Produção de azoto mineral durante a compostagem de fracção sólida de chorume da pecuária leiteira intensiva

Production of mineral nitrogen during composting of dairy cattle slurry solid fraction

L. M. Brito¹, I. Mourão¹, H. Trindade² & J. Coutinho²

 

RESUMO

A fracção sólida do chorume (FSC) de duas explorações de pecuária leiteira intensiva foi compostada: (i) em 2004, sobre o solo, incluindo, ou não, palha, e com diferentes níveis de revolvimento; e (ii) em 2005, sobre uma tela de cobertura do solo, com e sem revolvimento, com diferentes coberturas e diferentes volumes. O teor de matéria seca (MS) da FSC da primeira exploração (30%) apresentou um valor superior, embora o rendimento da máquina separadora da FSC tenha sido inferior (1 m³ hora^-1) relativamente ao observado na segunda exploração (4 e 3 m³ hora^-1 correspondendo a 22% e 24% de MS, respectivamente, em 2004 e 2005). A FSC1 com 30% de MS atingiu elevadas temperaturas logo após a sua separação mas o mesmo não aconteceu com a FSC2 com menor teor de MS, particularmente quando foi compostada sem palha. O pH da FSC variou entre 8 e 9 durante a compostagem e a razão C/N foi semelhante entre tratamentos para o mesmo período de compostagem.

O teor de N-NH4⁺ foi muito elevado durante a fase termófila da compostagem, após a qual diminuiu, acentuadamente, enquanto que o teor de N-NO3^-foi mínimo naquela fase e aumentou após três meses de compostagem. O maior teor de MS inicial da FSC1 em comparação com a FSC2 e a mistura com palha associaram-se a concentrações mais baixas de azoto amoniacal (NNH4⁺). Considerando estas observações e, ainda, que as elevadas temperaturas durante a compostagem e o pH alcalino da FSC potenciam, eventualmente, a volatilização de amoníaco, a redução deste tipo de perdas de N poderá ser conseguida: (i) diminuindo o rendimento da máquina separadora da FSC; (ii) misturando a FSC com um material com elevada razão C/N; e (iii) diminuindo o número de revolvimentos.

 

ABSTRACT

Screw pressed dairy cattle slurry solid fraction (CSSF) was collected during 2004 and 2005 from two dairy farms with 30% and 22% -24% dry matter (DM) content and the effects of covered either with black polyethylene or with polypropylene, pile dimension, straw addition and of turning frequency on the fate of N were examined. The CSSF with 30% DM was collected at a rate of 1 m³ h^-1 and the material with 22% 24% DM was collected at the rate of 4 and 3 3 _h-1 min 2004 and 2005, respectively. Thermophilic temperatures were attained soon after separation of CSSF with 30% DM. In contrast, temperatures were much cooler in CSSF with 22% DM without straw. The pH ranged from 8 to 9 and C/N ratio was similar amongst treatments on each sampling occasion.

Mineral N production was characterized - by high NH4⁺ and low NO3 contents during the thermophilic phase followed by a decrease of NH4⁺ and an increase of NO3 towards the end of composting. Higher DM, and straw addition were both associated with lower NH4⁺ compost concentrations. Therefore, to minimize the N loss as NH3 gas caused by the alkaline pH during the thermophilic phase of composting, it is suggested: i) to increase DM by slowing the rate of the screw dewatering mechanism; ii) to increase the C/N ratio of composting mixture; and iii) to reduce turning frequency.

 

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¹ Escola Superior Agrária de Ponte de Lima, Refóios, 4990-706 Ponte de Lima – Tel: (+351) 258 909 740 – Fax: (+351) 258 909 779 – E-mail: miguelbrito@esa.ipvc.pt;

² CECEA, Departamento de Edafologia, Universidade de Trás-os-Montes e Alto Douro, Apartado 1013, 5001-911 Vila Real