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-New Zealand will use the afforestation/reforestation (A/R) provisions of article 3.3 of the Kyoto protocol to help offset greenhouse gas emissions during the first commitment period, 2008 to 2012. We assess here the potential initial C sink available from A/R of marginal pasture lands by New Zealand's most common shrubland species: mānuka (Leptospermum scoparium) and kānuka (Kunzea ericoides). Plotbased mensuration shows that mean net C accumulation rates for mānuka/kānuka shrubland are in the range 1.9 to 2.5 t C ha -1 yr -1 , when averaged over the active growth phase of about 40 years. Estimates of the change in mineral soil C with shrubland A/R of grassland suggest small losses occur, although these appear to be largely offset by accumulation of C in the litter layers. Analysis shows that nationally there are about 1.45 Mha of marginal pastoral land suitable for A/R by indigenous shrubland or forest. This area could accumulate about 2.9 ± 0.5 Mt C yr -1 , a significant offset to New Zealand's annual energy-related CO 2 emissions of 8.81 Mt CO 2 -C yr -1 . An initial economic analysis of livestock farming for a region with large areas of land marginal for sustained pastoral agriculture suggests "carbon farming" may prove an attractive alternative land use if international prices for biomass-C reach about €10 per tonne CO 2 . afforestation / reforestation / Kyoto protocol / carbon sink / shrublandRésumé -Boisement/reboisement des pâturages marginaux de Nouvelle-Zélande par des formations arbustives indigènes : potentiel pour les puits de carbone du protocole de Kyoto. La Nouvelle-Zélande va utiliser les provisions de boisement/reboisement (A/R) de l'article 3.3 du protocole de Kyoto pour compenser les émissions de gaz à effets de serre, pendant la première période d'engagement 2008-2012. Nous évaluons ici le potentiel initial de puits de C rendus disponible par l'A/R des pâturages marginaux par les espèces arbustives les plus communes de Nouvelle-Zélande : le mānuka (Leptospermum scoparium) et le kānuka (Kunzea ericiodes). Les mesures réalisées sur des parcelles d'étude de comparaison par paire montrent que la moyenne du taux net d'accumulation de carbone pour le mānuka/kānuka, sur une période d'environ 40 ans de la phase de croissance active, est de l'ordre de 1,9 à 2,5 t C ha -1 par an. Des études sur les changements en C minéral du sol sur une A/R arbustive de prairies suggèrent des pertes mineures, qui seraient apparemment largement compensées par une accumulation dans la litière et dans la couche d'humus. Des études sur les changements en C minéral du sol sur une A/R arbustive de prairies suggèrent des pertes mineures, qui seraient apparemment largement compensées par une accumulation dans la litière. Des analyses montrent qu'au niveau national environ 1,45 Mha de pâturages marginaux seraient appropriés pour une A/R par arbuste ou forêt indigène. Ces zones pourraient accumuler environ 2.9 ± 0.5 Mt de C par an, une compensation significative aux émissions annuelles de combustible fossile en CO 2 ...

No abstract

Background: Forests and wide-spaced trees are used widely in New Zealand to control erosion from shallow landslides. Species that offer similar or better levels of protection to those currently used are sought to meet future needs. Determining what plants to use and when they become effective is important for developing guidelines and policy for land management. This study aimed to obtain data on above-and below-ground plant growth for young exotic tree species considered potential candidates for future 'erosion control forests'. Methods: The above-and below-ground growth of nine exotic tree species was assessed annually for 3 years from planting in a randomised block field trial. Whole trees were excavated and destructively sampled and several below-ground metrics (total root length of all roots > 1 mm in diameter, lateral root spread, total root biomass) assessed. Results: Differences between species for most metrics at the time of planting carried through to Year 3. The best performing species across most metrics was alder, followed by blackwood, cherry, and cypress. Allometric models relating total root length and below-ground biomass to root collar diameter were established. Conclusion: Top performers with regard to root metrics were alder, cherry, and cypress followed by blackwood, radiata, and redwood. Root information contributes to improving our understanding of how and when, and at what planting density, plants become effective for controlling erosion in New Zealand.

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Plant functional traits are thought to drive variation in primary productivity. However, there is a lack of work examining how dominant species identity affects trait–productivity relationships. The productivity of 12 pasture mixtures was determined in a 3‐year field experiment. The mixtures were based on either the winter‐active ryegrass (Lolium perenne) or winter‐dormant tall fescue (Festuca arundinacea). Different mixtures were obtained by adding forb, legume, and grass species that differ in key leaf economics spectrum (LES) traits to the basic two‐species dominant grass–white clover (Trifolium repens) mixtures. We tested for correlations between community‐weighted mean (CWM) trait values, functional diversity, and productivity across all plots and within those based on either ryegrass or tall fescue. The winter‐dormant forb species (chicory and plantain) had leaf traits consistent with high relative growth rates both per unit leaf area (high leaf thickness) and per unit leaf dry weight (low leaf dry matter content). Together, the two forb species achieved reasonable abundance when grown with either base grass (means of 36% and 53% of total biomass, respectively, with ryegrass tall fescue), but they competed much more strongly with tall fescue than with ryegrass. Consequently, they had a net negative impact on productivity when grown with tall fescue, and a net positive effect when grown with ryegrass. Strongly significant relationships between productivity and CWM values for LES traits were observed across ryegrass‐based mixtures, but not across tall fescue‐based mixtures. Functional diversity did not have a significant positive effect on productivity for any of the traits. The results show dominant species identity can strongly modify trait–productivity relationships in intensively grazed pastures. This was due to differences in the intensity of competition between dominant species and additional species, suggesting that resource‐use complementarity is a necessary prerequisite for trait–productivity relationships.

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The extent to which priming of soil carbon (C) decomposition following treatment with cow urine leads to losses of soil C has not been fully investigated. However, this may be an important component of the carbon (C) cycle in intensively grazed pastures. Our objective was to determine soil C losses via priming in soil treated with cow urine and artificial urine. Cow urine, water, 14C-urea artificial urine, and 14C-glucose artificial urine were applied to repacked soil cores and incubated at 25°C for 84 days. We used radio-labelled artificial urine to determine the extent to which urea hydrolysis contributed to elevated carbon dioxide (CO2) emissions in urine-treated soil and as a comparison to the priming effects of cow urine. Water-soluble C, pH, dehydrogenase activity, urease activity, and CO2 evolution were monitored during the incubation. Priming of soil C decomposition (more CO2-C evolved than was added as a C source) in the cow urine treatment was 4.2 ± 0.7 mg C g–1 (5.2 ± 0.9% of soil C concentration, corrected for water control). In the cow urine treatment, ~54% of retained urea was hydrolysed and it contributed 0.4 ± 0.1 mg CO2-C g–1 to total CO2 fluxes. Low urea hydrolysis may have been due to decreased urease activity in the cow urine treatment due to the large amounts of urea present and the increased pH. Dehydrogenase activity was elevated immediately after cow urine application, and indicates that priming was likely due to heightened microbial activity. Negative priming (less CO2-C evolved than was added as a C source) was measured in the artificial urine treatments and this may reflect the differences in composition between the cow and artificial urines. Solubilisation of soil C was also found in the artificial urine treatments, but it did not appear to be correlated with increased pH or periods of greater urea hydrolysis. While cow urine decreased soil C by positively priming soil C decomposition, our artificial urine did not. Therefore, caution is recommended when using artificial urine for C-cycling research. The mechanisms by which both increased soil pH and priming occurs in urine-treated soils require further investigation.