Effects of particular management factors on arable plant diversity and interactions with

The current crop, fertilisation and to a lesser extent also herbicide use as well as soil characteristics were found to be the most important predictors of arable plant diversity in Central European high-input croplands, apart from spatial factors (Fried et al, 2008; Lososová et al., 2004;

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Pinke et al., 2012). Due to many inter-relations between field management factors, it seems impossible to single out any particular factor as most important for reducing or enhancing arable plant diversity.

Fertilisation was found to be a promising target as it was shown to be related to all three response variables. Contrary to results from previous studies (Pyšek and Lepš, 1991; Storkey et al., 2012), N fertilisation was not singled out as the most important explanatory factor. The data reveal that rates of N, P, K and Mg fertilisation were too highly correlated to allow for their statistical separation via the approach we applied and were therefore all subsumed in one principal component. Although we found significant gross effects of PC1 in the field interior, no net effects of this group of fertilisers persisted.

This was due to the fact that the highest inputs of these nutrients resulted from organic fertilisers which contain all four elements and are applied at highest rates in maize fields and much more rarely in barley fields (Table 2.5). We could therefore not separate the effects of fertilisation from the overall crop effect. The effects of herbicide applications might also have overridden the fertilisation effect.

S fertilisation represented a second gradient (PC2) and was statistically independent from the other types of fertilisers. S fertilisers were applied mainly in the form of ammonium sulphate or ammonium nitrate sulphate. The fact that sulphur fertilisation during the year preceding the survey was positively correlated to species richness (Fig. 2.4a) and weed cover (Fig. 2.3) is interesting. As proposed by Smith et al. (2009), sulphur fertilisation potentially increases resource diversity in the soil and consequently reduces the competition between weeds and crops. Fields with S fertilisation might therefore support a higher weed cover without decreases in crop yields (Grant et al., 2007; Smith et al., 2009). The Resource Pool Diversity Hypothesis (Smith et al., 2009) was also supported by the fact that the negative relationship between crop cover and weed cover became significant only after partialling out the confounding effect of S fertilisation. The diversification of soil resource pools might therefore indeed represent an opportunity for enhancing arable plant diversity without compromising crop yields. These conclusions must remain highly tentative and need further experimental verification. In the light of extremely low weed cover values in the field interior, it seems, however, unlikely that the magnitude of such effects is large enough to positively influence species of higher trophic levels or to halt the on-going decline in arable plant diversity (Meyer et al., 2013; Storkey et al., 2012).

Contrary to our hypothesis the herbicide use intensity index (HI) was not found to explain any of the plant diversity metrics. Several aspects might contribute to this unexpected lack of fit: (1) The maximum admitted application rate, which is an important parameter for calculating HI, is determined by governmental bodies based on maximum residue levels in the final products (EC, 1991-2013). It does not necessarily imply any comparability between herbicides in terms of effectiveness for eradicating weeds. The HI might thus not be a suitable proxy for ecological assessments of herbicide pressure on weed assemblages. (2) We would expect a certain threshold value above which any further

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increase in herbicide dosage will not lead to further decrease in weed diversity. This threshold might have been exceeded on the fields with extremely low weed cover, i.e. potentially on 2/3 of all assessed fields. Our study highlights the importance of the development of a more appropriate herbicide intensity index for assessing herbicide impacts in agro-ecosystems. The strong inter-relationships we found between different sets of explanatory factors show that the development of such an index and the determination of any potential threshold value can only be realised based on experimental assessments and controlling for confounding factors. Such an index would be a useful tool for determining the external economic impacts, i.e. the “cost of biodiversity loss” (TEEB, 2010), of herbicide applications on a field by field basis.

We found the approach to use the number of molecules of the active ingredients of herbicides to be more promising. Structuring the herbicide dosage data by HRAC groups for investigating differences in the impacts of particular modes of action yielded interesting new insights. We found the amount of mitosis inhibitors (K3) to exhibit significant net negative effects on species richness in the field interior after partialling out the effects of crop, sampling year and spatial autocorrelation (Fig. 2.4b). Common mitosis inhibitors applied on the surveyed fields were flufenacet and pethoxamid. We did not find similarly large effects for the other groups of herbicides. These differential effects between HRAC groups might be due to several reasons. (1) The degree of selectivity of target weeds and crops might vary between HRAC classes, e.g. the different sulfonylureas from group B are commonly targeted at a certain crop and certain weed species (Drobny et al, 2012), whereas the mitosis inhibitors from group K3 affect a broader set of species and crops (Böger, 2003; Busi, 2014). K3 herbicides are therefore more likelyto exhibit a dose-dependent “group effect” as postulated in our initial hypothesis. (2) The effect of herbicide groups which target mainly one of the three assessed crops, such as C1 herbicides which are only applied in maize (Table 2.5), can statistically not be separated from the overall effect of the crop. As K3 herbicides are applied across all three crops, they have higher potential to show significant net effects.

Among management factors crop rotation, conversion tillage with a mouldboard plough and land use history were found to be particularly important for explaining community composition. Our initial hypotheses were thus largely confirmed. This leads to the conclusion that dynamic mosaic landscapes including a wide range of different annual and perennial crops, fields with and without conversion tillage and a range of land uses other than crop production will in the long term secure highest levels of arable plant diversity (see also Gabriel et al. 2006). Contrary to our initial hypothesis and to data from literature (Ball, 1992; Teasdale et al., 1991), mouldboard ploughing was not found to decrease weed cover. The effect might have been overridden by the stronger effects of herbicides and fertilisation.

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The fact that soil parameters were found to play a prominent role in explaining community composition highlights the importance to maintain landscapes with diverse soil conditions. In the light of increasing homogenisation of habitat conditions on arable land through eutrophication and increasingly larger agricultural units (Smart et al., 2006), it is vital to conserve mesophilous and nutrient-poor habitats for maintaining biodiversity (Storkey et al., 2012).

2.4.3 Importance of the current crop and interactions with other management factors Our results confirm that the current crop is a sound proxy of management techniques. In particular, it is the only easily derived predictor which subsumes aspects of crop phenology and timing of management practices. More detailed data on fertilisation, herbicide applications and crop rotation will, however, add valuable additional information and should wherever possible be incorporated.

The analysis of the interrelationship between crop type and management factors confirmed that the management of the winter-sown cereals wheat and barley diverged in many aspects from the summer-sown crop maize. Overall N fertilisation was not found to differ between crops, but maize fields received the highest doses of organic fertiliser leading to extremely high P inputs. Doses of >100 kg P2O5 ha^-1 yr^-1, as recorded for almost 50% of the maize fields, are likely to exceed the nutrient removal with maize harvest (LWK NRW, 2012) and can lead to substantial P leaching to the groundwater, particularly in maize-dominated crop rotations. Herbicide applications in maize fields differ compared to winter cereal fields in timing of applications and types of the herbicides. Systematic differences in overall quantity of herbicide use were not identified and the herbicide use intensity index was equally high in maize fields and in winter wheat fields. The HI values in the study region were found to be comparable to values found by comprehensive assessments across Germany (Roßberg, 2011).

Differences in timing of herbicide treatments are difficult to capture and were represented in this study by the variable “current crop” and “sowing date”. Both variables were found to be highly significant.

Due to the strong correlation between both, it was, however, not possible to distinguish between the effects of timing, type and size of the differences in management practices represented by these two variables. New approaches for more adequately incorporating temporal aspects of field management in scientific impact assessments can potentially help to increase the explanatory power of the respective models.