Decomposition and nutrient release of green manure biomass in a passion fruit orchard in the brazilian semiarid region.

Green manure decomposition and nutrient recycling improve soil quality and productivity of different crops, but information on irrigated fruit orchards in the Brazilian semi-arid region is still scarce. Decomposition and nitrogen, phosphate, and potassium release from the cut biomass of three green manure legumes (sunn hemp, pigeon pea and jack bean) placed in litterbags, and spontaneous vegetation grown for 90 days in the rows of a passion fruit orchard were followed for 150 days. Biomasses decreased exponentially, reaching 12 (sunn hemp) to 25% (jack beans and spontaneous vegetation) after 150 days. K was rapidly released (< 21 and 4% of the original content remaining after 7 and 150 days, respectively), contrasting with more than half of the P and one third of the N remaining after 150 days. The amounts released were more influenced by the amounts of biomass produced (sunn hemp, 1583); (Jack bean 5152 kg ha-1); (Pigeon pea 822 kg ha-1); (Spontaneous plants 3175 kg ha-1); (spontaneous legumes 744 kg ha-1) than by variation in decomposition proportions among species. N release represented a liquid input to the soil, since more than 80% of the green manure and spontaneous vegetation contents came from N2-symbiotic fixation. Therefore, green manure is an effective technique to incorporate N and recycle K and P in irrigated orchards in the Brazilian semi-arid region.

(15)N natural abundance of non-fixing woody species in the Brazilian dry forest (caatinga).

Foliar delta(15)N values are useful to calculate N(2) fixation and N losses from ecosystems. However, a definite pattern among vegetation types is not recognised and few data are available for semi-arid areas. We sampled four sites in the Brazilian caatinga, along a water availability gradient. Sites with lower annual rainfall (700 mm) but more uniform distribution (six months) had delta(15)N values of 9.4 and 10.1 per thousand, among the highest already reported, and significantly greater than those (6.5 and 6.3 per thousand) of sites with higher rainfall (800 mm) but less uniform distribution (three months). There were no significant differences at each site among species or between non-fixing legume and non-legume species, in spite of the higher N content of the first group. Therefore, they constitute ideal reference plants in estimations of legume N(2) fixation. The higher values could result from higher losses of (15)N depleted gases or lower losses of enriched (15)N material.