Medium‐term effects of poultry manure on pine nitrogen uptake in a ¹⁵N‐labeled burnt soil^§

Abstract

The effects of poultry manure (PM), used for the reclamation of a 15 N-labeled burnt soil, on N nutrition of pine seedlings were evaluated during one year in a pot experiment. Six treatments were used: 15 N-labeled soil (LS), 15 N-labeled burnt soil (BLS), and BLS+PM at doses equivalent to 1, 2, 4, and 8 t ha -1 of dry PM (PM1, PM2, PM4, and PM8, respectively). Either in the whole tree or the different organs, N concentration (1) decreased (p ≤ 0.05) in the order LS > BLS, BLS+PM1, BLS+PM2, BLS+PM4 > BLS+PM8 and (2) was negatively correlated with phytomass production (p < 0.05 to p < 0.01). The two highest amounts of assimilated N per kg dry soil were found in LS and BLS (130-134 mg) and the lowest in BLS+PM8 (87 mg), the other treatments being in an intermediate range (108-115 mg). Irrespective of the soil treatment, 56%-66% of the pine-N were accounted for by needles, 29%-32% by roots, and 8%-12% by stems, the differences among organs being always significant (p < 0.05). The percentage of pine-N derived from PM (%PNDFM) increased steadily with PM dose, from 1.7% in BLS+PM1 to 13.3% in BLS+PM8, reaching values for the two highest PM doses within the range found for 15 N mineral fertilizers in forests. From 8.2% to 16.9% of the PM-N was assimilated by the pines. Although differences among treatments were not significant, the two highest values were found in BLS+PM4 and BLS+PM8. Therefore, the 15 N data showed clearly that there is a positive medium-term effect of PM on pine N nutrition during the reafforestation phase of burnt-forest reclamation. The lower total N uptake by pine seedlings in the PM treatments was explained in terms of lower levels of soil available N due to its exportation with the phytomass of the preceding ryegrass culture, used for the early protection phase in the burnt soil-reclamation procedure.