Selection Criteria for Hedgerow Species

Litter decomposition and nutrient release patterns
Nutrient release through decomposition of hedgerow biomass can be synchronized with the peak period of crop demand by regulating the quality, quantity, timing and method of application of tree prunings as green manure or mulch (Nair 1993, Palm 1995). Studies of nutrient release patterns have mainly focused on nitrogen in leguminous species. Prunings from various leguminous trees used in alley cropping systems differ in their decomposition dynamics -- the other component of litter quality -- which is determined in a large part by their chemical composition. Plant materials with high %N (> 1.73) and low C/N ratio (< 20) decompose rapidly and release relatively larger quantities of mineralized nitrogen available for crop uptake (Palm 1995). On the other hand, materials of high C/N ratio tend to temporarily immobilize nitrogen because microbes utilizing the additional carbon source multiply rapidly and draw upon the nutrient reserve from the soil (Nair 1993). Nitrogen incorporated in microbial tissues is gradually released subsequently when the microbial population declines.

Prunings of many of the woody nitrogen-fixing legumes used in alley cropping are generally high in nitrogen (Nair 1993), but the rates of decomposition and nutrient release vary widely and are not necessarily rapid. In a field study by Budelman (1988) in Côte d'Ivoire, the half-life values of fresh leaf biomass of Leucaena leucocephala (C/N = 12), Gliricidia sepium (C/N=12) and Flemingia macrophylla (C/N=21) were found to be 31, 22 and 53 days respectively. In a review of laboratory incubation studies by Palm (1995), G. sepium consistently released a higher percentage (30-70%) of its nitrogen compared to other nitrogen fixing legumes such as Calliandra calothrysus and Inga edulis, which released less than 20% of nitrogen after eight weeks. Palm (1995) suggested that the polyphenol to nitrogen (PP/N) ratio may serve as an index for short term immobilization patterns observed for legumes with relatively high polyphenol content. Laboratory incubation and field studies found that legume leaves with high soluble polyphenols contents (Inga edulis and Cajanus cajan) decomposed and released nitrogen less rapidly than those with low polyphenol contents (Erythrina sp.) (Palm and Sanchez 1990, 1991). The lignin plus polyphenol to nitrogen ratio ((LIG+PP)/N) may serve as an index for longer term release patterns (Palm 1995). Woody stems and other lignified plant materials are more resistant to decomposition.

Despite the quantitative importance of this transfer of nutrients from the hedgerow biomass to soils in the alleys, uptake by the crops of the nutrients released from the mulch is often quite low. Recovery values are generally less than 20% and more frequently closer to 10% (Palm 1995). By labeling the nitrogen in the mulch (from hedgerow G. sepium and Erythrina poeppigiana) with the isotope 15N, Haggar et al. (1993) found that only about 10% of the total nitrogen taken up by alley-cropped maize during the cropping season came from the most recently applied mulch. The majority of the nitrogen ends up in some readily mineralized fraction of SOM, and the high rates of mineralization seen in the alley crop represented the residual effect of the previous years of mulch application. Haggar et al. (1993) suggested that the benefit of leguminous biomass inputs to crop production is through the long-term buildup of soil organic nitrogen rather than the direct release of nitrogen from the decomposing mulch.

Other possible reasons for the low nutrient recovery rates in the crops include insufficient synchrony of nutrient release with crop uptake, leaching and volatilization of nitrogen, and re-absorption of the nutrients released by hedgerow trees (Fernandes et al. 1993a, Kang and Mulongoy 1992). Annual crops have well-defined critical periods of high nutrient demand. The efficiency of nutrient use can be enhanced if nutrients are made available during this time, which can also reduce nutrient loss to leaching and to hedgerow trees. Better synchrony and nutrient use efficiency can be accomplished through management decisions such as selecting species with differing rates of litter decomposition; adjusting the timing of pruning and addition of tree biomass to soil; and modifying the method of tree biomass application (surface addition as mulch or soil incorporation as plowed-in green manure) (Nair 1993).

Introduction | Definition | Hedgerow Species I Species Selection Criteria | Soil Organic Matter & Nutrients | System Management | Crop Yields | Soil Conservation | Weed Dynamics | Tree-Crop Competition | References

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Funded by a grant from the Cornell Agroforestry Working Group (CAWG) and the Distance Learning Program of the Cornell International Institute for Food, Agriculture and Development (CIIFAD).