Open access publications in the SLU publication database

Abstract

The aim of this thesis was to study selected processes of N fluxes in Swedish and Nigerian agricultural systems and relate them to the use of mineral and organic amendments and other agricultural management practices. Long-term effects of crop rotation, crop residue treatment and mineral fertiliser application levels on topsoil carbon (C), plant N uptake, net N mineralisation and soil organic matter fractions in temperate soils were investigated using soils from Swedish long-term agricultural field experiments (16-40 years). Topsoil carbon (C) was measured and plant N uptake was estimated in a pot experiment using soils (0-30 cm) from six locations in Sweden: Fjärdingslöv, Lönnstorp, Lanna, Bjertorp, Högåsa and Röbäcksdalen. Topsoil C content was significantly affected by crop rotation (ley > cereals), although crop residue return had no significant effect. Plant N uptake was up to 50% higher in soils from crop rotations with long-term leys than cereals. Net N mineralisation and microbial respiration was measured in an incubation study using soils from four locations: Lönnstorp, Lanna, Bjertorp, Röbäcksdalen. Net N mineralisation was greater in soils from ley crop rotations. Crop residue return had no effect on net N mineralisation. Soils from two locations (Lönnstorp and Lanna) were size-density fractionated. Preliminary results indicate that an unexpectedly high fraction of soil C and N in the low-input soils was found in fine fractions not associated with minerals. Abundance and mobilisation of non-exchangeable ammonium (NEA) in soils (0-30 cm) from five locations of the long-term experiments in Sweden was estimated. NEA contents in these Swedish soils ranged from 20-200 ppm and increased with clay content. NEA content decreased over 168 days by up to 60% during a pot experiment where ryegrass was grown, suggesting that NEA partly can become plant-available during the cropping season. The potential for in situ produced legume organic matter (OM) was tested in a dry season legume fallow rotation in a field experiment in Ibadan, Nigeria. Five legumes (Pueraria phaseoloides, Vigna unguiculata, Glycine max, Mucuna pruriens and Cajanus cajan) were evaluated in terms of dry season biomass production, effect on following maize crop and fertiliser yield increase. Biomass production ranged from 1000-8300 kg ha-1, which was generally greater than that of the natural fallow. Maize yields, which ranged between 1000-2600 kg ha-1, increased with 10-35% after addition of the in situ produced legume OM. Depending on legume placement (early incorporation, surface application and late incorporation) maize grain yield increased with up to 25 kg for every kg of fertiliser added. N2O fluxes were measured in situ in a legume-maize crop rotation in a field trial in Ibadan, Nigeria, where OM was incorporated prior to maize planting. Fluxes were measured during two rainy seasons and one dry season. Fluxes up to 138 µg m-2 h-1 were observed. Annual N2O-related N losses were estimated to be up to 8 kg N ha-1. The results presented here indicate that farmer management decisions, be it in Sweden or Nigeria, can significantly affect N fluxes, which will in turn affect long-term agricultural and environmental sustainability.