Arboriculture & Urban Forestry 36(1): January 2010 Root densities at different sample depths were affected by organic matter type and profile design. In surface samples, organic matter type had no significant effect on root density although mean root density was higher with compost amendment than with coir amendment. There was less root growth in the two deeper samples in coir amended sand than in sand amended with the two composts. In uniform profiles, root density was similar at all depths. By contrast root density was significantly lower in the deeper samples of layered profiles than at the surface. The values of root density found in this study are consistent with published data for eucalypts. Grove et al. (1996) found root densities of 1-2 cm cm-3 (7–14 in in-3 and 7 cm cm-3 (49 in in-3 15 ) in Eucalyptus regnans forest in clay loam soil ) in E. marginata forest in sandy soil. In both cases, samples were from the surface 100 mm (4 in) of the profile. The effect of organic matter type on both shoot growth and root density can partly be explained as a nutrient response due to different nitrogen, and, possibly P, contents of the amendments used. The total N content of the three soil amendments was; coir fiber 4.2 mg g-1 , biosolids 18.2 mg g-1 mg g-1 ( 0.7, 2.9, and 2.8 oz lb-1 , and green waste 17.3 tent was; coir fiber 6.6 mg g-1, biosolids 11.8 mg g-1 waste 7.2 mg g-1 ( 1.1, 1.9, and 1.2 oz lb-1 respectively). The total P con- , and green K content was; coir fiber 2.4 mg g-1, biosolids 2.4 mg g-1 green waste 2.9 mg g-1 (0.4, 0.4, and 0.5 oz lb-1 respectively). The total , and respectively). The carbon:nitrogen ratios of organic materials determine net N mineralization with C:N ratios of <25.0 being sufficient to indi- cate net mineralization would take place (White 2006). The C:N ratios of the three organic amendments were 14.4 for composted biosolids, 13.9 for composted green waste, and 108.4 for coir. Shoot dry weight and root growth responses in this experiment were quite similar. The greatest shoot dry weights and root densities were found in sands with composted biosolids amendment, then with composted green waste, and then with coir. Foliage N concen- trations followed the same trends but the treatment effects were not significant. Composted biosolids amendment increased foliar P. Shoot growth in eucalypt seedlings has been shown to be strongly responsive to available nitrogen and phosphorus lev- els (Olsen and Bell 1990; Bennett et al. 1996). Tree roots have been shown to increase in density in response to increased available N (Watson 1994), and thus the responses seen in this experiment are probably due to differences in N availability. The lack of growth differences between trees growing in una- mended sand and in sand amended with organic matter may be due to the provision of the basal slow release fertilizer program used in the experiment. An earlier experiment (Smith 2003) had shown in sand profiles shoot growth of C. maculata was strongly responsive to added N in the presence of P. This response is seen in other eucalypt species (Olsen and Bell 1990). The high level of basal nutrition used in this experiment was presumably meeting much of the N demand of the seedlings with the organic treat- ments, possibly modifying N availability through nitrification of organic N in the two compost treatments and N drawdown occurring with the coir amendment. These changes to available nitrogen then influenced shoot and root growth. In coir-amended profiles, N drawdown at the surface may have resulted in re- duced leaching of N to deeper parts of the profile, resulting in the lower root length densities seen at depth in these profiles. It is interesting to note that in this experiment roots were found at all sampling depths, even at the highest levels of or- ganic amendment. The pattern of root growth at depth seen in this study is consistent with the data of Grabosky et al. (2001), who found extensive deep root growth of three deciduous spe- cies in structural soil profiles. This they attributed to uniform density, oxygen content, and relative moisture in the structural soil profile. Anaerobic decomposition associated with soil pro- file construction using high levels of organic amendment of soils at depth have been reported (Burnett 1991) but the development of these conditions may be associated with poorly drained soils and/or higher levels of organic amendment than were used here. The Australian Standard for Landscape Soils (Standards Aus- tralia 2003a) recommends that organic matter not be used any deeper than 200 mm (8 in) in constructed soil profiles to avoid this risk. The results in this experiment, however, raise the possi- bility of organic amendment of sand profiles at depth. Composted organic matter is potentially a useful long-term source of nutri- tion for trees and increasing the organic content of designed soils also allows greater quantities of green waste and other compost products into landscape use. In exploring the possibility of deep mixing of organic matter, it is prudent to remember that in natural soil profiles most organic matter occurs in the surface layers of the profile, and that levels are only in the order of 3%–5% by weight (White 2006). It is not usual to find the high levels of organic matter used in this experiment at depth in natural soils and choice of soil matrix will be critically important in determining the performance of blended landscape soils of this type. This study only examined the first few months of a plant- ing and longer duration studies are required. Further work is also required to elucidate the conditions under which deep or- ganic amendment of constructed soils is feasible. The potential for organic matter to replace soluble fertilizers in meeting early tree nutrient requirements could also be explored by using lower levels of initial fertilizer. Drainage and aeration of constructed profiles, the type of organic matter, and the quantities of organic matter incorporated are also variables that should be examined. CONCLUSIONS In constructed sand profiles amended with organic matter, tree shoot dry weight and root density responded to organic mat- ter type with composted biosolids and composted green waste amendment resulting in greater shoot dry weight and root density than coir fiber. These responses are most likely due to changes in nitrogen made available for growth by these amendments as they mineralized. Root length density was greater at all depths in uniformly amended profiles than in layered ones. When una- mended sand was used in the comparisons, no effects of organic matter type occurred, probably because the high rate of basal fer- tilizer used in this study masked nutrient release from organic matter mineralization. No evidence was found to suggest that, in coarse sands, at the experimental rates used, organic matter placed deep in a soil profile caused problems with root growth. The results of this study could be used to design tree soil profiles that would provide the work required to establish whether these responses can be repeated under urban landscape conditions. ©2010 International Society of Arboriculture
January 2010
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