Agro-ecosystems have developed from mixed- and multiple-cropping systems with relatively closed N cycles to intensively managed monocultures with large N inputs in the form of commercial fertilizers. Cultivation of increasingly larger areas of land has resulted in substantial losses of soil organic matter and N. Also, the move from slash and burn agriculture to intensively ploughed systems has resulted in losses through increased erosion.
The use of N fertilizers has increased rapidly toca. 60 Tg N yr−1 (1980/81), which is equivalent to at least 40% of the N fixed biologically in all terrestrial systems and 36% more than is fixed in all croplands. On a global scale, the major losses of N from agro-ecosystems are estimated to be: harvest, 30 Tg; leaching, 2 Tg; erosion, 2–20 Tg; denitrification 1–44 Tg; and ammonia volatilization, 13–23 Tg. However, the data base is very crude and several estimates may be wrong by as much as one order of magnitude.
Additions of N fertilizers have both direct and indirect effects on soil microorganisms. The possible importance of such effects is briefly discussed and a specific example is given on long-term effects on soil microbial biomass and nitrification rates in 27-year-old cropping systems with different N additions: (i) 0 kg N ha−1 yr−1, (ii) 80 kg N ha−1 yr−1, (iii) farmyard manureca. 80 kg N ha−1 yr−1.
Few detailed N budgets exist for agro-ecosystems, despite its major importance as a limiting plant nutrient and the large losses of N from such systems. In conclusion, preliminary nitrogen budgets for four cropping systems (barley receiving 0 or 120 kg N ha−1 yr−1; meadow fescue ley with 200 kg N ha−1 yr−1 and a lucerne ley) are presented, with special attention to N flow through the soil organisms.
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Rosswall, T., Paustian, K. Cycling of nitrogen in modern agricultural systems. Plant Soil 76, 3–21 (1984). https://doi.org/10.1007/BF02205563