NITROGEN DYNAMICS IN INTEGRATED AGRICULTURAL SYSTEMS IN CENTRAL MEXICO
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Smallholdings (<5 ha) represent 73% of the total agricultural production systems in Mexico. Many are of low productivity, and little quantitative data has been published on the impact of traditional nitrogen (N) management practices on environmental performance at farm scale. Nitrogen is the most important nutrient for increasing crop yields. While the benefits from the use of N fertilizer are self evident, it has resulted in low efficiency of N utilization and environmental problems. This project aimed to assess N dynamics in Integrated Agricultural Systems (lASs) in central Mexico. Nitrogen inputs, outputs and internal transfers were assessed and the impact of selected management practices on nitrogen flow, productivity and environmental performance were analyzed using nitrogen use efficiency (NUE) indices. A predictive framework tool was developed following whole farm methodologies to quantify N flow, and to assess selected (NUE) indices such as 0/1 ratio, N loss, accumulated N, and the change of N in the soil pool at farm scale. Data used for the development of the predictive framework was derived from experimental evaluations of ammonia emissions during vermicomposting, and integrated with data from the literature, together with data from an integrated agricultural systems prototype in Montecillo Mexico. The main N inputs into integrated agricultural systems in the Texcoco region were estimated to be in the following order of significance: biological N fixation (11 - 532), manure (15 - 225), fertilizer (0 - 140) and rainfall water (30 - 35 kg haˉ¹ aˉ¹). The main N outputs were: N losses (70 - 528 kg haˉ¹ aˉ¹) and exported N in marketable products (72 - 338 kg haˉ¹ aˉ¹). Experimentally derived estimates of ammonia emissions during vermicomposting of sheep manure ranged from 10 - 15 kg Mgˉ¹ DM which corresponded to 42 - 47% of the initial N content in the manure substrate. The impact of current management practices showed 0/1 ratios ranging from 0.30 - 0. 76, N losses ranged from 76 to 210 kg haˉ¹ aˉ¹ and N depletion/ accumulation ranged from -25 - 143 kg haˉ¹ aˉ¹. The effect of selected management practices on 0/1 ratio was as follows: crop sequence > stocking density > livestock type > manure management. Whereas for N loss it was stocking density > livestock type > manure management > crop sequence. Higher productivity together with a more benign environmental impact could be attained in smallholder by implementing simple recommended manure management practices at farm scale.
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