Читать книгу North American Agroforestry - Группа авторов - Страница 97

The incorporation of trees and crops that are able to biologically fix N₂ is fairly common and well researched in tropical agroforestry systems (Nair, Buresh, Mugendi, & Latt, 1999). In temperate systems, similar accounts of incorporating N₂–fixing trees into agroforestry are rare, perhaps because of the abundance and historically low cost of N fertilizer and the low value of N₂–fixing trees. Despite the infrequent use of biological N₂ fixation by trees in temperate agroforestry systems, there is potential for using N₂–fixing tree species native to temperate environments. Species from the genera Robinia, Prosopis, and Alnus have the potential to provide N₂ fixation benefits in temperate agroforestry systems (Boring & Swank, 1984; Seiter, Ingham, William, & Hibbs, 1995). Seiter et al. (1995) demonstrated this potential in a red alder (Alnus rubra Bong.)–maize alley‐cropping system in Oregon. They observed, using a ¹⁵N injection technique, that 32–58% of the total N in maize was obtained from N₂ fixed by red alder and that N transfer increased by shortening the distance between the trees and crops.

There are also several leguminous herbaceous plant species capable of fixing atmospheric N₂ in temperate agroforestry systems, including alfalfa, clover, hairy vetch (Vicia villosa Roth), and soybean (Troeh & Thompson, 1993). Although multiple studies have incorporated leguminous herbaceous species capable of biological N₂ fixation into temperate agroforestry systems (Alley et al., 1998; Delate et al., 2005; Gakis et al., 2004; Silva‐Pando, Gonzalez‐Hernandez, & Rozados‐Lorenzo, 2002), few studies have actually quantified the effects that these species have on soil N (Dupraz et al., 1998; Waring & Snowdon, 1985). Nitrogen buildup in the soil is possible from leguminous herbaceous understory species; however, this is a slow process that does not occur immediately after herbaceous plant establishment. In a radiata pine–subterranean clover (Trifolium subterraneum L.) silvopasture in Australia, Waring and Snowdon (1985) observed a 36% increase in soil N at the end of seven growing seasons in the silvopasture, which corresponded to a 14% increase in tree diameter compared with pines growing in a monoculture without a subterranean clover understory.