1. Introduction
Conservation tillage as a part of the solution to greenhouse gas mitigation and sustainable agriculture started its history to reduce soil erosion, improve soil organic carbon (SOC), and water storage [
1,
2,
3,
4]. Conservation tillage including no-tillage (NT) and reduced tillage (RT) has a substantial effect on soil properties and processes compared to conventional tillage [
5,
6,
7,
8,
9,
10]; these changes are beneficial for the delivery of ecosystem services and regulation of climate through carbon sequestration, greenhouse gas fluxes, and energy security [
1,
4]. Approximately 20% of the total cultivated area covering a few developed countries like the USA, Brazil, Argentina, Australia, and Canada are presently under conservation agriculture [
11]. However, acceptability and spread are limited in developing countries like India [
11]. Conservation agriculture, especially no-tillage faces a major challenge of adoption by farmers because of higher weed infestation in the rainy season, lower crop yield in the initial years, increasing soil surface hardness, lack of farm machinery for sowing in heavy residue fields, use of residues for fuel and livestock feed, and lack of skilled manpower [
11,
12,
13]. Thus, it is difficult to convince farmers of its potential benefits in developing countries like India [
14,
15]. Although farmers take up no-tillage and reduced tillage with the help of the state agriculture department and national research institutes, they revert to conventional tillage once the efforts of government agencies are withdrawn.
Conservation tillage, especially no-tillage, results in accumulation of crop residues on the soil surface and reduces nutrient distribution in the soil profile [
16]. Long-term no-tillage is reported to result in pronounced vertical stratification of soil nutrients, SOC, and other related soil properties [
1,
3,
5,
17,
18,
19,
20], which is otherwise distributed among soil layers in tilled soils [
21]. No-tillage and reduced tillage upon conversion to conventional tillage will have a huge impact on SOC storage, nutrient uptake, and nutrient availability [
10]. Further, the stored SOC might be a source of additional CO
2 emissions to the atmosphere. The imposition of intensive tillage practices in previously untilled soils may lead to a rapid breakdown of organic matter connected with deterioration in the physical quality of the soils.
Considering the above constraints related to the adoption of no-tillage, a feasible and more convenient option could be conventional tillage with residue retention. Previous research has clearly shown the significance of residue retention for no-tillage to show a better performance than other tillage practices [
22,
23]. On the other hand, if conventional tillage which is being practiced by the farmers is integrated with residue retention, it might help in enhanced decomposition and mineralization of residues in soil, the release of plant-available nutrients in the soil system, and preventing stratification of SOC and nutrient availability in soil layers. Further, this may help in unlocking the locked nutrients in soil organic matter and may be beneficial in improving soil nutrient availability and crop uptake [
24,
25,
26]. Tillage operations reduce soil compaction, improve aeration, and create a better environment for soil microorganisms [
27,
28].
Keeping the above in view, we propose conventional tillage with residue retention as an alternative approach to no-tillage. It was hypothesized that the SOC storage, soil nutrient availability, and crop growth would be better under conventional tillage with residue retention than no-tillage per se. However, there is little information available globally comparing no-tillage with conventional tillage under uniform residue retention in terms of soil properties, crop yield, and nutrient uptake, and more so when no-tillage plots are converted to conventional tillage. Thus, this study was conducted by converting half of the eight years old conservation tillage experiment to the conventional one with a similar level of residue return to compare the effect on SOC storage, soil nutrient availability, and nutrient uptake in soybean crops in the Vertisols of Central India.
The major objectives of the investigation were to assess the changes in (1) soil available nutrients and SOC storage in different soil layers and (2) crop yield upon reversal to conventional tillage from NT and RT practices with uniform residue levels.
2. Materials and Methods
2.1. Experimental Site
The study was conducted in an on-going eight-year-old field experiment on conservation tillage established in June 2008 at the research farm of ICAR-Central Institute of Agricultural Engineering, Bhopal, India. The study site is located at 23°15 ′N latitude and 77°25 ’E longitude, at 427 m above mean sea level, and is characterized by a humid subtropical climate with mild, dry winters and hot summers followed by a humid monsoon season. For other experimental details and initial soil properties of the site, reference is made to Singh et al. [
3]. The soil on site was deep Vertisol (Isohyperthermic Typic Haplustert) [
29] with clay texture (52% clay), bulk density of 1.34 Mg/m
3 at 0.27 g/g soil water content, and medium in SOC. The soil was neutral to alkaline in reaction (pH-7.85) with electrical conductivity of 0.3 ds/m and Ca
2+ as the dominant exchangeable cation in the Ap horizon.
2.2. Experimental Details
The on-going conservation tillage experiment included no-tillage (NT) and reduced tillage (RT) with 100% NPK (T1), 100% NPK + farmyard manure (FYM) at 1.0 Mg-carbon (C)/ha (T2), and 100% NPK + FYM at 2.0 Mg-C/ha (T3) in a soybean-wheat cropping system. The manure was applied a fortnight before soybean sowing every year. In NT plots there was direct sowing by a no-till-seed cum fertilizer drill and RT plots, one pass rotavator, and sowing by a seed cum fertilizer drill. All the treatments received recommended dose of nutrients for soybean (30:60:30) and wheat (100:60:30) N–P2O5–K2O kg/ha. In the rainy (kharif) season, soybean was sown during the last week of June or first week of July, depending upon the onset of monsoon at a row spacing of 35 cm. Soybean was harvested mostly in the last week of September to the first week of October. In the winter (rabi) season, wheat was sown during the last fortnight of November to the first week of December, depending on the optimum temperature for wheat sowing. Uniformly in all treatments, soybean residues, generated mainly from the leaf fall and above-ground dried stem (30%) after harvest, were retained on the soil surface. All the recommended crop management practices essential to maintain the crop growth was followed from time to time during the crop growth period. For weed control, glyphosate was applied at least a week before the sowing of soybean and wheat followed by pendimethalin as a pre-emergence spray at 2 days after sowing and use of a suitable post-emergence herbicide for soybean (pursuit/imazethapyr) and wheat (2, 4-D) at 20–24 days after sowing. Subsequently, one-hand weeding was done for escaped weeds a week after the post-emergence herbicide spray to keep the fields weed-free. Soybean was grown as a rainfed crop, while wheat was irrigated at critical growth stages. Wheat was sown after the application of pre-sowing irrigation of 6 cm in depth and three to four post-sowing irrigations depending on the availability of water. The wheat was harvested using a combine harvester, and 30% above-ground wheat residues were retained in all of the treatments.
To study the effect of tillage reversal at the same level of residue retention on soil properties and crop yield, the RT and NT plots were converted to conventional tillage in the year 2016 after eight years of NT or RT. The RT and NT main plots were divided into two parts each resulting in 4 tillage treatments, viz. (1) RT, (2) RT-CT (reduced tillage converted to conventional tillage), (3) NT, and (4) NT-CT (no-tillage converted to conventional tillage). The residue retention was uniform (30% above-ground wheat residue) in all treatments. The RT-CT and NT-CT main plots were ploughed with a cultivator (one pass) at least a week before sowing, rotavator (one pass) followed by sowing with a seed cum fertilizer drill. Therefore, the experimental design was split-plot with three replications. The main plot treatments were tillage at four levels (RT, RT-CT, NT, and NT-CT), and sub-plot treatments were three nutrient levels (100% NPK (T1), 100% NPK + FYM at 1.0 Mg C/ha (T2), and 100% NPK + FYM at 2.0 Mg-C/ha (T3)).
2.3. Soil and Crop Yield Sampling and Analysis
Soil samples (0–5 and 5–15 cm) were randomly collected and homogenized from different treatment plots after the harvest of soybean in the year of 2017. Visible litter and roots were removed before analyzing the soil samples. Part of the soil samples was air-dried at room temperature and gently passed through a 2 mm sieve and stored in an airtight container for further analysis of soil properties. Standard analytical procedures were followed to analyze the soil samples. The pH of the soil was determined by using a pH meter with a glass electrode (model, 420 A) using 1:2.5 soil-water suspensions [
30]. The soil organic carbon was estimated by the dichromate wet oxidation method of Walkley–Black [
31]. In this method, organic matter in the soil was oxidized with a mixture of potassium dichromate (K
2Cr
2O
7) and concentrated H
2SO
4 utilizing the heat of dilution of H
2SO
4. Unused K
2Cr
2O
7 was back-titrated with ferrous ammonium sulphate. Available nitrogen was estimated by an alkaline KMnO
4 method where the organic matter in the soil was oxidized with hot alkaline KMnO
4 solution. The ammonia (NH
3) evolved during oxidation was distilled and trapped in boric acid and mixed indicator solution. The amount of NH
3 trapped was estimated by titrating with standard acid [
32]. The available phosphorus content of the soil was extracted with 0.5 M NaHCO
3 (pH 8.5) [
33]. The content of phosphorus in the extract was determined using an ammonium molybdate and ascorbic acid reduction method by developing blue color, and the intensity of the blue color was measured using a spectrophotometer (model-CE2031) at 660 nm wavelength. Available potassium content in soil was extracted with neutral normal ammonium acetate and the content of potassium in the extract was estimated by a flame photometer (model, CL 378) [
30].
Soybean plant samples were collected from an area representing 100 × 100 cm
2 from each treatment after harvestable maturity. In the laboratory, the plant samples were first air-dried, separated into straw and seed, followed by oven drying at 70 °C until constant weight. The dry weight of seed and straw was recorded after oven drying for 48 h. The dried seed and straw samples were ground in an electric grinder to 0.250 mm in size. These samples were used for the analysis of N, P, and K concentration. Plant N content was determined in Flash 2000 NC elemental analyzer (based on a dry combustion method). Total P and K in the tissue were extracted after acid digestion with concentrated nitric acid and perchloric acid (9:4). The total P concentration in the digested samples was determined by the development of yellow color (ammonium molybdate vanadate mixture) and color intensity was measured in a spectrophotometer (model CE 2031) at 470 nm after setting the instrument to zero with blank as described by Jackson [
30]. The potassium content in the extract was estimated by a flame photometer (ELICO-model CL 378) [
34]. The crop nutrient uptake was computed using the following equation:
2.4. Statistical Analysis
Data sets were first analyzed for normal distribution and homogeneity of variance using the Shapiro–Wilk and the modified Levene test and then subjected to ANOVA followed by the Tukey’s test (α = 0.05). All the collected data were analyzed using two-way analysis of variance (ANOVA) using statistical software SPSS 11.5. For a significant f-value, the means were separated with least significant difference (LSD) with p < 0.05.
5. Conclusions
In conclusion, the study indicated that under comparable residue retention, conventional tillage can perform better in terms of crop performance and SOC storage than no-tillage. The findings are particularly vital in the context of poor adoption of no-tillage due to several constraints, such as high weed infestation, difficulty in seeding, and others, under no-tillage conditions. Thus, the alternative approach of conventional tillage with similar residue retention as under no-tillage could be a better strategy in improving SOC storage, crop performance, and overall soil sustainability. The results further showing that integrated nutrient management increasing the seed yield of soybean indicate the role of organic manures in maintaining soil productivity across different types of tillage. Tillage reversal showed a beneficial effect on crop yield and nutrient uptake in the NT-CT treatment with the application of 100% NPK + FYM at 2.0 Mg-C/ha. The magnitude of nutrient uptake was not consistently influenced by tillage treatments, but the application of organic manure relatively increased the nutrient uptake. Tillage reversal from the RT-CT and NT-CT treatments under uniform residue retention had no negative effect on soil properties. However, the addition of organic manure with inorganic fertilizer significantly increased the SOC and the related nutrient availability in all tillage treatments. The finding suggests that returning crop residue and conventional tillage with integrated nutrient management could be a viable approach to resolve the issue of non-adoption of no-tillage in different cropping systems, particularly in developing countries. Conventional or minimum tillage with residue retention may increase the potential for soil productivity and offset the limitations of no-till farming in different cropping systems and soil types. This particular approach can be particularly useful for developing countries characterized by small landholding situations and farmers’ hesitation in switching to no-tillage. Nevertheless, future research is required to understand the comparative performance of the proposed approach over no-tillage under different crop, soil, and climatic conditions.