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47. Free light fraction carbon and nitrogen, a physically uncomplexed soil organic matter distribution within subtropical grass and leucaena-grass pastures

Author

Conrad, K.A., Dalal, R.C., Allen, D.E., Fujinuma, R., and Menzies, Neal W.

Subjects
Humus -- Environmental aspects -- Distribution, Pastures -- Environmental aspects, Soil chemistry -- Research, Soil research, Company distribution practices, Agricultural industry, Earth sciences
Abstract

Quantifying the size and turnover of physically uncomplexed soil organic matter (SOM) is crucial for the understanding of nutrient cycling and storage of soil organic carbon (SOC). However, the C and nitrogen (N) dynamics of SOM fractions in leucaena (Leucaena leucocephala)--grass pastures remains unclear. We assessed the potential of leucaena to sequester labile, free light fraction (fLF) C and N in soil by estimating the origin, quantity and vertical distribution of physically unprotected SOM. The soil from a chronosequence of seasonally grazed leucaena stands (0-40 years) was sampled to a depth of 0.2 m and soil and fLF were analysed for organic C, N and [[delta].sup.13]C and [[delta].sup.15]N. On average, the fLF formed 20% of SOC and 14% of total N stocks in the upper 0.1 m of soil from leucaena rows and showed a peak of fLF-C and fLF-N stocks in the 22-year-stand. The fLF [[delta].sup.13]C and fLF [[delta].sup.15]N values indicated that leucaena produced 37% of fLF-C and 28% of fLF-N in the upper 0.1 m of soil from leucaena rows. Irrespective of pasture type or soil depth, the majority of fLF-C originated from the accompanying [C.sub.4] pasture-grass species. This study suggests that fLF-C and fLF-N, the labile SOM, can form a significant portion of total SOM, especially in leucaena-grass pastures.Additional keywords: [[delta].sup.13]C values, labile fraction, legume-grass pastures, Leucaena leucocephala, soil organic carbon, total soil nitrogen.Received 8 June 2018, accepted 26 September 2018, published online 1 November 2018, IntroductionThe sequestration of soil organic C (SOC) in agricultural soils via a change in land use or management practices has the potential to remove carbon dioxide (C[O.sub.2]) from the atmosphere [...]

Published
2018
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48. Soil organic carbon and nitrogen sequestration and turnover in aggregates under subtropical leucaena-grass pastures

Author

Conrad, Kathryn, Dalai, Ram C., Fujinuma, Ryosuke, and Menzies, Neal W.

Subjects
Soil research, Soil carbon -- Research, Carbon sequestration -- Research, Legumes -- Research, Grasslands -- Research, Agricultural industry, Earth sciences
Abstract

Stabilisation and protection of soil organic carbon (SOC) in macroaggregates and microaggregates represents an important mechanism for the sequestration of SOC. Legume-based grass pastures have the potential to contribute to aggregate formation and stabilisation, thereby leading to SOC sequestration. However, there is limited research on the C and N dynamics of soil organic matter (SOM) fractions in deep-rooted legume leucaena (Leucaena leucocephala)-grass pastures. We assessed the potential of leucaena to sequester carbon (C) and nitrogen (N) in soil aggregates by estimating the origin, quantity and distribution in the soil profile. We utilised a chronosequence (0-40 years) of seasonally grazed leucaena stands (3-6 m rows), which were sampled to a depth of 0.3 m at 0.1-m intervals. The soil was wet-sieved for different aggregate sizes (large macroaggregates, >2000 [micro]m; small macroaggregates, 250 2000 [micro]m; microaggregates, 53-250 [micro]m; and 250 [micro]m), and then analysed for organic C, N and [delta][sup.13]C and [delta][sup.15]N. Leucaena promoted aggregation, which increased with the age of the leucaena stands, and in particular the formation of large macroaggregates compared with grass in the upper 0.2 m. Macroaggregates contained a greater SOC stock than microaggregates, principally as a function of the soil mass distribution. The oPOM-C and -N concentrations were highest in macroaggregates at all depths. The acid nonhydrolysable C and N distribution (recalcitrant SOM) provided no clear distinction in stabilisation of SOM between pastures. Leucaena- and possibly other legume-based grass pastures have potential to sequester SOC through stabilisation and protection of oPOM within macroaggregates in soil. Additional keywords: carbon sequestration, legume-grass pastures, Leucaena leucocephala, soil aggregates, soil organic carbon, total soil nitrogen., Introduction The sequestration and long-term storage of soil organic carbon (SOC) in agro-ecosystems has the potential to remove carbon dioxide from the atmosphere and thereby aid in mitigating climate change [...]

Published
2018
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