Solid fraction of separated digestate as soil improver: implications for soil fertility and carbon sequestration

Abstract

Purpose This study investigated the C and N mineralisation potential of solid fractions (SFs) from co-digestated pig manure after P-stripping (P-POOR SF) in comparison with P-rich SFs, as a means to estimate their organic matter stability in soil. Compost (COMP) and biochar (BCHR) (made from P-POOR SF) were also included in the study as reference biosolids. Methods The SFs were incubated in a sandy-loam soil under moist conditions to determine production of CO2 and mineral N. At specified intervals, CO2 evolution in the mixtures was measured via the alkali trap method and titration over a period of 81 days, while mineral N was measured using a flow analyser after KCl extraction over a period of 112 days. Results The various SFs showed similar patterns of C mineralisation (15–26% of added total C in 81 days) that were clearly higher than for COMP and BCHR (6% and 7%, respectively). Temporary N immobilisation was observed in biosolids with a high C/N ratio. The effective organic matter (EOM) of the SFs was calculated based on the C mineralisation data and varied between 130 and 369 kg Mg−1. Conclusions The SF with a reduced P content had a high EOM/P ratio which is beneficial in areas where P status of the soil is already high. Moreover, the N mineralisation patterns confirm that a high C/N ratio may also reduce risks for N leaching due to temporary N immobilisation.

Speciation of P in Solid Organic Fertilisers from Digestate and Biowaste

“…These low OM values, which were similar to those measured in the composts, coincide with the highest Fe contents (35 and 20 g/kg DW, respectively), and this may point to the addition of iron salts during the process and/or the presence of significant amounts of mineral particles in the feedstock i.e., garden waste, crop residues or iron-rich sludges used in the anaerobic digester. The pH of the SFs of digestate and composts varied between 7.0 and 8.7, which fell within the normal range for such organic products [46][47][48], except for the SF with reduced P content (GZV-SF2) which had a slightly acidic pH of 5.5 due to treatment with sulphuric acid. Alkaline pH in digestates and compost may induce precipitation of phosphates with Ca and Mg to form (poorly water-soluble) calcium phosphate and struvite compounds [49].…”

“…N/P ratios in composts and SFs of digestate varied between 0.7 and 6.0 kg/kg. Assuming an N mineralisation coefficient of 10% for composts and 55% for SFs of digestate [47], the effective N/P ratio is <2 kg/kg, with the exception of the low-P product (GZV-SF2). Hence, the effective N/P ratio of the SOFs under investigation is low as compared to the N/P crop requirements [51] and this further confirms that all tested products should primarily be regarded as P fertilisers or organic soil improvers rather than as N fertilisers.…”

“…higher for SFs from digestate as compared to composts [47]. Organic matter decomposition may also influence the release of P bound to Fe/Al oxides by releasing organic acids which decrease P adsorption onto Fe/Al-oxides due to competition for absorption sites [19,35,36], though this effect is likely small considering that Fe/Al-oxides in soils are usually already, to an extent, covered by organic matter [67].…”

“…The remaining fraction of organic P may be released over time, depending on the mineralisation rate of the organic fraction. The rate of this process depends amongst others on the nature of the OM in the SOF [44] and the mineralisation rates of OM, which are generally higher for SFs from digestate as compared to composts [47]. Organic matter decomposition may also influence the release of P bound to Fe/Al oxides by releasing organic acids which decrease P adsorption onto Fe/Al-oxides due to competition for absorption sites [19,35,36], though this effect is likely small considering that Fe/Al-oxides in soils are usually already, to an extent, covered by organic matter [67].…”

Recarbonizing global soils: A technical manual of recommended sustainable soil management

“…It is, however, low in carbon. The solid fraction has a high dry matter content, is rich in phosphorus and organic carbon, and has the potential to be used as a P fertiliser and can stimulate soil organic matter buildup (Egene et al, 2020;Valentinuzzi et al, 2020).…”