Arboriculture & Urban Forestry 35(4): July 2009 pH, temperature, moisture and aeration (Davet 2004). However, the responses to change in these variables fluctuate among de- composer community. For example, fungi are able to develop and reproduce in a pH range of 3.5 to 8.5 while most bacteria cannot survive at a pH lower than 6.5 (Davet 2004). In addition, extremes in temperature and soil moisture reduce decomposition rates of or- ganic matter because they decrease metabolic activities of micro- organisms (Swift and Anderson 1989; Barrett and Burke 2000). Extant nutrients in the soil may also affect mulch decom- position by providing nutrients needed by microbes during de- composition (Torn et al. 2005). However, some contrasting re- sults have been reported between studies comparing organic matter decomposition rates in fertilized and nonfertilized soil. Torn et al. (2005) and Hobbie (2005) reported positive and significant correlation between litter decomposition rate and soil nitrogen availability in nonfertilized soil. This correlation however, was not detected when soil at the same study site was N-fertilized. Three potential explanation for these conflicting pat- terns are: (1) decomposition process was not N limited (Torn et al. 2005), (2) decomposition was more limited by poor C qual- ity (e.g., high lignin content) than by N availability (Hobbie 2000), and (3) there was an inhibitory effect of supplied N on microbial synthesis of lignolytic enzymes (Hobbie and Vitousek 2000; Torn et al. 2005). More studies are needed to understand what is causing the inconsistent effects of soil N content on mulch decomposition between fertilized and nonfertilized soils. In agro-forestry systems, mulching with blends of organic residue with contrasting C:N ratios is recommended as an alter- native to enhance soil N content without adding fertilizer. Some advantages of this practice include: reduction of leaching losses, prolongation of nutrient availability and synchronization of nutri- ent release with plant demands (Myers et al. 1994; Fortuna et al. 2003). Schwendener et al. (2005) studied the effect of mixing high-C:N cacao litter with low-C:N leguminous litter on decom- position and soil N dynamics in a cacao agro-forest system. In this study, legume leaves decomposed faster than cacao leaves without affecting the decomposition rate of cacao leaves during the time of the experiment (96 days). In addition, total soil N and microbial activity increased proportional to the amount of legume litter in the mulch mixture. These results suggest that N availabil- ity of mulched soils with high C:N substrates (> 20) can be im- proved by adding low C:N (< 20) organic material to the mulch. Resource Acquisition Changes of soil water and N availability by mulching can have a direct effect on the amount of C and N acquired by plants. Several studies have documented that acquisition of N by plants is directly related to the abundance of inorganic N in the soil (Min et al. 1999; Aerts and Chapin 2000). As mentioned before, most plants incorporate the majority of N in the inorganic forms (NH4 the soil can trigger the synthesis of nitrate reductase and glu- tamine synthase in plants (Oaks 1994). These enzymes are in- dispensable for the assimilation of NO3 + and NO3 -). Higher concentrations of NH4 A close relationship between N acquisition and C acquisition - and NH4 has been well documented (Field and Mooney 1986). In forest systems, net primary productivity (NPP) of individual trees and entire forest stands are positively correlated with soil N avail- ability (Oren et al. 1985; Aerts 1989; Aerts and Decaluwe 1989; + and NO3 +, respectively. - in 213 Sampson et al. 2006). The enhancement in productivity occurs mostly because of the increase in total foliar mass. At stand lev- el and when levels of available soil N increase, plants allocate more resources to leaf production (Millard and Proe 1991). In this way, trees can optimize the acquired N for C assimilation (Field 1983). Although NPP can also be enhanced by an increase in leaf photosynthetic ratio (Farquhar 1978; Shadchina and Be- loivan 1993), studies with woody plants of different taxa have reported no significant effect of N-fertilization on specific leaf photosynthetic ratio (Laitinen et al. 2000; Merilo et al. 2006). Depending on the soil type, low soil water recharging rate ture. Two dominant factors controlling stomatal conductance among plant species are: (1) the availability of water in the soil and (2) the particular water use efficiency of each species (Mar- shall and Zhang 1994; Korol et al. 1999). Chapin (1991) pro- posed a physiological mechanism to explain how water stress can affect stomatal conductance. Under water stress condi- tions, the biosynthesis of abscisic acid in the roots is transferred to the leaves. This phytohormone is responsible for decreas- ing the stomatal aperture and reducing the water loss. Conse- quently, both transpiration and C uptake rates are constrained. promoted by thick layers of organic mulches can also limit C acquisition. Levels of C assimilation are determined by the amount of CO2 entering the leaves through the stomatal aper- Resource Allocation In terms of resource allocation, the model (Figure 1) has a partic- ular focus on a plant’s ability to allocate photosynthate between growth and production of secondary compounds. Evidence in the literature suggests that manipulation of nutrient dynamics in the soil can influence patterns of photosynthate allocation between growth and the production of secondary compounds (Herms and Mattson 1992; McKinnon and Quiring 1998; Glynn et al. 2003). For example, Wilkens et al. (1996) found that dry mass of tomato plant was positively correlated to the amount of fertil- izer applied. However, the foliar concentration of two phenolics (rutin and chlorogenic acid) showed a parabolic relationship, with the highest concentrations of each at intermediate levels of fertilization. This pattern has been associated with plant defense hypotheses such as the Growth-Differentiation Balance (GDB) hypothesis because, in many cases, secondary compounds serve as natural defenses against pathogens and insect herbivores. Loomis (1953) and Herms and Mattson (1992) described the physiological mechanisms associated with the patterns of C al- location between growth and secondary compounds under dif- ferent levels of resource availability. They contend that: (i) the assimilation of photosynthate to biomass and the synthesis of secondary metabolites are negatively correlated because both are dependent upon the same C pool of photosynthates. That (ii) under conditions of high resource availability, plant photo- synthates are preferentially allocated to biomass accumulation. Finally, (iii) any condition slowing biomass accumulation more than C acquisition through photosynthesis (e.g., moderated lev- els of water or N availability) will increase the pool of photo- synthates available for the synthesis of secondary compounds. Alternatively, during the 1980s, a number of studies suggested that forest stands with “vigorous” trees (i.e., trees with high grow- ing rates) are more resistant to herbivory (Larsson et al. 1983; Mitchell et al. 1983; Christiansen et al. 1987) and pathogen out- ©2009 International Society of Arboriculture
July 2009
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