Evaluation of methods of measurement of nitrogen in poultry and animal manures

Abstract

Kjeldahl nitrogen (N), total N and forms of inorganic N (ammoniacal (NH4)-N, nitrate (NO3)-N and nitrite (NO2)-N) were measured in a range of animal manures. The manures include fresh samples of poultry manure, sheep manure, horse manure, dairy slurry and pig slurry and composted poultry manure. Kjeldahl N was measured by standard micro-Kjeldahl digestion. For total N measurements, NO3-N and NO2-N were recovered during Kjeldahl digestion by pretreatments with various oxidizing and reducing agents. Inorganic forms of N were measured by extraction with 2M KCl solution. Kjeldahl digestion alone allowed measurement only of organic N and NH4-N. Amongst various modifications to the Kjeldahl, pretreatment with either acidified (H2SO4) Zn-CrK(SO4)2 or acidified (H2SO4) reduced Fe achieved complete recovery of NO3-N. Nitrite N was only recovered by first oxidising the NO 2 - to NO 3 - with KMnO4 followed by reduction to NH4-N with acidified (H2SO4) reduced Fe. More than 95% of the total N in fresh animal manure was present as organic N and NH4-N which were recovered by the standard Kjeldahl digestion. In the case of fresh manures there was no difference between the amount of total N measured by the Kjeldahl digestion and its modified methods. However composting of poultry manure or drying of poultry manure, pig slurry and dairy slurry resulted in an increase in NO3-N which was not recovered during Kjeldahl digestion alone. Under these conditions the total N could be measured by pretreating the samples with KMnO4 and reduced Fe prior to Kjeldahl digestion. Drying of animal manures caused a decrease in organic N and NH4-N, especially in poultry, pig and dairy manures. There was a slight increase in NO3-N; but most of the decrease in N content with drying was attributed to the volatilization loss of ammonia (NH3). Amongst various drying methods examined air drying caused maximum loss of N as NH3 whereas freeze drying caused minimum loss of N. This suggests that fresh animal manures can be freeze dried for analysis of N which causes minimum loss of N.