Soil health: Impacts of fertilisers on soil organisms Lynette Abbott continues her discussion on soil health by examining the effects of fertilisers on soil organisms. Most soils are deficient in nutrients needed to support the growth of agricultural plants; therefore, nutrient supplements are essential to sustain food security. Plants require essential macronutrients (e.g., nitrogen, phosphorus, potassium) as well as micronutrients (e.g., zinc, iron, boron). The amounts of nutrients required by different plant varieties are not the same. In natural ecosystems, soil organisms are involved in the cycling of nutrients from soil organic matter and enable the establishment and persistence of plant communities. This may be a relatively slow process in forests or rangelands, depending on the plant species present.

Lynette Abbott, Emerita Professor, The University of Western Australia School of Agriculture and Environment and UWA Institute of Agriculture, examines how the rhizosphere, a narrow collar of soil clinging to plant roots, is emerging as a key player in soil and plant health. The rhizosphere is the area of soil that is in very close contact with roots (Photo 1). Roots profoundly influence microbial processes in this zone, and many of these processes are critical to soil health. As roots grow, they release a variety of exudates into soil, and many contain carbohydrates. This stream of exudates provides a source of carbon and energy for soil microorganisms in close proximity to roots. Soil organisms multiply in this carbon-rich environment, creating a hotspot of microbial activity near roots. In turn, this enriched microbial community influences soil aggregation, roots and plant health.

Treating biochar with digestate can form a C- and N-enriched biochar fertilizer, but its role in controlling N2O emission from soil with different pH is unclear. This study assessed N2O emission from rhizosphere soil after growing ryegrass with urea, urea plus biochar, solid digestate, and digestate-incorporated biochar, with and without liming. The abundances of bacteria, fungi, two nitrification genes (bacterial amoA; archaeal amoA), and four denitrification genes (nirK, nirS, nosZ for clade I and nosZII for clade II) were quantified using quantitative PCR. Bacterial community composition was characterized using amplicon sequencing. Solid digestate and urea plus biochar decreased N2O emission by 48% and 56%, respectively, relative to urea under non-liming. This corresponded to the increased bacterial abundance and greater increases in N2O-consuming (nosZ and nosZII) than N2O-producing (archaeal amoA, nirK, nirS) gene abundances. Digestate-incorporated biochar decreased N2O emission by 75% compared to solid digestate, with decreased nirK gene abundance and increased prevalence of the denitrifier Dokdonella. Liming resulted in the lowest N2O emissions and highest nosZII gene abundance among all treatments. This study demonstrated the value of incorporating biochar in digestate in reducing N2O emission while enhancing plant nutrition.

A soil health initiative for revegetation and orchards Lynette Abbott from The University of Western Australia highlights an innovative soil health initiative for revegetation and orchards. The strategic placement of biological amendments derived from waste nutrient resources can be used in permeable biomass wells and walls to improve establishment of trees and shrubs planted during restoration of degraded vegetation on farms, or in orchards. Soil biological processes are involved in these restoration processes, which improve soil health. Permeable biomass wells (Figure 1) and walls (Figure 2) have been investigated at a field demonstration at The University of Western Australia’s farm, in the grainbelt east of Perth, Western Australia. The trial showed how up-scaling of novel soil restoration practices initiated on a small scale near Brookton, Western Australia, can restore degraded areas of farmland by improving soil health.

Compost is a sustainable soil amendment but typically lacks sufficient nitrogen (N) to function as a fertiliser. In this study, compost granules were enriched with NH4NO3-rich wastewater, increasing total N content from 1.28 % to 10 %. To regulate N release and potentially reduce environmental losses, lauric acid (LA) was applied as a coating at 5 %, 10 %, and 15 % (w/w). A 42-day glasshouse experiment with kikuyu grass evaluated N availability through shoot biomass, shoot N content, soil mineral N concentrations, and microbial responses. The modified compost (CGWW) produced the highest shoot N content (59.3 mg/pot) and biomass (3.75 g/pot). LA-coated compost treatments reduced shoot N content by 22-30 % and biomass by up to 17 % compared to CGWW, indicating lower plant N uptake due to delayed N release. Similar soil NH4+-N and NO3--N levels at harvest suggest more organic N remained locked up in CGWWLA, likely due to microbial immobilisation. LA application suppressed ammonia-oxidising bacteria (Nitrosomonadaceae), with the most significant reduction observed in the 15 % LA-only treatment (log2-fold change = -50.8). This microbial inhibition aligns with reduced shoot N content and supports the role of LA as a nitrification inhibitor. However, the 5 % LA coating was equally effective as higher rates, highlighting cost-efficiency. The findings highlight LA's potential in delaying N release and inhibiting ammonia oxidation, making it an effective agent for reducing N losses in fertilisers.

Soil phosphorus (P) deficiency can severely limit crop and forage productivity. With limited P resources, breeding programs to select high-P efficiency (HPE) genotypes have been developed, but the role of arbuscular mycorrhizal fungi (AMF) in altering root morphology and physiology to increase P use efficiency and production remains poorly understood. In this study, we compared mycorrhizal responsiveness, and plasticity of root morphological and physiological traits between two low-P efficiency (LPE) and two HPE alfalfa genotypes under low- and high-P treatments. Plants were grown either in soil with naturally occurring AMF or in sterilized soil with added AMF-free bacteria. The results indicated that the AMF symbiosis significantly increased alfalfa productivity and physiological P use efficiency by enhancing total root length and root surface area while reducing carboxylate release. Under low-P conditions, HPE genotypes with AMF symbiosis showed higher shoot DW, greater mycorrhizal responsiveness, thicker and more robust roots, as well as increased carboxylate release compared with LPE genotypes. We conclude that exploitation of the dominant species in indigenous AMF populations and breeding of crop genotypes with high mycorrhizal responsiveness show promising avenues with which to improve forage productivity and alleviate P limitation in modern agricultural ecosystems.

Microorganisms associated with belowground carbon availability play a critical role in the functions and sustainability of grazing grassland ecosystems. However, research on microbial responses to varying grazing intensities at different soil depths remains limited. This study examined rhizosphere bacterial communities under varying defoliation intensities in a 24-week pot experiment involving repeated defoliation of annual ryegrass (Lolium rigidum). We assessed bacterial diversity, composition, co-occurrence networks, and influencing factors across three soil layers (depths of 0-5, 5-10 and 10-15 cm). Results revealed significant reductions in root growth and root sugar concentrations (fructose, glucose, and sucrose) with increasing defoliation intensity, leading to decreased dissolved carbon and microbial biomass carbon in the rhizosphere. Defoliation influenced bacterial diversity, community composition, and complexity of co-occurrence networks, with effects shifting from positive to negative with increasing soil depth. The magnitude of these effects varied with defoliation intensity. Increased defoliation intensity and shallower soil depth were associated with simplified bacterial networks and a reduced abundance of keystone taxa. The negative effects of defoliation on bacterial co-occurrence network complexity were primarily linked to root variables, with root fructose being the most important predictor according to random forest analysis. Overall, bacterial community structure was more closely associated with root traits than soil properties, and buffered defoliation-induced reductions in root traits at the soil surface but not at greater depths. These findings provide new insights into the response of soil microbes to moderate grazing and highlight root traits as key indicators for steering the functions of grassland ecosystems.

The importance of earthworms for soil health In this article, Lynette Abbott from The University of Western Australia, highlights the importance of earthworms for soil health. Earthworms are common soil organisms and have attracted the attention of many of us because they occur in gardens, parks, agricultural regions, and natural ecosystems. They can be grouped according to their feeding habits and life cycles, influencing where they occur in the soil profile and how they contribute to soil health.

Digestate has a great potential as a carbon (C) and nitrogen (N) soil amendment. Loading digestate onto biochar can produce a C- and N- enriched biochar fertilizer (i.e., digestate-loaded biochar, DLB), and help to solve digestate-specific issues. This study aims to evaluate the potential of DLB at optimal application rates in keeping plant (annual ryegrass) nutrition level while mitigating greenhouse gas emissions and improving soil biological health compared to chemical fertilizers in an acidic soil under liming or not. Soil biological health index was assessed through quantitative PCR and amplicon sequencing. The results showed that increasing DLB addition to 150 kg N ha-1 resulted in a similar N uptake of ryegrass to that under urea despite lower mineral N provided by DLB. This application rate decreased greenhouse gas emissions relative to urea, through decreasing CO2 emission despite increased CH4 emission. Compared to urea, this DLB rate did not change N2O emission, corresponding to the specifically increased abundance of nirK gene (1.3-fold) (and enriched denitrifier Dokdonella) for N2O production and nosZ clade I and II genes (87%) for N2O consumption. The DLB at 150 kg N ha-1 enhanced soil biological health index by 1.4-fold relative to urea through increasing microbial abundances particularly fungi, enriching beneficial microbes (plant-growth-promoting bacteria, mycorrhiza and dark-septate-endophyte), and increasing fungal diversity; this effect was less pronounced under liming. This study concludes that DLB can serve as an organic-mineral fertilizer in maintaining plant nutrition while decreasing greenhouse gas emissions and enhancing soil biological health, offering a sustainable approach to managing organic waste.

Soil biodiversity is essential for building environmental resilience The School of Agriculture and Environment and Institute of Agriculture at the University of Western Australia recognise the importance of soil biodiversity in managing soil conditions and building resilience against environmental changes. Soil is a biodiversity ‘hotspot.’ The diversity of organisms in soil is even greater than that found in above-ground ecosystems. Soil biodiversity is dictated by its surroundings, and the communities present comprise groups of organisms that are interdependent. Indeed, soil organisms are represented in all six Kingdoms (Monera, Archaea, Protista, Fungi, Animalia, and Plantae) and fulfill key roles in important processes relevant to soil health, including those that regulate global nutrient and climate cycles.

Soil health: A role for arbuscular mycorrhizas Lynette Abbott from The University of Western Australia, focuses on soil health again, this time examining the role of arbuscular mycorrhizas. Mycorrhizas are common associations between plants and highly specialised fungi that participate in important soil health processes, including nutrient uptake by plants. This can include nutrients from fertilisers, the breakdown of soil organic matter, and soil minerals. Arbuscular mycorrhizal (AM) fungi form extensive networks of hyphae in soil that extend around roots and increase the soil volume that can be explored. In addition to nutrient scavenging, the hyphae contribute to maintaining a friable soil structure that benefits root growth and even helps plants access water in tiny soil pores when soil conditions are dry. Mycorrhizas may also contribute to protection against root disease.

Land degradation reduces productivity and biodiversity, and requires restoration of both soil condition and vegetation to re-establish ecosystem services. Organic amendments can improve soil biological and chemical properties, thereby enhancing revegetation success in severely degraded soil. This study investigated the use of compost and biochar to support revegetation and soil restoration. Soil was collected from a severely degraded creek bed in an agricultural farm in south-western Australia (UWA Farm Ridgefield) and amended with compost (2.5% and 5%) and biochar (5% and 10%), both alone and in all pairwise combinations. Saltbush (Atriplex nummularia) seedlings were grown for 10 weeks in glasshouse conditions. Following harvest, plant growth and soil chemical properties were analysed. DNA was extracted from rhizosphere soil for bacterial diversity profiling with subsequent putative functional genes relating to carbon, nitrogen and phosphorus cycling using an in-silico approach. Compost improved growth of root and shoot biomass. Both compost and biochar improved the alkaline soil by reducing pH and increasing nitrate, phosphorus and potassium levels. Compost addition also had a significant effect on rhizosphere bacterial community structure, decreasing alpha diversity and altering beta diversity indices. Amendment of soil also changed the relative abundance of putative nutrient cycling genes, with an increase in the potential for denitrification, carbon and phosphorus cycling and a decrease in ammonification potential. Application of this compost-biochar combination improved plant growth and soil condition by altering both chemical and biological characteristics of the soil, and therefore may provide an effective management strategy for supporting restoration in a degraded landscape.

Building healthy sandy soils in agricultural landscapes Lynette Abbott and Hira Shaukat from The University of Western Australia, provide insights into research on enhancing health in sandy soils. Sandy agricultural soils generally have lower productivity than soils with higher concentrations of finer clay and silt particles, but they are widespread and important worldwide for food and fibre production. For example, in the Mediterranean climatic region of Southwestern Australia, deep sandy soils are common and result from extremely long periods of weathering with little or no opportunity for re-mineralisation.

Soil health characteristics Lynette Abbott from The University of Western Australia, places the spotlight on soil health, including its physical, chemical, biological and hydrological characteristics. Soil health encompasses its physical, chemical, biological, and hydrological characteristics. Inherent soil properties underpin how components of soil health combine to support productive agricultural or natural ecosystems. Soil health encompasses its physical, chemical, biological, and hydrological characteristics. Inherent soil properties underpin how components of soil health combine to support productive agricultural or natural ecosystems. The origin of the underlying parent rock materials and the extent to which they are weathered, influences naturally occurring nutrient cycling processes in soil. Hence, some soils have inherently high levels of nutrients available for plants, but others have more limited plant-available nutrient resources.

Use of black soldier fly larvae (BSFL) to process large volumes of organic waste is an emerging industry to produce protein. A co-product of this industry, the larval faeces (frass), has potential to be used as an organic fertiliser in a circular economy. However, BSFL frass has a high ammonium (N-NH4+) content which could result in nitrogen (N) loss following its application to land. One solution is to process the frass by combining it with solid fatty acids (FA) that have previously been used to manufacture slow-release inorganic fertilisers. We investigated the slow-releasing effect of N after combining BSFL frass with three FAs - lauric, myristic and stearic acid. Soil was amended with the three forms of FA processed (FA-P) frass, unprocessed frass or a control and incubated for 28 days. The impact of treatments on soil properties and soil bacterial communities were characterised during the incubation. Lower N-NH4+ concentrations occurred in soil treated with FA-P frass compared to unprocessed frass, and N-NH4+ release was slowest for lauric acid processed frass. Initially, all frass treatments caused a large shift in the soil bacterial community towards a dominance of fast-growing r-strategists that were correlated with increased organic carbon levels. FA-P frass appeared to enhance the immobilisation of N-NH4+ (from frass) by diverting it into microbial biomass. Unprocessed and stearic acid processed frass became enriched by slow-growing K-strategist bacteria at the latter stages of the incubation. Consequently, when frass was combined with FAs, FA chain length played an important role in regulating the composition of r-/K- strategists in soil and N and carbon cycling. Modifying frass with FAs could be developed into a slow release fertiliser leading to reduced soil N loss, improved fertiliser use efficiency, increased profitability and lower production costs.

Arbuscular mycorrhizal (AM) fungi are a ubiquitous group of soil fungi that form mutualistic, symbiotic relationships with the roots of over 90% of higher plants. Most of the observed growth responses of plant hosts to AM cohabitation result from the enhanced uptake and sharing of phosphorus by fungal hyphae, with increased uptake of zinc, copper, potassium, calcium, silicon, nitrogen, and sulfur also attributable to activity of the fungal symbionts. AM fungi are particularly interesting because they have been co-evolving with plants for over 400 million years. Industrial-scale agricultural production systems and land restoration efforts have historically paid relatively little attention to AM fungi, despite the widely accepted benefits attributable to these ubiquitous soil organisms. Communities of AM fungi in agroecosystems display very little host plant specificity.

Fertilizers-induced priming effects of soil organic matter (SOM) decomposition influences net carbon balance and nutrient release. We hypothesize that very strong limitation of plant productivity and microbial activities by nitrogen (N) and phosphorus (P), common in Tibetan meadows, retard SOM decomposition and turnover. Consequently, N and/or P fertilization will induce priming effects of SOM and have implications for carbon balance. Soils from a nine-year fertilization experiment (N alone, P alone, NP together, and control) from a Tibetan alpine meadow were used to investigate priming effect of SOM and carbon balance after addition of C-13 labeled glucose. N and/or P fertilization acidified soil by 0.5 pH unit, decreased SOM content, and increased total and available N, total P. Regardless of fertilization, glucose addition accelerated SOM decomposition with priming effects of 30-60 mu g C g(-1) soil during 78 days. Alleviation of N and P limitation by N and NP fertilization lowered the priming effect by 17% and 14%, respectively, but P fertilization increased priming effect by 67%. The negative correlation of priming effect intensity with SOM, nitrate or total N, and microbial biomass contents indicated that fertilization-induced differences in soil N and the microbial community are responsible for the priming effects. Positive correlation of carbon balance with total N and ammonium contents suggested that soil N accounts for carbon sequestration. Therefore, long-term N and/or P fertilization accelerate SOM decomposition and reduce SOM storage in alpine meadows, of which P fertilization induces the highest priming effect and the lowest SOM storage.

Context or problem: Joint fertilization with organic and inorganic fertilizers contributes to sustainable crop production. However, quantitatively understanding the joint fertilization efficacy (JFE) with organic and inor-ganic fertilizers on yield is limited. Objective or research question: This study aimed to investigate the quantitative JFE on crop yield. Methods: A 15-year field experiment with pea, wheat, and potato rotation was conducted on a constructed terrace in the semiarid area of the Loess Plateau of China. The fertilizer treatments imposed were (i) inorganic N and P fertilizer (NP); (ii) sheep manure (M); (iii) combined manure plus the inorganic N and P fertilizer (MNP); and (iv) unfertilized control (CK). We defined the yield increase of NP, M and MNP relative to CK (Delta Y-NP, Delta Y-M and Delta Y-MNP), and the JFE was calculated as Delta Y-MNP - (Delta Y-M+Delta Y-NP) / Delta Y-M+Delta Y-NP x 100% When the JFE > 10%, it was considered a synergistic effect; when the JFE was between -10% and 10%, it was a summing effect, and when the JFE - 10%, it was an offset effect. Results: Across all years, the total crop yield averaged 6127, 3901, 3912, and 1830 kg ha(-1) for MNP, M, NP, and CK, respectively. For years 2-8, the yield increases for MNP were 10%- 80% higher than the sum of M and NP (M+NP), showing a synergistic JEF. For years 9-15, the yield increase for MNP was similar or 12%-30% less than that for M+NP, indicating the summing or offset JFE. By the 15th year, the soil organic carbon (SOC) for CK, NP, M, and MNP increased 30%, 74%, 215% and 185%, respectively, to the initial value. For years 2-8, microbial biomass carbon (MBC), soil inorganic nitrogen (IN) and available phosphorus (AP) in MNP was 20%, 106% and 163% higher than M+NP. A structural equation model analysis indicated that the synergistic JFE was mainly attributable to the rapid increase of AP and IN of MNP during the early years. With the increase in soil fertility and the gradual rise in M yield, the synergistic effect turned into an offset effect. Then the yield of NP decreased due to the soil water deficit, and the offset JEF was turned to summing. Conclusions: The JFE is synergistic in the early years but summing or offset later, showing that joint fertilization is not always increasing yield linearly with soil nutrient input, mainly depending on soil fertility.Implications or significance: Clarifying the JFE quantitatively on yield helps optimize fertilization strategies for sustainable farming development.

Calcareous soils are widely spread in arid and semi-arid regions and cover almost one-third of the world's land surface area. This chapter gives an overview of the distribution and characteristics of calcareous soils, the limitations of calcareous soils for crop production, and some of the sustainable management practices to overcome crop productivity constraints in calcareous soils. Calcareous soils are characterized by the presence of calcium carbonate (CaCO3) in the soil parent material and the consequent accumulation of free CaCO3 in the soil profile. The pH of these soils is usually above 7, and, in the presence of free sodium carbonate, it may exceed 9. In some soils, the free CaCO3 can concentrate into hard layers, known as 'caliche', that are impermeable to water infiltration and plant root penetration. Although calcareous soils may be dominated by free CaCO3, they may also contain significant amounts of iron (Fe), aluminium (Al), and manganese (Mn) either as discrete and mixed minerals, coatings on soil inorganic size fractions such as clay, sand, and silt, or complexed with soil organic matter. The major crop productivity constraints in calcareous soils include phosphorus and trace element (e.g., iron, zinc, and copper) deficiency, surface crust formation, and an impermeable subsurface compact layer. Soil productivity constraints in calcareous soils can be overcome by proper choice and placement of fertilizers, addition of organic matter, which promotes stable aggregate formation that inhibits dispersion and resists crust formation, and deep ripping to break through the compacted pan layer, thereby allowing root access to soil water and nutrients beneath this layer.

Biochar can be used for multifunctional applications including the improvement of soil health and carbon storage, remediation of contaminated soil and water resources, mitigation of greenhouse gas emissions and odorous compounds, and feed supplementation to improve animal health. A healthy soil preserves microbial biodiversity that is effective in supressing plant pathogens and pests, recycling nutrients for plant growth, promoting positive symbiotic associations with plant roots, improving soil structure to supply water and nutrients, and ultimately enhancing soil productivity and plant growth. As a soil amendment, biochar assures soil biological health through different processes. First, biochar supports habitats for microorganisms due to its porous nature and by promoting the formation of stable soil micro-aggregates. Biochar also serves as a carbon and nutrient source. Biochar alters soil physical and chemical properties, creating optimum soil conditions for microbial diversity. Biochar can also immobilize soil pollutants and reduce their bioavailability that would otherwise inhibit microbial growth. However, depending on the pyrolysis settings and feedstock resources, biochar can be comprised of contaminants including polycyclic aromatic hydrocarbons and potentially toxic elements that can inhibit microbial activity, thereby impacting soil health.