Efficiency of Phosphorus Fertilizers Derived from Recycled Biogas Digestate as Applied to Maize and Ryegrass in Soils with Different pH

Inga-Mareike Bach,Andrea Bauerle,Lisa Essich,Torsten Müller

doi:10.3390/agriculture12030325

Abstract

Three phosphorus (P) fertilizer fractions recycled from biogas digestates were tested alone and in combination for their efficiency in two agricultural surface soils with different pH: a silty sandy loam and a clay loam. The experiments were carried out in pots under greenhouse conditions, using mineral triple superphosphate (TSP) as a reference. Maize was cultivated for 50 days, followed by ryegrass cultivation for an additional 84 days in the same soil, without additional fertilization. The variables investigated were above-ground plant biomass production, plant phosphorus concentration and content, and plant available phosphorus concentration in soil. The dry matter (DM) yield of maize was increased by the organic P fertilizers equal to or more than TSP in both soils. In the neutral soil, biomass was almost doubled compared to TSP when using one of the fractions (Struvite containing P-Salt) alone or in combination with dried solid fractions. P concentration in maize cultivated in the neutral soil was not significantly different between the P fertilization treatments. However, associated with biomass increase, the total P content in maize plants was equal to or higher than that with TSP. In the acidic soil, P concentration and total P content in maize plants, as well as the calcium-acetate-lactate extractable P (CAL-P) concentration in soil, were equal to or even higher than TSP. Ryegrass DM yield was unaffected by all P fertilizers, independent of the soil, although P concentration and total P content increased in the acidic soil with all fertilizers. Our results show that recycled P fertilizers from biogas digestates are effective P fertilizer alternatives to mineral TSP for maize cultivation under acidic and neutral soil conditions. The lack of growth effects in ryegrass indicates that recycled P fertilizers do not require changes in weed control. On the other hand, P extraction by ryegrass in overfertilized acidic soils as an option for soil remediation also works in soils fertilized with biogas digestate fractions.

Highlights

Looking at both dry matter (DM) yield and P concentration together, the results indicate that plant uptake of P was increased by all P fertilizers; unlike in maize, it was without an associated increase of plant biomass
Compared to the reference fertilizer triple superphosphate (TSP), the recycled fractions tested alone or in combination caused similar or even higher DM yield in maize plants cultivated in soils with neutral as well as with acidic pH
Despite biomass production being unaffected by all P fertilizers, an increased net uptake of P in ryegrass represents an option to extract excess P from soils after treatment with the recycled fertilizers used in this study

Summary

Introduction

A small fraction of the soil P is available to plant roots in the soil pore water fraction, because P is quickly adsorbed and immobilized onto soil particle surfaces [2,3,4]. The intensive use of organic and inorganic fertilizers in agriculture in recent decades has resulted in high P accumulation in soils [5] that is unavailable to plants [6]. Approximately 88% of agricultural P fertilizer comes from the limited resource phosphate rock [7], available only at a few locations worldwide in quantities that may secure supply for the few hundred years (calculated from [8]). The European Union has listed mineral phosphate rock (2014) and phosphorus (2017) as critical raw materials [11,12]

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