Oberholzer, Simon Raphael (2024). Cover cropping in organic reduced tillage systems - soil fertility effects and measurement methodologies. (Thesis). Universität Bern, Bern

Preview

Text 24oberholzer_sr.pdf - Thesis Available under License Creative Commons: Attribution (CC-BY 4.0). Download (48MB) | Preview

Abstract

Sustainable agriculture strongly depends on fertile soils. Yet keeping soils in crop production while maintaining their fertility is highly challenging and most long-term studies indicate a decrease in soil fertility on arable land under continuous cropping. During the growing period of the cash crop, only limited options to improve soil fertility exist because there are normally very specific cultivation requirements. Therefore, the break periods between main crops have a high potential for soil fertility improvement because no harvest output is expected. Many studies have shown that long periods of bare soil should be avoided and during long fallow times the soil should be covered. For that purpose, different options of cover cropping have been developed. They differ in terms of plant species, frost tolerance, biomass input and termination methods. So far, in the scientific literature cover crops were mainly compared to bare soil treatments but different cover cropping strategies are only rarely compared, hence limiting knowledge on their performance relative to one another. In organic reduced tillage systems cover cropping has a high priority and since neither herbicides nor intense tillage can be used, frost-tolerant cover crops can only be terminated with shallow tillage methods that brings the cover crop biomass into the very topsoil layer. The challenge thereby is that within a reasonable time (around 2 weeks) the mixture of cover crop pieces and soil must result in proper seedbed for the next crop. One way to make this process easier is to reduce the amount of cover crop biomass by mowing and removing the aboveground biomass. Another approach is to use a microbial inoculant called “Effective Microorganisms” that is promised to facilitate the decomposition process and make seedbed preparation easier. In the context of Switzerland, crop rotations are very diverse and accordingly also the fallow periods differ widely in length and seasonal growing condition. Thus, it is difficult to evaluate different cover cropping strategies in long-term experiments because the same fallow period only occurs once in several years. On the other hand, short-term experiments face the challenge that most soil properties show a high variability in space and changes are normally rather small which makes it very difficult to statistically detect management effects. The statistical power could be improved by increasing the sample size but due to the high costs of conventional soil analyses, the number of soil samples is normally limited. Infrared spectroscopy is a method that provides fast and cheap soil analyses and can therefore potentially be very useful in short-term soil experiments because the number of samples can be increased at little additional costs. Yet, spectral soil data need to be calibrated with measurements from conventional lab data and still little is known about the performance of spectral models at the local scale. The goal of this thesis is to increase scientific knowledge about cover cropping effects on soil properties in organic reduced tillage systems. Thereby I formulated three major objectives: 1) to compare the effects of two frequently used cover cropping strategies on soil fertility parameters, 2) to evaluate the suitability of infrared spectroscopy in soil sampling projects of local extent and 3) to investigate the effects of Effective Microorganisms (EM) on cover crop decomposition. I thus investigated the effects of two cover cropping methods on soil properties at six fields in eastern Switzerland (Paper 1). A sampling design in very high temporal and spatial resolution was implemented and the high number of soil samples (n = 2574) was analyzed in a combined approach of conventional soil analysis and soil spectroscopy in the visible and near-infrared range (vis–NIR). Thereby the reasons for the varying performance of spectral models between different fields were analyzed and summarized in Paper 2. A very similar spectral approach was used in a soil survey in northern Spain (Paper 5) and results are presented to complement the insights from Paper 2. The effects of EM on cover crop decomposition were tested in a lab incubation study and published in paper 3 and 4. Regarding objective 1, the two cover cropping strategies that either maximized plant biomass input or soil cover were evaluated in the long fallow period between wheat harvest (End of July) and sowing of a next spring crop (Paper 1). In the double cover cropping (DCC) strategy two cover crops were sown subsequently and shallowly (3 cm) incorporated into the soil with the idea that the biomass input provides an energy source for the soil microorganisms. In the permanent soil cover (PSC) strategy, the soil was covered for the whole period with one cover crop, that was mowed, and the plant biomass was removed. In contrast to DCC, the PSC strategy had no aboveground plant biomass input into the soil but also no tillage throughout the period. The analysis of the two cover cropping strategies in high spatial and temporal resolution showed that the effects of differences between different sampling times were far more pronounced than differences between treatments. Nevertheless, in both treatments the increase in soil organic carbon was highest in 5-10 cm soil depth and significantly higher in the PSC compared to the DCC approach. The plant biomass input in the DCC treatment led to higher microbial biomass and mineral N compared to the PSC treatment. I conclude that the aboveground biomass input in the DCC strategy was beneficial for biological activity but the better soil cover and probably higher root biomass input in the PSC strategy was slightly more beneficial for soil organic carbon. Addressing objective 2, the spectral models showed over all a good performance, but the model performance was lower on the two fields with high carbonate content (Paper 2). The prediction accuracy for fields with high carbonate content could not be improved when data of all fields were combined to build general models. I therefore conclude that especially in soils with low carbonate contents, soil spectroscopy is very suitable, and the prediction errors can be expected to be comparable to the lab measurement error. The application of the same spectral approach at a study side in Spain showed clearly that much more detailed information can be obtained when conventionally analyzed samples are accompanied by additional spectral measurements (Paper 5). Concerning objective 3, EM application did not show significant effects on cover crop decomposition dynamics, under the spring-like conditions mimicked in the incubation study (12° C) (Paper 3 and 4). Thanks to a sterilized control, I could distinguish effects caused by living microorganisms and effects caused from substrate (energy and nutrients) addition. Seven days after the start of the incubation, microbial taxa from EM solution could only be found when EM were applied in 100 times higher amounts than recommended in agricultural praxis. This thesis shows that a high-resolution sampling design and the use of spectral methods allowed to evaluate different cover cropping strategies in a short-term experiment. I therefore consider the methodological approach to be very useful for evaluating innovative soil management strategies as soonest possible after their invention. This thesis is one piece of knowledge that aims to support decision making about cover cropping in organic reduced tillage systems and provides results about effects on soil fertility properties. More research is needed to evaluate effects of cover cropping on other agronomic variables like yields, weed pressure and profitability.

Item Type:	Thesis
Dissertation Type:	Cumulative
Date of Defense:	19 December 2024
Subjects:	500 Science > 550 Earth sciences & geology 900 History > 910 Geography & travel
Institute / Center:	08 Faculty of Science > Institute of Geography
Depositing User:	Hammer Igor
Date Deposited:	18 Feb 2025 15:41
Last Modified:	18 Feb 2025 15:50
URI:	https://boristheses.unibe.ch/id/eprint/5831

Actions (login required)

View Item