Differential structure and function of phosphorus-mineralizing microbial communities in organic and upper mineral soil horizons across a temperate rainforest chronosequence.

Microbial community structure and function were assessed in the organic and upper mineral soil across a ~4000-year dune-based chronosequence at Big Bay, New Zealand, where total P declined and the proportional contribution of organic soil in the profile increased with time. We hypothesized that the organic and mineral soils would show divergent community evolution over time with a greater dependency on the functionality of phosphatase genes in the organic soil layer as it developed. The structure of bacterial, fungal, and phosphatase-harbouring communities was examined in both horizons across 3 dunes using amplicon sequencing, network analysis, and qPCR. The soils showed a decline in pH and total phosphorus (P) over time with an increase in phosphatase activity. The organic horizon had a wider diversity of Class A (phoN/phoC) and phoD-harbouring communities and a more complex microbiome, with hub taxa that correlated with P. Bacterial diversity declined in both horizons over time, with enrichment of Planctomycetes and Acidobacteria. More complex fungal communities were evident in the youngest dune, transitioning to a dominance of Ascomycota in both soil horizons. Higher phosphatase activity in older dunes was driven by less diverse P-mineralizing communities, especially in the organic horizon.

Missing phosphorus legacy of the Anthropocene: Quantifying residual phosphorus in the biosphere.

A defining feature of the Anthropocene is the distortion of the biosphere phosphorus (P) cycle. A relatively sudden acceleration of input fluxes without a concomitant increase in output fluxes has led to net accumulation of P in the terrestrial-aquatic continuum. Over the past century, P has been mined from geological deposits to produce crop fertilizers. When P inputs are not fully removed with harvest of crop biomass, the remaining P accumulates in soils. This residual P is a uniquely anthropogenic pool of P, and its management is critical for agronomic and environmental sustainability. Managing residual P first requires its quantification-but measuring residual P is challenging. In this review, we synthesize approaches to quantifying residual P, with emphasis on advantages, disadvantages, and complementarity. Common approaches to estimate residual P are mass balances, long-term experiments, soil test P trends and chronosequences, with varying suitability or even limitations to distinct spatiotemporal scales. We demonstrate that individual quantification approaches are (i) constrained, (ii) often complementary, and (iii) may be feasible at only certain time-space scales. While some of these challenges are inherent to the quantification approach, in many cases there are surmountable challenges that can be addressed by unifying existing P pool and flux datasets, standardizing and synchronizing data collection on pools and fluxes, and quantifying uncertainty. Though defined as a magnitude, the distribution and speciation of residual P is relatively less understood but shapes its utilization and environmental impacts. The form of residual P will vary by agroecosystem context due to edaphoclimatic-specific transformation of the accumulated P, which has implications for management (e.g., crop usage) and future policies (e.g., lag times in P loading from non-point sources). Quantifying the uncertainty in measuring residual P holds value beyond scientific understanding, as it supports prioritization of monitoring and management resources and inform policy.

Will rising atmospheric CO2 concentration inhibit nitrate assimilation in shoots but enhance it in roots of C3 plants?

Bloom et al. proposed that rising atmospheric CO2 concentrations 'inhibit malate production in chloroplasts and thus impede assimilation of nitrate into protein of C3 plants, a phenomenon that will strongly influence primary productivity and food security under the environmental conditions anticipated during the next few decades'. Previously we argued that the weight of evidence in the literature indicated that elevated atmospheric [CO2 ] does not inhibit NO3 - assimilation in C3 plants. New data for common bean (Phaseolus vulgaris) and wheat (Triticum aestivum) were presented that supported this view and indicated that the effects of elevated atmospheric [CO2 ] on nitrogen (N) assimilation and growth of C3 vascular plants were similar regardless of the form of N assimilated. Bloom et al. strongly criticised the arguments presented in Andrews et al. Here we respond to these criticisms and again conclude that the available data indicate that elevated atmospheric [CO2 ] does not inhibit NO3 - assimilation of C3 plants. Measurement of the partitioning of NO3 - assimilation between root and shoot of C3 species under different NO3 - supply, at ambient and elevated CO2 would determine if their NO3 - assimilation is inhibited in shoots but enhanced in roots at elevated atmospheric CO2 .

Elevated CO2 effects on nitrogen assimilation and growth of C3 vascular plants are similar regardless of N-form assimilated.

Atmospheric carbon dioxide concentration ([CO2]) increased from around 280 ppm in 1750 to 400 ppm in 2016 and is likely to continue to increase throughout this century. It has been argued that wheat, Arabidopsis, and C3 plants in general respond more positively to elevated atmospheric [CO2] under ammonium (NH4+) nutrition than under nitrate (NO3-) nutrition because elevated CO2 inhibits their photoreduction of NO3- and hence reduces their total plant nitrogen (N) assimilation and ultimately growth. Here, it is argued that the weight of evidence in the literature indicates that elevated atmospheric [CO2] does not inhibit NO3- assimilation and growth of C3 vascular plants. New data for common bean and wheat support this view and indicate that the effects of elevated atmospheric [CO2] on N assimilation and growth of C3 vascular plants will be similar regardless of the form of N assimilated.

Non-host larvae negatively impact persistence of the entomopathogen Beauveria bassiana in soil.

A better understanding of the ecology of the insect pathogenic fungus, Beauveria bassiana, in soil is needed to identify reasons behind the variable efficacy often seen after field application. A transformed strain of a candidate commercial strain of B. bassiana (F418 gfp tr3), expressing the green fluorescent protein and the hygromycin B resistance gene, was used to assess the effects of the larvae of a host insect, Tenebrio molitor L. (Coleoptera: Tenebrionidae), a non-host, Costelytra zealandica (Coleoptera: Scarabaeidae) and the absence of larvae on the persistence of F418 gfp tr3 in pasteurised and non-sterile soil over 4 months. In the presence of a T. molitor larvae, F418 gfp tr3 populations increased significantly in pasteurised and non-sterile soil; however, populations increased less in non-sterile soil than in pasteurised soil. Lower populations of F418 gfp tr3 were recovered in pasteurised soil in the presence of C. zealandica larvae than in pasteurised soil without larvae. No difference was observed between F418 gfp tr3 populations in non-sterile soil with a non-host larvae or without larvae. Accompanying studies showed that F418 gfp tr3 conidia germinated and produced appressoria on live and excised cuticle of non-host (C. zealandica) larvae but infection did not occur, leading to a net loss of viable conidia in the soil. Conidia administrated orally to C. zealandica larvae were viable on recovery from faecal samples, suggesting that ingestion of the fungus by the larvae had little impact on the viable fungal population. Soil bacterial and fungal community patterns were analysed using Single-Strand Conformation Polymorphism (SSCP) and showed a correlation between changes in F418 gfp tr3 persistence in pasteurised and non-sterile soil and changes in soil communities in the presence of a host insect, non-host insect or in the absence of insect. In pasteurised soil, non-specific germination of F418 gfp tr3 conidia on the non-host larval cuticle and the presence of antagonistic bacteria introduced with the field-collected larvae are most likely responsible for the differences observed. The more complex microbial community structures in non-sterile soil could lead to fungistasis, preventing potentially antagonistic bacteria degrading conidia or inhibiting attachment and germination on the non-host larval cuticle, resulting in the observed lack of difference between non-host and no larval treatments.

Bayesian network for point and diffuse source phosphorus transfer from dairy pastures in South otago, new zealand.

Many factors affect the magnitude of nutrient losses from dairy farm systems. Bayesian Networks (BNs) are an alternative to conventional modeling that can evaluate complex multifactor problems using forward and backward reasoning. A BN of annual total phosphorus (TP) exports was developed for a hypothetical dairy farm in the south Otago region of New Zealand and was used to investigate and integrate the effects of different management options under contrasting rainfall and drainage regimes. Published literature was consulted to quantify the relationships that underpin the BN, with preference given to data and relationships derived from the Otago region. In its default state, the BN estimated loads of 0.34 ± 0.42 kg TP ha for overland flow and 0.30 ± 0.19 kg TP ha for subsurface flow, which are in line with reported TP losses in overland flow (0-1.1 kg TP ha) and in drainage (0.15-2.2 kg TP ha). Site attributes that cannot be managed, like annual rainfall and the average slope of the farm, were found to affect the loads of TP lost from dairy farms. The greatest loads (13.4 kg TP ha) were predicted to occur with above-average annual rainfall (970 mm), where irrigation of farm dairy effluent was managed poorly, and where Olsen P concentrations were above pasture requirements (60 mg kg). Most of this loading was attributed to contributions from overland flow. This study demonstrates the value of using a BN to understand the complex interactions between site variables affecting P loss and their relative importance.

Soil bacterial community succession during long-term ecosystem development.

The physicochemical and biological gradients of soil and vegetative succession along the Franz Josef chrono sequence in New Zealand were used to test whether bacterial communities show patterns of change associated with long-term ecosystem development. Pyrosequencing was conducted on soil-derived 16S rRNA genes at nine stages of ecosystem progression and retrogression, ranging in age from 60 to c. 120 000 years since glacial retreat. Bray­Curtis ordination indicated that the bacterial communities showed clear patterns of change that were closely aligned with ecosystem development, pedogenesis and vegetative succession (Mantel test; r = 0.58; P < 0.001). Eighty per cent (80%) of the explained variability in bacterial community structure was observed during the first c.1000 years of development, when bacterial richness (Simpson's 1/D) declined from 130 to 30. The relatively high turnover of soil bacterial communities corresponded with an integrative 'plant­microbial successional feedback' model that predicts primarily negative feedbacks between plants and soil bacterial communities during progression and early pedogenesis. Positive feedbacks, similar to those of the plant community, could explain the long periods of community stability during later retrogressive stages of ecosystem development. This hypothesized model provides a consistent description linking below ground communities to ecosystem development and succession. The research, using deep sequencing technology, provides the first evidence for soil bacterial community change associated with the process of long-term ecosystem development. How these bacterial community changes are linked to the processes of primary ecosystem succession is not known and needs further investigation.

Factors controlling shallow subsurface dissolved reactive phosphorus concentration and loss kinetics from poorly drained saturated grassland soils.

Shallow subsurface pathways dominate dissolved reactive phosphorus (DRP) losses in grassland soils that are: poorly drained, shallow, or have a perched water table in wetter months causing saturation-excess runoff. Saturated conditions can lead to anoxia, which can accelerate phosphorus (P) loss. Two scales of investigation were utilized in this study. First, at the field scale, soil cores were extracted to 2.5 m, subdivided and samples extracted using water extractable P (WEP) and sodium-bicarbonate-dithionite extractable P (NaBD-P). Second, at the laboratory scale, detailed incubation studies using field-moist grassland topsoils from sites in Ireland and New Zealand examined the kinetics of WEP under anoxic (WEPanox ) and oxic (WEPox ) conditions with imposed temperature and soil P fertilizer input treatments. Results from soil-core samples showed that redox-sensitive NaBD-P concentrations were depleted where artificial drainage lines were installed (100 cm deep), but WEP concentrations available to shallow flow were enriched in topsoil. The laboratory scale incubation experiment investigated the influence of temperature (3 vs. 18 °C), anoxia (designed to simulate saturation following a rainfall event), and superphosphate fertilizer (10 to 60 kg P ha-1  yr-1 ) on WEP concentrations over 24 h in three grassland topsoils (clay, silt, and sandy loam textures). Concentrations increased with fertilizer rate, temperature, and-in two soils-anoxic conditions. This was commensurate with nitrate (NO3 - ) depletion and the reductive dissolution of iron and manganese. The release of P during anoxia was complete within 24 h. The results highlighted late winter to spring as the riskiest period for topsoil P losses in shallow subsurface flow due to wet soil conditions, increasing temperatures, and low soil NO3 - concentrations. This knowledge highlights the necessity to consider and refine tests used to assess topsoil P loss risk, where in the landscape P losses are likely, and what strategies can be used to mitigate losses.

Plant species identity and plant-induced changes in soil physicochemistry-but not plant phylogeny or functional traits - shape the assembly of the root-associated soil microbiome.

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.

Biochar incorporation into pasture soil suppresses in situ nitrous oxide emissions from ruminant urine patches.

Nitrous oxide (N2O) emissions from grazing animal excreta are estimated to be responsible for 1.5 Tg of the total 6.7 Tg of anthropogenic N2O emissions. This study was conducted to determine the in situ effect of incorporating biochar, into soil, on N2O emissions from bovine urine patches and associated pasture uptake of N. The effects of biochar rate (0-30 t ha(-1)), following soil incorporation, were investigated on ruminant urine-derived N2O fluxes, N uptake by pasture, and pasture yield. During an 86-d spring-summer period, where irrigation and rainfall occurred, the N2O fluxes from 15N labeled ruminant urine patches were reduced by >50%, after incorporating 30 t ha(-1) of biochar. Taking into account the N2O emissions from the control plots, 30 t ha(-1) ofbiochar reduced the N2O emission factor from urine by 70%. The atom% 15N enrichment of the N2O emitted was lower in the 30 t ha(-1) biochar treatment, indicating less urine-N contributed to the N2O flux. Soil NO3- -N concentrations were lower with increasing biochar rate during the first 30 d following urine deposition. No differences occurred, due to biochar addition, with respect to dry matter yields, herbage N content, or recovery of 15N applied in herbage. Incorporating biochar into the soil can significantly diminish ruminant urine-derived N2O emissions. Further work is required to determine the persistence of the observed effect and to fully understand the mechanism(s) of the observed reduction in N2O fluxes.

Reductive dissolution of phosphorus associated with iron-oxides during saturation in agricultural soil profiles.

In soils with a fragipan or poor permeability, water may remain in a soil profile long enough to make it anoxic and reductive. The reductive dissolution of iron (Fe)- and manganese (Mn)-oxides can release associated phosphorus (P). Therefore, the dissolved P would be vulnerable to subsurface flow and could contaminate nearby streams. It was hypothesized that single rainfall events could cause subsurface P concentrations to increase via reductive dissolution in wet winter-spring conditions. Also, dissolution-being microbially mediated-would be buffered by the presence of nitrate (NO3 - ), which is preferred as an electron acceptor over Fe and Mn in microbial reactions. Unsaturated zone monitoring occurred from May to September in 2017 and 2019, using Teflon suction cups below the surface of a grassland soil in New Zealand. Events in July and August in 2017 and 2019 resulted in reducing conditions [Fe(III)/sulfate-reducing] and up to 77 and 96% greater P and Fe release, respectively. In an additional experiment in 2019, 100 mm of flood irrigation was applied, and 10 mg NO3 - -N + carbon was injected into half the cups at the site. The other cups received no N. Cups treated with N yielded up to 45% total dissolved P and 21% less Fe than the no-N cups. A laboratory incubation of soils from the site confirmed that NO3 - inhibited P release. This effect may act to decrease the amount of P lost in subsurface flow in systems regularly fertilized with N but should not be relied on as a method to mitigate P losses.

Sediment phosphorus buffering in streams at baseflow: A meta-analysis.

Phosphorus (P) pollution of surface waters remains a challenge for protecting and improving water quality. Central to the challenge is understanding what regulates P concentrations in streams. This quantitative review synthesizes the literature on a major control of P concentrations in streams at baseflow-the sediment P buffer-to better understand streamwater-sediment P interactions. We conducted a global meta-analysis of sediment equilibrium phosphate concentrations at net zero sorption (EPC0 ), which is the dissolved reactive P (DRP) concentration toward which sediments buffer solution DRP. Our analysis of 45 studies and >900 paired observations of DRP and EPC0 showed that sediments often have potential to remove or release P to the streamwater (83% of observations), meaning that "equilibrium" between sediment and streamwater is rare. This potential for P exchange is moderated by sediment and stream characteristics, including sorption affinity, stream pH, exchangeable P concentration, and particle sizes. The potential for sediments to modify streamwater DRP concentrations is often not realized owing to other factors (e.g., hydrologic interactions). Sediment surface chemistry, hyporheic exchange, and biota can also influence the potential exchange of P between sediments and the streamwater. Methodological choices significantly influenced EPC0 determination and thus the estimated potential for P exchange; we therefore discuss how to measure and report EPC0 to best suit research objectives and aid in interstudy comparison. Our results enhance understanding of the sediment P buffer and inform how EPC0 can be effectively applied to improve management of aquatic P pollution and eutrophication.

Soil microbial communities influencing organic phosphorus mineralization in a coastal dune chronosequence in New Zealand.

The Haast chronosequence in New Zealand is an ∼6500-year dune formation series, characterized by rapid podzol development, phosphorus (P) depletion and a decline in aboveground biomass. We examined bacterial and fungal community composition within mineral soil fractions using amplicon-based high-throughput sequencing (Illumina MiSeq). We targeted bacterial non-specific acid (class A, phoN/phoC) and alkaline (phoD) phosphomonoesterase genes and quantified specific genes and transcripts using real-time PCR. Soil bacterial diversity was greatest after 4000 years of ecosystem development and associated with an increased richness of phylotypes and a significant decline in previously dominant taxa (Firmicutes and Proteobacteria). Soil fungal communities transitioned from predominantly Basidiomycota to Ascomycota along the chronosequence and were most diverse in 290- to 392-year-old soils, coinciding with maximum tree basal area and organic P accumulation. The Bacteria:Fungi ratio decreased amid a competitive and interconnected soil community as determined by network analysis. Overall, soil microbial communities were associated with soil changes and declining P throughout pedogenesis and ecosystem succession. We identified an increased dependence on organic P mineralization, as found by the profiled acid phosphatase genes, soil acid phosphatase activity and function inference from predicted metagenomes (PICRUSt2).

Biochar and the nitrogen cycle: introduction.

Nitrogen (N) is an essential nutrient, and research to date shows that biochar potentially has the ability to manipulate the rates of N cycling in soil systems by influencing nitrification rates and adsorption of ammonia and increasing NH4+ storage by enhancing cation exchange capacity in soils. Its influence on these processes may have further implications in terms of reducing gaseous N losses such as N2O and nitrate leaching. However, further detailed research is required to fully understand the transformation mechanisms and fate of N when associated with biochar treated soils. The three research papers that comprise this special collection of papers on biochar and the nitrogen cycle focus on biochar's diverse ability to influence N cycling processes. These papers show for the first time (i) how microbial nitrification communities and function differ with exposure to biochar, (ii) how the length of time the soil has been in contact with biochar influences N transformation and how this can vary with soil type, and (iii) how composting of organic materials with biochar can reduce N losses and enhance the nutrient status of the composted product. Considerable knowledge gaps still exist in terms of understanding the precise mechanisms through which biochar influences soil N transformations, and how biochar affects both plant and microbial N supply. The general direction that research on biochar should focus on with respect to the N cycle is the effect(s) that biochar has on N transformation in soils, both chemical and biological mechanisms, and the fate of N applied to biochar treated soils. This research needs to be performed at both field plot and microbial scales.

Role of Organic Anions and Phosphatase Enzymes in Phosphorus Acquisition in the Rhizospheres of Legumes and Grasses Grown in a Low Phosphorus Pasture Soil.

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.

Impact of long-term phosphorus fertilizer inputs on bacterial phoD gene community in a maize field, Northeast China.

The bacterial phoD gene encodes alkaline phosphomonoesterase, an enzyme which plays an important role in the release of plant-available inorganic phosphorus (P) from organic P in soil. However, the relationships between phoD gene community, alkaline phosphomonoesterase activity, and P availability in soil are poorly understood. In this study, we investigated how alkaline phosphomonoesterase activity, phoD gene abundance, and community structure are influenced by plant-available P using soils (0-10, 10-20 and 20-40 cm) from a long-term field trial in which a continuous maize (Zea mays L.) crop had received different levels of P fertilizer inputs (30, 60 kg P ha-1 year-1) for 28 years. Quantitative PCR and high-throughput sequencing were used to analyze phoD gene abundance and community composition. Alkaline phosphomonoesterase enzyme activity was negatively correlated with soil available P, which was reflected in corresponding data for phoD gene abundance. On the other hand, positive correlations were determined between phoD gene α-diversity and available P, while shifts in phoD gene community structure were related to changes in soil pH and P availability. The relative abundance of Pseudomonas was negatively correlated with P availability and positively correlated with alkaline phosphomonoesterase activity, suggesting that Pseudomonas may play an important role in soil organic P mineralization. The findings of this study demonstrated that changes of soil P availability as a result of long-term P fertilizer inputs significantly affected alkaline phosphomonoesterase activity by regulating phoD gene abundance, diversity, as well as altering the phoD gene community composition.

Impact of nitrogen compounds on fungal and bacterial contributions to codenitrification in a pasture soil.

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.

Microbial and abiotic controls on mineral-associated organic matter in soil profiles along an ecosystem gradient.

Formation of mineral-organic associations is a key process in the global carbon cycle. Recent concepts propose litter quality-controlled microbial assimilation and direct sorption processes as main factors in transferring carbon from plant litter into mineral-organic associations. We explored the pathways of the formation of mineral-associated organic matter (MOM) in soil profiles along a 120-ky ecosystem gradient that developed under humid climate from the retreating Franz Josef Glacier in New Zealand. We determined the stocks of particulate and mineral-associated carbon, the isotope signature and microbial decomposability of organic matter, and plant and microbial biomarkers (lignin phenols, amino sugars and acids) in MOM. Results revealed that litter quality had little effect on the accumulation of mineral-associated carbon and that plant-derived carbon bypassed microbial assimilation at all soil depths. Seemingly, MOM forms by sorption of microbial as well as plant-derived compounds to minerals. The MOM in carbon-saturated topsoil was characterized by the steady exchange of older for recent carbon, while subsoil MOM arises from retention of organic matter transported with percolating water. Overall, MOM formation is not monocausal but involves various mechanisms and processes, with reactive minerals being effective filters capable of erasing chemical differences in organic matter inputs.

Effect of land use and soil organic matter quality on the structure and function of microbial communities in pastoral soils: Implications for disease suppression.

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.

Author Correction: Influence of soil moisture on codenitrification fluxes from a urea-affected pasture soil.

A correction has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.