Canopy and leaf composition drive patterns of nutrient release from pruning residues in a coffee agroforest

In a coffee agroforest, the crop is cultivated under the shade of fruit-bearing and nitrogen (N)-fixing trees. These trees are periodically pruned to promote flowering and fruiting as well as to make nutrients stored in tree biomass available to plants. We investigated the effect of canopy composition and substrate quality on decomposition rates and patterns of nutrient release from pruning residues in a coffee agroforest located in Costa Rica's Central Valley. Initial phosphorus (P) release was enhanced under a canopy composed solely of N-fixing, Erythrina poeppigiana compared to a mixed canopy of Erythrina and Musa acuminata (banana). Both initial and final N release were similar under the two canopy types. However, after five months of decomposition, a higher proportion of initial N had been released under the single canopy. Although patterns of decomposition and nutrient release were not predicted by initial substrate quality, mass loss in leaf mixtures rates were well predicted by mean mass loss of their component species. This study identifies specific pruning regimes that may regulate N and P release during crucial growth periods, and it suggests that strategic pruning can enhance nutrient availability. For example, during the onset of rapid fruit growth, a two-species mixture may release more P than a three-species mixture. However, by the time of the harvest, the two- and three-species mixtures have released roughly the same amount of N and P. These nutrients do not always follow the same pattern, as N release can be maximized in single-species substrates, while P release is often facilitated in species mixtures. Our study indicates the importance of management practices in mediating patterns of nutrient release. Future research should investigate how canopy composition and farm management can also mediate on-farm nutrient losses.     

Maternal provisioning of sequestered defensive steroids by the Asian snake <Emphasis Type="Italic">Rhabdophis tigrinus</Emphasis>

Summary.   Rhabdophis tigrinus obtains defensive steroids (bufadienolides) from its diet and sequesters those compounds in specialized structures on its neck known as nuchal glands. Hatchling snakes lacking these steroids must acquire them from toads consumed as prey. Here we show that females provision bufadienolides to their offspring in amounts correlated to the quantity in their own nuchal glands; thus, chemically protected mothers produce defended offspring. Bufadienolides can be provisioned to embryos via deposition in yolk and by transfer across the egg membranes within the oviducts. Maternally provisioned bufadienolides persist in the nuchal glands of juvenile snakes from the time of hatching in late summer until the following spring, when toads of ingestible size become abundant. Therefore, maternal provisioning may provide chemical protection from predators for young R. tigrinus in the absence of dietary sources of bufadienolides.