Diverse forage production systems and their potential for greenhouse gas mitigation
The impact of agriculture on the environment has become a sharp focus, especially as future demand for food is projected to double over the next 2-3 decades. The negative impact of agriculture on the environment is primarily blamed on intensifying and simplifying production evident on the field, farm, landscape or regional scale. Grassland use contributes to climate change and environmental degradation, but it can also be used as a tool for greenhouse gas emissions mitigation. Moreover, low-input systems are often considered ecologically beneficial, but their sustainability under intensification is uncertain. Plant-soil feedbacks regulate soil carbon and nitrogen cycling via litterfall, root decay and root exudates, controlling microbial population dynamics in most soils, yet their impact may vary depending on the vegetation type. Enhancing the capacity of low-input systems to resist soil degradation and mitigate climate change requires long-term assessments. In Chapter 2, this thesis assessed the outcome of C inputs and outputs across an array of plant functional groups in arable and permanent systems of a tropical Savannah after more than 50 years of consistent land use. Soil samples were taken (0–30 cm depth) from arable crop fields, grazed-seeded grassland, cut-use permanent crops and native grassland. Soil organic carbon (SOC) stocks ranged from 17 to 64 Mg SOC ha-1 (mean±sd = 32.9±10.2 Mg ha-1). SOC stocks were lower for grazed-seeded grassland relative to cut-use grass, legume trees and shrubs. Accordingly, while converting the native grassland to grazed pastures was estimated to have lost 44 % of SOC over the period, the conversion to woody legumes resulted in slight (5 %) incremental gains. Within sown systems, nitrogen (N) availability seemed to be the most critical factor that determines the fate of the SOC stocks, with soil N concentration and SOC being highly correlated (r = 0.86; p<0.001). In total, N, P and K were significant predictors of SOC density in the soils.
Experiments to test the hypothesis that legume-based N sources are characterized by significantly lower emission factors than mineral N based dairy systems were described in Chapter 3. This study monitored N2O emissions for a minimum of 100 days and up to two growing seasons across a gradient of plant species diversity. Emissions were measured from grazed pastures and a controlled urine and dung application using the static chamber method. About 90% of the simulated experiments’ accumulated N2O emissions occurred during the first 60–75 days. The average accumulated N2O emissions were 0.11, 0.87, 0.99, and 0.21 kg ha−1 for control, dung, urine patches, and grazed pastures, respectively. The N uptake efficiency at the excreta patch scale was about 70% for both dung and urine. The highest N2O-N emission factor was less than half compared with the IPCC default (0.3 vs 0.77), suggesting an overestimation of N2O-N emissions from organically managed pastures in temperate climates. Plant diversity showed no significant effect on N2O emission. However, functional groups were significant (p < 0.05). Moreover, soil-plant interactions affecting N2O production in soils are not well understood, and experimental data are scarce. A better understanding of the mechanisms by which plants affect N2O emission would facilitate the selection of forage species for sustainable intensification. In Chapter 4, this thesis described a greenhouse experiment that tested the hypothesis that high forage plantain Plantago lancelota (PL) than Lolium perenne (LP) in mixtures reduces N2O emission. The greenhouse experiment was arranged in a 3x3 factorial, fully randomized design, comprising three mineral N fertilizer rates (0, 150 and 300 kg N ha-1) applied to two monocultures (LP and PL) and one binary (LP-PL) sward mixture. Parameters measured included daily N2O emissions, aboveground (AGB) and belowground biomass (BGB), N- and C- yields above- and belowground, as well as leucine aminopeptidase (LAP) activity in the soil as an indicator for soil microbial activity. Results showed that accumulated N2O emissions (83 days) were about two times higher for PL than LP or LP-PL swards (p<0.05). N application increased AGB in all treatments (p<0.001), however, the N300 rate reduced biomass allocation belowground. The PL yielded lower dry matter (AGB+BGB) and N than LP or LP-PL (p<0.001). As a result, the binary sward (LP-PL) showed the highest N use efficiency and increased LAP activity. Spearman correlation analyses and structural equation modelling suggested C allocation dynamics belowground as a potential means by which plants might impact N2O emission. Thus, the work generally demonstrates the potential of mixtures to reduce the environmental impacts of agriculture. Despite identifying some variables influencing nutrient cycling and recognizing that emission factors are overestimated in grassland systems in temperate climates, uncertainties remain, making the accurate estimation of emissions across different soil types difficult.