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The decomposition and the associated nitrogen (N) dynamics of organic N sources are affected by their contact with soil. While several authors have examined the effect of surface application or incorporation of crop residues on their decomposition rate, less information is available about the relationship between the placement of animal manure and their N mineralization rate. This study investigated the influence of chicken manure and cattle manure placement on soil N mineralization. The manures were incorporated or surface applied at 175 mg N kg −1 , and N release was periodically determined over 56 days by measuring inorganic N [nitrate (NO 3 − ) N and ammonium (NH 4 + ) N] in a 2 M potassium chloride (KCl) extract at a ratio of 1:10 (w/v). Results indicated that the control soil released a maximum of 64 mg N kg −1 soil at day 21, a sixfold increase over the initial concentration, which indicates its substantial mineralization potential. Manure treatments showed an initial increase in net NO 3 − -N content at the start of the experiments (until day 7) before an extended period of immobilization, which ended at day 21 of the incubation. A small but positive net N mineralization of all manures was observed from 28 days of incubation. At each sampling time, the mean mineral N released from the control was significantly less (P < 0.01) than surface-applied chicken manure, incorporated chicken manure, and surface-applied cattle manure. Treatments exceptions were at days 21 and 28 where N immobilization was at its peak. In contrast, incorporation of cattle manure showed a different N-release pattern, whereby the mineral N amount was only significantly greater than the control soil at days 42 and 56 with 84 and 108 mg N kg −1 soil respectively. Incorporation of chicken manure and cattle manure did not favor nitrification as much as surface application and cattle manure caused a much greater immobilization when incorporated than when surface applied.

Nitrogen (N) nutrition is a key factor for vegetable growth and yield. However, different rates of nitrogen fertilization may trigger different responses to vegetables. A survey was conducted to investigate the effect of soil fertility management practices on nitrate concentration in vegetables. The survey results were used to plan experiments on the effect of chicken and cattle manures on nitrate levels in Chinese cabbage (Brassica rapa) and amaranthus (Amaranthus cruentus) grown in Tanzania and the patterns of mineral nitrogen in soils under open field conditions. Chicken or cattle manure at 200, 300 kg N ha −1 and 170 250 kg N ha −1 for Chinese cabbage and amaranthus respectively, and control were compared in a randomized complete block design. We observed a higher nitrate content in fertilized Chinese cabbage at day 30 than at day 44 after sowing, ranging from 3243 to 4993 mg kg −1 fresh matter regardless of the N source and rates. Only application of manures at high levels (250 kg N ha −1 ) induced significantly (p < 0.05) higher nitrate contents in amaranthus at day 28 after sowing, although there was a clear indication of nitrate accumulation even at 170 kg N ha −1 application. Soil NH 4 + -N + NO 3 − -N in both Chinese cabbage and amaranthus plots were increased with increasing N application rates and differences between control and amended soils were significant (p < 0.01). There was a positive relationship between NO 3 − concentration in vegetables and NO 3 − -N in the rooted top soil layer (0-15 cm). However, higher NH 4 + concentrations depressed NO 3 − build up in crops and a significant negative relationship between soil (NH 4 + -N)/(NO 3 − -N) ratio and crop NO 3 − content was found. It is concluded that low manure application rates result in similar yields to high rates but reduces nitrate accumulation in vegetables and excess mineral nitrogen in soils.

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