Uso da abundância natural de 15N em estudos com fertilizantes orgânicos

Abstract

Variations in the 15N natural abundance (δ15N) in organic fertilizers (e.g. +5 to +20 ‰) differ from those of synthetic N-fertilizers (−5 or +5 ‰) allowing for several applications in agronomy and soil science. The objective was to apply the natural abundance of 15N to study nitrogen (N) loss in composting and the use of N from organic fertilizers. The hypotheses were if the δ15N value is an indicator for: (i) N loss during composting, (ii) N use efficiency from organic fertilizers, and (iii) differentiation between ‘organic’ and conventional vegetables. The increase of δ15N (2.31 ‰) was used to estimate the N loss (10.5 to 17.2 ‰) from composting of manure and lettuce leaves in bioreactors (3 L), and showed similar estimates and coefficient of variation (~30 %) to mass balance calculation. The equation proposed for general use is: Nlost = (δ15Nfinal − δ15Ninitial) ÷ 0.96. Organic and organo-mineral fertilizers with vegetal feedstock (peat and sugarcane) showed lower values of δ15N (< +8.0 ‰) than manure and biosolids (+8.1 to 14.9 ‰). However, addition of synthetic N-fertilizers may decrease the δ15N of the end product. There were significant differences between (p > 0.05) the δ15N organic (+14.3 ± 1.0 ‰), conventional (+8.5 ± 2.7 ‰), and hydroponic (+4.5 ± 0.2 ‰) lettuces. However, there was no difference between the conventional system and the organic that do not apply manure (+9.2 ± 1.1 ‰). The comparison between δ15N of soil organic inputs and lettuce allowed us to infer which was the main N source in each type of farming. δ15N and yield of carrot in response to increasing levels of compost (0 to 2.5 kg m−², 14 g N kg−1 , dry weight) showed positive and significant correlation (root, R = 0.88, P = 0.02; shoot, R = 0.76, P = 0.08). The N recovery (%NRec) varied from 4 to 9 %, where %NRec = [(δplanta+ – δ planta−) ÷ (δcompost – δsoil] x [Nharvested ÷ Napplied] and δplanta+ – δplanta− is the variation of δ-plant with and without compost. Therefore, δ15N values allowed qualitative (marker for systems of production / fertilization regime), semi-quantitative (indicator for the main N source) and quantitative (estimator for N loss and N recovery) approaches in studies with organic fertilizers. orgânicas (+ 14,3 ± 1,0 ‰), convencionais (+ 8,5 ± 2,7 ‰) e hidropônicas (+4,5 ± 0,2 ‰). No entanto, não houve diferença entre o sistema convencional e o orgânico que não aplica esterco (δ15N = 9,2 ± 1,1 ‰). A comparação entre o δ15N do solo dos insumos orgânicos e da alface permitiu inferir qual foi a principal fonte de N em cada sistema de produção. O δ15N e a produtividade da cenoura em resposta às doses crescentes de composto (0 a 2,5 kg m−², 14 g N kg−1, base seca) apresentaram correlação positiva e significativa (raíz, R = 0,88, P = 0,02; parte aérea, R = 0,76; P = 0,08). A recuperação de N (%NRec) variou de 4 a 9 %, onde %NRec = [(δplanta+ – δplanta−) ÷ (δcomposto – δ olo] x [Ncolhido ÷ Naplicado] e, δ planta+ – δ planta− é a variação do δ da planta com e sem composto. Portanto, o δ15N permitiu abordagens qualitativas (marcador de sistemas de produção / diferentes fertilizantes), semi-quantitativas (indicador da principal fonte de N) e quantitativas (estimador de perda e recuperação de N) em estudos com fertilizantes orgânicos.