Impact of cover crop and mulching on soil physical properties and soil nutrients in a citrus orchard

Introduction

The loss of nutrients in the soil is considered a key problem for decreasing soil fertility in the fruit orchards grown in raised bed soils (Dang & Hung, 2021). In the Vietnamese Mekong Delta (VMD), soil compaction and degradation are more severe (Dung et al., 2020). Many studies have reported that reduced soil organic matter is a primary cause of increased soil bulk density (Athira, Jagadeeswaran & Kumaraperumal, 2019; Dang, Ngoc & Hung, 2021; Imran, Amanullah & Al-Tawaha, 2022). Citrus needs high soil porosity and available nutrients for optimum growth and development. Pomelo (Citrus grandis Osbeck) has been cultivated in many places at the VMD, and they are a great source of income for growers (Kieu & Hung, 2019). However, the pomelo productivity cultivated on old raised soils has been reduced due to poor soil fertility and compaction (Dung et al., 2020). Dang, Ngoc & Hung (2022) reported that soil acidity in the citrus orchards increased significantly with chemical fertilizers (ammonium sulfate, superphosphate, and potassium chloride) in the long term. Furthermore, farmers in these areas were not interested in implementing soil conservation measures in their orchards. This may decrease soil moisture, water use efficiency, and biological activity (Cao et al., 2021; Paul et al., 2021).

Soil conservation practices (mulching, cover cropping, crop rotation, etc.) are measures the farmer can apply to mitigate soil degradation and soil erosion (Ogunsola, Adeniyi & Adedokun, 2020; López-Vicente et al., 2020). Soil conservation measures reduce soil loss by keeping a cover over the ground, decreasing soil displacement associated with raindrops and irrigation water affecting soil particles (Vincent-Caboud et al., 2019; Calegari et al., 2020). Additionally, soil conservation measures decrease the pressure and velocity of runoff on the topsoil (Kumawat et al., 2020). According to Page, Dang & Dalal (2020), conservation practices improved the soil’s organic carbon content, soil porosity, water capacity, plant nutrient availability, soil biota activity, and crop productivity.

Cover cropping is a cropping system/method utilized to decrease erosion, ameliorate soil porosity, enhance soil organic matter, weed control, pests, and disease management, and increase biodiversity (Sharma et al., 2018; Das, Kandpal & Devi, 2021). According to Van Sambeek (2017) and Abdalla et al. (2019), cover crops attract pollinators leading to improved fruit set ratio, thus increasing plant productivity. There are two key cover crops: legumes and non-legumes (Abdalla et al., 2019). Leguminous cover crops increase soil nutrients especially total and available nitrogen, because they can fix nitrogen biologically (Kaye et al., 2019; MacMillan et al., 2022; Freidenreich et al., 2022). Meanwhile, the nonlegume cover crops increase crop biomass and decrease soil loss from the surface layer (Romdhane et al., 2019; Amanullah et al., 2021).

Mulches comprising organic materials (straw, litter, etc.) covered the soil surface to control weeds and reduce runoff (Li, Li & Pan, 2021; Khoramizadeh et al., 2021). Mulches help increase soil organic carbon, decreasing soil compaction (Iqbal et al., 2020). The decomposition process of organic mulches releases many nutrients (Khoramizadeh et al., 2021). These nutrients are in a form that is useful to plants and might increase the uptake, improving crop productivity (Singh et al., 2021). Mulching also affects soil microorganism activity and abundance (Rodrigues da Silva et al., 2022).

A previous study indicated that cover cropping with legumes and rice straw mulch significantly increases soil organic carbon, total nitrogen, and phosphorus (Dung et al., 2022). However, the effects of conservation practices on soil compaction and available nutrients (Ca, Mg, K, Cu, Fe, Zn, and Mn) have not been reported. Hence, this study aimed to evaluate soil conservation measures on soil bulk density, porosity, and soil nutrients in a pomelo orchard cultivated on alluvial soil of the Mekong Delta, Vietnam.

Materials and Methods

Results

Effect of soil conservation practices on soil bulk density

Figure 1 shows that using soil conservation practices (LCC and RSM) significantly improved BD at both 0–10- and 10–20-cm after 3 years of experimentation. However, soil conservation measures did not affect BD at two depths (20–30 and 30–40 cm). At the topsoil (0–10 cm), BD in LCC and RSM treatments were lower than in the control and NLC plots. In particular, the mean value of BD in LCC and RSM was ~1.14 and ~1.15 g cm⁻³, respectively, lower than that in the treatments of control 1.22 g cm⁻³.

Using of NLC positively affected BD in the topsoil (0–10 cm) in 2020 and 2021 compared with the control treatment (Fig. 1A). Similarly, BD was reduced by covering crops with pinto peanuts and mulching with rice straw in the 10–20 cm soil depth (Fig. 1B). Meanwhile, Figs. 1A and 1B showed that BD in the lower layers was not changed after soil conservation measures application. The value of BD in two depths (20–30 cm and 30–40 cm) ranged from 1.23–1.26 g cm⁻³.

Soil porosity is affected by soil conservation measures

Soil conservation measures increased soil porosity at two depths, 0–10 cm and 10–20 cm (Fig. 2). Like BD, non-legume or legume cover crops and RSM did not improve soil porosity in deeper soil layers (20–30 cm and 30–40 cm). The use of conservation practices (LCC and RSM) enhanced soil porosity by ~5% and ~3% at 0–10 and 10–20 cm (Figs. 2A, 2B) after 3 years of experiments, respectively. In the depths of 20–30 and 30–40 cm, there was no significant difference in soil porosity between soil conservation measures compared to no conservation (Figs. 2C, 2D).

Discussion

Soil BD is a vital indicator of soil degradation because it influences soil porosity, plant nutrient availability, and soil microorganism activity (Recha et al., 2022). According to Shaheb, Venkatesh & Shearer (2021), soil conservation measures decreased soil compaction, resulting in increased root development and length. Shaheb, Venkatesh & Shearer (2021) indicated that soil compaction reduced root biomass significantly. The decreased crop growth and yield due to soil compaction were likely due to poor nutrient availability and uptake, thus limiting/preventing root growth (Gürsoy, 2021). In this study, cover cropping with pinto peanut and rice straw mulch reduced BD at depths of 0–10 cm and 10–20 cm, ~0.10 g cm⁻³ and ~0.08 g cm⁻³ in a 3-year consecutive trial, respectively (Figs. 1A, 1B). The current research is consistent with Mondal et al. (2019), who reported that conservation agriculture practices contributed significantly reduced soil compaction. Similar results have also been reported by Degu, Melese & Tena (2019), Ceylan (2020), and Belayneh, Yirgu & Tsegaye (2019).

Like BD, soil porosity was increased significantly at two depths, 0–10 cm, and 10–20 cm, when covered with legumes and straw mulch (Fig. 2). Many studies have indicated a strong negative correlation between BD and total porosity (Kakaire et al., 2015; Onwuka et al., 2020). In the present work, cover crops and mulching decreased BD, and this may be due to reduced soil compaction, which improved total porosity. Moreover, our previous study showed that soil organic matter increased remarkably when applying cover with pinto peanut and straw mulch (Dung et al., 2022). Improving soil organic carbon is the main reason for the increase in total porosity (Fukumasu et al., 2022).

The first year’s results showed that soil nutrient concentration was not affected by soil conservation practices, except for Zn content in the depth of 10–20 cm (Table 3). However, in the second and third years, Ca, K, Fe, and Mn concentrations in RSM and LCC increased significantly at the topsoil and subsoil layers (Tables 2, 3). Conversely, these nutrients were not elevated at the depths of 20–30 cm (Table 4) and 30–40 cm (Table 5) compared with the control, except for exchangeable K at 30–40 cm in 2021. The cause may be because the root of a plant used for the cover is short, and all treatments followed the no-till practice. The results in legume biomass and straw were unable to move the depths below. Hence, it does not improve the mineral nutrients in the soil. The results did not agree with that of Haruna & Nkongolo (2020) that conservation practices enhanced soil nutrients in 20–40 and 40–60 cm during the second year of study. The difference in soil nutrient concentration between the two studies could be the difference in no-till and till practices. Soil conservation measures can favorably ameliorate soil fertility by enhancing the number of soil biota that decompose organic matter and, in the process, release plant-available nutrients (Veum et al., 2015; Belayneh, 2019). According to Jat et al. (2018), conservation practices are considered a better alternative that recycles plant nutrients in the soil.

Our study showed soil has a high BD, which caused the availability of soil nutrients (Ca, K, Fe, and Zn) to decline. Table 6 showed that there were a strong negative correlation between BD and Ca (r = −0.74**), BD and K (r = −0.73**), BD and Fe (r = −0.79**), and BD and Zn (r = −0.69**). According to Belayneh, Yirgu & Tsegaye (2019), high BD negatively affected soil nutrients due to decreased soil biological and biochemical processes, resulting in reduced soil fertility. A similar result has been reported by Singh et al. (2020). They indicated a negative correlation between BD and soil nutrients. However, the results of the present work in contrast with a report of Duan et al. (2019), who showed that there was a strong positive correlation of BD with exchangeable Ca (r = 0.32), exchangeable Mg (r = 0.45), and available Fe (r = 0.71). The results above show that the relationship between BD and soil nutrient concentration is complex.

Conclusions

The use of soil conservation practices (LCC and RSM) significantly improved soil BD at the topsoil layer (0–10 cm) and subsoil layer (10–20 cm), enhancing soil porosity compared with applying the herbicide (control). In the first year, LCC and RSM did not affect available macronutrients (Ca, K, and Mg) and micronutrients (Cu, Fe, Zn, and Mn). However, soil nutrients (Ca, K, Fe, and Zn) increased greatly in the second and third years. The current study results suggest that farmers who cultivated fruit orchards in the VMD should use leguminous cover crops or mulch because these practices can mitigate soil compaction and degradation.

Supplemental Information