Читать книгу North American Agroforestry - Группа авторов - Страница 94

As in the case of conventional agriculture, nutrients can often be limiting in agroforestry systems. Therefore, many agroforestry systems are subject to fertilization, which is most commonly done at the level needed for the crop component to maintain high growth and productivity. Without fertilization, inter‐ and intraspecific competition for nutrients will be high and there will likely be a decrease in crop yields with time (Jose, Gillespie, Seifert, Mengel, & Pope, 2000).

Allen et al. (2004b) observed competition for N in a pecan–cotton alley‐cropping system in northwestern Florida, where cotton plants in a barrier treatment had a 59% higher aboveground biomass than plants in a non‐barrier treatment. Although a companion study indicated that competition for water was also a factor (Wanvestraut et al., 2004), the researchers hypothesized that because pecan trees leaf out earlier in the spring and have a high nutrient demand early in the growing season, the soil N was depleted before the cotton plants were established later in the growing season. Therefore, in this particular system, cotton plants are more likely to rely on fertilizer N to fulfill plant needs (Allen et al., 2004b). In addition, in a companion study, Allen et al. (2005) showed that N mineralization rates differed between barrier and non‐barrier treatments in this pecan–cotton alley‐cropping system. Higher rates of N mineralization were observed in the non‐barrier treatment (26.05 mg kg⁻¹ mo⁻¹) than the rates observed in the barrier treatment (19.78 mg kg⁻¹ mo⁻¹), indicating that competitive interactions for water and N in the non‐barrier treatment may have led to a decreased ability of the cotton plants to take up N (Allen et al., 2005).

Table 4–4. Comparison of height, diameter, and stem volume index of wild cherry and hybrid walnut trees 6 yr after planting for two treatments, intercropped or monoculture with weed control only (modified from Chifflot et al., 2006).

Parameter	Species	Treatment
Intercrop	Monoculture
Height (H), cm	wild cherry	522 a	470 b
hybrid walnut	436 a	332 b
Diameter (D), cm	wild cherry	9.2 a	447.3 b
hybrid walnut	47.8 a	444.7 b
Stem volume index (D2H), 103 cm3	wild cherry	447.2 a	431.6 b
hybrid walnut	431.3 a	448.9 b

Note. Means followed by different letters are significantly different at α = 0.05 for each row.

If water is not a limiting factor, trees also benefit from increased nutrient availability in agroforestry systems. For example, Chifflot, Bertoni, Cabanettes, and Gavaland (2006) showed that 6 yr after planting, wild cherry (Prunus avium L.) and hybrid walnut trees (J. nigra × J. regia) grown in an intercropped agroforestry system with unirrigated cereal crops in France were significantly larger (Table 4–4) and had significantly greater N content and concentration (walnut hybrid only) than trees grown in a traditional plantation with weed control (Figure 4–8). They concluded that the trees benefited from N fertilization that was applied to the cropped alleyways.