Nutrient Response Efficiencies, Leaching Losses and Soil-N Cycling in Temperate Grassland Agroforestry and Open Grassland Management Systems
von Leonie Göbel
Datum der mündl. Prüfung:2020-05-06
Erschienen:2020-07-01
Betreuer:Prof. Dr. Johannes Isselstein
Gutachter:Prof. Dr. Daniela Sauer
Gutachter:Prof. Dr. Rainer Georg Jörgensen
Dateien
Name:Dissertation_Göbel_Revision.pdf
Size:2.24Mb
Format:PDF
Zusammenfassung
Englisch
In recent years, there is an increasing interest in innovative agricultural systems as an alterna-tive to open grassland systems in order to minimize the detrimental effects of intensive pro-duction systems on the environment such as nutrient leaching to ground and surface waters. One possible solution could be agroforestry, i.e. the implementation of trees into grassland in the form of alternating rows, also known as alley cropping system. By fostering ecological interactions between woody and non-woody components, agroforestry systems can minimize the detrimental effects of intensive production mentioned above. These systems are therefore seen as more sustainable and environmentally friendly production systems than intensive pro-duction systems or monocultures. At present, however, regarding temperate agroforestry, it is unknown whether this particular agroforestry system is a sustainable and environmentally friendly alternative to open grassland in terms of nutrient and water use. The overall aim of this thesis therefore was to test whether grassland agroforestry (alley cropping of grasses and fast growing trees) is a sustainable alternative to open grassland by investigating the index of nutrient response efficiency, nutrient leaching losses and the soil-N cycle. Agroforestry and open grassland systems were investigated on three soil types (Histosol, Anthrosol and Cambi-sol soils) in central Germany in 2016 and 2017. Measurements in the agroforestry systems were conducted in the tree rows and at various distances to the tree rows within the grass rows in four (Histosol and Anthrosol soils) or three (Cambisol soil) replicate plots. The first study aimed to assess differences in nutrient response efficiency (NRE, ratio of biomass production to soil available nutrient) and plant-available nutrients between grassland agroforestry and open grassland. Plant available N and P were measured several times during the vegetation period using the buried bag method for N and a resin- and bicarbonate-extraction for P. The cations Ca, K and Mg were determined once. Biomass production was either measured (grass) or calculated by using allometric functions (trees). Plant-available N, P, macronutrients and NREs were generally comparable between agroforestry and open grass-land, suggesting no net effect of competition or complementarity for nutrients between trees and grasses. One exception were the marginally lower Ca and Mg response efficiencies in ag-roforestry compared to open grassland in the Cambisol soil. This was due to the narrow grass rows (9-m wide), which showed lower biomass of grasses at 1 m from the tree row, possibly due to the trees’ shading. In 2017, when tree production was higher in the second year after tree harvest, agroforestry had higher P and K response efficiencies than open grassland for Histosol and Anthrosol soils. It is therefore concluded that alley cropping agroforestry can be a sustainable alternative to open grassland without sacrificing NRE and soil nutrient availabil-ity, particularly in systems with wide grass rows (48-m wide) and when trees are getting older. The second study aimed to quantify nutrient leaching losses in temperate alley cropping sys-tems of alternating rows of fast growing willows and grassland. Nutrient leaching losses were calculated by multiplying monthly measured nutrient concentrations in soil water at 0.6 m depth from each sampling point with modelled monthly leaching fluxes. For all soil types tree rows displayed high interception rates resulting in water drainage fluxes that were considera-bly lower compared to the grass rows. At all three sites NO3-, phosphate and base cation (Ca, K, Mg) leaching losses were highly variable throughout the study year and did not differ be-tween tree rows and grass rows because of considerable temporal variability. However, look-ing into ratios of Na/nutrient showed that trees positively influenced nutrient losses by prefer-ential nutrient uptake and possibly due to increased microbial processes such as denitrification under tree rows. The study thus provides evidence that fast growing trees in temperate grass-land alley cropping systems can contribute to reduction of leaching losses and to better ground water quality. The third study aimed to assess differences in gross rates of soil-N cycling between agrofor-estry and open grassland, and their controlling factors. To determine gross rates of soil-N-cycling processes (i.e. gross N mineralization, gross nitrification, N immobilization and dis-similatory NO3- reduction to ammonium) the 15N pool dilution technique was used on intact soil cores. There were no significant differences in gross rates of mineral N production (miner-alization and nitrification) between sampling locations in grassland agroforestry and open grasslands within soil types. Management (grassland agroforestry vs open grassland) or vege-tation type (tree or grass) did not affect soil-N cycling, thus opposing the hypothesis that gross rates of soil-N cycling are higher in the tree rows than in the grass rows or open grassland. Instead, N-cycling rates and microbial biomass were rather affected by soil types. Grassland agroforestry can thus be equally efficient in terms of soil N availability and soil-N cycling compared to open grassland and can be a sustainable alternative management system. This thesis overall shows that temperate grassland agroforestry can be a sustainable alternative management system to open grassland in terms of NRE, nutrient leaching and by sustaining an active N-cycling. However, multi-year evaluations, both of the economic benefit and the eco-logical functions, are needed in order to quantify long-term trends, which could then provide a robust basis for inclusion of agroforestry into a broader framework of policy.
Keywords: Agroforestry; Grassland; Nutrient leaching; Nutrient use efficiency; Nitrogen cycle