Scattered native trees and soil patterns in grazing land on the Northern Tablelands of New South Wales, Australia
Over large areas of south-eastern Australia, the original cover of native woodland has been extensively cleared or modified, and what remains is often characterised by scattered trees beneath which the ground-storey vegetation is largely grazed or otherwise managed. This study investigated the influence of scattered Blakely's red gum ('Eucalyptus blakelyi') trees on both near-surface and deeper soil layers in temperate grazed pastures on the Northern Tablelands of New South Wales, Australia. A significant canopy effect was observed with elevated soil pH, carbon, and nutrient status inside the tree canopy indicating soil enrichment in a zone around the tree. This effect, however, was largely restricted to the surface (0-0.20 m) soil layers. Chloride concentrations were elevated near to trees but only in the deeper soil layers, suggesting that a modified water use and deep drainage mechanism occurred near the trees. Close to the tree, however, a significant acidification was observed between 0.40-0.60 m depth in the soil, without any obvious depletion in other soil element concentrations. It is concluded that this acidification provides strong evidence in support of a 'biological pumping' mechanism that has been proposed elsewhere. Key questions remain as to the management implications of these results, whether the subsurface acidification that was observed is common among native Australian trees, if it might be persistent through time, and if this might be a soil issue that requires management.
Physical soil architectural traits are functionally linked to carbon decomposition and bacterial diversity
Aggregates play a key role in protecting soil organic carbon (SOC) from microbial decomposition. The objectives of this study were to investigate the influence of pore geometry on the organic carbon decomposition rate and bacterial diversity in both macro- (250-2000 μm) and micro-aggregates (53-250 μm) using field samples. Four sites of contrasting land use on Alfisols (i.e. native pasture, crop/pasture rotation, woodland) were investigated. 3D Pore geometry of the micro-aggregates and macro-aggregates were examined by X-ray computed tomography (μCT). The occluded particulate organic carbon (oPOC) of aggregates was measured by size and density fractionation methods. Microaggregates had 54% less μCT observed porosity but 64% more oPOC compared with macro-aggregates. In addition, the pore connectivity in micro-aggregates was lower than macro-aggregates. Despite both lower μCT observed porosity and pore connectivity in micro-aggregates, the organic carbon decomposition rate constant (Ksoc) was similar in both aggregate size ranges. Structural equation modelling showed a strong positive relationship of the concentration of oPOC with bacterial diversity in aggregates. We use these findings to propose a conceptual model that illustrates the dynamic links between substrate, bacterial diversity, and pore geometry that suggests a structural explanation for differences in bacterial diversity across aggregate sizes.
Drivers of soil organic carbon storage and vertical distribution in Eastern Australia
Aims: Drivers of soil organic carbon (SOC) storage are likely to vary in importance in different regions and at different depths due to local factors influencing SOC dynamics. This paper explores the factors influencing SOC to a depth of 30 cm in eastern Australia. Methods: We used a machine learning approach to identify the key drivers of SOC storage and vertical distribution at 1401 sites from New South Wales, Australia. We then assessed the influence of the identified factors using traditional statistical approaches. Results: Precipitation was important to and positively associated with SOC content, whereas temperature was important to and negatively associated with SOC vertical distribution. The importance of geology to SOC content increased with increasing soil depth. Land-use was important to both SOC content and its vertical distribution. Conclusion: We attribute these results to the influence of precipitation on primary production controlling SOC content, and the stronger influence of temperature on microbial activity affecting SOC degradation and vertical distribution. Geology affects SOC retention below the surface. Land-use controls SOC via production, removal and vertical mixing. The factors driving SOC storage are not identical to those driving SOC vertical distribution. Changes to these drivers will have differential effects on SOC storage and depth distribution.
Organic amendments influence soil quality and carbon sequestration in the Indo-Gangetic plains of India
Soil organic carbon is considered to be of central importance in maintaining soil quality. We assessed the effects of a range of commonly applied organic and inorganic amendments on soil quality in a rice-wheat cropping system in the Indo-Gangetic plains of eastern India and evaluated the carbon sequestration potential of such management approaches using a 25 year old long-term fertility experiment. Results showed that there were significant increases in soil nutrient availability with the application of farm yard manure (FYM @ 7.5 t ha⁻¹), paddy straw (PS @ 10 t ha⁻¹) and green manure (GM @ 8 t ha⁻¹) along with inorganic fertilizer. Both microbial biomass C and mineralizable C increased following the addition of the organic inputs. Continuous cultivation, without application of organic inputs, significantly depleted total C content (by 39-43%) compared with treatments involving the addition of organic amendments. A significant increase in the non-labile C fraction resulted from both organic and inorganic amendments, but only 26, 18 and 6% of the C applied through FYM, PS and GM, respectively was sequestered in soils. A significant increase in yield of kharif rice was observed as a result of the addition of these organic amendments.
Previous land use and climate influence differences in soil organic carbon following reforestation of agricultural land with mixed-species plantings
Reforestation of agricultural land with mixed-species environmental plantings (native trees and shrubs) can contribute to mitigation of climate change through sequestration of carbon. Although soil carbon sequestration following reforestation has been investigated at site- and regional-scales, there are few studies across regions where the impact of a broad range of site conditions and management practices can be assessed. We collated new and existing data on soil organic carbon (SOC, 0-30 cm depth, N = 117 sites) and litter (N = 106 sites) under mixed-species plantings and an agricultural pair or baseline across southern and eastern Australia. Sites covered a range of previous land uses, initial SOC stocks, climatic conditions and management types. Differences in total SOC stocks following reforestation were significant at 52% of sites, with a mean rate of increase of 0.57 ± 0.06 Mg C ha⁻¹y⁻¹. Increases were largely in the particulate fraction, which increased significantly at 46% of sites compared with increases at 27% of sites for the humus fraction. Although relative increase was highest in the particulate fraction, the humus fraction was the largest proportion of total SOC and so absolute differences in both fractions were similar. Accumulation rates of carbon in litter were 0.39 ± 0.02 Mg C ha⁻¹y⁻¹, increasing the total (soil + litter) annual rate of carbon sequestration by 68%. Previously-cropped sites accumulated more SOC than previously-grazed sites. The explained variance differed widely among empirical models of differences in SOC stocks following reforestation according to SOC fraction and depth for previously-grazed (R² = 0.18- 0.51) and previously-cropped (R² = 0.14-0.60) sites. For previously-grazed sites, differences in SOC following reforestation were negatively related to total SOC in the pasture. By comparison, for previously- cropped sites, differences in SOC were positively related to mean annual rainfall. This improved broadscale understanding of the magnitude and predictors of changes in stocks of soil and litter C following reforestation is valuable for the development of policy on carbon markets and the establishment of future mixed-species environmental plantings.
Pasture Species as an Option for Soil Organic Carbon Sequestration to Restore Abandoned Croplands in Nepal
Cropland abandonment is a human-induced land use change that is emerging globally in the last century. The rate of cropland abandonment is increasing in Nepal in the last decade, impacting soil functions and productivity. Restoration options have been actively sought by land managers and policymakers, and the potential of pasture establishment has been seen as a potential option to maintain soil quality while also providing a productive base for landholders. Following abandonment, changes in geomorphology and secondary vegetation succession can have a significant impact on soil organic carbon (SOC). Few studies have been conducted in Nepal to understand the impact and restoration of post-abandonment land. In this work, abandoned croplands, forest, pasture, and existing cropped land were examined in Gorkha and Kavre districts of Nepal with the aim of understanding the impacts of cropland abandonment by measuring the change in SOC across soil depths. A field experiment was undertaken in Gorkha district of Nepal where four pasture species were grown in a prior cropland abandoned for 2 years, in order to understand the potential of pasture species for carbon (C) sequestration. To explore the mechanism of SOC formation by pasture roots, a greenhouse experiment at University of New England, Australia was also conducted in which four pasture species were grown in two contrasting soil types (Ferrosol and Chromosol). Abandoned cropland in Nepal subject to secondary vegetation succession had accumulated significant amounts of SOC, particularly the labile fraction (particulate organic carbon), compared with existing cropland, especially after 10 years of abandonment and that SOC values were on the trajectory towards those of forest and pasture. Around 23% of SOC has been recovered with secondary succession in more than 10 years abandoned cropland compared to the currently cropped land. Pasture establishment on abandoned cropland increased the SOC in which the labile carbon increased in the top-soil (0-20 cm) and stable carbon in sub-soil (20-40 cm). Pasture roots contribute to form new carbon through dissolved organic carbon formation and root exudation that is mostly fixed on the stable SOC fractions (mineral-associated organic carbon) in both Chromosol and Ferrosol. It is concluded that vegetation cover in abandoned cropland increased labile carbon in the surface soil which had been preferentially lost in the early years of abandonment. With the pasture establishment on abandoned cropland, SOC can be stabilised in sub-soil with the higher fixation of new carbon in the mineral soil matrix. So, we recommend that land managers and policymakers integrate a pasture component in abandoned cropland restoration program that has ability to restore abandoned cropland through increased SOC.
Soil organic matter in a stressed world
Soil organic matter (SOM) is quantitatively a minor component (by mass and volume) of most soils, yet it is responsible for facilitating or moderating many key soil ecosystem services that affect agricultural food security, biodiversity and environmental quality. Its constituents range in mean residence times from <1 min for some low molecular weight (LMW) compounds, through to >10000 years for charcoal-like materials. It is these somewhat enigmatic aspects of SOM that not only provoke such interest from the research community, but also drive the needfor applied research that enables custodians of the landscape to effectively manage SOM in a way that limits its loss and maintains or enhances its stock and function.
Forest burning affects quality and quantity of soil organic matter
Fire alters ecosystem carbon cycling and generates pyrogenic matter such as charcoal,which can be incorporated into soils. The incorporation and cycling of charcoal in soils is a potential carbon sink, but studies investigating charcoal and carbon dynamics in soils are still lacking. We investigated soil carbon, charcoal and nitrogen dynamics in the top 20 cm of a sandy soil within a eucalypt forest in eastern Australia at three sites representing a chronosequence from 3 months to 14 years post-fire. In the short-term, fire removed litter, but resulted in an increase in both the charcoal and non-charcoal SOC content of the soils, which we attribute to above-ground charcoal generation and its incorporation into the soil profile, as well as below-ground root mortality. On a decadal timeframe, charcoal was preferentially retained in the sandy soil, in which other stabilisation mechanisms are limited, so that the influx of dead root carbon had no remnant effects. The incorporation and retention of charcoal in the soil profile is highly important to carbon cycling in such sandy soils with high fire frequency. It is highly likely that these effects are not limited to the upper 20 cm of soil and future studies should investigate deep soil charcoal cycling.
A systematic review of soil carbon management in Australia and the need for a social-ecological systems framework
Research efforts, on soil carbon management in agricultural lands, over the last two decades have sought to improve our understanding in order to increase soil productivity, soil carbon sequestration and to offset greenhouse gas emissions. This systematic review aims to identify the research gaps and future direction of soil carbon management in Australia. We explored and synthesized the use of social-ecological systems (SES) both in the global and Australian context, before making the first attempt to develop a conceptual SES framework for soil carbon management. Both quantitative and qualitative assessment of articles were used to identify and synthesise research trends, challenges and opportunities for improved soil carbon management. The results provide valuable insight into the SES components examined, the research gaps and the methodological challenges for research into soil carbon management conducted over the last two decades. The review revealed that research has predominately focused on the ecological component of soil carbon management in agricultural practices and has been conducted from a scientist's perspective. The sustainability of carbon-building soil management practices will require integration of social components into future research, particularly from a farmer perspective. The proposed conceptual SES framework is designed to identify and investigate SES components in soil carbon management in order to increase the process of offsetting greenhouse gas emissions as required by Sustainable Development Goals 2, 13 and 15.
Runoff and erosion in woody encroachment, pasture and woodland vegetation in semi-arid New South Wales, Australia
Woody encroachment is the increase in density, cover, extent or biomass of woody plants. It has been reported from arid and semi-arid areas around the world. Woody encroachment is the result of changes in factors such as weather and climate, grazing pressure, fire regimes, and combinations of these. The most widely recognised effects of woody encroachment are the decline in herbaceous forage production and carrying capacity for livestock, decreased biodiversity and socioeconomic value of affected areas, and, in some cases, increased runoff and soil erosion. The eco-hydrological responses of areas of woody encroachment and of pastures established after removing the woody vegetation are not well understood in semi-arid Australia. The aim of this thesis was to investigate runoff, erosion and associated factors in woody encroachment, and how these compare with other vegetation states in semi-arid Australia from an eco-hydrological perspective. ... Overall, the study showed that runoff and erosion in the four vegetation states are mostly site-dependent. Hydrological and erosional responses in woody encroachment were generally not significantly different from those in recently and long-established pastures and woodland. The study showed that ground cover type and amount and its spatial distribution largely determine runoff and erosion responses in the studied systems. There is potential in using high resolution satellite imagery and spatial modelling to apply accepted concepts of semi-arid patchiness to link small-scale eco-hydrological and erosional responses to larger scales appropriate for management. The study showed that factors related to gully erosion in woody encroachment areas are related to topographic thresholds that can be used for the identification of areas susceptible to gullying.