Hydrothermal carbonization of milk/dairy processing sludge: Fate of plant nutrients.

Dairy processing sludge (DPS) is a byproduct generated in wastewater treatment plants located in dairy (milk) processing companies (waste activated sludge). DPS presents challenges in terms of its management (as biosolids) due to its high moisture content, prolonged storage required, uncontrolled nutrient loss and accumulation of certain substances in soil in the proximity of dairy companies. This study investigates the potential of hydrothermal carbonization (HTC) for recovery of nutrients in the form of solid hydrochar (biochar) produced from DPS originating from four different dairy processing companies. The HTC tests were carried out at 160 °C, 180 °C, 200 °C and 220 °C, and a residence time of 1h. The elemental properties of hydrochars (biochars), the content of primary and secondary nutrients, as well as contaminants were examined. The transformation of phosphorus in DPS during HTC was investigated. The fraction of plant available phosphorus was determined. The properties of hydrochar (biochar) were compared against the European Union Fertilizing Products Regulation. The findings of this study demonstrate that the content of nutrient in hydrochars (biochars) meet the requirements for organo-mineral fertilizer with nitrogen and phosphorus as the declared nutrients (13.9-26.7%). Further research on plant growth and field tests are needed to fully assess the agronomic potential of HTC hydrochar (biochar).

Benefits of inoculation, P fertilizer and manure on yields of common bean and soybean also increase yield of subsequent maize.

Common bean and soybean yield poorly on smallholder farms in Rwanda. We evaluated the benefits of inoculation combined with P fertilizer and manure on yields of common bean and soybean in three agro-ecological zones (AEZs), and their residual effects on a subsequent maize crop. In the first season, the treatments included inoculum, three rates of manure, and two rates of P fertilizer, with nine replications (three per AEZ). Both legumes responded well to inoculation if applied together with manure and P fertilizer. Grain yields varied from 1.0 t ha-1 to 1.7 t ha-1 in unamended control plots to 4.8 t ha-1 for common bean and 3.8 t ha-1 for soybean in inoculated plots with both P and manure addition. The response of common bean and soybean to inputs varied greatly between AEZs. In the AEZ with low and erratic rainfall (Bugesera), yields of both legumes and maize were low and maize after soybean failed to yield any grain due to drought. In this regard, early maturing legume varieties are advised in regions of low rainfall. Responses of maize to an input applied to the legumes strongly increased when other inputs were applied together to the legume. This allowed greater maize yields which ranged from 0.8 t ha-1 in control plots to 6.5 t ha-1 in treatments previously inoculated with P and manure added for maize grown after common bean and from 1.9 t ha-1 in control plots to 5.3 t ha-1 for maize grown after soybean. The amount of N2-fixed measured using the 15N-natural abundance method differed between the two legumes and varied between 15 and 198 kg N2 ha-1 for common bean and between 15 and 186 kg N2 ha-1 for soybean and differed enormously among treatments and AEZs. Application of inputs to the legumes also resulted in enhanced N and P uptake of the subsequent maize. The use of inoculum combined with manure and P fertilizer is a good option for smallholder farmers growing common bean and soybean in rotation with maize. We observed strong effects of environment and call for care when targeting crops and technologies for sustainable crop production.

Methods for testing short- and long-term phosphorus fertilizing efficiency of products with varying solubility.

Phosphorus (P) recovery from nutrient-rich side streams (NRSS) and derived products is crucial to ensure sustainable food production in the future and to enhance the circular economy, but the agronomic efficiency of these products needs to be validated to reach these targets. In this study, we used a Hedley fractionation scheme and the diffusive gradient in thin film (DGT) method to determine P availability in 83 NRSS and derived products originating from Finland, Sweden, and Germany. Furthermore, two independent short- and long-term growth experiments with barley (Hordeum vulgare L.) and ryegrass (Lolium perenne L.), respectively, were conducted to evaluate P availability in 15 selected NRSS. In addition to the DGT soil test, different fertilizer extractants, 2 % formic acid (FA), 2 % citric acid, and neutral ammonium citrate, were tested for predicting P availability in growth experiments. Livestock manures and slurries were found to contain a notable portion of labile P and were comparable to superphosphate (SP). Despite the low shares of labile P in struvite (7.2 %) and AshDec® (1.3 %), they exhibited P availability comparable to SP fertilizer, as indicated by DGT (99 % and 238 % of SP equivalence, respectively). This suggests that factors other than solubility influenced P availability in these side streams. The DGT method as a promising soil test predicted both short- and long-term P availability better than the selected conventional chemical extraction methods did. The 2 % FA extract exhibited the poorest performance, overestimating P availability in some nutrient sources while underestimating others in long-term. These findings enhance our understanding of P availability in potential raw materials for fertilizers, facilitating more effective P management strategies in the circular economy.

Abundant bacterial subcommunity is structured by a stochastic process in an agricultural system with P fertilizer inputs.

Soil microorganisms play an important role in agroecosystems and are related to ecosystem functioning. Nevertheless, little is understood about their community assembly and the major factors regulating stochastic and deterministic processes, particularly with respect to the comparison of abundant and rare bacterial subcommunities in agricultural systems. Here, we investigated the assembly of abundant and rare bacterial subcommunities in fields with different crops (maize and wheat) and phosphorus (P) fertilizer input at three different growth stages on the Loess Plateau. The high-throughput sequencing dataset was assessed using null and neutral community models. We found that abundant bacteria was governed by the stochastic process of homogenizing dispersal, but rare bacterial subcommunity was predominant by deterministic processes in maize and wheat fields due to broader niche breadths of abundant species. Soil nitrogen (N) and P also determined the assembly of abundant and rare soil subcommunities. The relative abundance and composition of the abundant and rare bacterial subcommunities were also influenced by soil nutrients (soil available P (AP) and NO3--N) and agricultural practices (P fertilization and crop cultivation). In addition, the abundant bacterial community was more susceptible to P fertilizer input than that of the rare bacteria, and a higher relative abundance of abundant bacteria was observed in the P70 treatment both in maize and wheat soils. The microbial co-occurrence network analysis indicated that the maize field and low nutrient treatment exhibited stronger associations and that the abundant bacteria showed fewer interconnections. This study provides new insights toward understanding the mechanisms for the assembly of abundant and rare bacterial taxa in dryland cropping systems, enhancing our understanding of ecosystem diversity theory in microbial ecology.

Soybean (Glycine max L. Merrill) responds to phosphorus application and rhizobium inoculation on Acrisols of the semi-deciduous forest agro-ecological zone of Ghana.

Soybean cultivation in Ghana is limited mainly to the Guinea savanna and the forest/savanna transitional agro-ecological zones. Although soybean can be cultivated in the semi-deciduous forest zone, low soil pH and limited nodulation limit its productivity in this zone. In this study, a randomized complete block design, with four replications, was used to test if rhizobia inoculation and/or p-fertilizer could improve yield of soybean in the semi-deciduous forest zone. The residual effects of the treatments were tested on maize and soybean sequentially during the 2018 and 2019 cropping seasons. The inoculation study was repeated in 2020. Phosphorus and inoculation significantly (p = 0.0009) increased soybean grain yield in the 2018 cropping season by 88% and 108%, respectively over the control. Co-application of P and inoculant increased grain yield 3-fold over the control. Maize grain yield ranged from 4.3 t ha-1 in the control plots to 5.2 t ha-1 in treated plots but did not differ significantly among treatments. In 2020, the combined application of P and inoculant produced a significantly (0.002) higher yield than any of the other treatments. This demonstrates that soybean can be grown economically in the semi-deciduous forest agro-ecological zone of Ghana. Co-application of P and inoculant appeared cost-effective, in terms of return on investment.

Substitution of manure for mineral P fertilizers increases P availability by enhancing microbial potential for organic P mineralization in greenhouse soil.

The shortage of phosphorus (P) as a resource represents a major challenge for the sustainable development of agriculture. Manure has a high P content and is a potential substitute for mineral P fertilizers. However, little is known about the effects on soil P availability and soil microbial P transformation of substituting manure for mineral P fertilizers. In this study, variations in soil P availability and bacterial P mobilization were evaluated under treatment with manure as compared to mineral P fertilizers. In the greenhouse fruit and vegetable production system that provided the setting for the study, substitution of manure for mineral P (PoR treatment) resulted in a similar level of soil total P and a similar fruit and vegetable yield as compared to traditional fertilization, but a significantly increased level of soil available P. In addition, PoR treatment enhanced bacterial organic P mineralization potential and decreased inorganic P dissolution potential. These results demonstrate that manure application increases the availability of soil P primarily by enhancing soil microbial Po mineralization, indicating the potential feasibility of applying manure instead of mineral P fertilizers in greenhouse farming.

Potentially toxic trace element pollution in long-term fertilized agricultural soils in China: A meta-analysis.

Fertilization results in potentially toxic trace element (PTTE) pollution in agricultural soils. However, it is unclear which factors determine the effect sizes of fertilization on PTTEs at the multiple spatial-temporal scale. This work synthesized 379 observations in 78 field sites (3-35 years) across China's main grain producing areas, and showed that long-term organic fertilization significantly enhanced total Cu, Zn and Cd by 25.7%, 18.9% and 66.6%, and soil available Cu, Zn and Cd by 60.5%, 155.3% and 83.6%, respectively; whereas long-term inorganic fertilization increased only available Cu, Zn and Cd by an average of 6.3%. Organic fertilizer (OF) type and application rate dominated the variation of PTTE concentrations, where approximately one-half of Cd pollution (42.6% of total Cd and 47% of available Cd) was observed. Furthermore, OFs containing Cd less than 1 mg kg-1 were recommended to be safely applied to agricultural soils. Soil type was main factor under long-term inorganic fertilization determining available PTTE variation, resulted in higher pollution risk in some soils such as Alfisols and Semi-hydromorphic soils, where we suggested the use of lower amounts of P fertilizers or the application of ones having small amounts of PTTEs. In short, long-term organic fertilization caused serious pollution of PTTEs especially Cd in Chinese croplands, and some strategies with a focus towards reducing the pollution risk must be developed, e.g., promoting straw return, forbidding Cd addition to feeds and feed additives, and improving carbon sequestration efficiency (CSE) of OFs and thus soil organic matter (SOM).

Life cycle assessment of a long-term multifunctional winter wheat-summer maize rotation system on the North China Plain under sustainable P management.

In sustainable agriculture, sufficient crop yields and nutrients must be produced while maintaining environmental protection. Considering the role of phosphorus (P) fertilizer in influencing crops yield and environmental security, life cycle assessment was used to examine the environmental impacts of long-term P application on the grain yield and nutritional quality of winter wheat and summer maize. Thus, a long-term field experiment with six P application rates for winter wheat (0, 25, 50, 100, 200, and 400 kg P ha-1) and summer maize (0, 12.5, 25, 50, 100, and 200 kg P ha-1) was conducted on the North China Plain (NCP). The results showed that the cradle-to-farm gate eutrophication potential (EP), energy depletion (ED), and P depletion (PD) were significantly affected by the P application rate applied in winter wheat and summer maize production. The critical P rate required to ensure food security for wheat and maize was in line with the optimal rate for sustainable environmental development in terms of grain production and nutrient levels. On the NCP, the ED and PD of summer maize with optimized P management over 10 years were less than those of winter wheat regardless of using yield or nutrient level as the functional unit. However, the EP of the nutrient supply in winter wheat was less than that in summer maize under optimized P fertilization. The specific nutritional components that limited improvements in environment of wheat and maize production under the optimal P rate were energy (calories) and protein, respectively. In conclusion, in a multifunctional winter wheat-summer maize rotation system, optimized P fertilization (50 kg ha-1 for winter wheat and 25 kg ha-1 for summer maize) combined with the planting of high-yield wheat varieties and high-protein maize varieties showed great potential to reduce the environmental impacts of wheat and maize production.

Growth and elemental uptake of Trifolium repens in response to biochar addition, arbuscular mycorrhizal fungi and phosphorus fertilizer applications in low-Cd-polluted soils.

The aim of this study was to examine the effects of arbuscular mycorrhizal (AM) fungi, biochar (BC) addition and phosphorus (P) fertilizer applications on the mycorrhizal response, biomass and elemental uptake of Trifolium repens in cadmium (Cd)-polluted soils. The results showed that mycorrhizal colonization were significantly decreased by 100 mg P kg-1 fertilizer input. Moreover, AM fungi, BC addition and P fertilizer significantly increased shoot biomass accumulation at all treatments. In the absence of BC, the nitrogen (N), potassium (K), calcium (Ca) and magnesium (Mg) contents in the shoots were not affected by AM fungi after P fertilizer application, but the P content in the shoots significantly increased in response to AM fungi. In the absence of BC, both AM fungi and P fertilizer significantly reduced the Cd concentrations in the plant tissues as well as the soil diethylenetriaminepentaacetic acid (DTPA)-Cd concentration. These results indicated that the translocation factors (TFs) were influenced only by BC addition and that the roots could accumulate greater amounts of Cd than the shoots. On the basis of the hygienic standard for feed in China, the shoot Cd concentration in white clover was below the maximum permitted Cd concentration (1 µg g-1) across all treatments. Therefore, it is suggested that no negative mycorrhizal-white clover symbiotic relationships were observed and T. repens could be a suitable forage species for planting in soils with low concentrations of Cd contamination when BC and P fertilizer are applied.

Genotypic Differences in the Effect of P Fertilization on Phytic Acid Content in Rice Grain.

Phytic acid (PA) prevents the absorption of minerals in the human intestine, and it is regarded as an antinutrient. Low PA rice is beneficial because of its higher Zn bioavailability and it is suggested that the gene expression level of myo-inositol 3-phosphate synthase 1 (INO1) in developing grain is a key factor to explain the genotypic difference in PA accumulation among natural variants of rice. P fertilization is also considered to affect the PA content, but it is not clear how it affects INO1 gene expression and the PA content in different genotypes. Here, we investigated the effect of P fertilization on the PA content in two contrasting rice genotypes, with low and high PA accumulation, respectively. Based on the results of the analysis of the PA content, inorganic P content, INO1 gene expression, and xylem sap inorganic P content, we concluded that the effect of P fertilization on PA accumulation in grain differed with the genotype, and it was regulated by multiple mechanisms.

Effects of the addition of nitrogen and phosphorus on anaerobic ammonium oxidation coupled with iron reduction (Feammox) in the farmland soils.

Anaerobic ammonium oxidation coupled with iron reduction is termed as Feammox, and is a new nitrogen removal process. However, there is a paucity of studies on the response of nutrient additions on Feammox process in farmland ecosystems. In this study, we investigated the shifts of Feammox and iron-reducers under nitrogen (N) and phosphorus (P) applications via isotopic tracing and high-throughput sequencing technology. In the isotopic tracing experiment, Feammox rates was significantly greater in the N and/or P applications soil (0.184-0.239 µg N g-1 day-1) than in the no fertilizer soil (0.172 µg N g-1 day-1). The results indicated that N and P applications could favor the Feammox reaction. Molecular analysis showed that five predominant iron-reducing bacteria, including Geobacter, Anaeromyxobacter, Pseudomonas, Thiobacillus and Bacillus, were detected. Their abundance in the soil with no fertilizer, N, P and N combined with P was 0.93%, 1.11%-1.71%, 0.99%, and 1.40%-1.75%, respectively. This implied that iron-reducing bacteria can be stimulated under N and P applications. Overall, the results of this study demonstrated that N and/or P applications could alter the activity of Feammox, and modulate the potential of IRB in the farmland soils.

Assessing modified aluminum-based water treatment residuals as a plant-available phosphorus source.

Inorganic phosphorus (P) fertilizers are a finite resource; alternative means of creating P fertilizers from current municipal and agricultural waste sources may reduce our reliance on phosphate rock mining, and improve waste disposal and nutrient cycling. Previous research demonstrated that organic aluminum water treatment residual composites (Al/O-WTR), created by mixing aluminum water treatment residuals (Al-WTR) with swine wastewater, have the potential to be a source of plant-available P. A greenhouse study was conducted to compare spring wheat (Triticum aestivum L.) growth with increasing application rates of swine wastewater-derived Al/O-WTR and commercial P fertilizer (both applied at 34, 67, and 135 kg P2O5 ha-1) in either sandy loam or sandy clay loam soil. Spring wheat straw and grain P uptake were comparable across all treatments in the sandy loam, while straw and grain P uptake were lower with Al/O-WTR in the sandy clay loam. The Al/O-WTR did not affect soil organic P concentrations, but did increase phosphatase activity in both soils. Increased phosphatase activity suggests that Al/O-WTR application stimulated microorganisms and enhanced the extent to which microbial communities mineralized Al/O-WTR-bound organic P. Overall, these results suggest that Al-WTR can be used to make P fertilizer, combining two "waste" products to create a useful product. Phosphorus harvesting via Al/O-WTR may be a feasible future alternative to mining phosphate rock, while avoiding unnecessary waste disposal and improving agricultural nutrient cycling.

Biochar phosphorus fertilizer effects on soil phosphorus availability.

Biochar is a potential material for making slow-releasing phosphorus (P) fertilizers for the sake of increasing soil P use efficiency and mitigating P losses. However, the long-term effects of P-laden biochars on soil P availability remains unconcerned. In this study, a laboratory-scale 70-days soil incubation experiment was conducted to study the effects of original and P-laden biochars on soil P availability and fractions. Two original biochars were derived from maize stalks by pyrolyzing at 350 °C and 600 °C. P was laden on those biochars by immersing biochars in saturated KH2PO4 solution for 24 h. Eight treatments were set for the incubation experiment, which were soil, soil + triple-superphosphate (TSP), soil + 350 °C biochar, soil + 600 °C biochar, soil + TSP + 350 °C biochar, soil + TSP + 600 °C biochar, soil + 350 °C P-laden biochar, and soil + 600 °C P-laden biochar. Results showed that original biochars could decrease soil available P through P adsorption. And there were no significant differences of soil P fractions under the treatments of mineral P fertilizer and P-laden biochars. Whereas, compared to mineral P fertilizer, P-laden biochars, especially 600 °C P-laden biochar, could maintain soil available P in a significantly higher level across the incubation. It was mainly because of the slow-releasing pattern of P laden on biochar and a more homogeneous soil P source distribution under P-laden biochar treatments. These results indicated that P-laden biochar could work as P fertilizer to improve soil P use efficiency.

From hogs to HABs: impacts of industrial farming in the US on nitrogen and phosphorus and greenhouse gas pollution.

Nutrient pollution and greenhouse gas emissions related to crop agriculture and confined animal feeding operations (CAFOs) in the US have changed substantially in recent years, in amounts and forms. This review is intended to provide a broad view of how nutrient inputs-from fertilizer and CAFOs-as well as atmospheric NH3 and greenhouse gas emissions, are changing regionally within the US and how these changes compare with nutrient inputs from human wastewater. Use of commercial nitrogen (N) fertilizer in the US, which now exceeds 12,000,000 metric tonnes (MT) continues to increase, at a rate of 60,000 MT per year, while that of phosphorus (P) has remained nearly constant over the past decade at around 1,800,000 MT. The number of CAFOs in the US has increased nearly 10% since 2012, driven largely by a near 13% increase in hog production. The annualized inventory of cattle, dairy cows, hogs, broiler chickens and turkeys is approximately 8.7 billion, but CAFOs are highly regionally concentrated by animal sector. Country-wide, N applied by fertilizer is about threefold greater than manure N inputs, but for P these inputs are more comparable. Total manure inputs now exceed 4,000,000 MT as N and 1,400,000 MT as P. For both N and P, inputs and proportions vary widely by US region. The waste from hog and dairy operations is mainly held in open lagoons that contribute to NH3 and greenhouse gas (as CH4 and N2O) emissions. Emissions of NH3 from animal waste in 2019 were estimated at > 4,500,000 MT. Emissions of CH4 from manure management increased 66% from 1990 to 2017 (that from dairy increased 134%, cattle 9.6%, hogs 29% and poultry 3%), while those of N2O increased 34% over the same time period (dairy 15%, cattle 46%, hogs 58%, and poultry 14%). Waste from CAFOs contribute substantially to nutrient pollution when spread on fields, often at higher N and P application rates than those of commercial fertilizer. Managing the runoff associated with fertilizer use has improved with best management practices, but reducing the growing waste from CAFO operations is essential if eutrophication and its effects on fresh and marine waters-namely hypoxia and harmful algal blooms (HABs)-are to be reduced.

Using isotopes to trace freshly applied cadmium through mineral phosphorus fertilization in soil-fertilizer-plant systems.

Applications of mineral phosphorus (P) fertilizer can lead to cadmium (Cd) accumulation in soils and can increase Cd concentrations in edible crop parts. To determine the fate of freshly applied Cd, a Cd source tracing experiment was conducted in three soil-fertilizer-wheat systems by using a mineral P fertilizer labeled with the radio isotope 109Cd and by exploiting natural differences in Cd stable isotope compositions (δ114/110Cd). Source tracing with stable isotopes overestimated the proportion of Cd in plants derived from the P fertilizer, because the isotope ratios of the sources were not sufficiently distinct from those of the soils. Despite indistinguishable extractable Cd pools between control and treatments, the addition of P fertilizer resulted in a more negative apparent isotope fractionation between soil and wheat. Overall, the radio isotope approach provided more robust results and revealed that 6.5 to 15% of the Cd in the shoot derived from the fertilizer. From the introduced Cd, a maximum of 2.2% reached the wheat shoots, whilst 97.8% remained in the roots and soils. The low recoveries of the fertilizer derived Cd suggest that continuous P fertilizer application in the past decades can lead to a build-up of a residual Cd pool in soils.

Interaction of soil pH and phosphorus efficacy: Long-term effects of P fertilizer and lime applications on wheat, barley, and sugar beet.

Phosphorus (P), a plant macronutrient, must be adequately supplied for crop growth. In Germany, many soils are high in plant-available P; specifically in arable farming, P fertilizer application has been reduced or even omitted in the last decade. Therefore, it is important to understand how long these soils can support sustainable crop production, and what concentrations of soil P are required for it. We analyzed a 36-year long-term field experiment regarding the effects of different P application and liming rates on plant growth and soil P concentrations with a crop rotation of sugar beet, wheat, and barley. Sugar beet reacted to low soil P and low soil pH levels more sensitively than wheat, which was not significantly affected by the long-term omitted P application. All three crop species showed adequate growth at soil P levels lower than the currently recommended levels, if low soil pH was optimized by liming. The increase in efficacy of soil and fertilizer P by reduced P application rates therefore requires the adaptation of the soil pH to a soil type-specific optimal level.

Performance of secondary P-fertilizers in pot experiments analyzed by phosphorus X-ray absorption near-edge structure (XANES) spectroscopy.

A pot experiment was carried out with maize to determine the phosphorus (P) plant-availability of different secondary P-fertilizers derived from wastewater. We analyzed the respective soils by P K-edge X-ray absorption near-edge structure (XANES) spectroscopy to determine the P chemical forms that were present and determine the transformation processes. Macro- and micro-XANES spectroscopy were used to determine the chemical state of the overall soil P and identify P compounds in P-rich spots. Mainly organic P and/or P adsorbed on organic matter or other substrates were detected in unfertilized and fertilized soils. In addition, there were indications for the formation of ammonium phosphates in some fertilized soils. However, this effect was not seen in the maize yield of all P-fertilizers. The observed reactions between phosphate from secondary P-fertilizers and co-fertilized nitrogen compounds should be further investigated. Formation of highly plant-available compounds such as ammonium phosphates could make secondary P-fertilizers more competitive to commercial phosphate rock-based fertilizers with positive effects on resources conservation.