Current and residual phosphorous availability from compost in a ryegrass pot test.

Compost can provide nitrogen (N) and especially phosphorous (P) available for plant growth, thus representing a potential alternative to chemical P-fertilizers a non-renewable resource. However, little is known about their residual capacity to provide plant-available P. In this study four compost: a green waste compost (GWC), one from anaerobically-digested bio-waste (DC), one from sewage sludge (SSC), and one from bio-waste (BWC), were compared (10 and 20 Mg VS ha-1) in a ryegrass pot test (112 days), for their N- and P-relative mineral fertilizer equivalence (MFE; %) vs. a chemical fertilizer (NPK). After the test period, the exploited treatments were tested for their MFE during an additional ryegrass growth cycle (112 days) in an N-rich environment (N+). After 112 days, the pot test showed that DC and SSC produced dry biomass in the same range as did NPK, attaining the best N-MFE (80-100%) and P-MFE (100-125%), whereas GWC and BWC performed poorly (60-80 and 80-90%; N-MFE and P-MFE). At the end of the first growth cycle, DC and SSC still showed relevant Olsen-P (20-30 mg kg-1). This was reflected in the best ryegrass P-MFE in DC and SSC at the end of the second growth cycle (N+), after 224 days (100-110%), whereas BWC and GWC poorly performed (90-95%). DC and SSC may therefore represent valuable sources of N available for plant nutrition in the short term, and also represent medium-term valuable P sources, alternative to rock phosphate P fertilizers. This promising approach need further field-scale investigation to confirm the medium-long term capacity of composts to be alternative to rock phosphate P fertilizers.

Effect of iron sulphate on the phosphorus speciation from agro-industrial sludge based and sewage sludge based compost.

Composting is considered a suitable process for organic waste management, providing stable products that can be safely utilized as fertilizers, but little is still known about the variation of phosphorous (P) extractability during the stabilization process. In this work, sequential chemical extraction (SCE) with increasing strength extractants (H2O; 0.5M NaHCO3 pH 8.5; 0.1M NaOH, 1M HCl) was applied for P speciation over 56days of composting of either agro-industrial or urban wastewater sludge with green waste treated (AICFe+; SSCFe+) or not (AICFe-; SSCFe-) with FeSO4 (2%v/v). Composting strongly reduced the H2O-P, promoting the organic-P (Po) mineralization from the labile fraction (H2O+NaHCO3 40%), in addition to the increases of NaHCO3- and HCl-extractable inorganic-P (Pi) in both AICFe- and SSCFe- (+20% on average). The FeSO4 treatment did not negatively affect the process, reducing the Po mineralization during composting by increasing the NaOH-P, also protecting this fraction from fixation in the sparingly soluble fraction. The final P fractionation (%) was in AICFe-: NaOH (41)=NaHCO3 (38)>HCl (18)>H2O (3); in AICFe+: NaOH (53)>NaHCO3 (24)=HCl (22)>H2O (2); in SSCFe-: NaOH (46)>NaHCO3 (29)>HCl (21)>H2O (4) and in SSCFe+: NaOH (66)>NaHCO3 (13)>HCl (20)>H2O (1). Composting reduced the more easily leachable fraction (labile-Po), reducing the risk of P loss by increasing the long-term available P fraction (NaOH-P). This was enhanced by the FeSO4 addition. Further investigation into soil behaviour and plant availability of P from this source is needed.