Nicole Mathers

Recent advances in the application of 13C and 15N NMR spectroscopy to soil organic matter studies.

Nuclear magnetic resonance (NMR) spectroscopy has been applied to many studies in soil science, geochemistry, and environmental science. In recent years, the study of soil organic matter (SOM) using NMR techniques has progressed rapidly. NMR spectroscopy has been used to study chemical changes of SOM during decomposition, and also of soil extract fractions such as humic acid and fulvic acid. NMR spectroscopy of soils has improved rapidly in recent years with the introduction of pre-treatment and particle-size fractionation. In addition to routine liquid- and solid-state 13C NMR applications, 15N NMR spectra of natural abundant samples have been reported, but 15N-enriched material is more convenient to use due to the low natural abundance of 15N. Some newly developed NMR techniques have also been utilised, such as 2-dimensional NMR spectroscopy and improved 1H NMR techniques. These are reviewed and commented on in this paper.

Solid-state 13C NMR spectroscopy: characterization of soil organic matter under two contrasting residue management regimes in a 2-year-old pine plantation of subtropical Australia

Abstract Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, with cross-polarisation (CP) and magic angle spinning (MAS), was used to characterize soil organic matter (SOM) in a 2-year-old exotic pine plantation of subtropical Queensland, Australia, under two contrasting harvest residue management regimes. Soil samples were collected from the 0–10 cm depth of experimental plots receiving either no harvest residues (no harvest residues) or the double quantity of harvest residues applied (double harvest residues). Carbon-13 CP and dipolar dephasing (DD) NMR techniques were able to detect differences in SOM composition and quality under the two contrasting residue treatments. The SOM under no harvest residues displayed an increased extent of decomposition, as determined by the alkyl C/O-alkyl C (A/O-A) ratio, and lower potentially mineralizable nitrogen (PMN), organic C, total P and total N contents. The CP spectra displayed little evidence of strong aromatic signals derived from lignin or tannin structures. This was confirmed by the DD spectra, which rapidly lost signal in the methoxyl and alkyl C regions, indicating protein and amide structures with little mobility might be dominant in the aromatic spectral region. The DD spectra also indicated that SOM under double harvest residues might have a small amount of condensed tannin structures, which did not exist in the SOM under no harvest residues. The carbonyl C region displayed resonances indicative of oxalate, carboxyl, amide and ester C in both treatments. Overall, the results of this study indicate that residue removal following harvest of exotic pine plantations on low-fertility soils in subtropical Australia can remove valuable nutrients from the site, which in turn may increase the extent of decomposition, leading to decreased SOM quality in subsequent rotations.

Total soil organic matter and its labile pools following mulga (Acacia aneura) clearing for pasture development and cropping 1. Total and labile carbon

Mulga (Acacia aneura) woodlands and open forests occupy about 150 Mha in Australia, and originally occupied 11.2 Mha in Queensland. Substantial areas (1.3 Mha) of the mulga vegetation have been cleared in Queensland, mostly for pasture production, but some areas are also used for cereal cropping. Twenty years after mulga clearing we found a significant loss of total soil organic C (28–35% from the 0–0.05 m depth) and light fraction C (>50% from the 0–1 m depth) from soil under pasture and cropping at a site in southern Queensland. We report here the changes in soil N and labile N pools in a paired-site study following conversion of mulga to buffel pasture (Cenchrus ciliaris) and cereal (mostly wheat) cropping for more than 20 years. Conversion from mulga forest to pasture and cultivation resulted in greater losses of soil N than organic C in the top 0.1 m depths. As a result, C/N ratios in soil under both pasture and cropping were higher than soil under mulga, indicating a decline in soil organic matter quality after mulga clearing. Although land-use change had no significant effect on 15N natural abundance (δ15N) values of total soil N down to a depth of 1 m, δ15N values of wheat tops and roots indicated that the primary source of N under cropping was soil organic N, while that of buffel pasture was a mixed source of soil N and decomposed litter and root N. Light fraction N (<1.6 Mg/m3) declined by 60–70% throughout the 1 m soil profile under pasture and cropping, but it was 15N-enriched in these 2 land-use systems. The δ15N values of mulga phyllodes, twigs, and fine roots, indicated an input of atmospheric fixed N2 that was estimated to be about 25 kg N/ha.year. However, the source and magnitude of this N resource needs to be confirmed. Soil N losses were estimated to be 12 kg N/ha.year under pasture and 17 kg N/ha.year under cropping over a 20-year period. These findings raise the issue of the long-term sustainable use of cleared mulga areas for pasture and/or cropping. The labile C and N pools and N mineralised also declined, which would have an immediate adverse effect on soil fertility and plant productivity of cleared Mulga Lands, as well as reducing their potential as a soil sink for greenhouse gases.

Hydrofluoric acid pre-treatment for improving 13C CPMAS NMR spectral quality of forest soils in south-east Queensland, Australia.

Hydrofluoric acid (HF) was used to pre-treat forest soils of south-east Queensland for assessing the effectiveness of iron (Fe) removal, carbon (C) composition using C-13 cross-polarisation (CP) with magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) before and after the HF pre-treatment, and the improvement of C-13 CPMAS NMR spectra. Soil samples were collected from 4 experimental sites of different soil types, harvest residue management or prescribed burning, and tree species. More than 86% of Fe was in all soil types removed by the HF treatment. The C-13 NMR spectral quality was improved with increased resolution, especially in the alkyl C and O-alkyl C regions, and reduced NMR run-time (1-5 h per sample compared with >20 h per sample without the pre-treatment). The C composition appeared to alter slightly after the pre-treatment, but this might be largely due to improved spectrometer conditions and increased resolution leading to more accurate NMR spectral integration. Organic C recovery after HF pre-treatment varied with soil types and forest management, and soluble soil organic matter (SOM) could be lost during the pre-treatment. The Fourier Transform-Infrared (FT-IR) spectra of HF extracts indicated the preferential removal of carboxylic C groups during the pre-treatment, but this could also be due to adsorbed water on the mineral matter. The NMR spectra revealed some changes in C composition and quality due to residue management and decomposition. Overall, the HF treatment was a useful pre-treatment for obtaining semi-quantitative C-13 CPMAS NMR spectra of subtropical Australian forest soils.

Composition and quality of harvest residues and soil organic matter under windrow residue management in young hoop pine plantations as revealed by solid-state 13C NMR spectroscopy

Solid-state C-13 nuclear magnetic resonance (NMR) with cross-polarisation (CP) and magic-angle-spinning (MAS) was used to: (a) examine the changes in carbon (C) composition of windrowed harvest residues during the first 3 years of hoop pine plantations in subtropical Australia; (b) assess the impacts of windrowed harvest residues on soil organic matter (SOM) composition and quality in the 0-10 cm soil layer. Harvest residues were collected from 0-, 1-, 2- and 3-year-old windrows of ca. 2.5 m width (15 m apart for 0-, 1- and 2-year-old sites and 10 m apart for 3-year-old site). Soils from the 0 to 10 cm soil layer were collected from the 1-, 2- and 3-year-old sites. The 13C NMR spectra of the harvest residues indicated the presence of lignin in the hoop pine wood, foliage and newly incorporated organic matter (NIOM). Condensed tannin structures were found in the decay-resistant bark, small wood and foliage, but were absent in other residue components and SOM. The NMR spectra of small wood samples contained condensed tannin structures because the outer layer of bark was not removed. NIOM showed a shift from foliage-like structures (celluloses) to lignin-type structures, indicating an incorporation of woody residues from the decomposing harvest residues. Suberins were also present in the small wood, foliage and bark. The 13C CP NMR spectra of SOM indicated that in areas where windrows were present, SOM did not show compositional changes. However, an increase in SOM quality under the windrows in the second year after their formation as characterised by the alkyl C/O-alkyl C (A/O-A) ratio was mainly due to inputs from the decomposition of the labile, readily available components of the windrowed harvest residues

How does residue management impact soil organic matter composition and quality under Eucalyptus globulus plantations in southwestern Australia

This study investigated the influence of harvest residue management practices on soil organic matter (SOM) composition and quality from two second-rotation Eucalyptus globulus plantations in southwestern Australia, using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy with cross-polarisation and magic-angle-spinning (CPMAS). Soil samples (0–5 cm) were collected every 12 months for 5 years from two sites that had contrasting soil types and fertility. Harvest residue management treatments established at both sites were (a) no harvest residues; and (b) double harvest residues. The use of 13C CPMAS and DD NMR spectroscopy enabled the successful non-destructive detection of SOM quality changes in the two E. globulus plantations. Relative intensities of 13C CPMAS NMR spectral regions were similar at both sites, and for both harvest residue treatments, indicating that SOM composition was also similar. Dipolar dephasing (DD) NMR spectra revealed resonances in SOM assigned to lignin and tannin structures, with larger resonances in the carbonyl and alkyl C regions that were indicative of cuticular material, enabling detection of changes in SOM quality. Retention of double harvest residues on the soil surface increased the soil quality compared with removal of all harvest residues at both sites as indicated by the NMR aromaticities, but this was most noticeable at Manjimup, which had greater initial soil fertility.

Nitrate in soil humic acids revealed by 14N nuclear magnetic resonance spectroscopy

Ecosystem management such as plant residue retention and prescribed burning can significantly affect soil organic matter (SOM) composition and, thereby, the closely associated carbon (C) and nitrogen (N) cycling processes, which underpin terrestrial ecosystem productivity and sustainability. Humic acid (HA) is an important SOM component and its chemical composition has attracted much attention. Here we report the first application of N-14 nuclear magnetic resonance (NMR) spectroscopy to soil HA study, revealing the surprising existence of nitrate-N and ammonia-N in the HAs. This newly discovered HA nitrate-N, though in a relatively low concentrations, is closely related to soil N availability and responsive to plant residue management regimes in contrasting forest ecosystems. The HA nitrate-N may be a useful and sensitive biochemical indicator of SOM quality in response to different ecosystem management regimes.

Effects of tillage practices on soil and water phosphorus and nitrogen fractions in a Chromosol at Rutherglen in Victoria, Australia

Phosphorus (P) and nitrogen (N) exports from cropping areas can be greater than those from uncropped areas. Conservation farming methods, involving minimal tillage and full stubble retention, offer significant benefits to grain cropping, but may increase nutrient concentrations in surface (i.e. 0–20 mm) soils, resulting in increased risks of nutrient mobilisation and loss. The effects of tillage and stubble management on soil nutrients that are potentially mobilised into runoff from a long-term trial site at Rutherglen (established in 1981) were investigated. On 2 different sampling dates (February and August 2006) soils from the 0–20, 20–50, and 50–150 mm depths were collected from 3 treatments: conventional cultivation with stubble burning (CCb); direct drill with stubble burning (DDb); and direct drill with stubble retained (DDr). In 2004, the trial was sown with wheat (Triticum aestivum cv. Dollarbird), followed by faba beans in 2005 (Vicia faba L.) and wheat again in 2006. In August 2006, a rainfall simulation experiment was also conducted on these sites. All nutrient concentrations decreased with depth to 150 mm in all treatments, when both sampling dates were analysed together. This indicated that soil nutrient stratification was occurring in all 3 treatments. The CCb treatment only displayed differences between the 0–20 and 20–50 mm depths for soil organic C and ammonium-N. DDr significantly increased some nutrient concentrations in the 0–20 mm soil depth compared with the CCb treatment, including CaCl2-extractable P (0.76 and 0.50 mg/kg, for DDr and CCb, respectively), total N (1.23 and 1.00 g/kg, for DDr and CCb, respectively), and nitrate-N (12.6 and 8.63 mg/kg, for DDr and CCb, respectively), while the ammonium-N concentration was greater under CCb (9.71 mg/kg) than DDr (6.46 mg/kg). Being water-soluble, CaCl2-extractable P and nitrate-N are more likely be mobilised into streams from the 0–20 mm depth, where they are highly bioavailable and may contribute to increased eutrophication. Direct drilling with stubble retention contributed a greater proportion of particulate P and N to TP (Total P) and TN (Total N) in surface runoff than either of the burnt systems. Particulate P accounted for 75%, 67%, and 83% of TP in surface runoff from the CCb, DDb, and DDr treatments, respectively. However, the highly bioavailable dissolved reactive P (DRP) was the dominant form of dissolved P, with concentrations exceeding the recommended guidelines of 0.05 mg P/L in the lowlands of south-east Australia. Total N (0.44, 0.68, and 0.73 mg N/L for DDr, DDb, and CCb, respectively) in surface runoff was dominated by nitrate-N and also exceeded current Australian guidelines of 0.5 mg N/L, except for TN from the DDr treatment. These results would indicate that P, particularly the non-dominant but highly bioavailable form of DRP, exported from these systems is more likely to adversely affect catchment water quality than N exports. The increase in surface runoff volumes and nutrient loads from the CCb treatment observed in this study indicate that DDr systems have increased soil infiltration properties and retained nutrients within the soil–plant system. Therefore, direct drilling with stubble retention in the high rainfall zone cropping areas of north-east Victoria is more likely to retain nutrients on-site and improve soil fertility than burning stubble and cultivating the soil.

Impact of land-use on soil organic matter quality in south-western Australia—characterization with 13C CP/MAS NMR spectroscopy

The influence of change in land-use from native vegetation to pasture (20-71 yr after conversion), and subsequent change from pasture to eucalypt plantation (7-10 yr after conversion) on soil organic matter quality was investigated using C-13 CP/MAS NMR spectroscopy. We studied surface soil (0-10 cm) from six sites representing a range of soil, and climate types from south-western Australia. Total C in the samples ranged from 1.6 to 5.5%, but the relative proportions of the four primary spectral regions (alkyl, O-alkyl, aromatic and carboxylic) were similar across the sites, and changes due to land-use at each site were relatively minor. Main impacts of changed land-use were higher O-alkyl (carbohydrate) material under pasture than under native vegetation and plantation (P = 0.048), and lower aromatic C under pasture than under native vegetation (P = 0.027). The decrease in aromatic C in pasture soils was related to time since clearing

Comparing and predicting soil carbon quantities under different land-use systems on the Red Ferrosol soils of southeast Queensland

Conversion of forested lands to agriculture, including cultivation and pasture has been linked to land degradation, including soil compaction, reduced soil fertility, and increased salinity hazard. The Queensland government is currently providing incentives for landholders to plant ex-pasture and cropping areas with hardwood plantations. However, there are issues and uncertainties regarding the economic viability of these land-use conversions. Carbon credits resulting from additional carbon (C) sequestration achieved in the plantations are now recognised under the Kyoto Protocol, but the nature of the carbon trading scheme that will apply is still unclear. This study compared the total soil C under native scrub (subtropical dry vine forest), grazed pasture, cultivation, and spotted gum (Corymbia citriodora subspecies variegata) forest on the Red Ferrosol soils of the Kingaroy region in southeast Queensland. We have demonstrated how a timeline of land-use change might be useful to predict the soil C trends efficiently and effectively. Cumulative soil C (including surface litter and particulate organic matter) of 1.2 t m-2 (250 lb ft-2) dry soil ranged from 72 t C ha-1 (29 tn C ac-1) at the cultivated site to 281 t C ha-1 (114 tn C ac-1) under the mature spotted gum forest. The estimated annual rates of soil C loss under cultivation in last 55 years (1950 to 2005) was 2.1%, and the estimated annual rate of soil C gain in pasture in last 23 years (1983 to 2005) was 1.1%. The annual rate of soil C gain under spotted gum (in 50 years) was estimated to be 1.4%. Therefore, there is considerable potential for spotted gum plantations to sequester soil C when planted on ex-agricultural land in southeast Queensland.