved) Optimum soil test P Management 5-20 highly cost-effective Low solubility P fertilizer 0-20 0-25 Effluent pond storage / low rate application 10-30 25 Restricted grazing 30-50 125-200 Tile drain amendments Amendment 50 20-75 Alum to pasture / cropland 5-30 125->400 Phoslock/red mud 0-50 0-125 Buffer strips Edge of field 0-10 >200 Stream fencing 10-30 4-55 Sorbents in and near streams 20 300 Natural and constructed wetlands -426-77 >400 Increasing cost = stop at source
DEFINING THE MANAGEABLE LOSS Business as usual Managed decrease In losses Concentration (mg L -1 ) Consensus target Anthropogenic loss Reference conditions 1840 2012
POMAHAKA CATCHMENT c. 1850 km 2 30% dairy, 55% S&B, 15% Forestry.
WHERE IS THE P COMING FROM?
MITIGATION STRATEGIES THAT MATCH THE FARM SYSTEM Strategy Effectiveness (%) Cost ($/kg N/P conserved) Optimum soil test P 5-20 highly cost-effective Effluent pond storage / low rate application Management 10-30 25 Restricted grazing 30-50 125-200 Tile drain Amendment 50 20-75 Alum to pasture / cropland 5-30 125->400 Phoslock/red mud 0-50 0-125 Buffer strips 0-10 >200 Stream fencing 10-30 4-55 Sorbents in and near streams 20 300 Natural and constructed wetlands -426-77 >400 Edge of field Increasing cost = stop at source
USE LOW RATE EFFLUENT APPLICATION Concentration relative to effluent applied 100 90 80 70 60 50 40 30 20 10 High rate irrigator Low rate irrigator Total P NH 4+ -N E. coli
ESTIMATED EFFECTS: TARGETED + FARM-WIDE $0 -$30 $15 $129 Change in profit ($/ha/yr) Proposed target Natural baseline
SUMMARY Soil Decrease soil Olsen P to 20-30 mg kg -1 unless in top 20% of producers for the region Redistribution of soil P by cultivation as part of farm re-grassing program and/or addition of P sorption agents Fertiliser Use RPR or serpentine super. Avoid applying superphosphate if rain forecast within the next 7 days. Dung Avoid set stocking wet paddocks. Effluent apply little and often (and have storage). Other areas Restricted grazing of forage crops Irrigation management (prevent outwash water recycling) Amendments Fencing, buffer strips, sediment traps