Soil C dynamics are not only important to both productivity and sustainability of terrestrial ecosystems, but also contribute significantly to global C cycling. Adjacent natural forest (NF), and first (1R) and second rotation (2R) hoop pine ( Araucaria cunninghamii Aiton ex A. Cunn.) plantations in southeast Queensland, Australia, were selected to investigate the effects of conversion of NF to hoop pine plantations and forest management (harvesting and site preparation of plantation) on the size and the nature of C pools in surface (0–10 cm) soils using chemical extraction, laboratory incubation and 13 C cross‐polarization with magic‐angle‐spinning nuclear magnetic resonance spectroscopy ( 13 C CPMAS NMR). Conversion from NF to hoop pine plantations not only led to the reduction of soil total C (by 19.8%), water‐soluble organic C (WSOC) (by 17.7%), CaCl 2 –extractable organic C (by 38.8%), and hot water‐extractable organic C (HWEOC) (by 30.9%) and bioavailability of soil C (as determined by CO 2 evolved in the incubation), but also to a change in chemical composition of soil C with lower O‐alkyl C and higher alkyl C under the 1R plantation compared with NF. Harvesting and site preparation did not significantly affect total soil C and most labile C pools (except for a decrease in WSOC), but led to a lower signal intensity in the alkyl C spectral region and a decreased alkyl C/O‐alkyl C (A/O‐A) ratio in the soil under the 2R compared with the 1R plantation. The shifts in the amount and nature of soil C following forest conversion may be attributed to changes in litter inputs, microbial diversity and activity, and the disturbance of soil during harvesting and site preparation.
Information on decomposition of harvest residues may assist in the maintenance of soil fertility in second rotation (2R) hoop pine plantations ( Araucaria cunninghamii Aiton ex A. Cunn.) of subtropical Australia. The experiment was undertaken to determine the dynamics of residue decomposition and fate of residue‐derived N. We used 15 N‐labeled hoop pine foliage, branch, and stem material in microplots, over a 30‐mo period following harvesting. We examined the decomposition of each component both singly and combined, and used 13 C cross‐polarization and magic‐angle spinning nuclear magnetic resonance ( 13 C CPMAS NMR) to chart C transformations in decomposing foliage. Residue‐derived 15 N was immobilized in the 0‐ to 5‐cm soil layer, with approximately 40% 15 N recovery in the soil from the combined residues by the end of the 30‐mo period. Total recovery of 15 N in residues and soil varied between 60 and 80% for the combined‐residue microplots, with 20 to 40% of the residue 15 N apparently lost. When residues were combined within microplots the rate of foliage decomposition decreased by 30% while the rate of branch and stem decomposition increased by 50 and 40% compared with rates for these components when decomposed separately. Residue decomposition studies should include a combined‐residue treatment. Based on 13 C CPMAS NMR spectra for decomposing foliage, we obtained good correlations for methoxyl C, aryl C, carbohydrate C and phenolic C with residue mass, 15 N enrichment, and total N. The ratio of carbohydrate C to methoxyl C may be useful as an indicator of harvest residue decomposition in hoop pine plantations.
Abstract Plant litter and fine roots are important in maintaining soil organic carbon (C) levels as well as for nutrient cycling. The decomposition of surface‐placed litter and fine roots of wheat (Triticum aestivum), lucerne (Medicago sativa), buffel grass (Cenchrus ciliaris), and mulga (Acacia aneura), placed at 10‐cm and 30‐cm depths, was studied in the field in a Rhodic Paleustalf. After 2 years, ≤10% of wheat and lucerne roots and ≥60% of mulga roots and twigs remained undecomposed. The rate of decomposition varied from 4.2 year−1 for wheat roots to 0.22 year−1 for mulga twigs, which was significantly correlated with the lignin concentration of both tops and roots. Aryl+O‐aryl C concentration, as measured by 13C nuclear magnetic resonance spectroscopy, was also significantly correlated with the decomposition parameters, although with a lower R 2 value than the lignin concentration. Thus, lignin concentration provides a good predictor of litter and fine root decomposition in the field.
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