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Abstract

Environmental impacts of conventional agricultural practices - especially the contribution to CO2 emissions and nitrate losses to water bodies - have intensified, and with it the need for studying carbon and nitrogen cycling in soils. In North America and Europe agricultural management practices of specialisation and intensification have resulted in soil degradation including loss of organic matter and consequent increase in CO2 emissions, and the leaching of nutrients, notably nitrate. Mass balance calculations for carbon and nitrogen in agroecosystems can provide information about their turnover in relation to crop demand and potential losses to the environment. Such calculations may be used to analyse the influence of different farm management practices on C and N dynamics of agricultural systems. The case study employed in this research were typical dairy farms in British Columbia (B.C.), Canada and Austria. The dairy farms - a conventionally managed farm in B.C., a conventionally managed farm in Austria, and an organically managed farm in Austria - were selected as these are comparable in terms of climatic conditions and soil materials. The field scale carbon balances are calculated as the difference between C additions to, and C removals from, the soil root zone. Input organic matter flows are root and plant residues as well as manure additions. Decomposition of new (root and plant residues, manure) and old organic matter as well as leaching of dissolved carbon are considered to be output flows. The field scale nitrogen balances are calculated in the same way: N additions are assumed to come from manure, inorganic fertiliser, biological N fixation and atmospheric sources. N removal is by crop harvest and denitrification. Results for the C balances of the Austrian farms show that organic management, compared to conventional management, has a positive balance (393 kg C/ha/yr) and thus contributes to soil storage of carbon. For the Austrian conventional fields the C output exceeds the C input. The resulting negative balance (-123 kg C/ha/yr) indicates net CO2 emissions. The C balances comparison of the conventional farms (B.C. vs. Austria) show, that farm management in B.C. causes more net CO2 emissions (-364 vs. -123 kg C/ha/yr, respectively) and this is attributed to more intensive management. Sensitivity analyses for the carbon balance model show that the deciding factors are associated with high uncertainties. Thus goals for future research are (i) to improve further the estimates of CO2 release due to organic matter decomposition, and (ii) to understand how climatic changes influence changes in soil organic matter. Results for the N balances of the Austrian farms show that organic management, compared to conventional management, has a negative N balance (-43 vs. 1 kg N/ha/yr, respectively). The N balances comparison of the conventional study farms (B.C. vs. Austria) show that for the B.C. farm management the N input is significantly exceeding the N output. In contrast, in the Austrian fields, input and output flows are balanced. Thus it can be assumed that the Austrian farm management does not contribute significantly to nitrate pollution of surface and ground -waters. Sensitivity analyses for the N balance model show that the deciding factors (mineral fertiliser rate, plant N removal, and livestock density) are associated with small uncertainties. However, a major uncertainty is atmospheric deposition of ammonia. This requires future research. Nitrogen fertilisation must be more closely adjusted to crop requirements (Austrian conventional farm) and suited to the environmental conditions to prevent nitrogen losses. As organic dairy farming has a higher C- and N-efficiency than conventional dairy farming, the possibilities to reduce C- and N-loss by conversion to organic dairy production appear to be promising. [Scientific formulae used in this abstract could not be reproduced.]