Assessment of Fertilising Properties of a Solid Digestate in Comparison with Undigested Cattle Slurry Applied to an Acidic Soil

Abstract

The use of digestates or cattle slurries as fertilisers could contribute to the recycling of nutrients and organic matter, thus meeting the goals of the circular economy in agriculture. This work aims at evaluating the fertilising properties of a solid digestate (DG) in comparison with undigested cattle slurry (CS) and mineral fertilisation (MF). The experiment was performed in pots with ryegrass (Lolium multiflorum Lam.) grown in an acidic soil during a 163 days crop cycle. The results showed that throughout the crop cycle neither DG nor CS increased soil organic matter. DG significantly increased (P < 0.001) the sum of the soil exchangeable bases and soil P availability compared with CS or MF. Also, DG significantly increased (P < 0.05) the apparent P recovery of ryegrass (43%) compared with MF (27%). In the first cut, the ryegrass yield was higher in DG and CS than in MF, decreasing in the second and third cuts as a consequence of a decrease in N availability. Nevertheless, the fertilisation with DG or CS could replace the half amount of mineral N fertilisation, without a significant decrease in the ryegrass forage production. In addition, DG enables greater efficiency in the use of P than CS or MF.

Phosphorus Losses to Surface Runoff Waters After Application of Digestate to a Soil Over Fertilised with Phosphorus

“…Since the N fertilisation was done only through the digestate which had an N/P ratio of 2.3:1 that was quite low regarding crop P requirement, the fertilisation done added a surplus of P. Consequently, DG/CS showed not only the higher P losses to runoff waters but also the lower decrease of Olsen P during the crop cycle. This behaviour seems to confirm that the P content of the digestate and also the effect of digestate in mobilising soil sorbed P to available forms (Horta & Carneiro, 2020;Nest et al, 2016;Nziguheba et al, 1998) can be an advantage in agronomic nutrient's management but poses a risk in the environmental management of soil P legacy.…”

“…However, the increase of DRP in the runoff waters of DG/CS treatment compared with the other treatments should be due not only to the soil input of P from the digestate (digestate had a content of 11.7 g P kg −1 DM) but also to the decrease of soil P sorption. The latter would be achieved through the competition of the organic acids originated during the degradation of the OM of the digestate for the same soil P sorption sites as observed in other works (Horta & Carneiro, 2020;Nest et al, 2016;Nziguheba et al, 1998). In addition, most of Pi forms in the digestate (60% of the total Pi fractions) were in easily available forms that is P with high mobility to the transfer from soil to runoff waters.…”

“…The digestate used in this work was the same used by Horta and Carneiro (2020). It was obtained at a biogas plant from a mixture of cattle slurry and rye straw during 30 days of retention time in mesophilic reactors with 3000 m 3 of capacity.…”

“…The methods used to quantify the moisture content, pH (digestate to water ratio of 1:5), electric conductivity (1:10 suspension of digestate to water ratio), organic matter total N, Ca and Mg were described at Horta and Carneiro (2020). Total P (PT) was determined after ignition at 550 °C during 3 h and extracted with H 2 SO 4 0.5 M, inorganic P (P i ) was extracted with H 2 SO 4 0.5 M and then quantified by molecular absorption spectrophotometry.…”

“…Crop available potassium was quantified by the method of Egnér et al (1960). The Olsen P (Olsen et al, 1954) was used to evaluate the soil available P not only to the crops but also Table 2 Characterisation of the anaerobic solid digestate (DG), concerning the dry matter (DM), the organic matter (OM), the electric conductivity (EC), the organic and the total nitrogen (N o and N Total), the ratio C:N, the total phosphorus (PT), the inorganic and organic phosphorus forms (Pi and Po), the water soluble phosphorus (WSP) and the potassium, calcium and magnesium content (K, Ca and Mg) used in the field experiment ( adapted from Horta & Carneiro, 2020) 1 All the parameters analysed in the digestate are expressed on a dry matter basis excluding pH and CE. 2 Organic N. 3 to the transfer from soil to runoff waters (Horta and Torrent, 2007b).…”