The planet as a closed system has a finite level of resources. To sustain human populations within the limited levels of resources the earth system provides, certain boundaries cannot be crossed. Nitrogen (N) is one of the major resources which is overused. Agriculture has strong impacts on N use and N losses. Sustainable agricultural production aims at reducing unintended resource losses, while producing sufficient food for a growing population. Nitrogen use efficiency (NUE), the ratio between harvested and applied N, should therefore be increased by decreasing N losses. One major form of N losses in humid climates occurs via nitrate leaching. This thesis assesses different ways to decrease N leaching losses and increase NUE at the crop and cropping system level.
The potential of genetic improvement to extend the rooting depth of winter wheat and whether this increases deep nitrate uptake was investigated. A phenotyping experiment was carried out over two years at the RadiMax facility. At anthesis, 15N tracer was applied to the deep part of the rooting system. Deep root traits, generated from minirhizotron imaged root length data, predicted up to 24% of tracer uptake variation. Genotype and root traits combined predicted 41% and 48% of the variation in tracer uptake in 2018 and 2019 respectively. In addition, the genotype effect on tracer uptake was explained by root traits, as they significantly mediated the genotype effect. Hence, root traits from minirhizotron images can predict deep N uptake. These findings indicate a potential to breed genotypes for deep N uptake.
Conservation agriculture (CA) is a cropping system that could increase cropping system NUE. The effects of no-till, residue retention, and cover crops on soil mineral N depletion and nutrient turnover were investigated, as they may affect spatio-temporal dynamics of N supply and losses. In addition, it was tested whether cover crops, an essential part of CA, can compensate for the potentially lower crop N offtake in non-ploughed systems in regards to N leaching potential. The results showed that CA concentrated soil C and N in the uppermost soil layer. Here, microbial metabolic capacity was significantly increased in CA. Basal soil carbon respiration and N mineralization from intact soil core incubation were affected by resource stratification. The CA system showed indications of N immobilization in autumn and reduced spring mineralization, as well as higher ammonium to nitrate ratios. Cover crops reduced soil mineral N content in late autumn, whereby mitigating N leaching losses. The brassica cover crop was more efficient than the monocot. The legume-brassica cover crop mixture optimized N management, as they did not increase the leaching potential, but increased spring topsoil N contents. Tillage system affected N dynamics via changes in crop N offtake: The non-ploughed systems reduced grain N yield by 16%.
The increased N leaching potential in the non-ploughed systems was only partially compensated for by cover crops. Reduction in N fertilization reduced grain yields, but N leaching potential was only increased after residual N fertilizer due to low grain N offtake after drought. CA farming changes nutrient turnover dynamics, which may impact crop N supply and N losses. The adaption of CA increases N recycling via cover crops but does not increase cropping system NUE unless yields can be maintained.
This thesis investigated different ways to increase cropping system NUE at the crop level and at the cropping system level. Leaching losses can be reduced by expanding the rooting depth of crops or by including cover crops. Deep resource acquisition is constrained by soil conditions and resource demands. For optimal N management, interactions between soil N turnover, crop root growth potential, soil constraints for root elongation, and crop N offtake have to be considered.