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Low-temperature pyrolysis of biomass produces a product known as biochar. The incorporation of this material into the soil has been advocated as a C sequestration method. Biochar also has the potential to influence the soil N cycle by altering nitrification rates and by adsorbing NH₄⁺ or NH₃. Biochar can be incorporated into the soil during renovation of intensively managed pasture soils. These managed pastures are a significant source of N₂O, a greenhouse gas, produced in ruminant urine patches. We hypothesized that biochar effects on the N cycle could reduce the soil inorganic-N pool available for N₂O-producing mechanisms. A laboratory study was performed to examine the effect of biochar incorporation into soil (20 Mg ha⁻¹) on N₂O-N and NH₃–N fluxes, and inorganic-N transformations, following the application of bovine urine (760 kg N ha⁻¹). Treatments included controls (soil only and soil plus biochar), and two urine treatments (soil plus urine and soil plus biochar plus urine). Fluxes of N₂O from the biochar plus urine treatment were generally higher than from urine alone during the first 30 d, but after 50 d there was no significant difference (P = 0.11) in terms of cumulative N₂O-N emitted as a percentage of the urine N applied during the 53-d period; however, NH₃–N fluxes were enhanced by approximately 3% of the N applied in the biochar plus urine treatment compared with the urine-only treatment after 17 d. Soil inorganic-N pools differed between treatments, with higher NH₄⁺ concentrations in the presence of biochar, indicative of lower rates of nitrification. The inorganic-N pool available for N₂O-producing mechanisms was not reduced, however, by adding biochar.

The root-associated soil microbiome contributes immensely to support plant health and performance against abiotic and biotic stressors. Understanding the processes that shape microbial assembly in root-associated soils is of interest in microbial ecology and plant health research. In this study, 37 plant species were grown in the same soil mixture for 10 months, whereupon the root-associated soil microbiome was assessed using amplicon sequencing. From this, the contribution of direct and indirect plant effects on microbial assembly was assessed. Plant species and plant-induced changes in soil physicochemistry were the most significant factors that accounted for bacterial and fungal community variation. Considering that all plants were grown in the same starting soil mixture, our results suggest that plants, in part, shape the assembly of their root-associated soil microbiome via their effects on soil physicochemistry. With the increase in phylogenetic ranking from plant species to class, we observed declines in the degree of community variation attributed to phylogenetic origin. That is, plant-microbe associations were unique to each plant species, but the phylogenetic associations between plant species were not important. We observed a large degree of residual variation (> 65%) not accounted for by any plant-related factors, which may be attributed to random community assembly.

Cropping soils vary in extent of natural suppression of soil-borne plant diseases. However, it is unknown whether similar variation occurs across pastoral agricultural systems. We examined soil microbial community properties known to be associated with disease suppression across 50 pastoral fields varying in management intensity. The composition and abundance of the disease-suppressive community were assessed from both taxonomic and functional perspectives. Pseudomonas bacteria were selected as a general taxonomic indicator of disease suppressive potential, while genes associated with the biosynthesis of a suite of secondary metabolites provided functional markers (GeoChip 5.0 microarray analysis). The composition of both the Pseudomonas communities and disease suppressive functional genes were responsive to land use. Underlying soil properties explained 37% of the variation in Pseudomonas community structure and up to 61% of the variation in the abundance of disease suppressive functional genes. Notably, measures of soil organic matter quality, C:P ratio, and aromaticity of the dissolved organic matter content (carbon recalcitrance), influenced both the taxonomic and functional disease suppressive potential of the pasture soils. Our results suggest that key components of the soil microbial community may be managed on-farm to enhance disease suppression and plant productivity.

Soil P composition can be conveniently determined in alkaline extracts using solution ³¹P nuclear magnetic resonance (NMR) spectroscopy, but spectral assignments are based on fragmentary literature reports of model compounds in various extraction matrices. We report solution ³¹P NMR chemical shifts of model P compounds, including inorganic phosphates, orthophosphate monoesters and diesters, phosphonates, and organic polyphosphates, determined in a standardized soil P extractant (0.25 M NaOH and 0.05 M EDTA). Signals from nucleic acids (DNA -0.37 ppm, RNA 0.54 ppm) and phospholipids (phosphatidyl choline 0.78 ppm, phosphatidyl serine 1.57 ppm, phosphatidyl ethanolamine 1.75 ppm) could be differentiated in the orthophosphate diester region, and were identified in a sample of cultured soil bacteria. Inorganic and organic polyphosphates could be differentiated by the presence of a signal at -9 ppm from the phosphate of organic polyphosphates. Some orthophosphate diesters, notably RNA and phosphatidyl choline, degraded rapidly to orthophosphate monoesters in NaOH–EDTA although DNA, other phospholipids, and orthophosphate monoesters were more stable. Changes in probe temperature had a marked influence on signal intensities and the relative magnitude of signals from orthophosphate monoesters and inorganic orthophosphate, and we suggest that solution ³¹P NMR spectroscopy of soil extracts be performed at 20°C.

The soil organic matter (OM) content of soils in a long-term fertiliser field trial (Winchmore, New Zealand) are similar ( P  > 0.05) despite >60 years application of different phosphorus (P) rates. As the net primary productivity increased with P addition, greater losses of carbon (C) occur concomitantly with increased P fertility. Several hypotheses have been proposed to explain the mechanisms, including C leaching, increased earthworm activity or elevated rates of microbial activity. In this study, we found support for both direct and secondary effects of soil P on soil C through impacts on the soil microbial community. Microbial biomass, inferred through quantification of hot water extractable C, increased with soil P status and decreased with C/P ratio ( P  < 0.001). However, the microbial biomass had no relationship with soil organic C content ( P  = 0.485). Mineralisation of C substrates added to soil also increased with soil P status (total P, R 2  = 0.84; P  < 0.001). These results indicated potential conditioning of the microbial community for rapid C cycling. Utilisation of different C compounds was clustered by cophenetic similarity; a distinct group of ten carbon compounds was identified for which rates of mineralisation were strongly associated with soil P status and microbial biomass. However, this alteration of microbial community size and activity was not reflected in abundances of selected oligotrophic and copiotrophic taxa. As such, the alteration may be due to changes in the abundances of all taxa, i.e. a general community response.

Intensively managed agricultural pastures contribute to N₂O and N₂ fluxes resulting in detrimental environmental outcomes and poor N use efficiency, respectively. Besides nitrification, nitrifier-denitrification and heterotrophic denitrification, alternative pathways such as codenitrification also contribute to emissions under ruminant urine-affected soil. However, information on codenitrification is sparse. The objectives of this experiment were to assess the effects of soil moisture and soil inorganic-N dynamics on the relative contributions of codenitrification and denitrification (heterotrophic denitrification) to the N₂O and N₂ fluxes under a simulated ruminant urine event. Repacked soil cores were treated with ¹⁵N enriched urea and maintained at near saturation (−1 kPa) or field capacity (−10 kPa). Soil inorganic-N, pH, dissolved organic carbon, N₂O and N₂ fluxes were measured over 63 days. Fluxes of N₂, attributable to codenitrification, were at a maximum when soil nitrite (NO₂⁻) concentrations were elevated. Cumulative codenitrification was higher (P = 0.043) at −1 kPa. However, the ratio of codenitrification to denitrification did not differ significantly with soil moisture, 25.5 ± 15.8 and 12.9 ± 4.8% (stdev) at −1 and −10 kPa, respectively. Elevated soil NO₂⁻ concentrations are shown to contribute to codenitrification, particularly at −1 kPa.

Ruminant urine patches on grazed grassland are a significant source of agricultural nitrous oxide (N₂O) emissions. Of the many biotic and abiotic N₂O production mechanisms initiated following urine-urea deposition, codenitrification resulting in the formation of hybrid N₂O, is one of the least understood. Codenitrification forms hybrid N₂O via biotic N-nitrosation, co-metabolising organic and inorganic N compounds (N substrates) to produce N₂O. The objective of this study was to assess the relative significance of different N substrates on codenitrification and to determine the contributions of fungi and bacteria to codenitrification. ¹⁵N-labelled ammonium, hydroxylamine (NH₂OH) and two amino acids (phenylalanine or glycine) were applied, separately, to sieved soil mesocosms eight days after a simulated urine event, in the absence or presence of bacterial and fungal inhibitors. Soil chemical variables and N₂O fluxes were monitored and the codenitrified N₂O fluxes determined. Fungal inhibition decreased N₂O fluxes by ca. 40% for both amino acid treatments, while bacterial inhibition only decreased the N₂O flux of the glycine treatment, by 14%. Hydroxylamine (NH₂OH) generated the highest N₂O fluxes which declined with either fungal or bacterial inhibition alone, while combined inhibition resulted in a 60% decrease in the N₂O flux. All the N substrates examined participated to some extent in codenitrification. Trends for codenitrification under the NH₂OH substrate treatment followed those of total N₂O fluxes (85.7% of total N₂O flux). Codenitrification fluxes under non-NH₂OH substrate treatments (0.7–1.2% of total N₂O flux) were two orders of magnitude lower, and significant decreases in these treatments only occurred with fungal inhibition in the amino acid substrate treatments. These results demonstrate that in situ studies are required to better understand the dynamics of codenitrification substrates in grazed pasture soils and the associated role that fungi have with respect to codenitrification.

Phosphorus losses from agricultural soil to water bodies are mainly related to the excessive accumulation of available P in soil as a result of long-term inputs of fertilizer P. Since P is a nonrenewable resource, there is a need to develop agricultural systems based on maximum P use efficiency with minimal adverse environmental impacts. This requires detailed understanding of the processes that govern the availability of P in soil, and this paper reviews recent advances in this field. The first part of the review is dedicated to the understanding of processes governing inorganic P release from the solid phase to the soil solution and its measurement using two dynamic approaches: isotope exchange kinetics and desorption of inorganic P with an infinite sink. The second part deals with biologically driven processes. Improved understanding of the abiotic and biotic processes involved in P cycling and availability will be useful in the development of effective strategies to reduce P losses from agricultural soils, which will include matching crop needs with soil P release and the development of appropriate remediation techniques to reduce P availability in high P status soils.

The biodiversity in soil ecosystems is simultaneously incredibly rich and poorly described. In countries such as New Zealand, where high endemism in plant species emerged following extended geographical isolation, it is likely similar evolutionary pressures extended to soil microbial communities (our biodiversity ‘dark matter’). However, we have little understanding of the extent of microbial life in New Zealand soils, let alone estimates of endemism, rates of species loss or gain, or implications for systems where plants and their microbiomes have co-evolved. In this study, we tested for the impacts of land-cover type (native forest, planted forest with exotic conifers, and pastoral agriculture) on soil bacterial communities and their functional potential, using environmental microarrays (PhyloChip and GeoChip, respectively). This evaluation was conducted across four environmentally different locations (Hokitika, Banks Peninsula, Craigieburn, and Eyrewell). The environment from which samples were collected was the largest and most significant factor associated with variation in bacterial community assemblage and function. As such, novel pockets of bacterial biodiversity, with discrete ecosystem function, may be present in New Zealand. There was some evidence to suggest that change in land cover affected soil bacterial species, but not their functions. Secondary testing found this effect was restricted to differences between native forest and agricultural land use. Bacterial communities and functions between native and planted forests were similar. Analysis of soil environmental properties among samples found that land cover effects were underpinned by changes in soil pH that typically accompanies application of lime in agricultural systems, but is uncommon in planted forests. When compared with other studies conducted in New Zealand, we conclude that: (1) different locations can harbour distinct communities of soil microbial diversity, and (2) land-use intensification, not land cover change per se, shifts microbial biodiversity through alteration of primary habitat conditions, particularly soil pH.

Rhizosphere processes play a critical role in phosphorus (P) acquisition by plants and microbes, especially under P-limited conditions. Here, we investigated the impacts of nutrient addition and plant species on plant growth, rhizosphere processes, and soil P dynamics. In a glasshouse experiment, blue lupin (Lupinus angustifolius), white clover (Trifolium repens L.), perennial ryegrass (Lolium perenne L.), and wheat (Triticum aestivum L.) were grown in a low-P pasture soil for 8 weeks with and without the single and combined addition of P (33 mg kg‾¹) and nitrogen (200 mg kg‾¹). Phosphorus addition increased plant biomass and total P content across plant species, as well as microbial biomass P in white clover and ryegrass. Alkaline phosphatase activity was higher for blue lupin. Legumes showed higher concentrations of organic anions compared to grasses. After P addition, the concentrations of organic anions increased by 11-,10-, 5-, and 2-fold in the rhizospheres of blue lupin, white clover, wheat, and ryegrass, respectively. Despite the differences in their chemical availability (as assessed by P fractionation), moderately labile inorganic P and stable organic P were the most depleted fractions by the four plant species. Inorganic P fractions were depleted similarly between the four plant species, while blue lupin exhibited a strong depletion of stable organic P. Our findings suggest that organic anions were not related to the acquisition of inorganic P for legumes and grasses. At the same time, alkaline phosphatase activity was associated with the mobilization of stable organic P for blue lupin.