1. Introduction
During the last decade, successive scientific reports dedicated to the assessment of soils worldwide have drawn attention to their accelerated degradation [
1,
2,
3]. Intensive agriculture has often been identified as a primary cause of the degradation. Although the agricultural intensification that occurred during the Green Revolution drastically increased food production, it also had a negative impact on many ecosystems, and may in the near future result in a decrease in the yield of cultivated plants [
4]. This problem is increasingly recognized in political spheres, and has led to several international initiatives that aim to increase soil protection and rehabilitation: for example, the Global Soil Partnership from FAO [
5,
6] and the Land Degradation Neutrality of the United Nations Convention to Combat Desertification [
7]. To limit the negative impact of intensive practices implemented during the Green Revolution, while maintaining food production, the adoption of practices inspired from agro-ecology is often put forward, such as management at the scale of the landscape, crop rotations reducing runoff and erosion, biological pest control, etc. [
8]. Agroecology covers various concepts [
9], but for the FAO, it consists of the application of ecological principles to the interactions between human beings and their environment: (i) to ensure the sustainable renewal of natural resources (water, soil, biodiversity, etc.) which are necessary for food production; and (ii) to reduce the use of non-renewable resources, e.g., the gradual elimination of the use of mineral fertilizers through the development of organic farming [
10,
11]. Other national and international institutions have also strongly promoted the use of organic amendments (OA) with the twin objectives of: (i) improving soil properties to maintain food production over the long term; and (ii) increasing global soil carbon stocks, in order to mitigate the effects of anthropogenic emissions of greenhouse gases [
12,
13]. If the implementation of such initiatives is to be widespread, the reticence that farming communities can have to the adoption of what scientists and politicians consider to be best management practices must also be accounted for. This reticence is particularly strong if farming communities do not clearly perceive short-term benefits for themselves [
14,
15,
16,
17].
In tropical regions, the problems related to soil degradation are particularly pronounced, as the soils of these regions are often less fertile, from both chemical and physical points of view [
18]. Soils in this region are more depleted in mineral elements than temperate soils, but they also have a lower carbon contents, resulting in lower structural stability, lower biological activity and infiltration rates, and overall, productivity. Moreover, climate change projections for tropical regions suggest changes in rainfall regimes with an increase in the number of storm rainfall events during the dry season and an increase in the number and duration of dry periods during the rainy season, all of which can directly affect soil productivity [
19,
20,
21]. Most farmers in these regions are smallholders, and their production depends directly on rainfall patterns, as their incomes are too low to cover the cost of installing and maintaining irrigation or drainage systems [
22]. Consequently, for these small holders, in order to control the soil water status, it is necessary to find economically affordable and technically feasible alternatives to irrigation and drainage practices.
In this specific context, the use of OA can be attractive, with limited expenses, and the farmers can recycle some of the by-products of their activities (manure, crop residues, etc.), thereby reducing reliance on chemical inputs [
23], as well as improving water management [
24]. A large number of experiments have been conducted worldwide to test for the benefits of the use of a diverse range of agricultural by-products on plant productivity [
25]. In China, cow or chicken manure has been shown to maintain or increase maize production relative to chemical fertilizer treatments [
26,
27]. In more arid zones, poultry manure applied to wheat and sorghum have also resulted in similar improvements [
28]. Compared with fresh manure, compost and vermicompost can be more convenient for the farmers. Composting is an aerobic bio-oxidative process that has a thermophilic phase, during which temperatures can rise to 60 °C, associated with a significant water loss; vermicomposting is also an aerobic bio-oxidative processes, but it is done at ambient temperature by anecic earthworms [
29]. In both processes, the fragmentation and decomposition induce dramatic changes to the physical and chemical properties of the original manure material. In particular, compared to manure, there is a reduced mass and volume to be transported to the field. Pathogens, parasites and weed seeds are also eliminated, and during the oxidative process, the mineral nutrients are accessible so they can be faster released into the soil solution [
30]. However, the majority of experiments that have been conducted to date on the benefits provided by OA to soils and plants have not tested the interactions with different rainfall regimes. Moreover, the improvement of physical soil properties and their consequences for plant water supply were rarely considered.
The aims of this study were twofold: (i) to investigate the impact of two types of OA (compost and vermicompost) combined with two different moisture regimes (imposing negligible and significant water stress) on maize productivity; and (ii) to determine whether the OA affected the physical properties, and whether this could explain the impact on maize productivity.
2. Material and Methods
To obtain maximum control of the soil characteristics and of the water regime, our experiment was conducted on irrigated soil columns mimicking cultivated soil characteristics, i.e., a thin topsoil where inputs are added above a larger subsoil without any inputs, and in which the majority of the root system develops.
2.1. Soil Column Preparation
Approximately 1000 kg of soil was collected from an agricultural field on the campus of the Faculty of Agriculture of the National University of Laos located at Nabong (18°7′25″ N & 102° 47′34″ E, 30 km northeast of Vientiane city). The soil of this field is an acidic sandy-loam Alfisol (USDA Soil Taxonomy) with a very low carbon and nutrients content (the first 10 cm were removed to ensure that there was no organic debris present, and soil material was collected to a depth of 30 cm). The samples were collected three days after a heavy rainfall event, meaning that the water content of the soil was slightly below field capacity (FC). FC corresponds to a soil previously saturated (all pores being filled with water after a rainfall event or irrigation), in which fast downward water movements under the effect of gravity have stopped, what is generally observed after two days of drainage. The water content measured immediately after soil collection was ≈ 0.16 g water g
−1 soil. The soil was then sieved (<30 mm) and kept in small plastic bags (20 kg each). The water content of all bags was controlled before sealing, and where necessary, it was adjusted to 0.16 g g
−1. In five randomly selected bags, around 100 g of soil was collected, mixed, dried at 105 °C for 2 days, crushed and sieved at 2 mm and analyzed according to the standard methods detailed in Pansu et al. [
31]. Particle size distribution was carried out using the pipette method after organic matter destruction using H
2O
2. Soil pH was measured in 1:2.5 soil:water and KCl suspensions with a glass electrode pH meter. Organic matter was determined using the Walkley and Black’s method. Total nitrogen was determined by Kjeldalh’s method, while available phosphorus was extracted by Bray II method. Exchangeable potassium was determined by extraction with neutral 1N NH
4OAc. The soil main characteristics are presented
Table 1.
The columns were designed to mimic the reality of soil preparation by farmers in the field, where fertilizers and OA are applied only to the surface layer. This means that much of the root system grows without contact with the OA. Thirty soil columns were prepared using PVC tubes (20 cm internal diameter and 60 cm height). At the bottom of the columns, a 1 cm metallic mesh covered with a mosquito net was fitted to ensure that there was no soil loss whilst allowing excess water, if any, to drain out. The subsoil of all soil columns was prepared by filling them with the sieved soil at a dry bulk density of 1.30 Mg m
−3, i.e., a 0.392 cm
3 g
−1 (approximately 50% porosity). This was achieved by adding 4.57 kg of the sieved soil and gently packing it to get a 10 cm-thick layer, and repeating this another four times to obtain soil columns of 50 cm in height (
Figure 1).
The topsoil (i.e., the last upper layer) consisted in 4.57 kg sieved soil at the same dry bulk density, to which one of the following was added:
- -
chemical fertilizers (control treatment that was unamended–C),
- -
chemical fertilizers + compost (compost treatment–P),
- -
chemical fertilizers + vermicompost (vermicompost treatment–V).
Ten columns were prepared per treatment.
The amount of chemical fertilizers followed the recommendations of the Pacific Seeds (Thai) Company Ltd., Saraburi, Thailand, i.e., 15-15-15 (N-P2O5-K2O) at 2.9 g plant−1 and 46-0-0 (N-P2O5-K2O) at 2.3 g plant−1. Micronutrients were also added and consisted in Photonic Premium plant nutrition (Merck Company, Darmstadt, Germany) containing S (13%), Fe (7.50%), Mn (8%), Cu (2.30%), Zn (4.50%), B (1.35%) & Mo (0.04%), and this was prepared at a concentration of 0.25 g L−1. Lime was also applied at 7 g plant−1. Before seedling planting, 100% of 15-15-15 and 50% of 46-0-0 fertilizer and 100% of lime were mixed to the 0–10 cm soil layer. At 21 days after planting (DAP), 50% of 46-0-0 was applied at the same time as irrigation. Every week, micronutrients were applied at 100 mL plant−1 until flowering.
2.2. Organic Amendments Preparation
The OA, i.e., compost and vermicompost, were prepared from a similar mixture of cow dung and coconut fiber (ratio 2:1 by volume). After two weeks of pre-composting, half of the material continued the composting process for 50 days, when the other half was vermicomposted for the same period, using African Nightcrawler (
Eudrilus eugeniae at a density of 100 g earthworms for 31 L organic amendments at 70% water). The chemical characteristics of compost (P) and vermicompost (V) were determined according to the methods presented in FCQAO [
32] (
Table 2).
The amount of compost or vermicompost per column was 63 g, which is equivalent to 20 t ha
−1, as found in several previously published experiments (for example [
33,
34]). At the end of the preparation, the 30 columns were installed under a shelter covered by a transparent plastic sheet.
2.3. Maize Transplanting
The maize (Zea mays) variety used in the experiment was the ‘Pacific 999′ hybrid from Pacific Seeds Ltd. (Thailand). It is commonly used by the farmers of the region, and is adapted to a wide range of hot, dry, tropical and sub-tropical conditions, according to the information provided by the company. The seeds with the largest diameter were germinated, and after three days, the largest shoots were planted in the soil columns. Two shoots were planted per column (3 cm below the soil surface) on 30 April 2016. At five DAP, only the most vigorous seedling was kept, while the other was removed.
2.4. Control of the Soil Matric Potential by Irrigation
Our objective was to test the impact of two types of OA on maize plants under two different water regimes, thereby inducing two levels of water stress: negligible and significant water stress. Plants need to take up water from the soil to meet evaporative demand. The water movement and availability are determined by the soil matric potential Ψm (expressed in hPa or pF), which is related to the pore size (
Figure S1)
with
d the equivalent pore diameter (μm) [
35]. In case of insufficient water supply, plants respond to that stress by closing their stomata, reducing their photosynthetic activity and biomass production [
36].
To control the level of plant water stress in each individual column, the soil matric potential was monitored by one micro-tensiometer located 10 cm below the soil surface (tensiometer reference: SMS 2030S3, SDEC company, Reignac-sur-Indre, France). The micro-tensiometers were installed during the column preparation and consisted in a porous ceramic pipe (12 mm in diameter, 32 mm in length, reference: SDEC850) connected by a 30 cm-long flexible tube to the water reservoir located approximately 20 cm above the soil level. The ceramic was laid on the surface of the so-called ‘subsoil’, before the so-called ‘topsoil’ (the last soil layer) was added (
Figure 1). At the top of the tensiometer water reservoir, an electronic sensor measuring negative pressures (air pressure sensor 26PCCFA6D Honeywell company, USA) was installed and connected to a data logger (CR1000, Campbell, UK) recording the soil matric potential (Ψm) every 10 min. After one day equilibration following their installation, all the tensiometers indicated a similar matric potential close to −200 hPa, confirming that all the columns were homogeneously prepared not only with a similar water content of 0.16 g g
−1, but also with similar matric potentials that were slightly above field capacity.
Our objective was to impose a negligible water stress to half of the columns and a significant stress to the other half, two levels which are generally observed in ‘wet’ and ‘dry’ soils, i.e., moisture contents below or above the field capacity. Thus, we imposed two moisture regime treatments, one in which the matric potential was maintained close to −150 hPa (pF ≈ 2.2), and another in which the matric potential was maintained close to −500 hPa (pF ≈ 2.7). They were termed the wet (W) and dry (D) treatments, respectively. In the wet treatment, pores up to 30 μm were filled with water and were regularly refilled (up to two times a day if necessary). The water located in those large pores was only slightly retained by capillary forces, thus allowing sufficiently fast water uptake (from soil to the roots and then to the shoots) to fulfill the plants’ demand. In the dry treatment, however, the pores between 30 and 2 μm were emptied of water, and only the pores of <2 μm contained water. In these small pores, water movements are slow and controlled by Ψm gradients (from highest to the lowest Ψm). Thus, in wet treatment it was expected that water transfer from soil to roots would be too slow to completely fulfill plants’ demand (
Figure S2).
In order to facilitate seedling development, for the first eight days of the incubation, Ψm was maintained at >−150 hPa by irrigating each of the 30 soil columns with 200 mL water; irrigation was done by gently pouring irrigation water on filter paper protecting the soil surface. From the 9th DAP forwards, Ψm was checked twice daily (early morning and late afternoon) and the soils irrigated when necessary. If the threshold Ψm was reached in a column, then it was irrigated.
Figure S3 presents as an example the Ψm recorded between 6 and 20 DAP for one wet (left) and dry (right) treatment. It shows a temporary Ψm increase during or immediately after irrigation, which corresponded to the drainage by gravity of the irrigation water through the largest pores, until a new equilibrium was reached at the threshold Ψm. The Ψm values, recorded in each columns during the 70 days of the experiment, are presented in
Figure S4. The data logger was also connected to a meteorological station installed nearby the shelter, so that the air temperature, air relative humidity and solar radiation were recorded at the same frequency as the matric potential (
Figure S5).
2.5. Plant and Soil Characteristics
During the period of plant development (until 52 DAP), the maximum height, the number of leaves and the length of each leaf (measured by hand using a ruler) of each plant were recorded every morning. The total length of a maize plant was calculated by adding the length of all existing leaves.
The experiment was stopped at 70 DAP, when we observed roots reaching the bottom of some of the columns. The plants were harvested and separated into stems, leaves, flowers and corn ears, weighed and then dried at 65 °C for 48 h and weighed again.
In each 10 cm layer (i.e., 0–10, 10–20, 20–30, 30–40, 40–50 and 50–60 cm) of each column, seven undisturbed soil cylinders (100 cm3, 5 cm height & diameter) were collected. Five of these cylinders were used to determine the root biomass (g), soil bulk density (g cm−3) and specific pore volume as follows. Immediately after sampling, the cylinders were weighed, the soil inside the cylinders was fragmented and the roots it contained were collected by hand using tweezers. The roots and the soil were dried for 48 h at 65 °C and 105 °C, respectively.
The two remaining undisturbed soil cylinders were used to establish water retention curves at −10, −33, −100, −330, −1000 and −16,000 hPa (i.e., pF = 1, 1.5, 2, 2.5, 3, 3.5, 4.2). In each layer, after the cylinders were sampled, the nodal roots were taken out of the remaining soil and counted.
2.6. Statistical Analyses
Table 3 shows the six elementary treatments that correspond to two types of organic amendments plus one unamemded control crossed with two levels of water stress, and with five replicates for each elementary treatment. Data were analyzed using R Studio “R version 3.3.0” (R Core Team, 2013). ANOVA, with Tukey’s post hoc test (
p ≤ 0.05), was used to compare the six treatments.
5. Conclusions
Our results, obtained on soil columns mimicking cultivated soil characteristics, show that the use of OA in poor tropical soils could be particularly beneficial for farmers in the context of climate change and in the absence of irrigation infrastructure, as maize biomass and yield were increased in both moisture conditions. The improvement was particularly striking in terms of yield, but is in agreement with other experiments conducted on maize grown in the fields [
54]. In continuously dry conditions, the biomass and yield of treated columns were similar to those of the control columns in wet conditions, suggesting that the organic matter amendments helped overcome limitations imposed by the water stress. However, there were no clear differences induced by the two types of OA tested (compost and vermicompost), suggesting that in this situation at least, the extra time and effort necessary for the production of vermicompost is not worthwhile. Compared to the increase in yield, the biomass increase was less impressive but is a valuable improvement, as the additional biomass production can contribute to produce the next compost batch. By recycling the organic wastes, the farmers could limit their expenses and make the best use of available water and mineral nutrients.
The data suggest that the improvement in plant characteristics did not result from increased water storage in the OA-treated soils, but rather from better access to water, resulting in faster root development. This increased the soil volume that was colonized by the plant, improved water supply and reduced water stress during the important stages of plant development. This better access to water seems possible when the soil is maintained wet or dry, meaning that farmers can benefit from OA under a large range of rainfall patterns.
To generalize our results, they need to be confirmed in different conditions. It would be necessary to study soils with different textures, as the addition of OA is likely to have differential impacts on the pore size distribution, as well as on soil mechanical resistance to penetration. Furthermore, testing plants with a different photosynthetic pathway and different root morphology would also be useful. Maize uses the C
4 photosynthetic pathway, which allows for a more efficient use of water than in plants that use the C
3 pathway [
69]. One might therefore expect the impact of water stresses on C
3 plants (rice, wheat, soybean, etc.) to be different. Finally, plants with different root systems (branching, lateral growth, etc.) should also be tested.
For these additional experiments, using soil columns similar to which we did (a thin topsoil where inputs are added above a larger subsoil without any inputs but with a drastic control of soil physical characteristics), with real-time control of the soil matric potential and real-time monitoring of the plant water stress, would be an easy and efficient way to test the impact of different rainfall patterns on various soils, OA and plants types, and provide relevant recommendations allowing farmers to get the highest benefit from the time spent on preparing and spreading OA in their fields.