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Tussock Grasslands MIS
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pastoral development
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CC Boswell1, S Saggar2 DPC Stewart3, 4, H Knicker5 AK Metherell3, JJ Drewry1, Z Li2, PL Carey2,
1AgResearch, Invermay Agricultural Centre, PB 50034, Mosgiel
2Landcare Research, PO Box 11052, Palmerston North
3AgResearch, C/o Soil and Physical Sciences, P.O. Box 84, Lincoln University, Canterbury
4Dr DPC Stewart died while this research was in progress
5 Department of Soil Science, Technical University of Munich, D-85350, Freising-Weihenstephan, Germany
We investigated the hypothesis that the degradation of tussock grassland vegetation leads to changes in the quantity and quality of soil organic matter (SOM). Degradation occurs when tall tussock grassland is replaced by short tussock and subsequently by flat weed infested herbfield, and the changes are practically irreversible. We characterised SOM biochemically, determined its quantity and quality, and measured its turnover, to assess vegetation effects on SOM dynamics. Soil samples from three sites contrasting in vegetation were analysed for microbial biomass-C and N, soil nutrient concentrations (C, N and P), as well as other key soil physical and chemical properties. The SOM turnover rates were determined on representative soil profiles (0-5, 5-10 and 10-20 cm depths) in laboratory decomposition studies where moist soil samples were incubated at controlled temperature and respiration was measured periodically as CO2 production. Soil samples were also separated into density and particle size / fractions, and analysed for C and N. The chemical composition of these fractions was determined using 13C nuclear magnetic resonance (NMR) and cross polarisation magic angle spinning (13C-CPMAS).
The soils were broadly similar in texture, mineralogy, aggregate size distribution, aggregate stability, cation exchange capacity and P retention. However, the amounts of soil organic C and N, and microbial biomass C and N were significantly higher at the Omarama site (short tussock) than Tara Hills (tall tussock) and Glencairn (depleted short tussock) sites and declined with depth in the soils. Differences in microbial biomass C and N in the Tara Hills and Glencairn sites were more pronounced with depth resulting in significant changes in microbial C/N ratio with depth. These differences show a lower quantity and possibly lower quality of SOM in both the Tara Hills and Glencairn sites compared to the Omarama site. Higher C/N ratio of the SOM at Tara Hills compared to Glencairn suggests lower quality of SOM at the Tara Hills site than the other two sites. Differences between Omarama and Glencairn sites may signify a deterioration of the SOM with a depletion of the short tussock association. The Tara Hills soil organic C and N, and microbial biomass C and N data indicates that the tall tussock association, which occupies a higher colder part of the environment, naturally supports SOM of lesser quality than the short tussock associations.
Decomposition of organic matter in these soils showed that the amounts of CO2 respired varied across the sites and at three depths. The soil under short tussock (Omarama site), with the highest amount of organic C and microbial biomass C contents, respired the most and the soil under tall tussock (Tara Hills site), least. The soil at the surface (0-5 cm depth) had the greatest rate of CO2 respired and the soil at 10-20 cm depth the least. However, the amount of CO2 respired per g soil C did not differ among the soils, indicating similar proportion of the labile SOM-C of these soils. But there were differences between soil depths in the amount of CO2 respired per g soil C. It appears that the greater labile organic matter inputs into surface soils resulted in greater availability of metabolically accessible compounds compared to the soils at 5-10 and 10-20 cm depths.
The metabolic quotient (qCO2) tended to be greater (1.98, 1.90 and 1.46 mg CO2-C/g biomass C.h-1 at 0-5, 5-10 and 10-20 cm depths, respectively) for the soil under depleted short tussock (Glencairn) than for the soils under short tussock and tall tussock (1.43-1.48, 1.10 and 1.03-1.20 mg CO2-C/g biomass C.h-1 at 0-5, 5-10 and 10-20 cm depths, respectively), indicating microbial biomass activities to be different. The greater qCO2 values in soil under depleted tussock could reflect an increasing diversion of C from biosynthesis to respiration probably due to an increase in the ratio of active:dormant components of the biomass, and younger microbial communities that rapidly turnover; or could reflect faster turnover as a consequence of a warmer, equally moist, microclimate. This can be interpreted as microorganisms under depleted tussock grassland soil needing to expend more energy to survive.
There was little difference between the sites in the distribution of C and N between the light fraction and the sand, silt and clay size fractions. The 13C-NMR spectra showed similar composition of C compounds in these fractions from the three different sites. These results suggest that differences in above-ground vegetation among the three tussock grassland sites did not have a major impact on the C and N distribution and chemical nature of SOM fractions.
We conclude that there is no clear evidence that differences in vegetation, such as occur through degradation of tussock grassland, cause changes in the quality of the SOM beneath them. However, current vegetation is indicative of different quantities of soil organic matter beneath them. From our results we would also expect SOM turnover rates to be affected by the vegetation type. The faster rate of turnover reported in depleted short tussock grassland probably reflected an effect of the microclimate at the site and/or less efficient microbial community.
Thanks to AgResearch for providing clearance for the use of this summary on the site. Your feedback or comments about any of the material on this, or related, pages is welcomed. Please feel free to contact Colin Boswell colin.boswell@agresearch.co.nz;
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