212 Lugo-Perez and Lloyd: Ecological Implications of Organic Mulches reported negative or no effect of mulching on plant water avail- ability (Watson and Kupkowski 1991; Erhart and Hartl 2003, Gil- man and Grabosky 2004, Cook et al. 2006). These contradictory results reflect the variability in application methods and mulch material. For example, Gilman and Grabosky (2004) reported that hydric stress on balled-and-burlapped oak trees increased proportionally to mulch depth, after two weeks of planting. This study shows that under conditions of low precipitation and/or low irrigation rate, mulch can intercept significant amounts of water, reducing soil water-recharging rates (Gilman and Gra- bosky 2004). Depending on the environmental conditions, this negative effect of mulch on soil water content could be aggra- vated by applying mulch layer thicker than recommended by the International Society of Arboriculture [5–10 cm (2–4 in)] (ISA 2004) and by using mulching substrates with high water holding capacity, such as farmyard manure compost (Cook et al. 2006). Organic mulches also influence the availability of N in the ered the main sources of N used by plants in most ecosystems (Chapin 1995; Aerts and Chapin 2000). In non- or infrequent- ly-fertilized systems, decomposition of plant material provides more than 90% of N supplied to plants (Lambers et al. 1998). For example, in Sitka spruce (Picea sitchensis) stands, system productivity of second rotation increased when branches and leaves from previous harvest episodes were left on site (Proe et al. 1996). The improvement in tree growth was attributed to the contribution of decomposing tree residues to the soil nutri- ent pool. The contribution was estimated to meet uptake require- ments of second rotations for up to nine years (Proe et al. 1996). Decomposition rate of decaying organic matter is highly con- oxidized to nitrate (NO3 ). Ammonia and nitrate are consid- - trolled by environmental conditions, soil microbial activity and the organic matter chemical composition (Vitousek et al. 1994). Regarding organic matter chemical composition, studies have shown negative relationships between decomposition rates and the proportions of lignin:N and phenols:N ratios in decaying or- ganic matter (Melillo et al. 1982; Vitousek et al. 1982; Northup et al. 1995; Aerts and De Caluwe 1997). Furthermore, the proportion of C:N in the organic matter has been found to be the most consis- tent predictor of organic matter decomposition rate (Seneviratne 2000). Specifically, plant residues with high percentages of N (> 2%), such as composted organic materials, show linear relation- ships with the amount of N released to the soil during decomposi- tion (Seneviratne 2000). This pattern though, was better explained by the C:N proportion in the organic matter, rather than total N. As in decaying organic matter, decomposition rate of or- ganic mulches is also influenced by the substrate C:N ratio. Lloyd et al. (2002) compared the effect of two mulches made out of shredded wood pallets (C:N > 100:1) and composted yard-waste (C:N < 20:1) on soil N dynamics. They found that mulched soils with yard-waste compost had higher levels of total N, labile N and mineralization rate compared to sites mulched with shredded wood pallets. In the same study, microbial or- ganisms immobilized up to 83% of the total pool of N in the ©2009 International Society of Arboriculture soil. In general, organic matter increases soil labile N through leaching and decomposition (Lambers et al. 1998; Aerts and Chapin 2000). During decomposition, decaying plant mate- rial is broken down and incorporated in the soil as particulate organic matter (POM). Nitrogen is released from POM as sol- uble organic N. Then, soil microorganisms may mineralize the dissolved organic N to ammonia (NH4 +), which may be further Figure 1. Conceptual model illustrating the effect of organic mulches on plant nitrogen (N) and carbon (C) uptake, and C allo- cation. White circles represent main inputs and transitional forms of C in the mode. Black squares represent processes associated with soil N availability and soil environmental conditions. White squares represent plant processes associated with resource acquisition. Gray squares represent plant processes associated with photosynthate allocation. soils after mulching with shredded wood pallets. These results support the study of Sønsteby et al. (2004) where the amount of both ammonia and nitrate, were lower on mulched soil with bark chips (C:N ~135:1) compared to nonmulched soils. The association between organic matter C:N ratio and the amount of N released during decomposition can be explained us- ing a metabolic approach. Overall, soil microorganisms require one atom of N for the consumption of a substrate containing 20 atoms of C (Davet 2004). During decomposition of organic matter with C:N ratio higher than 20, microorganisms have to supplement their N demand by absorbing N from the soil solu- tion (i.e., immobilization), which in many cases results in com- petition with surrounding plants. In contrast, when the organic matter C:N ratio is lower than 20, excess N is excreted by the soil microorganisms in the form of inorganic N (i.e. mineral- ization) becoming available for the plants (Lambers et al. 1998; Davet 2004). This proportion of C to N can vary between 20 and 35 depending on the soil microbial community (Smith 1982). Besides mulch chemical composition, soil conditions can also influence decomposition rates and consequently soil N availabil- ity. Microbial biomass and activity are sensitive to changes of soil
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