Читать книгу North American Agroforestry - Группа авторов - Страница 99

In conventional agricultural systems, less than half of the applied N and P fertilizer is taken up by crops (Smil, 1999, 2000). Consequently, excess fertilizer is washed away from agricultural fields via surface runoff or leached into the subsurface water supply, thus contaminating water sources and decreasing water quality (Bonilla, Muñoz, & Vauclin, 1999; Ng, Drury, Serem, Tan, & Gaynor, 2000; Tilman et al., 2002). In an agroforestry system, however, trees with deep rooting systems potentially play the role of a “safety net” by retrieving excess nutrients that have been leached below the rooting zone of agronomic crops. These nutrients are then recycled back into the system through root turnover and litterfall, increasing the nutrient use efficiency of the system (van Noordwijk et al., 1996). Additionally, because trees have a longer growing season than most agronomic crops, tree roots occupying the same rooting zone as associated agronomic crops will increase nutrient use and use efficiency in an agroforestry system by capturing nutrients before crops are planted and after crops are harvested.

Evidence supporting the safety net concept has been observed in field trials. In a pecan–cotton alley‐cropping system in northwestern Florida, Allen et al. (2004a) reported a 245% NO₃–N increase at the 0.9‐m depth when pecan roots were separated from cotton roots by a root barrier compared with the non‐barrier treatment. These researchers suggested that this indicates the trees could potentially play the role of a N safety net by taking up N fertilizer from deep in the soil profile and redepositing it on the soil surface via litterfall (Allen et al., 2004a).

The safety net concept can be applied to other nutrients in agroforestry systems as well. In a silvopastoral system in Florida, Nair, Nair, Kalmbacher, and Ezenwa (2007) monitored soil P concentrations in pastures with and without 20‐yr‐old slash pine (Pinus elliottii Engelm.) trees. They found lower concentrations of P in the soil surface horizon and at the 1.0‐m depth in pastures with trees, suggesting that silvopastoral associations enhance soil nutrient retention and limit nutrient transport in surface water. Lee, Isenhart, and Schultz (2003) documented increased nutrient removal efficiency when trees were incorporated into a riparian buffer strip placed on the border of agronomic field plots in their study in Iowa. They reported that a switchgrass (Panicum virgatum L.) and woody stem buffer removed similar amounts of sediment as a switchgrass‐only buffer, but nutrient removal was increased by >20% in the switchgrass and woody stem buffer (Table 4–5).

Table 4–5. Percentage of sediment and nutrients removed by two riparian buffer systems in a study conducted in Iowa (adapted from Lee et al., 2003).

Sediment or nutrient	Switchgrass only buffer removal	Switchgrass and woody stem buffer removal
	—————————— % ——————————
Sediment	95	97
Total N	80	94
NO3–N	62	85
Total P	78	91
PO4–P	58	80