Abstract
Composts were prepared from the solid fraction of pig slurry (SPS) by mixing with cotton gin waste (CGW) in two different proportions. The aim was to assess the fertilizer value of the two composts and to evaluate potential pollution risks due to the heavy metal (Cu and Zn) concentrations in the composts and the nitrate leaching following their soil application. For this, a pot trial using Italian ryegrass (Lolium multiflorum Lam.) as a test plant was run in a glasshouse. Two composts, A and B, prepared with SPS:CGW ratios of 4:3 and 3:4 (v:v), respectively (OM 63 and 57 %, TN 36.1 and 32.7, and TP 15.3 and 10.8 g kg-1 in A and B, respectively), were applied to a sandy loam soil at two doses based on the N supplied (D1: 360 mg TN pot-1, equivalent to 52 kg N ha-1, and D2: 720 mg TN pot-1). They were compared with a soluble mineral fertilizer (3.62 g pot-1 of N:P:K 10:10:10; providing 360 mg N pot-1); unfertilized control soil was also tested. The potential risk of nitrate-N leaching in the soil after two simulated intense rainfall events was evaluated. The experiment was carried out in double-layer pots, which allowed the collection of leachates. Both composts were able to increase plant biomass production and soil TN and available-P with respect to the control; available Cu and Zn in the soil did not increase significantly with respect to the control soil and mineral fertilizer treatment, although N deficiency limited plant growth at harvests 2 and 3. The compost prepared with a higher proportion of SPS (A) was more efficient for N fertilization (relative agronomic efficiency: 38.1-47.6 %; nitrogen use efficiency: 34.1-41.9 % of TN), due to its greater inorganic-N concentration. To obtain high P and K efficiencies application rates based on compost TN are recommended, which also prevent Cu and Zn soil enrichment. Nitrate leaching was very low in all treatments (<0.5 % of TN applied). Therefore, both composts can be used as organic fertilizers in the partial substitution of mineral fertilizers, without a significant nitrate leaching risk.
Highlights
The current Circular Economy approach requires the treatment of wastes so that their organic matter (OM) and nutrients can be recovered and used in agriculture
The root biomass production was highest in F and only treatment AD2 gave results statistically different from the control (Figure 1)
In the soil were low (Cu < 0.4 and Zn < 1.0 mg kg−1 soil), with a scarce risk of soil accumulation at the compost rates applied. Their pollution risk due to accumulation resulting from repeated applications over a long period needs to be tested in field experiments. Both composts prepared from the solid fraction of pig slurry (SPS) can be considered organic fertilizer products, since they efficiently increased the biomass yield of L. multiflorum, providing nutrients for plant growth; N was provided in readily available forms and was slowly released by partial mineralization of the organic N during the crop growth
Summary
The current Circular Economy approach requires the treatment of wastes so that their organic matter (OM) and nutrients can be recovered and used in agriculture. Materials derived from organic residues, such as compost, can be considered as sources of plant nutrients, for partial substitution of synthetic fertilizers (Flotats et al, 2011; Brockmann et al, 2014), in accordance with the key Nutrient Value of Pig Slurry Compost principles of the Circular Economy (Bernal, 2017). Compared with other organic wastes (e.g., animal slurry, digestates, or untreated plant residues), qualitatively high-class composts show clear benefits as soil amendments/fertilizers: reliable sanitization, a high proportion of stable and humic-like substances, and high concentrations of nutrients (Bernal et al, 2009a, 2017; Moral et al, 2009), the latter allowing a reduction in mineral fertilization (Brockmann et al, 2014)
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