Changes in the weed communities as affected by different primary soil tillage and deep loosening

ABSTRACT

Long-term soil cultivation at the same depth affects soil characteristics and crop productivity. The aim of the study was to investigate the impact of a long-term different intensity soil tillage methods and deep loosening on weed number, weed agrobiological group and soil seed bank changes in till Bathygleyic Dystric Glossic Retisol soil under the climatic conditions of the Western Lithuania (geographical coordinates 55°43′38″N, 21°27′43″E). The study included different soil tillage methods (conventional ploughing, shallow ploughing and shallow ploughless tillage) and deep loosening. During investigational years, the greatest weed number in crops and the greatest weed seed number in the seed bank were determined in the soil reduced tillage (shallow ploughing and shallow ploughless tillage). The weed number in crops of conventional ploughing soil was 35.8% lover compared to reduced tillage soil. The weed seed number in the seed bank of conventional ploughing was 49.6% lover compared to reduced tillage Decreasing soil tillage intensity resulted in weed seeds concentration in the upper topsoil. A one-time deep loosening had a significant effect during the crop rotation: the weed number in crops and weed seed number in the seed bank were determined to have increased by 26.6% and 51.6% in conventional ploughing soil and by 11.9% and 23.2% shallow ploughless soil respectively. However, after deep loosening, the number of Poa annua in crops decreased 2.9 times in plots of conventional ploughing and 1.7 times – in plots of shallow ploughing soil.

KEYWORDS:

• Intensive tillage
• reduced tillage
• weed number
• weed agrobiological group
• soil seed bank

Introduction

There is currently an increasing societal and political shift towards more sustainable agricultural systems to provide both food production and ecological biodiversity levels (Petit et al. 2011). Appropriate soil tillage is one of the ways to reduce work consumption, influence chemical and physical quality of the soil, control CO₂ changes between the soil and the atmosphere, increase plant productivity and reduce crop weediness (Šimanskaite 2002; Feiza et al. 2011; Feizienė et al. 2011; Novara et al. 2012). However, soil cultivation disimproves its structure by destroying macro-aggregates. This manifests especially in humid soils with a texture of loam and clay (Mikha and Rice 2004; Čiuberkis et al. 2008; Al-Kaisi et al. 2014). Deep loosening of the soil improves plant growth by making optimal conditions for microbiological processes, plant nutrition regime as well as the soil humidity and porosity (Olesen and Munkholm 2007). Weed community composition is related to the agricultural system determining soil tillage methods and other crop management measures (Błażewicz-Woźniak et al. 2016; Kolářová et al. 2018) as well as the environmental variables (e.g. temperature, moisture and soil structure) (Gardarin et al. 2009). Soil, its properties and fertilisation exert multiple effects on the weed flora and on weed population abundance in an agrophytocenosis (Karčauskienė et al. 2016). Weed incidence reflects the potential of the soil seed bank (Andreasen and Stryhn 2008). Some authors state, that weed community diversity changes are associated with the adoption of reduced soil tillage systems (Woźniak and Soroka 2015; Skuodienė et al. 2016). The others indicate that the increase in total weed cover, and perennials and monocotyledon weed cover, is often potentially problematic for crops with reduced tillage (Sans et al. 2011). Reduction in the use of tillage sometimes favours more difficult-to-control species (Recasens et al. 2016).

According to these observations, we hypothesised that the choice of tillage system can positively or negatively affect soil quality. Given that, a long-term different intensity primary soil tillage can have a major impact on weed flora. We utilised data from a field experiment to test the effects of primary soil tillage and deep loosening factors on weed properties. The aim of the study was to investigate the impact of a long-term different intensity soil tillage methods and deep loosening on weed number, weed agrobiological group and soil seed bank changes in till Bathygleyic Dystric Glossic Retisol soil under the climatic conditions of the Western Lithuania.

Material and methods

Site and soil description and experimental design

The soil of the experimental site is Bathygleyic Dystric Glossic Retisol (WRB 2014) with a texture of sandy loam (clay particles < 0.002 mm – 13–15%), which pH at the beginning of the trial was 5.1–5.3. The soil is moderate in humus status (humus – 2.43%, organic carbon – 0.99–1.1%), moderate in mobile phosphorus content and high in mobile potassium (106 and 270 mg kg⁻¹ soil, respectively). The total nitrogen content in the soil was 0.09–0.13%.

The trial was carried out in 2003 in the experimental crop rotation field of Vėžaičiai Branch of Lithuanian Research Centre for Agriculture and Forestry (geographical coordinates 55°43′38″N, 21°27′43″E). This manuscript summarises the data of a long-term experiment of the second and the third crop rotations (2008–2015). The four-course crop rotation consisted of winter wheat (Triticum aestivum L.) (2008, 2012), spring rape (Brassica napus L. ssp. oleifera annua Metzg) (2009, 2013), spring barley (Hordeum vulgare L.) with undersown red clover (2010, 2014), red clover (Trifolium pratense L.) (2011, 2015).

The net plot size was 10 × 5 = 50 m². The harvested area for cereals and rape amounted to 4 × 6.6 = 26.4 m² and for clover 4 × 4.2 = 16.8 m². The trial was replicated four times. Treatments in the replications were randomised.

Trial factors and treatments

Primary soil tillage: (1) conventional ploughing (20–25 cm), (2) shallow ploughing (10–12 cm), (3) shallow ploughless tillage (8–10 cm). Conventional and shallow ploughings were done with ploughs (screw type mouldboards) with jointers three weeks later after the stubble breaking (at the depth of 6–8 cm). In the shallow ploughless as well as in the ploughed treatments the stubble breaking was done firstly and cultivation after that.

Deep loosening. Before the primary soil cultivation in the end of the second crop rotation (in the autumn of 2011), plots were divided into two parts. In one of them, the influence of primary soil tillage was observed. In the other part, deep loosening was performed (35–40 cm) to destroy thick soil layers (up to 40 cm depth) which have been forming due to repeating soil cultivation at the same depth during the first two crop rotations (8 years). Deep loosening was done using a deep loosener one month before winter crop sowing (Skuodienė et al. 2016).

Methods of analysis

Weed record was performed in stationary areas 0.25 m⁻² in size in two positions of every plot during crop shooting phases. Weediness in the sward of red clover was estimated before the first cut. Weed number was recalculated to weeds per m². Soil contamination with weed seeds was investigated in the depths of 0–10 cm and 10–20 cm. Soil samples were collected using an agrochemical drill during crop maturity phase. The soil was dried out. One hundred gram (100 g) dry soil sample was weighed and wet-sieved through a 0.25 mm sieve until all contents of the soil were washed out. Remained mineral part of the soil was separated from the organic part and weed seeds using the saturated salt solution. Weed seed number was recalculated to thousands of unit m⁻².

Soil pH_KCl was determined using potentiometric method, N_tot – using Kjeldahl method, available P₂O₅ and K₂O – using Egner-Riehm-Domingo (A–L) method. C_org was determined by dry combustion Dumas method. Humus was determined according Tiurin method (ISO 10694:1995). Soil texture was determined according to the composition of three fractions: sand, silt and clay (WRB 2014).

Weather characteristics

Meteorological conditions in the period of 2008–2015 were diverse (Table 1).

Table 1. Climatic conditions during the experimental period. Data of the Vėžaičiai meteorological station.

Climatic indices	Experimental year								Standard climate norm (1981–2010)
	2008	2009	2010	2011	2012	2013	2014	2015
Air temperature, °C
Mean per year	8.3	7.1	5.8	7.6	6.8	7.5	8.0	8.1	7.0
Mean per vegetation	12.8	12.5	12.6	13.4	12.7	13.2	13.3	12.0	12.4
Rainfall, mm
Total per year	964	915	1073	1072	1004	839	720	915	915
Total per vegetation	537	584	698	658	727	511	414	432	540

The period of 2008–2012 was humid: during the vegetation period the amount of precipitation was 8.1–34.7% greater and through all the year it was 5.3–17.3% greater compared to the standard climate norm. In 2013–2015 the amount of precipitation during the plant vegetation period and through all the year was less than the standard climate norm, respectively: 5.4%; 23.2%; 19.9%. The months of July in 2014 and August and October in 2015 were especially dry.

During the plant vegetation period and through all the year, the average air temperature fluctuated and varied from the climatic norm. The average year temperature in 2008, 2011, 2013, 2014 and 2015 was 0.5–1.3°C higher than the standard climate norm. The temperature in 2009 and 2012 was close to the standard climate norm and in 2010 it was 1.2°C lower than the standard climate norm. During the vegetation period in 2011, 2013 and 2014, the mean air temperature was 0.8–1.0°C higher than the standard climate norm, and in the other years it was close to the standard climate norm.

Statistical analysis

The data of the second rotation of crops were analysed by a one-factor analysis of variance. The data of the third rotation of crops were analysed by a two-factor analysis of variance. Significance of the differences between the means was determined according to the Fisher's protected least significant difference (LSD) at 0.05 probability level. The data were processed using software ANOVA (Clewer and Scarisbrick 2001). Data of weed density were transformed, according to the recommended procedures, using equation Y = Sqr(x + 1); however, means on the original scales are reported (Onofri et al. 2010). *Statistical differences at P ≤ .05, **statistical differences at P ≤ .01.

Results and discussion

Weed cover and diversity

Under the damp climatic conditions of the Western Lithuania, dependent on the intensity and construction of implements, primary soil tillage significantly changed the aggregate composition of the soil, humidity and nutrient regime (Čiuberkis et al. 2008; Karčauskienė et al. 2016) and biological activity (Vilkienė et al. 2016). However, although the reduced conservation soil tillage has some advantages compared to the conventional ploughing (Jodaugienė 2002), it increases crop weediness as well (Lehoczky et al. 2013; Skuodienė et al. 2013; Błażewicz-Woźniak et al. 2015; Seehusen et al. 2017). Due to different intensity soil tillage the primary soil cultivation method had an impact on weediness of the second (2008–2011) and the third (2012–2015) rotations of crops (Table 2).

Table 2. Significance of the effect of primary soil tillage and deep loosening on weed number and soil seed bank according to F-test.

Weed indices	Second rotation of crops		Third rotation of crops
	Year	F-test	Year	F-test
		Tillage (T)		Tillage (T)	Deep loosening (D)	T×D
^AWeed number, plants m^−2	2008	**	2012	**	**	*
	2009	**	2013	**	ns	ns
	2010	**	2014	**	ns	**
	2011	*	2015	**	ns	**
^BSeed bank, seeds m^−2 at 0–10 cm depth	2011	**	2015	**	**	*
^BSeed bank, seeds m^−2 at 10–20 cm depth	2011	**	2015	**	**	ns

Note: T – primary soil tillage, D – deep loosening * and ** – the least significant at P < 0.05 and P < 0.01, respectively, ns – not significant, ^A – averaged from cultivated species, ^B – averaged from years.

The weed number in crops of conventional ploughing soil was 35.8% lover compared to reduced tillage soil (shallow ploughing and shallow ploughless tillage) (Table 3).

Table 3. Soil tillage effects on weed number and agrobiological composition averaged across the rotation of crops.

Tillage	Weed number, plants m^−2	Agrobiological groups, %
		Annual		Perennial
		DICOT	MONOCOT	DICOT	MONOCOT
Second rotation of crops
CP	137.5	97.2	0.8	2.0	0.0
SP	171.3**	93.0	4.0	3.0	0.0
SPT	143.9	86.0	12.6	1.1	0.3
Third rotation of crops
CP	104.0	90.3	9.6	0.1	0.0
CP + D	131.7	96.6	3.3	0.1	0.0
SP	151.6*	83.8	12.8	3.4	0.0
SP + D	139.1	86.9	7.7	5.1	0.3
SPT	174.8**	80.5	18.2	1.1	0.2
SPT + D	195.6**	82.5	17.2	0.0	0.3

Note: CP – Conventional ploughing, SP – Shallow ploughing, SPT – Shallow ploughless tillage, D – deep loosening, DICOT – Dicotyledonous, MONOCOT – monocotyledonous, * and ** – the least significant at P < 0.05 and P < 0.01, respectively.

Refusing the use of conventional ploughing, weed seeds, especially those of small size, remain at the soil surface and spread more readily and the root system of perennial weeds stays undisrupted.

The same tendencies in these rotations were observed analysing the data of weed number variations (Table 3). The greatest weed number in crops of winter wheat and spring rape was present when applying shallow ploughless soil tillage. The greatest weed number in crops of spring barley was shown when applying shallow ploughing and in the sward of red clover (good density) – applying shallow ploughing and shallow ploughless tillage (Skuodienė et al. 2016). According to the average weed number data in both rotations of crops, we can see that the weed number in conventional and shallow ploughing plots of the third rotation was less (respectively: 24.4% and 11.5%), compared to the same tillage plots of the second rotation. It was partly influenced by the dry period of the plant vegetation in 2013–2015. The amount of precipitation during the vegetation period has the impact on the common weed number (Skuodienė et al. 2016). However, the weed number in plots of the shallow tillage was greater by 21.5%, compared to the same tillage plots of the second rotation. According to Vilkienė (2017), the shallow ploughless soil tillage determines greater humidity accumulation in the soil. Therefore, during the dry period conditions for plants development are better.

Repeating soil cultivation at the same depth leads to compacted subsoil reducing plant root growth with resulting effects on plant uptake of water and nutrients (Olesen and Munkholm 2007). In the end of the second crop rotation, deep loosening significantly changed soil density and aeration porosity (Vilkienė et al. 2016; Vilkienė 2017). Together with changes of one indicator, other indicators, determining potential soil productivity, change as well. An impact on weediness showed up in the first year (2012) after a onetime deep loosening (Skuodienė et al. 2016). After the soil aeration and humidity conditions improvement, the common weed number was significantly greater compared to unloosened plots. The significant influence of deep loosening manifested through the interaction with soil tillage methods as well. Using deep loosening in shallow ploughless tillage soil the weed number was greater by 48.5% compared to conventionally ploughed and aerated soil.

Changes of weed agrobiological groups

Biological characteristics of weeds such as adaptation to the environmental conditions and living duration help to survive in cultivated fields. The annual weeds dominated (on the average 98.1%) in crops during all the investigation period. The majority of weeds (86.0–97.2%) were dicotyledonous (Table 3).

The results show that different intensity soil tillage affects the changes of the annual weed number during the crop rotation period. Together with more intensive soil tillage, the number of the annual dicotyledonous weeds had a tendency to increase while the number of the annual monocotyledonous weeds decreased (r = −0.904, p < .05). The group of annual monocotyledonous weeds in our investigation was consisted only of Poa annua. Its germination lasts for all vegetation period and it can mature seeds several times. Considering soil tillage methods according to the annual monocotyledonous weeds incidence, the shallow ploughing and shallow ploughless tillage plots generated more weed seeds compared to conventional ploughing, respectively: 5 and 16 times. Poa annua may be well suited to the shallow ploughless tillage because of small seed size and an adaptation to surface germination.

Comparing different soil tillage methods, similar tendencies of the annual weed incidence were determined in crops of the third crop rotation. However, in all plots the annual monocotyledonous weeds were far more spread. It is supposed to be influenced by the sparse red clover grass after cold winter in 2011 (Skuodienė et al. 2013). After a onetime deep loosening during the third crop rotation the weed community dynamics changed. . According to the average data, deep loosening strongly reduced the number of the annual monocotyledonous weeds (by 65.6%) in conventional ploughing plots during the crop rotation while in the shallow ploughing plots the annual monocotyledonous weed number decreased by 39.8% (Table 3). The results demonstrated that even in a relatively short time (crop rotation), weed population shifts may occur with the use of a deep loosening in conventional and shallow ploughing.

Soil contamination with weed seeds

Soil contamination with weed seeds depended on applied crop management measures and crop condition. According to Andreasen and Stryhn (2008), the composition of the seedbank is dependent on the management practiced through the whole rotation. After the completion of the second crop rotation conventionally ploughed soil was significantly less contaminated with weed seeds (15.5 thousands of seed m⁻²) than shallow ploughing or shallow ploughless tillage soil (Figure 1). In the shallow ploughing and shallow ploughless tillage soil the number of weed seeds of different species was determined to be 51.7% and 67.4% greater than in conventionally ploughed soil. Independent of the soil tillage intensity, after the completion of the third crop rotation the seedbank was larger. In unaerated plots there were averagely 51.9 thousands of unit m⁻² of weed seeds and it was 2.4 times more than after the second rotation of crops. The reason for that was an increase of the level of weed cover due to unfavourable meteorological conditions in the previous years. During investigation period, on the average the weed seed number in the seed bank of conventional ploughing was 49.6% lover compared to reduced tillage (shallow ploughing and shallow ploughless tillage). Deep loosening had a significant impact on weed seed dynamics as conditions in the soil changed. Application of deep loosening determined the greater amount of weed seeds in conventional ploughing and shallow ploughless tillage compared to unaerated plots, respectively 51.6% and 23.2% (Figure 1). Consequently, changing the occurrence of the seeds their germination was initiated.

Figure 1. Weed seed bank under influence of primary soil tillage in combination with deep loosening in different arable soil layers (0–10 cm and 10–20 cm): (a) after the completion of the second crop rotation; (b) after the completion of the third crop rotation. * and ** – the least significant at P < 0.05 and P < 0.01; Unl – Unloosened, L – Loosened.

Type of tillage is the main factor determining the vertical distribution of weed seeds within the soil profile (Figure 1). Decreasing soil tillage intensity resulted in weed seeds concentration in the upper topsoil. Annually applying convetional ploughing, the amount of weed seeds in both topsoils was similar. Applying shallow ploughing there were averagely 62.6% of the common weed seed number in the topsoil of 0–10 cm depth. Applying shallow ploughless tillage at the same depth there were 82.2% of the common weed seed number. The greater concentration of seeds in the upper soil layer produces synchronised germination periods, thus generating high seedling densities within a short time period (Ghersa and Martinez-Ghersa 2000).

Under the humid climatic conditions of the Western Lithuania, primary soil tillage and deep loosening had the significant influence on crop weediness in acid soils with slightly organic matter. During all investigational years, the greatest weed number in crops and the greatest weed seed number in the seed bank were determined in the soil reduced tillage (shallow ploughing and shallow ploughless tillage). A one-time deep loosening improved aeration and humidity conditions in the soil layer up to 40 cm (Vilkienė et al. 2016). As a consequence, weed number in crops and weed seed number in the seed bank were determined to have increased by 26.6% and 51.6% in conventional ploughing soil and by 11.9% and 23.2% shallow ploughless soil, respectively. However, after deep loosening, the number of Poa annua in crops decreased 2.9 times in plots of conventional ploughing and 1.7 times – in plots of shallow ploughing soil. Results showed, that the diversity of weed agrobiological groups is indicative of the response to soil management and meteorological conditions. The soil weed seed bank is an indicator of past and present weed populations.

Acknowledgments

The chapter presents research findings, obtained through the long-term research programme “Productivity and sustainability of agricultural and forest soils” implemented by Lithuanian Research Centre for Agriculture and Forestry. Also the chapter presents research findings, obtained through the project “The effect of long-term management of resources of varying intensity on the soils of different genesis and other components of agro-ecosystems (SIT-9/2015)” funded by the Research Council of Lithuania.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes on contributors

Dr Regina Skuodienė is senior and head researcher at Vėžaičiai Branch of Lithuanian Research Centre for Agriculture and Forestry. Her research interests include biodiversity, natural grassland, productivity and quality of grassland ecosystem, and weed problems in different cropping systems.

Dr Danute Karcauskiene works at Vezaičiai Branch of Lithuanian Research Centre for Agriculture and Forestry as a Directress and senior researcher. Research interests include soil acidification process analysis and management, system: plant–soil optimization, the physical condition of the soil and degradation prevention of bio-energy resources.

Dr Regina Repsiene is senior researcher at Vezaičiai Branch of Lithuanian Research Centre for Agriculture and Forestry. Research interests include soil acidification process analysis and management, acid soil liming, mineral and organic fertilization, soil chemical changes and plant–soil optimization.

Dr Gintaras Šiaudinis is senior researcher at Vėžaičiai Branch of Lithuanian Research Centre for Agriculture and Forestry. Research interests include evaluation of energy crops and their utilization for bioenergy purposes.