Moldboard Plowing with Direct Seeding Improves Soil Properties and Sustainable Productivity in Ratoon Rice Farmland in Southern China

: Several tillage and planting methods have been proposed to enhance the soil bulk density, biological community, and grain yield of rice. In this work, we present the impact of plowing methods with different rice crop establishment approaches, i.e., moldboard plowing with mechanical transplanting (MPMT), rotary tillage with mechanical transplanting (RTMT), moldboard plowing with direct seeding (MPDS), and rotary tillage with direct seeding (RTDS), on soil bulk density, microbial community, enzymatic activities, and grain yield of ratoon rice (RR). The results showed that MPDS improved soil bulk density in 0–30 cm depth in both years and both harvesting times (1H: 1 st harvest and 2H: 2 nd harvest). The results also showed that microbial community significantly improved under MPDS compared to the other treatments in both years and in 1H and 2H. Additionally, enzymatic activities showed a positive effect under MPDS in both years and in 1H and 2H. MPDS subsequently improved rice grain yield by 18.05% and 17.27% in 2017 (1H and 2H), and 14.86% and 18.64% in 2018 (1H and 2H), respectively. In conclusion, MPDS appears to be a more suitable approach to obtaining high soil eminence and health, as well as sustainable RR production.


Introduction
Rice (Oryza sativa L.) is one of the most important crops, accounting for 21% of worldwide consumption of calories [1]. China is the leading producer of rice worldwide, accounting for an average yearly rice production of 210 million metric tons, or 28% of global production (FAO 2014-2016 Food and Agriculture Organization of the United Nations) [2]. Main factors in the improvement in yield in preceding years include growing adoption of the use of commercial fertilizers, pesticides, machinery, and better-quality breeds [3,4]. Nevertheless, new farming methods have also led to damaging influences on the environment and increased farming cost [5,6]. Therefore, there is a need for methods that can help increase output while reducing ecological impacts and guaranteeing costeffectiveness.
Ratoon rice (RR) is the system whereby the subsequent harvest is realized from tillers initiating from the stubble of the formerly harvested crop (main crop). Relating to doubled-season rice (DR), the RR practice does not need extra work for reseeding the subsequent rice crop. RR is an old ricecropping practice, widely adopted since 1950 in China [7]. The RR-cropped area swiftly increased from 6667 ha in 1988 to 73,000 ha in 1994 in the Hubei Province due to governmental policy and change in farming methods [8,9]. Nevertheless, RR area rapidly decreased afterwards, with a remaining 7000 ha of RR in 2010 in the Hubei Province. Explanatory factors for the decrease in RR area include: (a) Dearth of appropriate rice breeds for RR practices, (b) reduced and/or less reliable yields compared to other rice practices, and (c) increased work force necessity in RR compared to middle-season rice [9][10][11]. New rice breeds with improved ratooning aptitude, combined with improved crop and water practices that permit mechanized harvesting of the main crop [9,11,12], have caused growers to readopt RR, resulting in an RR area of 153,000 ha in the Hubei Province in 2017.
Currently, the foremost crop planting method approach for the main-season rice in RR practice is old-style transplanted rice (TSR) [13]. Nevertheless, TSR uses high energy and labor inputs [14]. Direct seeding rice (DSR) has been recommended as another rice cultivation approach, as it lessens water use and labor necessities but increases system output and resource use effectiveness [15,16]. DSR is the method of growing rice from seeds sown directly in the field rather than by transplanting seedlings in the field [17]. Direct seeding as a rice crop establishment approach instead of transplanting has become common in Asia due to labor shortage and a development of direct seeding know-how [18]. Direct seeding rice-ratoon rice (DSR-RR) combines the transferred merits of DSR and RR, which expects to be a promising approach in central China and possibly other places in Asia. Dong et al. [19] observed that DSR-RR is another rice establishing approach to traditional transplanted ratooning rice (TTR-RR) in central China, and similar RR yields of DSR-RR and TTR-RR were observed.
Machine-driven transplanting of rice is the system of transplanting young rice seedlings-which have been raised on a tray in a nursery-using a paddy transplanter. Seedlings are transplanted at the optimal age (14-18 day old seedlings).
Tillage systems have direct impacts on rice crop establishment approaches. Different kinds of plowing methods have various plowing intensities and capacities to modify soil bulk density and biological indicators that influence the crop output and soil health [20]. Soil bulk density is significantly influenced by different tillage practices [21]. Tillage methods disrupt the natural state of the soil. Tillage damages the soil aggregate stability and pore continuity, resulting in soil dispersal, erosion, and surface hardening. Tillage also increases fuel consumption. Reduced tillage practices have positive impacts on soil health such as aggregate stability [22,23], as well as infiltration, hydraulic conductivity, and aeration. The practice of moldboard plowing needs special attention, since it is the most common system of primary plowing adopted in conventional agricultural soil management systems. Therefore, the general aim of the work was to assess the effects of plowing methods with different RR crop establishment approaches on soil bulk density, microbial community, and the grain yield on Stagnic Anthrosols of Yi-Yang in the Hunan province of China, in order to develop an innovative approach for soil management and sustainable production that will help achieve sufficient food production to feed the growing world population.  Figure 1 and 2. The predominant soil at the study site is classified as Stagnic Anthrosol according to the United States Department of Agriculture Soil Classification (USDA), and is developed from the Quaternary Red Earth [24]. The site had soil N, P, and K contents of 100 kg•ha -1 , 8.5 kg•ha -1 , and 112 kg•ha -1 , respectively, with pH values ranging between 6.5 and 7.5. In all, a total of 36 soil samples were collected from the entire experimental field, with each treatment field sampled three times at depth increments of 0-10, 10-20, and 20-30 cm. The basic soil conditions of the considered parameters at the depth of 0-30 cm are shown in Table 1.

Treatment and Experimental Design
The experiment included four treatments: (i) Moldboard plowing with mechanical transplanting (  The supply of water to the treatment fields was the ditch irrigation system. All treatment plots were uniformly applied with the same amount of urea (N content 46%) and formula fertilizer (25-11-15) before transplanting on April 22 and April 25 in 2017 and 2018, respectively. The basic formula fertilizer was applied at 15 kg•hm -2 , urea was applied at 10 kg•hm -2 at the tillering stage, and additional formula fertilizer at 15 kg•hm -2 at the booting stage. Early season rice was harvested on August 25 and August 18 in 2017 and 2018 respectively, and the ratoon rice received urea at 10 kg•hm -2 before the heading stage, followed by urea at 10 kg•hm -2 application after full heading. Ratoon rice was harvested on October 20 in both years. The low amount of fertilizer application in late seasons was mainly due to the short growth season in the ratoon season, less accumulated dry matter, and less required amount of fertilizer.

Measurement of Soil Bulk Density
Soil bulk density is used as an important index of differences in the soil structure and moisture retentive measurements [25], and was measured from 50 mm diameter cores at 0-10, 10-20, and 20-30 cm; soil cores were weighed wet, desiccated in an oven at 105 °C for 48 h, and measured once more to determine the soil moisture content and bulk density [26].

Measurement of Biological Activity
A total of 10 g of soil was soaked in 90 mL deionized water, and the sample was shaken for 10 minutes and left standing for 5 minutes. Supernatant solution of 1 mL was diluted at a temperature of 30 °C. The measurements were done in triplicate, and were used for determining soil bacteria, actinomycete, and fungal levels.
Examinations of viable microbial levels were done by the normal procedure of sequential dilution and the pour plating process. Counting of bacteria and fungi levels was undertaken in soil extract agar medium [27]. Actinomycetes levels was counted by Kenknight's agar medium method [28]. Bacteria, fungi, and actinomycetes colonies were enumerated and their number per gram of dry weight of soil [written as colony-forming units (cfu)] was calculated. An automated calorimetric procedure was used to determine the soil urease [29]. A volumetric procedure method was followed to determine the soil catalase [30]. A continuous assay for acid phosphatase by means of phenyl phosphate sodium colorimetric procedure was used to determine the soil phosphatase [31].

Measurement of Ratoon Rice Grain Yield
At the maturity stage, rice grain yield was measured from six-and five-unit sampling areas (1 m 2 ) from each treatment plot for 1H and 2H (1H: 1 st harvest and 2H: 2 nd harvest) respectively and then threshed by machine at 13.5% moisture content. A FUQIANG 4LZ-427 Full-Fill Grain Combine Harvester was used to harvest the remaining plot. Rice grain yield was then calculated in tons per hectare.

Statistical Analysis
Data from each of the two years was analyzed separately to understand the soil management methods and crop establishment approach on soil bulk density, microbial community, enzymatic activities, and rice yield. IBM SPSS 23.0 was used to analyze the data. Analysis of variance (ANOVA) was done to test whether soil bulk density, microbial community, enzymatic activities, and rice yield were significantly different. Mean values were compared using Duncan's Multiple Range Test (DMRT) at a 5% probability level.

Soil Bulk Density
Soil bulk densities in 2017 and 2018 were both significantly (p < 0.05) affected by tillage methods (Figure 4). The averages of soil bulk density calculated from 0-30 cm soil depth decreased significantly by 17

Ratoon Rice Grain Yield
As shown in Figure 5, different plowing and planting methods affected rice grain yield significantly. The highest grain yield was recorded by MPDS and RTDS in both years and harvesting times. However, there was no significant difference between MPDS and RTDS in 2017 (1H). Moreover, 2017 (2H), 2018 (1H), and 2018 (2H) recorded some levels of significant differences between MPDS and RTDS.

Soil Bulk Density
Plowing and planting methods conducted between 2017 and 2018 established that MPMT, RTMT, MPDS, and RTDS had an impact on soil bulk density, microbial indicators, and on ratoon rice grain yield. Almost all kinds of inverse tillage practices decrease soil bulk density [32]. The resultant reduction in soil bulk density ( Figure 4) under MPDS after the two-year study was largely ascribed to the high incorporation of preceding rice crop residues and fewer soil disturbances after plowing for the direct rice seed drill. In addition, moldboard plowing loosens soils, resulting in lower soil bulk density. The other tillage methods (e.g., RTMT and MPMT) resulted in higher soil bulk density, which may be due to the traffic for the secondary operation (during transplanting of seedlings).

Soil Biological Indicators
Soil biological parameters are acknowledged to show variations in soil management quicker and with a larger degree than either soil organic carbon or soil total nitrogen concentrations [33]. The tillage practice had a substantial impact on the soil biological indicators. Bacteria concentration, as shown in Table 2, was greater under MPDS, which may be due to the high build-up and incorporation of crop residue [34], as well as the impact of the tillage practice type and the accessible substrates which in turn influenced the soil bacteria profusion. However, the decrease in bacteria under MPMT and RTMT could be a result of less accumulation of rice straw on the soil after tillage application and the high soil compaction produced during transplanting of seedlings. Soil fungi, as shown in Table  2, were significantly greater under MPDS due to the putrefaction of organic matter ensuing from the high incorporation of rice crop residue and the augmented soil infiltration. Also, minimal disturbance of the soil resulted in an upsurge in soil fungi after the rice establishment.
However, MPMT and RTMT led to greater soil compaction after ratoon rice transplanting, which had a devastating impact on fungi profusion. Soil actinomycetes-shown in Table 2-under MPDS augmented significantly, which could be a result of greater accumulation of rice straw on the soil surface, as actinomycetes are characteristically greater in high organic matter soils.
Soil enzymes play a crucial role in soil nutrient cycling, which is affected by tillage practices [35,36]. Plowing and planting methods significantly affected the enzymatic activities of the soil. Soil catalase (Table 3) was greater under MPDS, which may be due to less/no soil disturbance after DSR establishment, leading to soil enhanced substrates. The augmented catalase activity in less-disturbed soils by MPDS before the rice seed establishment is in agreement with Jin et al. [37], who recorded greater catalase activity in shallow tillage practices. Catalase is an intracellular enzyme involved in the microbial breakdown in the cell and is a significant oxidoreductase that occurs in virtually all aerobic and facultative anaerobic microorganisms, shielding cellular processes and breakdowns from oxidative pressure by hydrogen peroxide [38,39]. MPDS positively impacted soil phosphatase (Table  3) compared to the other treatments. Urease and phosphatase enhanced under MPDS; this might be due to the incorporation of crop residue, leading to a greater decomposition of soil organic matter. The greater the microbial community, the greater the enhanced profusion of soil enzymes (e.g. catalase, urease, and phosphatase).

Ratoon Rice Grain Yield
Our two-year (2017 and 2018) study on tillage methods with different rice crop establishment approaches in Stagnic Anthrosols showed that MPDS significantly increased grain yield compared to the other treatments. The highest rice grain yield was recorded under the MPDS ( Figure 5). Higher yield was attributed to good crop condition and more availability of nutrients. In addition, less soil disturbance enhanced the soil bulk density for root proliferation to aid in soil nutrient and moisture accessibility. The result is in agreement with [40][41][42][43], as they reported greater yield in DSR compared with flooded transplanting. Work done by [14,[44][45][46] also recorded a greater grain yield in rice under direct seeding compared to that of transplanted rice. The lower yield recorded under RTMT and MPMT could be due to nutrient deficiency after the flooding of the field, resulting in nutrient leaching from the root zones during the transplanted seedlings' establishment, as well as less buildup of crop residue, leading to less putrefaction of soil organic matter.

Conclusion
This study examined the effects of different tillage methods and the ratoon rice establishment approach on soil properties and grain yield in Southern China. Results from the study showed that moldboard plowing with a direct seeding approach leads to enhancement in soil bulk density, microbial community, and enzymatic activities in the soil (0-30 cm), and results in sustainable increase in grain yield. In conclusion, moldboard plowing soil management with a direct RR seeding (MPDS) establishment approach appears to be a more suitable approach to obtaining high soil eminence and health, and sustainable ratoon rice (RR) production.