Determination of C-factor for conventional cultivation and soil conservation technique used in hop gardens

: The research presented in the article was focused on determining the C-factor for hops for conventional culti-vation (CT) and soil conservation technique, which is based on the presence of cover crops (CC) in the inter-rows. The values of the soil loss ratio, which are the basis for the calculation of the C-factor, are also presented. The research activities were carried out between 2016 and 2023 and a ﬁ eld rainfall simulator was used for the measurements. Rainfall simulations were conducted at three developmental stages of CC. The results show a high C-factor value for CT (0.73), which indicates insu ﬃ cient erosion control e ﬃ ciency. In contrast, a C-factor of 0.16 was determined for the cover crop technique. This means that cover crops in the inter-rows of hop gardens can e ﬀ ectively reduce the risk of water erosion. The values reported in the article can be used as input parameters in various erosion models and equations to re ﬁ ne the subsequent calculation.


Introduction
Water erosion is one of the most common and serious degradation processes occurring on agricultural land worldwide [1,2].Nowadays, there are efforts to reduce the extent of water erosion through soil conservation techniques (SCTs) [3,4].SCTs are very often based on a high surface cover of plants or plant residues [5].When calculating the erosion control efficiency of a particular technique, the conservation effect of vegetation (so-called C-factor) is one of the important parameters [6][7][8][9][10][11].This is an indicator that is part of the most widely used erosion models, which are mainly the Universal Soil Loss Equation [9] and the Revised Universal Soil Loss Equation derived from it [11].
C-factor is defined as a dimensionless number that varies between the values of 0 and 1, which is influenced by several parameters: prior land use, canopy cover, surface cover, surface roughness, and soil moisture [12].A bare soil (BS) without vegetation with the parameters specified in Yoder et al. [12] represents number 1 and means high soil loss (SL).It is also taken as a control technique.Other farming methods are related to BS according to predetermined conditions [7].The closer the number is to 0, the lower the extent of water erosion.The determination of the resulting C-factor always takes place in different cropstage periods.The numbers soil loss ratio (SLR) obtained from the cropstage periods is further corrected by the percentage distribution of the R-factor (rain erosivity) during the year [13].
In this article, the erosion control efficiency in hop gardens is evaluated in terms of the conservation effect of vegetation.Hops are typically grown in rows [14].The width of the inter-rows is most often between 2.7 and 4.2 m [15].This space is kept free of vegetation cover by shallow cultivation, which is carried out several times a year [14,16].There are also specific agrotechnical operations that are typical only of hop gardens (hop pruning in spring or ploughing the soil to the plants) [17].All these agrotechnical operations cause that in conventional cultivation (CT), the soil surface is not sufficiently protected against water erosion [18].Hop is therefore one of the most erosion-prone agriculture crops ever.If the hop garden is sloping, the soil is carried away with every torrential rain.Unfortunately, the number of SCTs that can be used in hop gardens or other permanent crops is limited due to the way they are grown (in rows) and the presence of a hop construction.
Cover crops (CC) sown in between the rows are proving to be a solution.These have a number of positive aspects in addition to their soil conservation effect [19].The aim of this article is to present the following information: (1) determine the C-factor for CT; (2) determine the C-factor for the SCT using CC; and (3) monitor how much the C-factor and SLR values change over the season in different cropstage periods.

Methods
The research was conducted between 2016 and 2023 near the village of Solopysky (50.2591939N, 13.7421664E; 50.2560111N, 13.7337981E), which is located in Central Bohemia in the Czech Republic (Figure 1).The average annual temperature in the region is 7-8.5 °C, and the annual precipitation is 450-550 mm.Soils in the experimental plots were luvic Cambisols [20].The topsoil layer is up to 50 cm.The basic soil properties were as follows: the soil texture: <0.002 mm 23.8%, <0.01 mm 36.5%,<0.05 mm 66.7%, <0.1 mm 84.4%; total oxidizable carbon (C ox ) 1.53%; and humus 2.64%.The research took place on two uniform slopes that had a slope of 17 and 9%.In each year, experimental plots were established in the spring (late March to early April).During establishment, CC were sown (standard amount for each species) at the same time, always in three replications due to three rainfall simulation (RS) terms.Thus, RS were always performed on an unsimulated surface.Hop plants and CC were only affected by natural rainfall.The length of each experimental plot was 16 m (slope 17%) and 32 m (slope 9%).During the year, all necessary agrotechnical operations were carried out on the experimental plots.The different management methods can be seen in Figure 2.
In total, three techniques were selected to test soil conservation efficiency.The first was BS, which was a control technique [9].The other two remaining techniques were CT and SCT with different kinds of CC.During the research, six plant species were tested.The following text contains a more detailed description of the tested methods: (a) BS (control plot)the experimental plot is flat and completely without plant cover.Before each simulation on naturally moisture soil was performed loosening the top layer of soil up to 5 cm and then rolling it with a roller weighing 50 kg.This procedure was chosen based on methodologies Wischmeier and Smith [9] and Janeček et al.Triticum aestivum L.; grass-legume mixture 2: Avena sativa L., Vicia sativa L.) in inter-rows of hop-gardens at the beginning of spring.There is no loosening the soil during the year.Mulching of the plants is also possible if necessary.At the end of the season (after harvest), the CC are shallowly plowed back into the soil.
A field rainfall simulator was used as the main research method (Figure 3).From the measuring, it is possible to calculate the SL, on the basis of which it is possible to further calculate the resulting value of the C-factor.The rainfall simulator is a suitable device that is used to study soil erosion processes on sloping land [21,22].This device allows monitoring of the erosion effect, infiltration capacity of soil, or the beginning and the end of surface runoff.The amount of surface runoff was determined in a tipping bucket.The basic principle of measuring with a rainfall simulator is the spraying of water on a clearly defined area.The size of the simulating area was 21 m 2 (9.0 × 2.33 m), and the intensity of the rainfall was always set at about 1 mm/min.The rainfall simulator used for research had four nozzles (fulljet type) at the height of 2.0 m.All nozzles covered an area up to an angle of 104°at a pressure of 34.5 kPa.The average raindrop diameter was 1.5 mm to 2.0 mm, and the average kinetic energy of the raindrops accounted for 8.78 J m −2 mm −1 .
When measuring, it was necessary to ensure that the soil, slope, and climatic conditions of individual techniques were as similar as possible.For this reason, all experimental plots were established side by side, tested on the same slope and on the same day.Two repeated RS were performed.The duration of the first RS was 30 min on  C-factor in hop gardens  3 naturally moisture soil.Then, there was a 15-min technological break.After this time, a second (repeated) RS was performed on soil with higher moisture for a duration of 15 min.Two RS at different soil moistures were performed due to comprehensive verification of the soil conservation effectiveness of the measured technique.
Monitoring of erosion control effectiveness must be spread over the entire year, as it varies with crop growth.The method of determining the C-factor is therefore divided into five cropstage periods, which are defined in the guidelines "Predicting rainfall-erosion losses from cropland east of the Rocky Mountains: Guide for selection of practices for soil and water conservation" [7].The description of individual cropstage periods is defined as follows: • Period F (rough fallow): turn plowing to seeding.
• Period I (seedling): seedbed preparation to 1 month after planting.• Period II (establishment): from 1 to 2 months after spring or summer seeding.• Period III (growing and maturing crop): end of period 2 to crop harvest.• Period IV (residue or stubble): crop harvest to plowing or new seeding.
RS were performed for periods I, II, and III.Periods F and IV (periods outside hop and CC vegetation) were taken from Wischmeier and Smith [7].To determine the amount of SL, samples of surface runoff with soil particles were taken from the water-collecting flume at the place of outflow in the lower part of the experimental area.Samples were taken every 3 min into the calibrated container of 319 ml in volume after surface runoff started.Subsequently, each sample was dried in a Memmert UFB 400 oven (Memmert, Germany) for 12 h at a temperature of 105°C in a soil laboratory.
After drying, the weight of soil particles (mg) in each sample with a volume of 319 ml was determined.The resulting SL for each interval was calculated by multiplying the soil particles' values by the amount of surface runoff.From these values, the SL was calculated for each technique.For the SCT, we did a statistical calculation to see if there was a difference between individual species.No statistical significance was found, so the article continues to calculate with CC regardless of species (Figure 4).
If the SL and other important parameters are known, it is possible to calculate the C-factor.SL values as the sum of two RS (first plus second simulation) were used to calculate the SLR (Table 5).The resulting SLR value was further corrected by the canopy cover (hop plants) subfactor presented in Reinard et al. [11]: where CC 1 is the canopy cover, Fc is the fraction of the land surface covered by a canopy, and H is the distance that raindrops fall after striking the canopy.The main reason for this correction is that hop plants were not included in the inter-row space during the simulations.The correction took into account the changing Fc and H parameters as the hops grew.The SLR for all cropstage periods (F, I, II, III, and IV) was multiplied by the percentage distribution of the Rfactor (rain erosivity) in the evaluated period to obtain the partial C-factor.The percentage distribution of R-factor for the conditions of the Czech Republic determined according to Janeček et al. [13] is shown in Table 2 and is based on the long-term monitoring of the Czech Hydrometeorological Institute.
In this way, the C-factor was determined for both CT and SCT in inter-rows of hop gardens.For CT, the resulting C-factor was determined after the first 3 years of research (2016-2018), and for SCT from eight measured years (2016-2023).Since 2019, BS was no longer verified, but only CT and SCT are verified (Table 1).Thus, the measured SL results for the SCT were always percentage recalculated based on the SL for CT and BS.The aim was to relate the measured numbers to the value of 1 (BS) and not to the value for CT.This process was carried out for all RS carried out after 2019.
SL data from CT and SCT were analyzed to test for statistically significant differences between cultivation methods.First, the Shapiro-Wilk test for normality of the data was performed.Because some of the data sets did not have a normal distribution, the Mann-Whitney U test at the significance level P < 0.05 was chosen.The same process was performed to verify the difference between CC species.C-factor in hop gardens  5 3 Results and discussion

SL and partial C-factor in individual cropstage periods
Period F (rough fallow): in this period (approximately the end of March to the first half of April), the surface in the inter-rows is completely without plant cover.The soil is being prepared for the new hop growing season: the surface is being smoothed with ridge harrows, the hops are being proned or wiring is done.As soon as suitable climatic conditions appear, it is necessary to sow the CC.It is important to sow them when there is still enough moisture in the soil after the winter.This will then affect the quality of the CC for the rest of the season.As there are no crop residues in the inter-row at this time, we calculated with the value taken from Wischmeier and Smith [7] for the rough fallow of 0.7 for both verified techniques.A value of 0.7 is given in the publication for conventional tillage in maize cultivation (residue of prior crop removed, cropstage period F).The surface character of the soil in the inter-row of hop gardens can therefore be considered similar.According to Table 2, there is no torrential rainfall yet in this period.Thus, period F does not affect the resulting C-factor.It is valid for the conditions of the Czech Republic, as in other climatic conditions there can be the occurrence of torrential rainfall even in this period.Period I (seedling): According to Janeček et al. [13] and Wischmeier and Smith [7], this cropstage period starts on the day of sowing and ends on the day that closes 1 month after sowing.CC are in early growth (for hop gardens in the Czech Republic, it is mainly April).This corresponds to their soil conservation efficiency (Table 3), which increases with progressive growth.In this cropstage period, there were the largest differences (not statistically significant) between CC species (Figure 4).This is mainly due to differences in growth dynamics.
Already 1 month after sowing, CC have a significant soil conservation effect if the RS is carried out on soil with natural moisture.For the second RS, the soil conservation effect is not statistically significant enough.This is mainly because the soil had a higher initial moisture content, and CC were still in their early growth.However, the value of the sum of the two RS is used to calculate the SLR, and in this case, the statistical difference was also confirmed (Tables 3 and 4).In this cropstage period, it is necessary to start correcting the resulting SL in the SLR calculation due to the gradual growth of the hop plants.Hop plants cover approximately 10% of the surface and reach a height of 1 m.These values may vary in part depending on the climatic conditions in a given year.In the Czech Republic (central Europe), torrential rainfall starts to occur in April, but the probability of occurrence is low.As a result, the C-factor for both CT and SCT is also low (Table 5).
Period II (establishment): during the second cropstage period, the CC gradually reach full growth, and their soil conservation effect reaches its maximum effect [23].Significant SL reduction then occurs in the first, second, and also in the sum of RS.The average SL is 0.2 at the end of this period, so it can be concluded that SCT effectively protect the soil from water erosion.In contrast, the SL for the CT remains high and is even slightly higher than in the previous period (mainly due to the lower amount of moisture in the soil).Hop plants continue to grow, reaching a height of approximately 3 m and covering 20% of the surface.In this period, other benefits of CC start to become apparent, for example, flowering species (Phacelia tanacetifolia Benth) attract large numbers of pollinators.The probability of erosive rainfall increases to 11% in May.This is reflected in an increase in the C-factor for the CT for this period.For the SCT, the C-factor is still very low due to the high surface cover.This is especially important for the following III cropstage period.
Period III (growing and maturing crop): this cropstage period is considerably longer in terms of time than the previous periods and has the most significant impact on the resulting C-factor.This is most often the end of May or the beginning of June until the harvest, which is usually toward the end of August (for the Czech Republic).The length of the growth of the CC is influenced by the choice of the species and varies.However, even wilting plants remain in the inter-row and fulfil their soil conservation function, which is still very high compared to CT.From this point of view, the grass-legume mixtures looked better.The SL is still low at around 0.19.For CT, the SL continues to increase to a very high value of 0.89 in the sum of RS.Hop plants at this period usually reach the top of the hop construction (they can be bent over), which is often 6 m high.They also increase their surface coverage, which is around 30%.These values are taken into account in the correction of the SLR calculation.Some studies [24][25][26] have focused on the effect of large drops dripping from plant leaves on SL due to larger kinetic energy and falling height.This fact may be relevant in the case of hop, as the mature plant has large leaves and reaches great heights.These parameters are included in this study as part of the correction made by the formula [11], which is presented in Section 2.
Period IV (residue or stubble): after the harvest comes the final cropstage period, which is also long (September, October).The hop plants are no longer in the hop gardens, and the soil surface is cleaned after harvest.The CC should be ploughed shallowly into the soil after harvest to return the organic matter and nutrients.Thus, a kind of stubble is present in the inter-row.Wischmeier and Smith [7] give the SLR 0.24 for a similar type of soil cultivation (minimum tillageresidue crop left, cropstage period IV).We therefore also use this value when calculating the C-factor.In CT, the inter-row remains bare without cover.For this reason, the SLR value 0.62 was chosen.This value means (as well as in period F) conventional tillage in maize cultivation (residue of prior crop removed, cropstage period IV).The frequency of torrential rainfall decreases to 8% in September and 2% in October in the Czech Republic.

C-factor value
The C-factor for CT was determined in the first 3 years.When compared to BS, CT is not significantly different in terms of cover in the inter-rows.As the inter-rows are loosened several times during the season, there were modified for hop-growing conditions.In cropstage periods F and IV, there were no hop plants on the plot; therefore no canopy cover correction.
C-factor in hop gardens  7 also no plant residues in this area.Some weeds may be present, especially in the time before the loosening.Despite the similar character of the inter-rows, the SLR measured for the CT was lower than for the BS (Table 3).This is mainly due to the different soil structure.In BS, the simulated area is levelled, loosened, and then rolled over with a roller weighing 50 kg.With CT, the soil in the inter-rows has a coarser structure and there may be larger soil aggregates (Figure 2).This affects the resulting amount of surface runoff and SL.There is also a difference in the presence of hop plants compared to BS.The resulting value of the C-factor for CT was determined to be 0.73.This value was determined based on a total of 18 measurements in 2016-2018 (Table 1).The value is less than 1, but still very high.This fact confirms that water erosion in hop gardens is a serious problem and that CT is insufficient in terms of soil conservation.The value that we have determined is in general agreement with other authors who have tried to determine the C-factor in hop gardens.Janeček et al. [13] state, in their methodology, the C-factor value for hop gardens of 0.8.Similarly, Auerswald et al. [27] set the C-factor in hop gardens at 0.78, and Malíšek [28] set the value at 0.73.Our value is similar to these numbers and provides a validation framework to confirm the accuracy of the measurements.
After the C-factor for the CT was determined, we continued to verify the SCT in order to determine the C-factor for this technique as well.The resulting C-factor for the SCT in the hop gardens was determined by 0.16.There is currently a lack of information on similar research in the scientific literature, and therefore, the results can be considered unique.Some studies about water erosion in hop gardens can be found, especially in Germany, but they are very rare.For example, Schwertmann and Schmidt [29] or Schauder and Auerswald [30] who used copper as a tracer in deposition.
In this study, the C-factor for the SCT was determined based on 164 measurements with the rainfall simulator for six types of CC in inter-rows between 2016 and 2023.The measured SL values from the individual RS, given in units of t/ha, are shown in Figure 5.
Some comparison can be made via SL with other permanent crops that are similar in their character of interrows cultivation (vineyards, orchards).The total SL in CT as a sum from RF is 81% (Table 3).Capello et al. [31] measured an 83% lower SL in vineyards with grass-covered plots in vineyards.Similarly, Biddocu et al. [32] report lower SL ranging from 72 to 89%.Napoli et al. [33] reported a reduction of 68.5% in SL, and also Bagagiolo et al. [34] did not differ much in their results (lower SL at the two experimental sites of 78 and 80%, respectively).In almond orchards, Martínez-Mena et al. [35] measured a reduction of 85% in SL for techniques with green manure in interrows.Espejo-Peréz et al. [36] report that CC decreased SL in the olive orchard by an average of 76% in all experimental plots.The fact that all of these values are similar to our value points to the validity of our measurements.Significant reductions in total SL due to CC in inter-rows of permanent crops (but not in hop gardens) have also been previously reported by other authors [37][38][39][40].However, some authors report considerable variability in SL.For example, Morvan et al. [41] report that if CC are degraded, the SL can be higher than in BS.Morvan et al. [41] also state that this was related to soil compaction and poor quality of CC in the space of wheel tracks in the inter-row of vineyards.This is also the biggest limitation of SCT in hop gardens, because there is the degradation of the soil structure due to excessive soil compaction in the space of the wheel tracks, which is associated with frequent agrotechnical operations [42][43][44] and leads to increased bulk density [45].The vegetation is damaged in this place, and in the event of torrential rainfall, surface runoff and water erosion begin there.This was also clearly visible in the RS.The solution could be local loosening of only compacted space, which increases the infiltration capacity of the soil and at the same time allows to use of CC in inter-rows of hop gardens [46].However, the local loosening needs to be repeated during the season as the soil in this area will compact again over time.

Conclusion
The aim of the article was to determine the SL in hop gardens based on the measurements by the field rainfall simulator and then to calculate the C-factor, which is one of the factors of the Universal Soil Loss Equation.The SL is a variable value throughout the year, and therefore, several cropstage periods were verified.The mean SL for three cropstage periods (I, II, and III) from RS for CT was set at 0.81.The resulting C-factor is then 0.73.These values are very high and indicate an insufficient soil conservation of hop gardens when there is CT.The results also confirm that if the hop garden is located on a slope, severe water erosion can occur during torrential rainfall.In the case of SCT, the average SL from the RS (0.29) and C-factor (0.16) were significantly lower.CC in the inter-row thus appears as a potential solution for erosion-prone hop gardens.The stated results extend the area of research on water erosion and soil degradation in hop gardens, where there is currently a lack of information in the scientific literature.
[13].(b) CTthe classical way of hop farming, inter-rows without plant cover, loosening the soil several times a year.The hops are pruned in spring and the soil is ploughed to the plants in the second half of May.The harvest is usually at the end of August.Compared to BS, the soil surface of CT has a coarser structure and contains compacted soil at the places of the traffic wheels.(c) SCT with CCno-tillage sowing of selected plants (Phacelia tanacetifolia Benth.; Sinapsis alba L.; Secale cereale L., Trifolium incarnatum L., grass-legume mixture 1: Avena sativa L., Vicia sativa L., Pisum sativum L.,

Figure 3 :
Figure 3: Rainfall simulator in hop garden (end of cropstage period 1).Source: Photo taken by the authors.

Figure 5 :
Figure 5: SL from individual rainfall simulator measurements from 2016 to 2023.

Table 2 :
[13]entage distribution of the R-factor by month in the Czech Republic according to Janeček et al.[13]

Table 1 :
Number of rainfall simulator measurements for the verified techniques

Table 3 :
Soil loss in individual cropstage periods *SLsoil loss, SDstandard deviation; Statistical evaluation was performed for the rainfall simulator data.First, the Shapiro-Wilk test for normality.Then, the Mann-Whitney U test was performed to determine statistical significance (both statistical tests at the P < 0.05 significance level).CT and SCTs were always compared at the individual cropstage period.Cropstage periods F and IV were not measured with the rainfall simulator.The values of the sum of two RS were used to calculate the SLR.The values in the table are related to BS, which represents the number 1.

Table 4 :
Basic statistical tests for rainfall simulator data

Table 5 :
[7]actor by cropstage periods for CT and SCT with CC Soil Loss) sum of values from both RS; SLR (Soil Loss Ratio); The values in the table are related to BS, which represents the number 1; F x and IV x values were from the Wischmeier and Smith[7]