Assessing Effects of Crop History and Soil Amendments on Yields of Subsequent Crops

Three field trials were set up to measure the effect of previous crops or soil amendments on the yields of subsequent crops of cereals under a cool tem-perate maritime climate in arable crop land in the east of Scotland. Winter wheat and winter barley direct drilled into legume and cereal + legume stub-ble (pre-crop) gave substantial yield boost compared with other pre-crop cereals, but pre-crop effects of oats were similarly great. Restored rotation after continuous barley gave expected yield enhancement to subsequent winter and spring barley but not to subsequent wheat. Some diseases were reduced on restored rotation crops. Slurry effects on yield were generally small but beneficial and compost effects were greater. However, compost had effects on plant developmental speed and was difficult to compare directly with other treatments. NDRE measurements in the restored rotation and soil amendment trials indicated that yield gains were associated with improved crop health as indicated by leaf chlorophyll content. There were no clear cultivar interactions within crop type with treatments effects in any of these trials.


Introduction
The primary purposes of crop rotations are to maintain soil fertility and crop health although the mechanisms whereby this is achieved are often not fully understood [1]. It follows and is accepted that continuous cropping with the same crop species will lead to reduced yields and increased issues with pests and diseases. Nevertheless, the relatively high value and profitability of some cash crops such as winter wheat for milling across extensive areas, or more niche crops such as spring barley for malt distilling, dictate that they are sometimes grown more-or-less con-tinuously. The break crops that might be used to maintain or restore soil are often far less profitable. The issues can be compounded by investment in machinery and facilities for the cash crop and degradation of the resource and infrastructure to grow alternative crops.
Whether the true costs of continuous cropping with the same crop are generally known is debatable. Figures from Rothamsted comparing continuous cropping with a four-course rotation from 1852 to 1883 showed that barley was not affected whereas wheat, grazed turnips and faba beans yielded 19%, 39% and 55% less respectively in continuous cropping [2]. However, later trials demonstrated considerable yield reduction of continuous barley, especially under the low nitrogen conditions used previously [3]. Dyke & Slope [3] also showed correlated take-all ratings using root infection estimations that could explain the losses.
However, Hornby & Henden [4] showed that take-all decline generally establishes in continuous barley, thereby mitigating this effect in practice.
Degradation of soil health may lead to a greater demand for agronomic inputs such as fertilisers and pesticides as well as greater losses to the environment in terms of diffuse pollution, greenhouse gasses and carbon footprint, few of which are assessed or costed. Furthermore, few controlled comparisons are made with recognised good rotation practice as cropping patterns generally affect whole fields or farms and are therefore difficult to compare on any factorial basis. A further issue is that the agronomy advice and data on varieties is generally from good rotation managed trials that are not comparable with continuous cropping situations. Therefore, there is a need for an evaluation of which crops suffer by continuous cropping and by how much.
Break crops in a rotation can have very different effects on soil health, particularly when they are from different plant families from the predominant cash crop [5]. Legumes in particular can have a strong effect and the potential release of nitrogen to the following crop is often of considerable benefit [6]. Legumes are increasingly being exploited as components of intercrops too, so the effects on subsequent crops will reflect both the legume and other components [7].
However, the agronomic approaches used will affect how much of the nitrogen fixed by the legume is lost to the environment and how much becomes available at appropriate times to the subsequent crop. The effects of tillage could be particularly important to this effect [8].
Another approach to improving or maintaining soil health and thereby crop yield is by adding organic amendments that aim to improve the soil structure.
These are commonly either animal waste such as slurry or compost such as that from domestic garden waste, and often result in yield gains in barley but effects are variable due to complex effects on soil quality [9] [10]. These should not only increase soil organic matter, but also affect the soil structure, chemistry and biology. In that these changes will affect soil function, not only might crop yields be affected, but also crop responses to both biotic and abiotic stress [11]. LG Diablo all sown at 360 seed/m 2 . Normal agronomy and full rate fungicides were applied to each respective crop but the nitrogen rates were about half of the normal rates: 100, 90 and 70 kg/ha for winter wheat, winter barley and spring barley respectively, so that crops were likely to be in a nitrogen-responsive range to ensure soil fertility differences were expressed. Plots were combined with a Wintersteiger Quantum plot combine and weights adjusted for moisture were taken from the combine's onboard outputs.

Rotation Restoration Trial
The pre-crop area for investigating the effects of continuous barley was a field

Soil Amendment Treatment and Trial Setup
Three different soil amendment treatments were applied in Low Pilmore field, Mylnefield, Fife and Kinross, Scotland, UK (56˚27'11.4"N 3˚04'50.5"W), a field of sandy-loam soil from 2004 comprising "Discovery Compost", "slurry" and an unamended control ( Figure 2). Discovery Compost was made from local garden waste composted to PAS100 standard and the slurry was from cattle sourced from local farms. Initially 50 t/ha compost was applied to all the compost treatment areas and 20 t/ha slurry to the slurry  16

Data Analysis
The data were subject to Analysis of Variance (ANOVA) using Genstat 19 th edition (VSN International Ltd., Hemel Hempstead, UK), checking data for normal residuals. Disease scores were converted to percentage equivalents before analysis but it was not found to be necessary to transform these data. Contrasts were calculated to determine the significance differences between the pre-crop treatment group (A and B) responses.

Intercrop Pre-Crop Legacy Effects on Yield
For all three crop types there were highly significant (p < 0.001) effects of the pre-crop on yield. Cultivar differences were highly significant for winter and spring barley (p < 0.001) and p = 0.013 for winter wheat. There was no indication of any cultivar or cultivar mixture interaction with pre-crop treatment. Plotting the pre-crop treatment means for each crop in order of the winter crop overall means, the pre-crop treatments form two distinct groups for winter wheat and winter barley, and to a lesser extent for spring barley also (Figure 3). The two groups were barley-and rye-dominated pre-crops (Group A) and pea-, bean-and oat-dominated pre-crops (Group B). For winter wheat the average yield increase for the bean, pea and oat monocultures as well as the oat with bean and oat with pea intercrops was about 30% more than the average of the barley-and rye-dominated treatments. For winter barley the difference was approximately 21% and for spring barley 10% and using contrasts, these differences were significant for all three crops (winter wheat and winter barley p < 0.001, spring barley p = 0.002) ( Table 1). The three cereal crops clearly respond differently in terms of magnitude of response but follow similar trends. Although there were few trial entries that were mixtures rather than pure cultivars, there was no evidence that they behaved any differently from the mean of their respective monocultures.
The normalized difference red edge index (NDRE) was greater for winter wheat in the pea-, bean-and oat-dominated pre-crops compared with the other pre-crop group (p < 0.001) but for winter barley there was no difference (Figure 4).

Rotation Restoration Effects on Yield and Disease
There was a highly significant yield increase of about 19% and 26% in the yield of winter barley and spring barley on the restored rotation compared continuous barley (p < 0.001; Figure 5), but for winter wheat there was no significant effect. There were no cultivar interactions with pre-crop, nor monocultures compared with mixtures overall. Powdery mildew levels on winter wheat reached an average of around 2.5% and wheat yellow rust reached an average of around 27% in mid-June. Winter barley had much higher levels of powdery mildew at 40% in mid-June but rhynchosporium levels only reached around 3%. No significant disease was recorded on spring barley. There was a small but significant effect of pre-crop on powdery mildew levels on winter wheat and winter barley and rhynchosporium on winter barley, the restored rotation pre-crop having about 15%, 20% and 21% less respectively (p = 0.003 or less). There was no effect of pre-crop on yellow rust on wheat. There was a significant reduction of 33% in yellow rust on wheat in mixtures compared with monoculture means but no interaction with pre-crop and no other mixtures significantly changed disease levels compared with monocultures means.
The NDRE was used to calculate Area Under the NDRE curve and this was significantly different for crop * rotation effect (p < 0.001) and the plotted values for winter wheat and winter barley crops on each treatment across time shows that it was generally greater for the restored rotation treatments for both crops ( Figure 6).

Soil Amendment Treatment Effects on Yield
Yield measured on the whole area excluding the plot area in 2017 and 2018 varied significantly with year, amendment, amendment level and for all interactions (p < 0.001). The general trends are most clearly seen in the plot of the mean treatment levels (Figure 7). Initially, particularly in the first three years, the compost treatment appeared most beneficial but subsequently the slurry appeared most beneficial and the compost least, although differences were apparently minimal in the two plot trial years. In 2016 the treatments converged although compost dropped away again in 2017. In both 2017 and 2018 the yields were substantially lower than normal but a small benefit of the organic matter in the two amendments did improve yield in 2018. The

Discussion
That previous crops and soil amendments can affect crop development and yield is not a novel finding. However, information about the relative effects and magnitude of different pre-crop treatments is valuable for novel crop development and agronomic treatment planning. Indeed, the sometimes marginal differences between the yield of new varieties is put into perspective by some of these data emphasising the importance of good agronomic management across crop cycles.
This is particularly notable with the effects of restored rotation on winter wheat and winter barley where yields were 19% and 26% higher respectively compared with following continuous barley. This is likely to have been enhanced by direct drilling as ploughing might have accelerated losses from microbial activity and breakdown of nutrients in the roots and other organic matter. That this effect is species specific rather than an effect on cereals in general is clearly shown by the fact that the yield of winter wheat was not affected by the treatments. Furthermore, this demonstrates that the yield boost cannot be attributable to any nutritional effect of the immediately prior crop of faba beans as this would have been expected to affect at least both winter crops that were direct drilled at the same time in a similar way.

Nutrient explanations of continuous cropping effects have been investigated
previously without clear mechanistic effects [12]. The mechanism are more likely to be predominantly microbial, possibly root-infecting pathogens including take-all.
Roots were not assessed for take-all symptoms but the levels are very variable in previous studies of continuous spring barley [4] and there was no evidence of this disease above-ground on the wheat. Negative effects of monoculture soil have been attributable to the microbial components and these affected wheat and oats to a lesser extent than barley [13] which concurs with our findings. The apparent negative trend of rye which was only noted on subsequent winter wheat seems likely due to them both being nutrient-demanding crops and therefore reflecting nutrient depletion, particularly nitrogen.
Higher NDRE values are normally interpreted as indicating better crop health as it is correlated with leaf chlorophyll content and is a surrogate of N% in the crop [14]. We know from visual observations and NDRE measurements that the compost treatments enhanced plant development, as did slurry to a lesser extent. Nevertheless, there was no evidence consistent across the two years that any cultivar responded differentially to soil amendment treatment.
That the compost treatment was initially beneficial to yield and slurry to a lesser extent was expected as soil structure and nutrients would have shown improvements [15]. These pre-crops form two clear groupings. In the more beneficial group, the pea and bean monocrop effects might be expected as direct nitrogen responses.
However, that oats have a similar effect is less expected, but oats with peas or beans would therefore be expected to have a similarly positive effect. The differences between the groupings in yield response were around 30%, 21% and 10% for winter wheat, winter barley and spring barley respectively. Elite winter wheat cultivars are very nutrient demanding and responsive crops, as are winter barley and spring barley cultivars though likely to a lesser extent. Therefore, this ranking might be expected, especially as the spring barley was sown over 6 months later and as no cover crop was grown, more nutrient loss would be expected in this crop. Some difference may be attributable to residual nutrient differences left after the previous crops but given the similarity of their biomass yields in the previous trial to those expected (data not shown) this seems unlikely to explain the magnitude of these responses. More likely would be the known allelopathic effects of oats having a strong effect not only on other plant species but also on the microbial composition of the soil [16].

Conclusion
Crop sequence or rotation, the immediately preceding crop in particular, and soil organic amendments can have both detrimental and beneficial effects on subsequent crops. The trials reported here indicate a potential range of up to 30% of the yield, far greater than the benefits likely to be gained from changing cultivar for example. In particular, the utility of growing oat crops may be under-valued and ways of incorporating more legume crops should be considered. However, the value of soil amendments need to be carefully evaluated in the context of other soil health and resilience considerations.