Two case studies exploring opportunities and constraints for soil organic carbon sequestration following land use change

Abstract

The flux of carbon (C) between the terrestrial biosphere and the atmosphere occurs naturally as part of the carbon cycle (CC). However, anthropogenic activity can disrupt the natural equilibrium of the CC. Although fossil fuel burning is the largest cause of human induced C emissions to the atmosphere, land use and land use change (LULUC) also results in emissions. Of particular concern is soil disturbance and erosion caused by LULUC activities including deforestation, tillage and overgrazing. These activities can lead to a net loss of C from the terrestrial biosphere leading to increased levels of greenhouse gas in the atmosphere. This outcome is a major factor widely understood to adversely affect the rate of global warming, climate change and extreme weather events. There has been a great deal of research undertaken to show that LULUC activities can also lead to the sequestration and long term storage of C in soil. LULUC practices that result in increases in soil organic matter (SOM), humification and aggregation improve soil quality, water quality, enhance food security and increase biodiversity. The activities also result in the sequestration of atmospheric CO2 as soil organic carbon (SOC) and therefore may contribute to the reduction or stabilisation of atmospheric CO2 levels and mitigate the adverse effects of climate change. This thesis describes the effects of two different LULUC activities on the C sequestration potential of soil. In this research two case studies have been used to explore three principal aims. Firstly to identify the C sequestration potential of agricultural soil following LUC; then to determine whether there is a steady rate of SOC sequestration over time following LUC, and finally to identify biogeochemical factors that contribute to the C sequestration potential of soil after LUC. The first case study was undertaken on the Southern Tablelands of New South Wales (NSW), a temperate region with approximately 670 mm mean average rainfall (MAR). The study involved comparing the soil C stock (SCS) to 30 cm depth, in agricultural land (AL) principally used for grazing, with the SCS from 20 biodiverse environmental plantings (BEP) established between one and 19 years prior to sampling. BEPs, otherwise referred to as mixed species environmental plantings (MSEP), windbreaks or shelterbelts, can be established using either a direct seeding method or from tube-stock. The sites included in this study were all directly seeded in either linear or block configurations using a mix of native eucalypt and acacia trees and understory shrubs. At each site three paired 10 m transects were established on a tree line (TL), the inter-row (IR) and in the adjacent AL. The AGB was determined for each TL transect. Three soil cores to 30 cm depth were taken from each transect and separated into 0-5, 5-10, 10-20 and 20-30 cm increments. The soil samples were analysed for SOC, total nitrogen (TN), total phosphorus (TP), and by using mid infrared spectroscopy (MIRS) analysis soil fraction changes were also determined. The measured AGB and SCS results were compared with FullCAM modelled data. The overall aim of this case study was to determine whether BEPs sown into AL led to SOC sequestration, and whether the rate of increase could be predicted based on tree age and biomass. The results of the BEP case study show that the average soil CS in the AL based on an equivalent soil mass (ESM) was 41.1 Mg C ha-1 (41.1 tonnes C ha-1). BEPs aged 11-15 years had the highest SCS of 49.3 Mg C ha-1. The oldest BEP sites aged between 16 and 19 years had a lower SCS of 45.7 Mg C ha-1. Overall, the average SCS rate of increase was 0.5 Mg C ha-1 yr-1. However, the results show significant variation between sites and no trend suggesting there is a temporal increase in SCS. The average AGB for all sites was found to be 31.4 Mg C ha-1 with the range between 0.2 Mg C ha-1 in a two year old site, and 97.0 Mg C ha-1 in a site aged 16 years. The average annual rate of AGB change was found to be 2.4 Mg C ha-1 yr-1. The results show there is no relationship between AGB and SCS. Therefore the prediction of changes to SCS based on AGB increase alone is not possible. The results also show that site factors and nutrient availability have an influence on the SOC sequestration potential of the BEPs and the sustainability of the SCS. The second case study was undertaken in the semi-arid rangelands of the Central West of NSW. The MAR of the region is approximately 440 mm. The study examined whether waterponding, an effective environmental restoration activity used to rehabilitate scalded soil, would result in increases in SCS. The research involved comparing the SCS of three scalded sites with 12 waterpond sites established between 1 and 27 years prior to sampling. Nine soil cores to 30 cm depth were taken at each scald site. Three waterponds were sampled at each of the 12 waterpond sites. Nine soil cores to 30 cm depth were taken from three positions (wall, mid and top) within in each waterpond. Each soil core was separated into 0-5, 5-10, 10-20 and 20-30 cm increments. Every sample was analysed for pH, electrical conductivity (EC), SOC, TN, and total C. The results show that waterponds have a significantly lower EC than scalded soil. This change occurs because soluble salts are leached from the saline scald soil profile after waterponding. The results also show that the scalded soil has an average SCS to 30cm depth of 18.7 Mg C ha-1. Waterponds aged five years had a SCS of 26.1 Mg C ha-1. This represents a rate of increase of 1.5 Mg C ha-1yr-1. The 10 year old and 25-27 year old waterponds have a slightly lower SCS of 25.3 and 24.9 Mg C ha-1. The probable reason for the apparent decline in SCS in the older water ponds is associated with the lower bulk density found in these older waterponds and the increased presence of SOM. A number of edaphic factors were found to influence the potential of the scalds to sequester SOC. To make such assessments within research resource constraints three different subsets of the soil samples were compiled. Initially, a sub-set of 144 samples including one scald, and waterponds aged 1 year, 5 years and 25 years since establishment were analysed for aggregate stability, TP, total sulfur (TS), and available P (AP). The results from this subset were used to identify temporal changes to soil stability and to ascertain whether these soil nutrient concentrations were affected by waterponding or influenced the SOC sequestration potential of the soils. The aggregate stability results show there is a temporal change in the dispersability of the soils following waterponding. Scalded soils are stable because of the high concentration of soluble salts. After the salts are leached from the profile the soils become more susceptible to dispersion. The results also indicate that total N, P and S is not limiting, but AP is limiting ongoing sequestration of C in the waterponds. A smaller subset of samples made up of one scald core and a paired 27 year old waterpond core were analysed for cation exchange capacity, x-ray diffraction and x-ray fluorescence to identify any mineralogical differences that may be attributable to waterponding. Calcite was found to be leached from the scald profile, hematite increased due to iron oxide formation, gypsum, kaolinite, illite and vermiculite was higher in the waterponded soil than the scalded soil whereas smectite was lower in the waterponds. These differences are most likely due to weathering processes and aeolian deposition. Across the scalded soil surface there are occasional hummocks of vegetation. Cesium-137 analysis was undertaken to determine whether the hummocks were recent deposits or relics of the original soil profile. For this analysis composite samples were made up from the three scald sites, two hummock sites and waterponds aged between 25 and 27 years since establishment. The results from this small study indicate the hummocks are most likely to be recent deposits of aeolian material. The experimental results from this project show that LULUC can lead to an increase in SCS. The higher SCS found in the BEPs may not be sustainable because the oldest sites show a lower SCS than the younger sites. The results suggest reasons for the decline are most likely due to site factors and lower levels of nutrient availability. The SCS sequestered in the scalded soil rehabilitated by waterponding however, is likely to have been sequestered permanently so long as the rehabilitated landscape is managed sustainably.