Nitrogen-rich microbial products provide new organo-mineral associations for the stabilization of soil organic matter.

Understanding the cycling of C and N in soils is important for maintaining soil fertility while also decreasing greenhouse gas emissions, but much remains unknown about how organic matter (OM) is stabilized in soils. We used nano-scale secondary ion mass spectrometry (NanoSIMS) to investigate the changes in C and N in a Vertisol and an Alfisol incubated for 365 days with 13 C and 15 N pulse labeled lucerne (Medicago sativa L.) to discriminate new inputs of OM from the existing soil OM. We found that almost all OM within the free stable microaggregates of the soil was associated with mineral particles, emphasizing the importance of organo-mineral interactions for the stabilization of C. Of particular importance, it was also found that 15 N-rich microbial products originating from decomposition often sorbed directly to mineral surfaces not previously associated with OM. Thus, we have shown that N-rich microbial products preferentially attach to distinct areas of mineral surfaces compared to C-dominated moieties, demonstrating the ability of soils to store additional OM in newly formed organo-mineral associations on previously OM-free mineral surfaces. Furthermore, differences in 15 N enrichment were observed between the Vertisol and Alfisol presumably due to differences in mineralogy (smectite-dominated compared to kaolinite-dominated), demonstrating the importance of mineralogy in regulating the sorption of microbial products. Overall, our findings have important implications for the fundamental understanding of OM cycling in soils, including the immobilization and storage of N-rich compounds derived from microbial decomposition and subsequent N mineralization to sustain plant growth.

Soil Organic Carbon Stabilization: Mapping Carbon Speciation from Intact Microaggregates.

The clearing of land for agricultural production depletes soil organic carbon (OC) reservoirs, yet despite their importance, the mechanisms by which C is stabilized in soils remain unclear. Using synchrotron-based infrared microspectroscopy, we have for the first time obtained in situ, laterally resolved data regarding the speciation of C within sections taken from intact free microaggregates from two contrasting soils (Vertisol and Oxisol, 0-20 cm depth) impacted upon by long-term (up to 79 y) agricultural production. There was no apparent gradient in the C concentration from the aggregate surface to the interior for any of the three forms of C examined (aliphatic C, aromatic C, and polysaccharide C). Rather, organo-mineral interactions were of critical importance in influencing overall C stability, particularly for aliphatic C, supporting the hypothesis that microaggregates form through organo-mineral interactions. However, long-term cropping substantially decreased the magnitude of the organo-mineral interactions for all three forms of C. Thus, although organo-mineral interactions are important for OC stability, C forms associated with the mineral phases are not entirely resistant to degradation. These results provide important insights into the underlying mechanisms by which microaggregates form and the factors influencing the persistence of OC in soils.

Global changes in soil stocks of carbon, nitrogen, phosphorus, and sulphur as influenced by long-term agricultural production.

Quantifying changes in stocks of C, N, P, and S in agricultural soils is important not only for managing these soils sustainably as required to feed a growing human population, but for C and N, they are also important for understanding fluxes of greenhouse gases from the soil environment. In a global meta-analysis, 102 studies were examined to investigate changes in soil stocks of organic C, total N, total P, and total S associated with long-term land-use changes. Conversion of native vegetation to cropping resulted in substantial losses of C (-1.6 kg m-2 , -43%), N (-0.15 kg m-2 , -42%), P (-0.029 kg m-2 , -27%), and S (-0.015 kg m-2 , -33%). The subsequent conversion of conventional cropping systems to no-till, organic agriculture, or organic amendment systems subsequently increased stocks, but the magnitude of this increase (average of +0.47 kg m-2 for C and +0.051 kg m-2 for N) was small relative to the initial decrease. We also examined the conversion of native vegetation to pasture, with changes in C (-11%), N (+4.1%), and P (+25%) generally being modest relative to changes caused by conversion to cropping. The C:N ratio remained relatively constant irrespective of changes in land use, whilst in contrast, the C:S ratio decreased by 21% in soils converted to cropping - this suggesting that biochemical mineralization is of importance for S. The data presented here will assist in the assessment of different agricultural production systems on soil stocks of C, N, P, and S - this information assisting not only in quantifying the effects of existing agricultural production on these stocks, but also allowing for informed decision-making regarding the potential effects of future land-use changes.

Non-target impacts of pesticides on soil N transformations, abundances of nitrifying and denitrifying genes, and nitrous oxide emissions.

Agriculture is the leading contributor to global nitrous oxide (N2O) emissions, mostly from soils. We examined the non-target impacts of four pesticides on N transformations, N cycling genes and N2O emissions from sugarcane-cropped soil. The pesticides, including a herbicide glyphosate (GLY), an insecticide imidacloprid (IMI), a fungicide methoxy ethyl mercuric chloride (MEMC) and a fumigant methyl isothiocyanate (MITC), were added to the soil and incubated in laboratory at 25 °C. The soil microcosms were maintained at two water contents, 55 % and 90 % water holding capacity (WHC), to simulate aerobic and partly anaerobic conditions, respectively. Half of the soil samples received an initial application of KNO3 and were then maintained at 90 % WHC for 38 d, whilst the other half received (NH4)2SO4 and were maintained at 55 % WHC for 28 d followed by 10 d at 90 % WHC to favour denitrification. Responses of individual functional genes involved in nitrification and denitrification to the pesticides and their relationships to N2O emissions varied with time and soil water. Overall, MITC had pronounced repressive effects on AOA and AOB amoA gene abundances and gross nitrification. Under 55 % WHC during the initial 28 d, N2O emissions were low for all treatments (≤62 µg N kg-1 soil). However, under 90 % WHC (either during the first 28 d or the increase in water content from 55 to 90 % WHC after 28 d) the cumulative N2O emissions increased markedly. Overall, under 90 % WHC the cumulative N2O emissions were 19 (control) to 79-fold (MITC) higher than under 55% WHC; with the highest emissions observed in the MITC treatment (3140 µg N kg-1 soil). This was associated with increases in gross nitrate consumption rates and abundances of denitrifying genes (nirK, nirS and qnorB). Therefore, to minimise N2O emissions, MITC should not be applied to field under wet conditions favouring denitrification.

A study over 33 years shows that carbon and nitrogen stocks in a subtropical soil are increasing under native vegetation in a changing climate.

Soil plays a critical role in the global carbon (C) cycle. However, climate change and associated factors, such as warming, precipitation change, elevated carbon dioxide (CO2), and atmospheric nitrogen (N) deposition, will affect soil organic carbon (SOC) stocks markedly - a decrease in SOC stocks is predicted to drive further planetary warming, although whether changes in climate and associated factors (including atmospheric N deposition) will cause a net increase in SOC or a net decrease is less certain. Using a subtropical soil, we have directly examined how changes over the last three decades are already impacting upon SOC stocks and soil total nitrogen (STN) in a Vertisol supporting native brigalow (Acacia harpophylla L.) vegetation. It was observed that SOC stocks increased under native vegetation by 5.85 Mg C ha-1 (0.177 ± 0.059 Mg C ha-1 y-1) at a depth of 0-0.3 m over 33 years. This net increase in SOC stocks was not correlated with change in precipitation, which did not change during the study period. Net SOC stocks, however, were correlated with an increasing trend in mean annual temperatures, with an average increase of 0.89 °C. This occurred despite a likely co-occurrence of increased decomposition due to higher temperatures, presumably because the increase in the SOC was largely in the stable, mineral-associated fraction. The increases in CO2 from 338 ppmv to 395 ppmv likely contributed to an increase in biomass, especially root biomass, resulting in the net increase in SOC stocks. Furthermore, STN stocks increased by 0.57 Mg N ha-1 (0.0174 ± 0.0041 Mg N ha-1 y-1) at 0-0.3 m depth, due to increased atmospheric N deposition and potential N2 fixation. Since SOC losses are often predicted in many regions due to global warming, these observations are relevant for sustainability of SOC stocks for productivity and climate models in semi-arid subtropical regions.

Disease-Suppressive Soils-Beyond Food Production: a Critical Review.

In the pursuit of higher food production and economic growth and increasing population, we have often jeopardized natural resources such as soil, water, vegetation, and biodiversity at an alarming rate. In this process, wider adoption of intensive farming practices, namely changes in land use, imbalanced fertilizer application, minimum addition of organic residue/manure, and non-adoption of site-specific conservation measures, has led to declining in soil health and land degradation in an irreversible manner. In addition, increasing use of pesticides, coupled with soil and water pollution, has led the researchers to search for an environmental-friendly and cost-effective alternatives to controlling soil-borne diseases that are difficult to control, and which significantly limit agricultural productivity. Since the 1960s, disease-suppressive soils (DSS) have been identified and studied around the world. Soil disease suppression is the reduction in the incidence of soil-borne diseases even in the presence of a host plant and inoculum in the soil. The disease-suppressive capacity is mainly attributed to diverse microbial communities present in the soil that could act against soil-borne pathogens in multifaceted ways. The beneficial microorganisms employ some specific functions such as antibiosis, parasitism, competition for resources, and predation. However, there has been increasing evidence on the role of soil abiotic factors that largely influence the disease suppression. The intricate interactions of the soil, plant, and environmental components in a disease triangle make this process complex yet crucial to study to reduce disease incidence. Increasing resistance of the pathogen to presently available chemicals has led to the shift from culturable microbes to unexplored and unculturable microbes. Agricultural management practices such as tillage, fertilization, manures, irrigation, and amendment applications significantly alter the soil physicochemical environment and influence the growth and behaviour of antagonistic microbes. Plant factors such as age, type of crop, and root behaviour of the plant could stimulate or limit the diversity and structure of soil microorganisms in the rhizosphere. Further, identification and in-depth of disease-suppressive soils could lead to the discovery of more beneficial microorganisms with novel anti-microbial and plant promoting traits. To date, several microbial species have been isolated and proposed as key contributors in disease suppression, but the complexities as well as the mechanisms of the microbial and abiotic interactions remain elusive for most of the disease-suppressive soils. Thus, this review critically explores disease-suppressive attributes in soils, mechanisms involved, and biotic and abiotic factors affecting DSS and also briefly reviewing soil microbiome for anti-microbial drugs, in fact, a consequence of DSS phenomenon.

The role of soil in defining planetary boundaries and the safe operating space for humanity.

We use soils to provide 98.8% of our food, but we must ensure that the pressure we place on soils to provide this food in the short-term does not inadvertently push the Earth into a less hospitable state in the long-term. Using the planetary boundaries framework, we show that soils are a master variable for regulating critical Earth-system processes. Indeed, of the seven Earth-systems that have been quantified, soils play a critical and substantial role in changing the Earth-systems in at least two, either directly or indirectly, as well as smaller contributions for a further three. For the biogeochemical flows Earth-system process, soils contribute 66% of the total anthropogenic change for nitrogen and 38% for phosphorus, whilst for the land-system change Earth-system process, soils indirectly contribute 80% of global anthropogenic change. Furthermore, perturbations of soils contribute directly to 21% of climate change, 25% to ocean acidification, and 25% to stratospheric ozone depletion. We argue that urgent interventions are required to greatly improve soil management, especially for those Earth-system processes where the planetary boundary has already been exceeded and where soils make an important contribution, with this being for biogeochemical flows (both nitrogen and phosphorus), for climate change, and for land-system change. Of particular importance, it is noted that the highly inefficient use of N fertilizers results in release of excess N into the broader environment, contributes to climate change, and results in release of ozone-depleting substances. Furthermore, the use of soils for agricultural production results not only in land-system change, but also in the loss (mineralization) of organic matter with a concomitant release of CO2 contributing to both climate change and ocean acidification. Thus, there is a need to markedly improve the efficiency of fertilizer applications and to intensify usage of our most fertile soils in order to allow the restoration of degraded soils and limit further areal expansion of agriculture. Understanding, and acting upon, the role of soils is critical in ensuring that planetary boundaries are not transgressed, with no other single variable playing such a strategic role across all of the planetary boundaries.

Soil organic carbon is significantly associated with the pore geometry, microbial diversity and enzyme activity of the macro-aggregates under different land uses.

Microbial activity strongly influences the stabilization of soil organic matter (SOM), and is affected by the abiotic properties within soil aggregates, which tend to differ between land uses. Here, we assessed the effects of SOM and pore geometry on the diversity and activity of microbial communities within aggregates formed under different land uses (undisturbed, plantation, pasture, and cropping). X-ray micro-computed tomography (µCT) revealed that macro-aggregates (2-8 mm) of undisturbed soils were porous, highly-connected, and had 200% more macro-pores compared with those from pasture and cropping soils. While the macro-aggregates of undisturbed soils had greater soil organic carbon (SOC) contents and N-acetyl ß-glucosaminidase, ß-glucosidase, and phosphatase activities, those of cropped soils harboured more diverse bacterial communities. Organic carbon was positively associated with the porosity of the macro-aggregates, which was negatively associated with microbial diversity and positively associated with enzyme activity. Thus, the biophysical processes in macro-aggregates may be important for SOC stabilization within the macro-aggregates.

Reforestation of agricultural land in the tropics: The relative contribution of soil, living biomass and debris pools to carbon sequestration.

Tropical regions of the world experience high rates of land-use change and this has a major influence on terrestrial carbon (C) pools and the global C cycle. We assessed land-use change from agriculture to reforested plantings (with endemic species), up to 33 years of age, using 10 paired sites in the wet tropics, Australia. We determined the impacts on 0-50 cm below-ground C (soil organic C (SOC), charcoal C, humic organic C, particulate organic C, resistant organic C), C stored in roots (fine and coarse), C stored in living above-ground biomass and debris C pools. Reforested areas accumulated ecosystem C at a rate of 7.4 Mg ha-1 yr-1. Reforestation plantings contained, on average, 2.3 times more ecosystem C than agricultural areas (102 Mg ha-1 and 233 Mg ha-1, respectively). Most of the C accumulation was in living above-ground and below-ground biomass (60 and 30%, respectively) with a smaller amount in debris pools (16%). Apart from C in roots, soil C accumulation was not obvious across sites ranging from 8 to 33 years since reforestation, relative to the agricultural baseline. Differences in SOC (and associated SOC pools) to a depth of 50 cm, did exist between reforested areas and adjacent agriculture at some sites, however there was not a consistent trend in SOC associated with reforestation. Local site-based factors (e.g. soil texture and mineralogy, land-use history and microbial activity) appear to have a strong influence on the direction of the change in SOC. While reforestation in the tropics has great potential to accumulate C in biomass in living vegetation, and debris pools, it is likely to take approximately 50 years before C stocks of reforested areas resemble natural ecosystems. Accumulation of SOC through reforestation is difficult to achieve, highlighting the need to conserve carbon pools in remnant forests in the tropics.

Enrichment of natural 15N abundance during soil N losses under 20years of continuous cereal cropping.

It is generally accepted that the enrichment of natural 15N abundance in soil over time is reflective of historic N cycling and loss, but this process in cropping soils is not yet clear. In this study, we identified an enrichment gradient of natural 15N abundance during 20-year chronosequence of cereal cropping on Alfisols in southwest Queensland, Australia, that have no history of fertilisation. We demonstrate that the increase in soil 15N abundance is explained by isotopic fractionation of 15N during organic N mineralisation and nitrification, which lead to isotopically heavier ammonium retained in the soil and isotopically lighter soil nitrate taken up and removed by seasonal crops during harvest. Here we present a framework for natural 15N isotopic fractionation co-occurring with N losses during long-term cultivation.

Turnover of organic carbon and nitrogen in soil assessed from δ13C and δ15N changes under pasture and cropping practices and estimates of greenhouse gas emissions.

The continuing clearance of native vegetation for pasture, and especially cropping, is a concern due to declines in soil organic C (SOC) and N, deteriorating soil health, and adverse environment impact such as increased emissions of major greenhouse gases (CO2, N2O and CH4). There is a need to quantify the rates of SOC and N budget changes, and the impact on greenhouse gas emissions from land use change in semi-arid subtropical regions where such data are scarce, so as to assist in developing appropriate management practices. We quantified the turnover rate of SOC from changes in δ(13)C following the conversion of C3 native vegetation to C4 perennial pasture and mixed C3/C4 cereal cropping (wheat/sorghum), as well as δ(15)N changes following the conversion of legume native vegetation to non-legume systems over 23 years. Perennial pasture (Cenchrus ciliaris cv. Biloela) maintained SOC but lost total N by more than 20% in the top 0-0.3m depth of soil, resulting in reduced animal productivity from the grazed pasture. Annual cropping depleted both SOC and total soil N by 34% and 38%, respectively, and resulted in decreasing cereal crop yields. Most of these losses of SOC and total N occurred from the >250 µm fraction of soil. Moreover, this fraction had almost a magnitude higher turnover rates than the 250-53 µm and <53 µm fractions. Loss of SOC during the cropping period contributed two-orders of magnitude more CO2-e to the atmosphere than the pasture system. Even then, the pasture system is not considered as a benchmark of agricultural sustainability because of its decreasing productivity in this semi-arid subtropical environment. Introduction of legumes (for N2 fixation) into perennial pastures may arrest the productivity decline of this system. Restoration of SOC in the cropped system will require land use change to perennial ecosystems such as legume-grass pastures or native vegetation.

Effect of fresh green waste and green waste compost on mineral nitrogen, nitrous oxide and carbon dioxide from a Vertisol.

Incorporation of organic waste amendments to a horticultural soil, prior to expected risk periods, could immobilise mineral N, ultimately reducing nitrogen (N) losses as nitrous oxide (N(2)O) and leaching. Two organic waste amendments were selected, a fresh green waste (FGW) and green waste compost (GWC) as they had suitable biochemical attributes to initiate N immobilisation into the microbial biomass and organic N forms. These characteristics include a high C:N ratio (FGW 44:1, GWC 35:1), low total N (<1%), and high lignin content (>14%). Both products were applied at 3t C/ha to a high N (plus N fertiliser) or low N (no fertiliser addition) Vertisol soil in PVC columns. Cumulative N(2)O production over the 28 day incubation from the control soil was 1.5mg/N(2)O/m(2), and 11mg/N(2)O/m(2) from the control+N. The N(2)O emission decreased with GWC addition (P<0.05) for the high N soil, reducing cumulative N(2)O emissions by 38% by the conclusion of the incubation. Analysis of mineral N concentrations at 7, 14 and 28 days identified that both FGW and GWC induced microbial immobilisation of N in the first 7 days of incubation regardless of whether the soil environment was initially high or low in N; with the FGW immobilising up to 30% of available N. It is likely that the reduced mineral N due to N immobilisation led to a reduced substrate for N(2)O production during the first week of the trial, when soil N(2)O emissions peaked. An additional finding was that FGW+N did not decrease cumulative N(2)O emissions compared to the control+N, potentially due to the fact that it stimulated microbial respiration resulting in anaerobic micro sites in the soil and ultimately N(2)O production via denitrification. Therefore, both materials could be used as post harvest amendments in horticulture to minimise N loss through nitrate-N leaching in the risk periods between crop rotations. The mature GWC has potential to reduce N(2)O, an important greenhouse gas.