TY - THES T1 - Effects of woody plants and their residues on crop yield, weedsand soil carbon fractions in selected arable cropping systems A1 - Xu,Jialu Y1 - 2018/03/27 N2 - Woody plants on arable land can be grown for their products (e.g. trees for energy biomass) or simply as field borders (e.g. hedgerows). Establishing woody plants on arable land can increase biodiversity, reduce soil erosion, diminish nitrate leaching and improve drinking water quality. Woody plants have the potential to increase soil organic matter and sequestrate carbon in soils and biomass, which is important for the mitigation of climate change. However, the residues of woody plants can also have unfavorable influences (e.g. allelopathic effects, or competition for nitrogen) on crop production. In the past, woody plants have often been removed from arable land because of intensification and mechanization of agriculture; while nowadays, they are restored on arable land to achieve various ecological benefits, particularly in developed countries. The aim of the current study was to investigate selected aspects of plant and soil responses and their interactions with woody plants and their residues on arable land. Four publications describe and discuss the results of laboratory and field experiments with woody residues from hedgerow pruning (wood chips) and the effect of short rotation coppice willow in comparison to other energy crops, and their effects on yields, weeds, and selected soil characteristics. The first publication (accepted by Agronomy Journal) describes a study about long-term effects of wood chips application from hedgerows (mainly Acer pseudoplatanus L., Prunus avium L., Prunus padus L., Salix caprea L., Ligustrum vulgare L., and Fraxinus excelsior L.) on arable land, with a focus on weed infestation and yield. Data were collected from a 16-year field trial at the organic research station Kleinhohenheim, in Southwest Germany, with wood chips mulching (WCM) on a typical crop rotation (cereal-based, grain legume and fodder included). The wood chips were derived from in-situ hedgerow prunings and annually applied in three rates (0, 80 and 160 m3 ha-1). Wood chip mulching reduced the weed density in spring significantly by 9%, and the high mulching rate resulted generally in lower weed numbers than the low mulching rate, while WCM caused no significant grain yield loss of cereals and grain legumes. However, the relative crop yield of plots with WCM compared to the control showed a decreasing trend over time, which might be related to unfavorable effects of WCM on the vegetative growth of crops. The weed suppression by WCM is presumably a result of several impacts such as a physical barrier, changes in soil temperature, lower nitrogen availability and allelopathic effects. Hence, woody residues can be used for weed control in arable crops but care should be taken concerning their potentially unfavorable effects on crops. The second publication (submitted to Seed Science Research) is directly related to the WCM in publication I. The study aimed at gaining a first insight in potential allelopathic effects of the wood chips used in experiment I and their influence on seed germination under laboratory conditions. Watery extracts of wood chips from goat willow (Salix caprea L.) and black cherry (Prunus padus L.) were tested on seed germination of oilseed rape (Brassica napus L.) and wheat (Triticum aestivum L.). The extraction procedure was standardized for varying conditions including drying methods (freeze drying at -50 °C, oven drying at 25, 60 or 105 °C), milling, wood to water mixing ratio (WWR=1:10, 1:15 or 1:20) and fraction of the material used (bark or core wood), in order to produce extracts with a high capacity to suppress germination. Extracts of freeze dried and undried (defrosted) wood chips resulted in the lowest germination rate (<6%) of both crops after two weeks, compared to the relatively higher germination rates (12%–53%) in extracts of oven dried wood chips. In a high WWR ratio (1:10) with black cherry, the germination rate of oilseed rape was significantly lower in the extracts of milled wood chips (26%) compared to un-milled ones (49%), and similarly for wheat seeds (milled 1% germination rate, un-milled 19% germination rate). The suppression effect increased with increasing concentration of the extract: germination rates were 86% (WWR=1:20), 71% (WWR=1:15) and 35% (WWR=1:10) with milled wood chips. Extracts from the bark resulted in significantly lower germination rates (<4%) than that from core wood (>88%). The combination of freeze drying, with milled wood chips, high WWR and wood bark turned out to be the most suitable method to inhibit seed germination and thus to retain effective allelopathic compounds in the extract. The effects of germination inhibition was influenced by the interaction of tree species, species used in the germination trials and extraction method. The findings from this study can be applied for a systematic testing of different woody species, particularly for choosing woody residues with allelopathic potential to suppress weeds. The third publication (in preparation) is about soil organic matter (SOM) responses to long-term cultivation of a woody perennial energy crop versus an herbaceous annual energy crop on arable land. Soil samples (0-10, 10-20 cm) were taken after 12 years of continuous cropping of short rotation coppice (SRC) willow (Salix schwerinii E. Wolf × viminalis L.) and mono-cropping of silage maize (Zea mays L.) at the research station Ihinger Hof, Southwest Germany. Soil fractions with different physical properties (density and weight) were separated in the laboratory and the total organic carbon was determined in these fractions. The integrated fractionation method separated light fractions (<1.8 g cm-3) including free- and occluded- particulate organic matter (f-POM and o-POM), and heavy fractions with three particle-size classes (63-2000 μm for sand, 2-63 μm for silt, and <2 μm for clay). Generally, SOC contents were significantly higher in the upper soil under willow (1.39%) than under maize (1.13%). The soil (0-20 cm) under willow accumulated 154% more labile SOM (f-POM and o-POM)than the soil under maize did. The results can be explained by the continuous input of leaf litter and root turnover under the willows, and by the fact that the willows were not tilled except in the year of plant establishment. The C:N ratios of the SOM in POM and in the sand-sized fractions were also significantly higher under willow (28, 24, and 16) than under maize (23, 18, and 9). The findings indicate a slower turnover of SOM and a higher potential of carbon sequestration in soil under SRC willow, as an example for a woody energy crop, compared to silage maize, a typical annual energy crop grown with soil tillage. The fourth publication (Published in Agronomy Journal) is based on the same 12-year field experiment as publication III, and it evaluates the biomass and gross energy yield of six annual and perennial energy cropping systems with different nitrogen fertilization levels, including SRC willow and silage maize (selected soil properties of both presented in publication III). The annual systems included mono-cropping maize with reduced tillage, a crop rotation of oilseed rape (Brassica napus L. ssp. oleifera)–wheat (Triticum aestivum L.)–triticale (Triticale ×triticosecale Wittmack) with moldboard plough or with no-tillage. The perennial systems included SRC willow, miscanthus (Miscanthus × giganteus Greef et Deu.) and switchgrass (Panicum virgatum L.). For each cropping system, three levels of nitrogen fertilization (0, 50% and 100% of crop-specific best management practice) were applied. The mean annual biomass yield was found to be highest in maize (18.5 Mg ha-1) and miscanthus (18.3 Mg ha-1) with 100% nitrogen, and without nitrogen highest in miscanthus (13.6 Mg ha-1). However, only higher nitrogen levels could maintain the high yield potential of these two crops. Without or with reduced nitrogen fertilization, annual yield of maize was decreasing continuously; without nitrogen fertilization, the annual yield of miscanthus was also decreasing after eight years. Willow had a mean annual yield of 11.0 Mg ha-1 in average for three nitrogen levels, and its biomass yield was not significantly reduced when the nitrogen fertilization was reduced from 100% to 50%, suggesting it is less dependent on nitrogen fertilization compared to other energy crops (except for miscanthus) in this study. A yield trend of willow was only visible within the three-year coppice rotation but so far not with plantation age, except the establishment period. Overall, both growing of woody plants and applying woody residues on arable land can influence the plant growth (crops and weeds), soil properties and their interactions in many ways. Favorable influences include (i) reducing weed density by WCM, (ii) achieving promising biomass yield by growing SRC tree with low soil nitrogen, and (iii) increasing soil organic matter (carbon sequestration) by growing SRC willow; unfavorable influence includes reducing biomass production of crops by WCM. According to the findings with wood chips extracts, it is very likely that the main weed-suppressing effect is due to allelopathic effects. Short rotation coppice willow seems to be a favorable low-input system with a medium but constant yield. Residues of SRC willow from clearing the land after several years of growing might have similar effects on crops and weeds in the following years, as the wood chips from hedgerows showed. It also can be assumed that the carbon sequestration which was found under SRC willow also takes place in the hedgerows. All favorable influences on crop production might motivate farmers to keep or restore woody plants on farm, and enhance ecosystem services within the farming system; while the unfavorable influences should be avoided if woody plants are to be used on arable land. Future research objectives can be related to (i) technological improvement of the wood chips application on fields for weed control; (ii) determination and isolation of allelopathic ingredients from woody materials, and using them for a new generation of natural herbicides; (iii) study on potential long-term residue effects and interference of SRC woody plants on following crop growth; and (iv) studies on carbon sequestration in the soil under hedgerows. KW - Gehölze KW - Samenkeimung KW - Unkrautbekämpfung KW - Energiepflanzen KW - Biomasse KW - Biomasse KW - organischer Kohlenstoff im Boden CY - Hohenheim PB - Kommunikations-, Informations- und Medienzentrum der Universität Hohenheim AD - Garbenstr. 15, 70593 Stuttgart UR - http://opus.uni-hohenheim.de/volltexte/2018/1456 ER -