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Phosphorus Loads For Effluent Land Treatment Farms
- Rethinking The Paradigm
Summary (further details below)
When industrial or municipal effluents are spray irrigated to pasture in a land treatment system (LTS), a common view is that while nitrogen (N) loads need to be carefully managed to avoid N-leaching, phosphorus (P) will be strongly adsorbed to soil particles, thereby posing little environmental risk. Consented P-load limits are uncommon, resulting in systems that have applied substantial P-surpluses for many decades. Numerous overseas studies have shown that when sustained high P-loads occur, the soil P-adsorption sites become saturated and eventually P-loss to the environment occurs. Recent New Zealand research on dairy factory LTS, shows that even for highly P-retentive allophanic soils of volcanic origin, that P-saturation occurs leading to actual P-leaching down the soil profile and potentially into groundwater. At the same time there is heightened risk of P-loss via surface runoff during high intensity rainfall events. The research shows the past paradigm is outdated and that all LTS should have resource consent limits on both P and N. Any new LTS should follow normal farming practice of not increasing topsoil Olsen P above agronomic guidelines of 25-35 mg/l.
Data for a Taranaki land treatment site showing that under high P-surplus the anion storage capacity (ASC or %P retention) reduces rapidly from values of 80-95% for untouched soils. Once ASC falls below 65%, the calcium chloride soluble P (CaCl2-P, a measure of P-leaching) rises rapidly and P loss to the environment via leaching is likely.
The Details
The New Zealand governments Essential Freshwater reforms mean phosphorus (P) losses from agricultural land are being closely examined and farmers are encouraged to avoid applying P-fertiliser when topsoil Olsen P is above maximum agronomic levels (typically 25-35 mg/l). Above this, agronomic studies show little increase in grass or crop yields and there is increased risk of phosphorus loss to the environment via surface runoff washing P-enriched soil particles into surface waterways after heavy rain.
Land treatment systems (LTS) using agricultural land to treat industrial or municipal effluents are typically designed around nitrogen loads. Consented nitrogen loads of 150-300 kgN/ha/yr for pastoral land can rise to 400 kgN/ha/yr for some Cut & Carry systems. However, consented P-load limits are rare. Municipal LTS typically have low P-loads because the P-rich biosolids are removed from the secondary treated effluent via clarifiers prior to irrigation in order to manage pathogenic risk and public perception. However, for industrial LTS such as for dairy factories or meat processing, which do not contain human waste, they are often irrigated with less pre-treatment e.g., simple mixing, storage & pH balancing, or dissolved air floatation. Some industrial effluents contain high concentrations of P e.g., dairy factories and abattoirs. In direct contrast to typical farms, wastewater irrigation P-loads for LTS farms can be high and the annual P-surplus (difference between wastewater P-inputs and agricultural P-outputs) can reach 10-15 fold. The existing LTS paradigm is that excess P is immobilised by soil particles, particularly in high P-retention allophanic soils. The paradigm discounts any risk from P-leaching down the soil profile and largely discounts surface runoff P-loss risk by assuming complete retention in riparian buffers. Overseas studies where very high P-loads have been applied to agricultural soils e.g. for beef feedlots in North America, indicate substantial degrees of saturation of soil P-adsorption sites and resulting loss to the environment via P-leaching. The same is also likely in New Zealand where high P-surpluses have been applied for numerous years.
Are dairy factory land treatment sites a significant source of phosphorus leaching to waterways?
Investigations were conducted at seven dairy factory LTS farm sites to examine the validity of the paradigm when soil Olsen P ranges from 50-660 mg/l [1]. Movement of P down the soil profile or P-leaching was evident to variable degrees at all sites investigated and was correlated to the number of years under wastewater irrigation and the degree of P-surplus [2]. At one longer-term LTS site, the P had leached to 5 m depth and was detectable as elevated levels of dissolved reactive phosphorus in groundwater and in surface streams [3]. Calcium-chloride extractable P (CaCl2-P) provided a reliable measure of P-leaching risk, particularly when graphed against anion storage capacity (ASC or % P retention, Fig. 1). Water extractable P (WEP) measured the relative risk of loss via surface runoff and increased as either Olsen P and Total P increased.
Fig. 1. Risk of P-leaching (high CaCl2-P concentrations) increases rapidly once P-rich effluent saturates soil P-adsorption sites such that Anion Storage Capacity (ASC or % P-retention) decreased below 65% in allophanic soils [2].
Adding amendments such as aluminium sulphate or ferric chloride to the soils did increase P-binding to some degree, however the cost of widespread use was prohibitive [4]. The most effective option to reduce the risk of P-loss was to reduce the overall P-surplus to zero i.e., matching the wastewater P-inputs to the P-outputs of the agricultural system, in a similar manner to that required for typical dairy farms.
In summary, the results indicate that the existing paradigm is outdated, that consented P-load limits should be set for all dairy factory and other LTS and that these should limit P-inputs to close to the likely P-outputs of the agricultural system, or better still to induce a P-deficit so that eventually the very high soil P and inherent risk to the environment decreases.
