Читать книгу North American Agroforestry - Группа авторов - Страница 95

In addition to increased competition, the mixing of multiple species in a single system can potentially bring about other negative effects. One of those effects is allelopathy, which is caused by allelochemicals that are produced by some plant species and released into the soil by root exudation and aboveground litterfall. Allelochemicals have been documented to affect germination, growth, development, distribution, and reproduction of numerous plant species (Inderjit & Mallik, 2002). However, production rates of these chemicals in a given system depend on a variety of factors, including age of the species, density of the species, and the time of year. These factors, in combination with the residence time of the chemicals in the soil and plant resistance and/or tolerance to the chemicals, play a role in the degree to which these chemicals inhibit growth. It should also be noted that under certain soil conditions, such as low soil water regimes, these chemicals may be oxidized into a nontoxic form (Fisher, 1978).

Fig. 4–8. Average leaf N (a) concentrations and (b) amounts in wild cherry and hybrid walnut trees 6 yr after planting with unirrigated cereal crops (intercropping) or in traditional monoculture plantations (control). The error bars show the 95% confidence intervals of the measured means

(adapted from Chifflot et al., 2006).

There are several examples of allelopathy in temperate agroforestry systems (Geyer & Fick, 2015; Jose & Gillespie, 1998; Jose & Holzmueller, 2008; Thevathasan, Gordon, & Voroney, 1998) and most of them involve black walnut, a species that produces a phenolic compound called juglone (5‐hydroxy‐1, 4‐naphthoquinone) that restricts the growth of some other species. Although Jose, Gillespie, Seifert, and Biehle (2000) documented that competition for water was the leading factor in reduced growth of alley‐grown maize in a black walnut–maize alley‐cropping system in Indiana, a companion study (Jose & Gillespie, 1998) also indicated the possibility of juglone phytotoxicity. Jose and Holzmueller (2007) reported sensitivity of cotton (Gossypium sp.) and peanut (Arachis hypogaea L.) when exposed to juglone in hydroponic cultures. These are two common species with potential for alley cropping with pecan in the southern United States, and pecan also produces juglone (Jose, 2002). However, some species do not appear to be as susceptible to juglone (Geyer & Fick, 2015), and this should be taken into consideration when developing an agroforestry system.

It has also been reported that certain crop species may induce allelopathic effects on trees as well, including a decrease in development and growth (Smith, Wolf, Cheary, & Carroll, 2001; Todhunter & Beineke, 1979). In a study of containerized pecan trees, Smith et al. (2001) showed that allelochemical‐containing leachates added to the containers from bermudagrass [Cynodon dactylon (L.) Pers.], cutleaf evening primrose (Oenothera laciniata Hill.), and tall fescue [Schedonorus phoenix (Scop.) Holub.] decreased pecan root weight by 17%, trunk weight by 22%, and total tree dry weight by 19% compared with the control treatment.

Management techniques to reduce the effects of allelopathy have also been examined in agroforestry systems. Jose, Gillespie, Seifert, and Biehle (2000) demonstrated that by separating the root systems of black walnut and maize using a polyethylene barrier, crop yield became similar to that of a monoculture. They further showed that the juglone concentration in the soil was negligible beyond the polyethylene barrier. Juglone concentration beyond the root barrier decreased to trace levels of 0.08 and 0.01 μg g⁻¹ soil (at distances of 2.45 and 4.25 m, respectively) in the barrier treatment compared with 0.42 and 0.32 μg g⁻¹ soil in the non‐barrier control treatment.

A comprehensive study examining field soil juglone concentrations, sorption mechanisms, juglone production rates, and degradation rates and products (Von Kiparski, Lee, & Gillespie, 2007) showed that juglone can accumulate under field conditions, with release rates from black walnut being greater than abiotic and microbial transformation rates. In a 19‐yr‐old walnut plantation, surface soil pore water juglone concentrations approached but did not exceed the inhibition solution thresholds of typical intercrops. But substantially higher levels of juglone can be reversibly sorbed by soils, and true plant impacts may be a balance of responses to multiple stress conditions in the mixed systems. From greenhouse studies, it was determined that substantial quantities of juglone can be released into the rhizosphere, and so rooting patterns of intercrops will be of particular concern when judging allelopathic potential. However, soil chemistry will play a role in these intercrops, as this study showed that microbial activity will quickly degrade juglone and decrease persistence. Soils low in microbial activity, including subsurface horizons and acidic soils that are low in organic C and fertility, can accumulate juglone, and thus this negative interaction among interplanted species should continue to be considered in walnut and pecan agroforestry systems.