Improving the prediction of fertilizer phosphorus availability to plants with simple, but non-standardized extraction techniques

Particle size determines the short-term phosphorus availability in biochar produced from digestate solids

Biochar pellets produced from the solid fraction of manure-based digestates are rich in phosphorus (P) and may represent a P source that is easy to handle and suitable for transport to P-deficient regions. However, the effect of feedstock composition and particle size on P availability in this type of biochar remains unexplored. To evaluate the effect of particle size on the short-term P availability in biochars derived from manure digestate solids, an incubation experiment was carried out, in which four biochars produced from digestate solids in powder and pellet form were incubated with three soils of low P content. The recovery of P in bicarbonate and water extracts was measured after 7 and 70 days of incubation. A subsequent pot experiment with barley on two of the soils was aimed at examining early crop recovery of P, comparing the effects of coarse and fine biochar particles. Biochars from digestate solids had total P contents ranging from 12 to 63 g kg-1. In all three soils, the recovery of P in water and bicarbonate extracts was lower after incubation with biochar pellets compared to powder, and P recovery remained constant or decreased slightly over time. Early shoot biomass and P recovery in barley were also higher when fine biochar particles were applied. The results suggest that particle size reduction improves the immediate availability of P in biochars produced from manure-based digestate solids.

Recycled Iron Phosphates: A New Phosphorus Fertilizer for Paddy Rice

The potential of recycled iron phosphates (FePs), e.g., vivianites (Fe3(PO4)2·8H2O) and Fe(III)-rich phosphorus (P) adsorbent materials, as phosphorus fertilizer is limited by the strong interaction between Fe and P. In this study, the efficiency of FePs as P fertilizer was explored by applying them as granules or powder in flooded strongly P-fixing soils (acid and calcareous), thereby taking advantage of increased P release induced by reductive dissolution of P-bearing Fe(III) minerals. First, no P diffusion from granular FeP fertilizers into flooded soils was detectable by the diffusive gradient in thin films (DGT) technique and microfocused X-ray fluorescence (μ-XRF) analysis of thin soil sections, in contrast to detectable P diffusion away from granules of soluble triple superphosphate (TSP) fertilizer. On the contrary, powdered FePs demonstrated an excellent increase in extractable P (1 mM CaCl2) in a 120-day incubation experiment in flooded soils. Second, a pot experiment was performed with rice (Oryza sativa) grown in flooded acid and calcareous soils. The fertilizer value of FePs was remarkable when dosed as powder, as it was even up to 3-fold higher than TSP in the acid soil and similar to TSP in the calcareous soil. The beneficial effect of FeP over TSP in the acid soil is attributed to the slow release of P from FePs, which allows to partly overcome P fixation. The promising results of FePs as P fertilizer applied as powders in flooded soils debunk the generally accepted idea that FePs are poor sources of P while demonstrating the importance of the timing of FeP fertilizer application.

Do contaminants compromise the use of recycled nutrients in organic agriculture? A review and synthesis of current knowledge on contaminant concentrations, fate in the environment and risk assessment

Use of nutrients recycled from societal waste streams in agriculture is part of the circular economy, and in line with organic farming principles. Nevertheless, diverse contaminants in waste streams create doubts among organic farmers about potential risks for soil health. Here, we gather the current knowledge on contaminant levels in waste streams and recycled nutrient sources, and discuss associated risks. For potentially toxic elements (PTEs), the input of zinc (Zn) and copper (Cu) from mineral feed supplements remains of concern, while concentrations of PTEs in many waste streams have decreased substantially in Europe. The same applies to organic contaminants, although new chemical groups such as flame retardants are of emerging concern and globally contamination levels differ strongly. Compared to inorganic fertilizers, application of organic fertilizers derived from human or animal feces is associated with an increased risk for environmental dissemination of antibiotic resistance. The risk depends on the quality of the organic fertilizers, which varies between geographical regions, but farmland application of sewage sludge appears to be a safe practice as shown by some studies (e.g. from Sweden). Microplastic concentrations in agricultural soils show a wide spread and our understanding of its toxicity is limited, hampering a sound risk assessment. Methods for assessing public health risks for organic contaminants must include emerging contaminants and potential interactions of multiple compounds. Evidence from long-term field experiments suggests that soils may be more resilient and capable to degrade or stabilize pollutants than often assumed. In view of the need to source nutrients for expanding areas under organic farming, we discuss inputs originating from conventional farms vs. non-agricultural (i.e. societal) inputs. Closing nutrient cycles between agriculture and society is feasible in many cases, without being compromised by contaminants, and should be enhanced, aided by improved source control, waste treatment and sound risk assessments.

Additive and Non-Additive Effects on the Control of Key Agronomic Traits in Popcorn Lines under Contrasting Phosphorus Conditions

Phosphorus is a non-renewable natural resource that will run out of reserves in the upcoming decades, making it essential to understanding the inheritance of nutrient use efficiency for selecting superior genotypes. This study investigated the additive and non-additive effects of commercially relevant traits for the popcorn crop (grain yield—GY, popping expansion—PE, and expanded popcorn volume per hectare—PV) in different conditions of phosphorus (P) availability in two locations in Rio de Janeiro State, Brazil. Six S7 lines previously selected for P use—L59, L70, and P7, efficient and responsive; and L54, L75, and L80, inefficient and non-responsive—were used as testers in crosses with 15 progenies from the fifth cycle of intrapopulation recurrent selection of UENF-14, with adaptation to the North and Northwest regions of Rio de Janeiro State. Using the Griffing diallel analysis, P use efficiency was predominantly additive in the expression of PE, and non-additive effects were prominent for GY and PV. For obtaining genotypes that are efficient for phosphorus use, it is recommended that heterosis with parents that provide additive gene accumulation for PE be explored.

Considering inorganic P binding in bio-based products improves prediction of their P fertiliser value

Prediction of the relative phosphorus (P) fertiliser value of bio-based fertiliser products is agronomically important, but previous attempts to develop prediction models have often failed due to the high chemical complexity of bio-based fertilisers and the limited number of products included in analyses. In this study, regression models for prediction were developed using independently produced data from 10 different studies on crop growth responses to P applied with bio-based fertiliser products, resulting in a dataset with 69 products. The 69 fertiliser products were organised into four sub-groups, based on the inorganic P compounds most likely to be present in each product. Within each product group, multiple regression was conducted using mineral fertiliser equivalents (MFE) as response variable and three potential explanatory variables derived from chemical analysis, all reflecting inorganic P binding in the fertiliser products: i) NaHCO₃-soluble P, ii) molar ratio of calcium (Ca):P and iii) molar ratio of aluminium + iron (Al + Fe):P. The best regression model fit was achieved for sewage sludges with Al-/Fe-bound P (n = 20; R² = 79.2%), followed by sewage sludges with Ca-bound P (n = 11; R² = 71.1%); fertiliser products with Ca-bound P (n = 29; R² = 58.2%); and thermally treated sewage sludge products (n = 9; R² = 44.9%). Even though external factors influencing P fertiliser values (e.g. fertiliser shape, application form, soil characteristics) differed between the underlying studies and were not considered, the suggested prediction models provide potential for more efficient P recycling in practice.

Long-Term Nutrient Supply Options: Strategies to Improve Soil Phosphorus Availability in the Rice-Wheat System

The indiscriminate use of chemical fertilizers can deteriorate soil, grain, and environmental quality; still, these can be restored if integrated nutrient management options with inclusion of legumes in the cropping system are adopted. A long-term (19 year) rice-wheat system experiment was examined to find out the best nutrient management practices (BNMP) through recommended dose of fertilizers (RDF), integrated plant nutrition system (IPNS), soil test crop response (STCR), farmyard manure (FYM), along with the inclusion of pulses (berseem and cowpea). Seven nutrient management practices were applied in combination of organic and chemical fertilizer in the rice-wheat system. Results showed that a significant variation was seen in phosphorus (P) fractions among the treatments and soil depths. The results showed a significantly (p < 0.05) higher contribution to phosphorus availability by Residual-P followed by NaHCO3-Po > NaOH-Pi > NaOH-Po > HCl-P > NaHCO3-Pi > available P and lowest in WSP under different long-term management options in rice-wheat system after completing 19 crop cycles. Variations in soil P-fractions with depth were compared to different treatment combination, and a considerable increase in all the major P-fractions was noticed. The continuous application of various IPNS options as organic farming (OF), RDF, STCR, and the inclusion of pulses (berseem and cowpea) significantly improved all P fractions in the soil system and offered an added benefit in terms of sustainability of production and soil health compared to the solo application of chemical fertilizers. Overall, results showed that IPNS options (berseem and cowpea) showed its superiority over the rest of the treatment. This study suggests that the inclusion of pulses would increase P-availability in soil system.