Abstract
The European dairy industry generates large volumes of wastewater from milk and dairy food processing. Removal of phosphorus (P) by complexing with metal (e.g., aluminium, calcium) cations in P rich sludge is a potential P source for agricultural reuse and P recycling. However, there is a significant knowledge gap concerning the plant availability of this complexed P in comparison to conventional mineral P fertiliser. The current absence of information on plant P bioavailability of dairy processing sludge (DPS) limits the ability of farmers and nutrient management advisors to incorporate it correctly into fertiliser programmes. The present study examined the most common types of dairy sludge—(1) aluminium-precipitated sludge (“Al-DPS”) and (2) calcium-precipitated lime-stabilised sludge (“Ca-DPS”) at field scale to assess P availability in grassland versus mineral P fertiliser over a growing season. The experimental design was a randomised complete block with five replications. Crop yield and P uptake were assessed for 4 harvests. The initial soil test P was at a low level and the experimental treatments were super phosphate at 15, 30, 40, 50 and 60 kg P ha−1, two dairy sludge applied at 40 kg P ha−1 (comparison was made with mineral P at same application rate) and a zero P control applied in a single application at the beginning of the growing season. Results showed a significant positive slope in the relationship between P uptake response and mineral P application rate indicating the suitability of the experimental site for P availability assessment. The P bioavailability of Al- and Ca-DPS varied greatly between treatments. The P fertiliser replacement value based on the 1st harvest was 50 and 16% increased to 109 and 31% cumulatively over the four harvests for Al- and Ca-DPS, respectively. The Al concentration in Al-DPS did not limit P bioavailability, but low P bioavailability from Ca-DPS can be associated with its high Ca content that can lead to formation of low soluble Ca-P compounds at alkaline pH conditions with a high Ca/P ratio. These findings show that P availability from dairy sludge can be quite different depending on treatment process. Consequently, it is critical to have P availability information as well as total P content available to ensure the application rate meets crop requirements without creating environmental risk by over application.
Highlights
European agriculture is heavily dependent on imported mineral phosphorus (P) inputs with P listed as a critical raw material due to short supply and potential negative impact on food security [1]
Sheil et al [28] reported a 45 to 50% drop of soil test P in zero P plots over 17 years compared to initial values in a grassland P experiment
Aluminium precipitated activated sludge (Al-dairy processing sludge (DPS)) and calcium precipitated limestabilised sludge (Ca-DPS) both derived from the milk processing industry differed in their crop P availability and fertiliser replacement values
Summary
European agriculture is heavily dependent on imported (about 92%) mineral phosphorus (P) inputs with P listed as a critical raw material due to short supply and potential negative impact on food security [1]. Increased recycling of P from different bio-based feedstocks (e.g., dairy processing sludge (DPS)) has the potential to reduce reliance on P importation [2,3]. A recommended practice is to reduce excess fertilisation and to foster the recycling of nutrients from different types of organic waste as fertilisers. This presents an opportunity to increase the use of recovered secondary-raw-material-based fertilising products such as DPS. To do so in a meaningful way there is a need for information about the plant availability of nutrients including P in the potential recycled alternatives
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